Summary

SUMMARY

In October 2001 the Highways Agency appointed WSP Group to undertake an environmental study to examine potential options to improve the A417 between Cowley Roundabout and the end of the Brockworth bypass, Gloucestershire. The aims were to identify the environmental constraints within a study area centred on the existing route, prepare a constraints map and identify if options were available to improve this section of road which did not have unacceptable environmental consequences. In addition, consultation with environmental statutory organisations and "other" local stakeholders was to be undertaken and a public open day organised to provide local people and interested parties an opportunity to discuss the results of the study process.

Background

This section of the A417 has a poor safety record and suffers from severe congestion. This section of the road is the only remaining single carriageway section on the A417/A419 route between the M4 (Jnc.15) and M5 (Jnc.11a). Traffic volumes are currently between approximately 28,000 and 31,000 vehicles per day of which approximately 12-14% are Heavy Goods Vehicles. Queues occur regularly at peak periods.

There are steep hills at Nettleton and Crickley Hill and slow moving Heavy Goods Vehicles (HGV) compound queuing issues. In places the road standard is generally poor with tight bends and narrow lanes. Vehicle shunts occur regularly at Air Balloon and there are accident clusters at Nettleton Bottom, Birdlip, Air Balloon and on Crickley Hill.

Three small scale improvement schemes have been completed recently to improve safety at the Golden Heart Public House, Air Balloon Roundabout and Birdlip junction. A number of additional small scale improvements are also planned in the next few years. However, it is recognised that a longer term solution is required.

Constraints

Environmental and engineering constraints have been identified and translated onto a constraints plan. Information was gathered from a wide range of organisations, from earlier studies and from limited walk-over surveys.

The entire study area falls within the Cotswolds Area of Outstanding Natural Beauty (AONB).

Option Development

Possible improvement options were developed using a staged approach in discussion with environmental statutory bodies, Local Authorities and other Stakeholders. A number of options were initially identified which have been systematically reduced on assessment of data collected or on the advice of those consulted.

As a result of this process three options remain. Two of these are on-surface options that only differ in their treatment of Crickley Hill and the Air Balloon Roundabout. The third is a tunnel option which would pass between Nettleton Bottom and the foot of Crickley Hill. The options are summarised below:

Option Assessment

Engineering Comparison

All three options are buildable, although Options 2 and 3 would cause less disruption during construction to vehicle flows up Crickley Hill. In addition remodelling of the Air Balloon roundabout under Options 1 and 2 would cause disruption while the schemes were built.

Options 1 and 2 would retain steep gradients on Crickley Hill (in the order of 10%). Option 3, the tunnel, would have an average gradient of 4.4%.

Option 1 would require departures from standards, however, there are safety concerns with the signalised junction which cannot be overcome by careful detailed design and for this reason Option 1 should be discarded. Departures from standards would be required on Option 2 but safety would not be compromised. Option 3 would not require departure from standards.

Temporary contractors camps would be required for all three options, although these would be potentially larger for Option 3. Option 1 would take less than 2 years to construct, Option 2 would take approximately 2.5 years, and Option 3, 3 years or more (depending on hydraulic conditions encountered).

Option 2 would have a severe impact on properties.

Economic Comparison

The preliminary economic assessment has identified that all options have positive Net Present Values (NPV's) at high growth. Options 1 and 2 have positive NPV's at low growth whereas Option 3 is negative. All three options contribute time savings and reduction in accidents.

Option 1 would be the least expensive option at approximately £40 million, but has the least benefits and would only provide a short to medium term improvement for relief of congestion. Option 2 represents best value for money as it has the highest NPV's and Benefit/Cost Ratio. This option would cost in the region of £60 million. Option 3, at a cost of approximately £200 million, has the greatest benefits but due to the anticipated tunnelling costs is also the most expensive.

Environmental Comparison

An environmental appraisal of the three options has been undertaken using the Guidance on the Methodology for Multi-Modal Studies (GOMMMS) approach.

In terms of landscape, Option 1 would have a moderate adverse impact on the AONB and landscape character, whilst both Options 2 and 3 would have large adverse impacts, the latter due to the link roads and portals particularly at the northern end.

Cultural and archaeological resources are many and varied within the study area. Options 1 and 3 would have slight adverse impacts whilst Option 2 would have a moderate adverse impact, mainly due to its affect on the context of Crickley Hill.

In terms of ecology, the two surface options largely avoid nationally and locally designated sites. However, dewatering for the tunnel may potentially severely affect Bushley Muzzard SSSI which relies on spring water. It is possible that mitigation measures could be designed to minimise impact. However, given the uncertainty at this time, there remains a high risk of permanent damage and a very serious adverse impact has been predicted for Option 3.

Water in the underlying Major Aquifer is a major issue in the study area and its presence has had a major impact on previous road schemes in the study area. Mitigation measures for Options 1 and 2 could reduce the impact on the water regime such that the long term impact on the ground and surface water regime is predicted to be slight adverse. However, there is considerably more risk about the potential impact of the tunnel and the Environment Agency have expressed serious concerns. The tunnel option, therefore, has been awarded a large adverse impact score.

In terms of local air quality, Options 1and 2 would increase localised air pollution although along the whole route there would be an overall decrease. Under Option 3 overall local air quality would improve, although the pollutants would be concentrated at the tunnel portals. Both Options 1 and 2 would also lead to an increase in the release of greenhouse gasses, whilst the tunnel option would lead to a decrease.

Up to 29 properties would experience an increase in noise levels under option 1 and 4 properties would experience a decrease in noise levels. Under Option 2, up to 32 properties would experience an increase and 6 a decrease in noise levels. Under Option 3, 12 would experience an increase in noise and 37 a decrease.

Public Open Day

A Public Open Day was held in Birdlip Village Hall on the 15th and 16th of February 2002. It was attended by approximately 200 people, including Local Councillors, MP's and local residents. Members of the consultants team and the Highway Agency were present to answer questions.

The issues raised and discussed related to:


Summary

1 introduction

1 INTRODUCTION

1.1 The safety and congestion issues on the A417 south of Cheltenham have long been recognised. The road is part of the core trunk road network and the remaining single carriageway section between Cowley (Nettleton) Roundabout and the bottom of Crickley Hill urgently requires improvement. It comprises the only single carriageway section on the route between the M5 at Junction 11a and the M4 at Junction 15.

1.2 Small scale improvements have been completed along the route since the opening of the Birdlip Bypass in 1995. Further small scale schemes are being planned over the next few years by the Highways Agency and the network maintenance contractor, RMS. These would provide short term amelioration of safety and congestion issues. However, a long-term solution is required for this section of the road.

1.3 Discussions with Local Authorities (County, Borough, District and Parish Councils) have resulted in all agreeing that the current situation is limiting economic growth in the area and resulting in significant congestion and safety problems along the route. They also agree that it is unacceptable not to improve the A417 within the study area.

1.4 Options developed in the past for a long-term road based solution have all resulted in the identification of unacceptable environmental impacts and so have not been taken forward.

Terms of Reference

1.5 In October 2001, the Highways Agency appointed WSP Group to undertake a study of the environmental and engineering issues related to the A417 between Cowley Roundabout and the bottom of Crickley Hill. The Terms of Reference were to consider if the route could be improved without unacceptable environmental impacts and to hold discussions with statutory bodies to ascertain their views on whether an option(s) could be taken forward for more detailed assessment. The Terms of Reference are given in Table 1-1.

1.6 The work undertaken comprises a pre-investment study. As such it utilises the "Plan" elements of the Guidance on Multi-Modal Methodology Studies (GOMMMS) Assessment and Stage 1 methodologies as given in the Design Manual for Roads and Bridges (DMRB).

Table 1-1 Study Terms of Reference
Terms of Reference. Cowley to Air Balloon Improvement. Environmental Study
  • The study should be conducted in close liaison with statutory consultees and should produce a comprehensive environmental constraints plan and environmental data report.
  • This should be used as a basis for considering whether environmentally acceptable improvements could be made to the trunk road to address the safety and congestion problems that currently exist on the A417.
  • Preliminary proposals for improvement that emerge should be discussed and agreed with statutory consultees.
  • Whether or not a tunnel is an appropriate solution should be given consideration.
  • The consultants should agree with the statutory consultees whether or not it will be necessary to carry out additional surveys on a particular issue and if so, a survey brief should be agreed with them. The consultant should advise the Agency when such surveys should be carried out, even when they could be outside the timescale for this study.
  • The consultants should take into account the views of local residents, local groups, representative bodies and other interested organisations.
  • The consultants should carry out an appropriate environmental and economic appraisal in accordance with GOMMMS and summarise the results in an Appraisal Summary Table (AST).
  • The final report should include comments made on the issues, so that Ministers will have sufficient information to be able to make informed decisions on whether an improvement scheme(s) could go forward and be added to the TPI programme.

Study Area

1.7 The study area is shown on Figure 1.1. The area is based on the line of the A417 from a point adjacent to the A46 junction in the west (at NGR SO 9100 1634) and a point near Highgate Farm, Elkstone in the east (at SO 9546 1264). The study area extends for 1 km in all directions from the road and covers approximately 14.5 km². The study area was extended to the west of the escarpment to cover possible offline corridors. The area is within the Gloucestershire civil parishes of Badgeworth and Great Witcombe in Tewkesbury Borough and Brimpsfield, Coberley, Cowley and Elkstone in Cotswold District. The entire area is within the Cotswold Area of Outstanding Natural Beauty and it also overlaps the Brimpsfield Conservation area.


1 introduction

2 traffic, congestion and safety issues

2 TRAFFIC, CONGESTION AND SAFETY ISSUES

2.1 The section of the A417 from Cowley Roundabout to the Brockworth Bypass, known as the "Missing Link", suffers from severe congestion and has a poor accident record. These problems are likely to get worse if, as predicted, future traffic levels increase. This section of road is single carriageway and its layout is below the acceptable standard for a road of its type and with its high traffic flows (see Figure 2.1).

2.2 The Personal Injury Accident record for this section is provided in Table 2-1 below. The figures refer to the number of accidents, rather than the number of casualties. Anecdotal evidence also suggests that there are a large number of near misses and minor damage only accidents along this section that do not find their way into official statistics. More recently, in May 2002, there was a fatal accident on Crickley Hill involving a car and a light goods vehicle.

Table 2-1 Personal Injury Accidents
Year Number of Personal Injury Accidents (NOT casualties)
Slight Serious Fatal Total
1996 7 2 1 10
1997 8 1 0 9
1998 13 1 1 15
1999 11 2 0 13
2000 13 5 1 19
Total 52 11 3 66

2.3 The national trunk road speed limit applies along the route with the exception of a 40 mph zone through Nettleton Bottom.

2.4 Congestion along the route (especially at Cowley and Air Balloon roundabouts) results in 'rat-running' on local roads and through villages including Birdlip, Elkstone and Syde. Local Parish Councils have also informed that Leckhampton is affected when problems occur on Crickley Hill and the Air Balloon roundabout.

2.5 A number of small scale improvements have recently been implemented along the route and these are described in the sections below. Although these schemes have provided short-term improvements all parties acknowledge that a long-term solution is required.

Cowley Roundabout

2.6 At Cowley Roundabout the dual carriageway terminates and the road becomes a single carriageway for the next 6km. This creates a traffic bottleneck resulting in severe peak hours congestion with traffic queues extending several kilometres to the south-east. There are approximately 28,000 vehicles daily using this section of the road, with approximately 12% being HGVs.

Nettleton Bottom

2.7 Nettleton is in a pronounced and localised dip in the trunk road, where forward visibility is not as good as it should be for this standard of road.

2.8 Vehicles tend to develop high downhill speeds, well in excess of the 40 mph speed limit, in an effort to build up speeds before the uphill section. The steep gradients entering the dip (approximately 10%) make vehicle control under braking more difficult.

2.9 The access to the Golden Heart Public House is located at the lowest point of the dip making right turns into and out of the car park a hazardous manoeuvre.

2.10 The high traffic volumes and speeds along this section create a high degree of severance and accessibility problems.

2.11 Recent safety improvements at Nettleton Bottom have been completed which include localised widening of the existing carriageway, a right turn lane into the Golden Heart Public House car park, traffic calming measures and the implementation of a 40 mph speed limit.

Stockwell Junction

2.12 Vehicles turning into and out of this junction can hold up through traffic on the A417. The volumes of traffic using this junction are low but there was a serious personal injury accident at this location in 1999.

A417/B4070 Birdlip Junction

2.13 Birdlip Junction is a major/minor junction where the B4070 terminates with the A417. The A417 is single lane dualled through the junction with a central island and a dedicated right turn lane. The majority of the accidents at the junction involve right-turning traffic out of the B4070 and a fatal personal injury accident occurred in August 2000.

2.14 Right-turning traffic from the A417 has difficulty crossing the northbound carriageway due to high traffic volumes and high vehicle speeds. There is also potential for vehicles waiting to turn right to queue back onto the running carriageway of the A417, obstructing through traffic.

2.15 There is also a problem with traffic turning right out of the B4070 local road from Birdlip Village. Visibility is restricted by the existing vertical alignment and by queuing right turning traffic. Drivers turning right out of the junction also misjudge speeds of vehicles coming up Birdlip Hill.

2.16 In order to improve the operation of the junction, the layout has recently been improved. A longer right turn lane on the A417, reflective marker posts, differential surfacing and improved road markings have been installed.

Shab Hill Lane

2.17 Vehicles turning into and out of Shab Hill Lane can hold up trunk road traffic. The turning vehicles are vulnerable to tail end shunts since they have to contend with high vehicle flows and high vehicle speeds.

Air Balloon Roundabout

2.18 Congestion often occurs at the junction during peak hours, with southbound traffic queues regularly extending for several hundred metres down Crickley Hill.

2.19 Vehicles have difficulty turning into the Air Balloon Public House and Crickley Hill private residence because of the fast moving trunk road traffic.

2.20 The horizontal alignment of the existing roundabout produces tight vehicle turning movements, particularly for vehicles travelling along the A417 from Cirencester and heading towards Gloucester. The turning circle for large vehicles can result in conflict with other vehicles approaching the roundabout from the Gloucester direction in the outside lane.

2.21 Vehicles continuing along the A417 heading to Cirencester have to undertake an approximate 300 degree turning manoeuvre to travel from Crickley Hill towards the Birdlip Bypass.

2.22 Recent improvements at the junction have extended the climbing lane and provided two right turn lanes on Birdlip bypass. The roundabout has also been widened which has increased the existing capacity and helped increase traffic flow. Accident figures show that there are no serious personal injury accidents at the junction with congestion being the major concern.

Grove Farm and Cold Slad Accesses

2.23 Both the Grove Farm and Cold Slad junctions have dangerous direct accesses onto the A417. Work is currently being undertaken to investigate whether it might be possible to carry out safety improvements at both junctions.

2.24 Contributing factors that make these junctions dangerous include high traffic volumes, high vehicle speeds (some exceeding the 60 mph speed limit) and the steep gradient of Crickley Hill (approximately 10%). Cold Slad also sits on the inside of the bend and has restricted visibility.

2.25 Right turning traffic from Gloucester into Grove Farm access wait in the fast outside lane to turn across the downhill lane. These turning vehicles risk shunts from behind and driver stress is increased whilst waiting for a gap in the high speed downhill traffic.

2.26 Articulated heavy goods vehicles from Air Balloon sometimes move over the central white line, into the line of onward uphill traffic, in order to negotiate the tight left turn into Grove Farm. A number of impatient drivers travelling downhill have also been reported to move into the uphill lane when overtaking vehicles slowing down to turn left into Grove Farm. Vehicles waiting to turn right into Cold Slad obstruct all downhill traffic causing congestion and the risk of shunt type accidents. Right turning vehicles out of both Cold Slad and Grove Farm have to cross lanes of fast moving traffic.

2.27 The historical accident record does not indicate a major problem but increased flows and evidence of many reports of near misses have given rise to the perception of the junction being dangerous. The high speeds especially of HGVs on the downhill (10%) approach to the access causes particular stress for drivers making left turns into Grove Farm access.

Crickley Hill

2.28 Crickley Hill has two lanes up (one being a climbing lane) and one lane down. The maximum gradient of the road is approximately 10% and there is no central reservation or safety barrier. The road layout is well below the acceptable standard for this type of road and the traffic volumes (nearly 31,000 vehicles per day and 14% HGV) are high.

2.29 Slow moving vehicles regularly restrict uphill and downhill traffic. This leads to driver frustration, with some drivers making the dangerous manoeuvre of overtaking slow moving vehicles by the crossing the solid white central lines.

2.30 Due to the steep gradient, vehicle brake failure is one of the main causes of accidents. HGV drivers travelling up Crickley Hill have also been seen to struggle to maintain control their vehicles.

2.31 Broken down vehicles are another consequence of the steep gradient. These cause severe congestion and pose a significant safety hazard.


2 traffic, congestion and safety issues

3 geotechnical issues

3 GEOTECHNICAL ISSUES

Background Information on Soils and Geology

3.1 The geology of the study area has been obtained from the British Geological Survey of Britain Sheet 234 (Solid and Drift).

3.2 Excerpts from the relevant 1:10560 geological mapping, Sheet SO 91 SW, have also been studied as part of this assessment.

3.3 The British Geological Survey borehole database has been consulted.

3.4 The records of Gloucester County Council were searched regarding this section of the A417. Some data was recovered regarding the improvements (provision of the third lane below the escarpment) carried out during the 1960s. However, the drawings are not dated.

3.5 A number of records were examined from the Local Studies Section of the Gloucester Library.

Previous Studies

3.6 Reports from previous studies in the area have been reviewed as part of this study. The reports studied include the following:

Topography and Outline Geomorphology

3.7 Within the study area the Cotswold Escarpment runs approximately SSW to NNE. To the east of the escarpment the land is formed by a plateau at an approximate elevation of between 240 and 290m AOD, whereas, to the west, the land drops away to an approximate elevation of 100m AOD. The lower slopes are generally at a shallower angle, ranging from 6º to 16º, while the higher slopes below the escarpment itself rise at between 30º to 45º.

3.8 In many places the slopes below the escarpment can be seen to exhibit evidence of past ground movements with features such as soil lobes, back scars and springs dominating. Many areas are hummocky in appearance indicating the possible existence of secondary landslipping masking the deeper-seated historic failures.

Geology

3.9 The study route commences at the top of the Cotswold Escarpment where the Inferior Oolite Group outcrops and descends to the Vale of Gloucester within the outcrop of the Lower Lias. In the south-east part of the study area further landslip material is encountered together with some deposits of Fullers Earth.

3.10 Almost all of the route below the escarpment to is within landslipped material (colluvium). Over this section of the study area the Upper and Middle Lias Formations, together with the upper horizons of the Lower Lias, are mantled by the colluvial deposits.

3.11 For details of the geological structure and information about soil types, see Appendix A.


3 geotechnical issues

4 environmental constraints

4 ENVIRONMENTAL CONSTRAINTS

4.1 Environmental background data were collected and collated for the study area in a desktop review exercise. Data was collected from a wide range of statutory and non-statutory sources and from previous studies undertaken for the Highways Agency on parts of the route. The sources contacted and a description of the data collected are given in Appendix B. In addition, a report containing all of the baseline information was prepared in November 2001 (the Environmental Data Report).

4.2 The data collected was used to prepare a constraints plan shown on Figure 4.1.

4.3 Several preliminary surveys have been undertaken by specialists within the team to supplement desktop data collected.

4.4 A summary of the environmental constraints is given below. Further details are available in Appendix B and in the Environmental Data Report.

Ecology and Nature Conservation

4.5 The study contains four sites designated at a National Level. These are:

4.6 At a Regional and Local level, Ullen Wood, Cowley Woods and Wards Woods are designated as Key Wildlife Sites. Ancient woodlands occur within the study area and are shown on Figure 4.1. These include Ullen Wood and Witcombe Wood.

4.7 Records show that a range of rare, nationally declining or protected species occur within the area. These include the Grey Partridge, Wood Warbler, Slow Worms and Adders. These are described in more detail in Appendix B.

4.8 The study area contains a range of habitat types which contribute to the nature of the Cotswolds. Field surveys have been undertaken as part of previous studies. These are described in Appendix B.

Landscape and Visual Impact

4.9 The entire study area falls within the Cotswolds Area of Outstanding Natural Beauty (AONB). Parts of the northern section of the study area also fall within the Cheltenham and Gloucester Green Belt. These are shown on Figure 4.1. There are no National Parks within the study area.

4.10 The National Trust own land at Crickley Hill. This is open to, and well used by, the public and is designated as a Country Park.

4.11 There are a number of Tree Preservation Orders (TPO's) concentrated in the northern section of the study area and these are shown in Figure 4.1.

4.12 Three areas of common land exist within the study area. These are along the western edge of Crickley Hill County Park, either side of Barrow Wake viewing area and at Birdlip. The latter includes Buckle, Cranham and Buckholt Woods. These are shown on Figure 4.1.

4.13 The most important feature of the study area is the escarpment slope which defines the eastern edge of the River Severn valley and the north-western boundary of the Cotswolds. From the top of the escarpment the plateau slopes gently to the east and is incised with a number of steep sided valleys, some containing spring fed rivers. The largest of these is the Churn valley in the north-eastern part of the study area.

4.14 Woodland cover is mainly restricted to the escarpment slope and the valleys, the plateau being largely open allowing long distance views.

4.15 There are no major settlements in the study area, the small villages of Birdlip and Brimpsfield are the largest settlements present. Elsewhere individual properties and small clusters of properties are scattered throughout the study area.

4.16 The landscape within the study area can be divided into three character zones which are discussed in detail in Appendix B. The three character zones are:

Cultural Assets and Archaeology

4.17 Over 400 important sites are listed on the Gloucestershire Sites and Monuments Register for the study area. Numerous archaeological surveys have been undertaken in the area as a result of previous developments. These surveys indicate a high potential for archaeological finds.

4.18 Six Scheduled monuments occur within the study area. These are:

4.19 A total of 31 listed buildings exist within the study area. These are detailed in Appendix B and are shown on Figure 4.1.

Land-Use and Material Assets

4.20 The majority of land within the study area is used for farming. The land primarily comprises Grade 3 with Grade 4 on steep slopes. Small areas of Grades 2 and 3a are present at the foot of the escarpment and around Birdlip.

4.21 There are two long distance footpaths, the Cotswold Way and Gloucestershire Way which cross the study area. These form part of a network of footpaths and bridleways across the study area. These are shown on Figure 4.1.

4.22 Two public houses lie adjacent to the existing route, the Golden Heart and the Air Balloon. The Royal George Hotel and Conference Centre is located in Birdlip and village halls exist in Brimpsfield and Birdlip.

4.23 Public Open Space occurs at the picnic area on Barrow Wake and at Crickley Hill Country Park.

Water Resources

4.24 The study area falls into the catchment of the rivers Thames and Severn. The eastern area drains into the River Churn which is largely spring fed in its upper reaches. The river suffers from low flow and has dried out in the past, most notably in 1995. A low flow alleviation plan for the river is currently in action with the Environment Agency. The catchment also provides head waters to the River Frome.

4.25 Public water supplies are drawn from the River Churn (the Baunton source) and are used to supply the Gloucestershire region including Cirencester. Water is also supplied to Swindon.

4.26 The slopes of the escarpment contain a large number of springs which cross the Vale of Gloucester towards the River Severn.

4.27 The chemical and biological quality of the River Churn is very good. Hatherley Brook and Horsbere Brook, both within the vale, have good to very good chemical quality but only fair biological quality.

4.28 The limestones comprising the upper strata in the escarpment contain a major aquifer and fall within a groundwater protection zone. Major drinking water abstractions lie within 4km of the current route.

4.29 The hydrological constraints are shown in Figure 4.2.

Air Quality

4.30 The study area is predominantly rural with no major polluting industries and so air quality is expected to be good. Concern over roadside air quality has been expressed by local residents, and was raised in Cotswold District Councils (CDC) Stage 1 Air Quality review due to the volumes of traffic using the A417. However, modelling carried out in CDC's Stage 2 assessment showed that air quality standards are not currently being exceeded and would not be exceeded in 2005 (the date by which Local Authorities have to show compliance with set Air Quality Standards). No further assessment was, therefore, warranted.

Noise and Vibration

4.31 Since the study area is predominantly rural the existing A417 is the dominant noise source in the area. A number of noise complaints have been received by Local Authorities relating to traffic flows along the A417. Many of these related to the opening of the Cirencester Bypass which terminates at Cowley Roundabout and are potentially the result of a concrete surface to this section of road. Noise was also cited as a major concern to the project team by the residents of Cold Slad.


4 environmental constraints

5 option development

5 OPTION DEVELOPMENT

5.1 Prior to assessment a process of option development was undertaken. This comprised:

Initial Reviews

5.2 The option development process began with two reviews. These were;

Environmental and Physical Constraints

5.3 A review of the information provided in Chapter 4 was undertaken to identify areas/sites that should be considered invaluable areas, which should be avoided wherever possible. The criteria used are given in Table 5-1. In addition to these environmental issues, the schemes would need to meet engineering and economic criteria.

5.4 In particular, the schemes should be:

Table 5-1 Criteria used in Option Development
1. Sites ring fenced and considered inalienable
  • SSSI's
  • The Scrubs Nature Reserve
  • Scheduled Monuments
  • Conservation Zones
  • Listed Buildings
  • Crickley Hill Country Park
  • The Star Centre
  • Ancient Woodland
2. Areas to be avoided wherever possible
  • Areas of archaeological potential
  • Trees and woods covered by TPO's
  • Local nature reserves/SNCI
  • Local properties - options which avoid or where possible, reduce existing impact preferred
  • Minimum impact on broad aquifers and surface waters (springs and rivers)
  • Minimum noise and local air quality issues
  • Minimum waste produced, or which generate opportunities for re-use or waste.
  • Avoid cutting local rights of way
  • Ensure access to local properties
3. Opportunities
  • Options which allow improved crossing on public rights of way
  • Options which decrease green house gas emissions from vehicles using the scheme

Review of Previous Options

5.5 A review of earlier studies undertaken for the Highways Agency in the study area was undertaken to assess if the initial assessments of these schemes could be revised due to:

5.6 Following the reviews a "long list" of initial potential options was developed. These are shown in Table 5-2.

Table 5-2 List of Options Initially Identified
Options Identified
  • On-line options within highway boundary
  • Mainly on-line options with widening/offline sections
  • Off-line options to the east of the current route alignment
  • Off-line options to the west of the current vents alignment
  • "short" tunnel option
  • "long" tunnel option
  • "very long" tunnel option
  • "toolkit" options
  • "do nothing" option

Initial Discussions with Environmental Statutory Consultees

5.7 Early in the study process a meeting was held with environmental statutory consultees. Those attending are listed in Table 5-3. A separate meeting was held with English Heritage. English Nature and the Environmental Agency were invited to the meeting but were unable to send representatives.

5.8 The meetings comprised a review of the environmental constraint procedure for the study and outline discussions on options for development. The following was agreed:

Table 5-3 Attendees at Initial Statutory Consultee Meeting
Attendees
  • Countryside Agency
  • National Trust
  • Tewkesbury Borough Council
  • Gloucestershire County Council
  • Cotswold National Trail
  • Cotswold District Council

Initial Option Development

5.9 Following the initial meeting with the statutory consultees the list of initial options was reconsidered and screened. Several options were ultimately discarded including:

5.10 Further work was undertaken on the remaining options and indicative corridor plans were prepared for consideration at the subsequent "optioneering" meeting.

Optioneering Meeting

5.11 An "optioneering" meeting was held at the Highways Agency's offices in December 2001. This considered the indicative options identified in light of the environmental and physical constraints. The meeting was attended by personnel from the Highways Agency, WSP, Gloucester Archaeology and Cooper Partnership.

5.12 The meeting confirmed the rationale used in discarding the "do nothing" option and options to the east and west of the existing route. Three further options were also discarded at this meeting. These were:

5.13 Three options were taken forward for further consideration and assessment:-

5.14 Each option is discussed in more detail in Chapter 6. Plans showing the three schemes are given in Figures 5.1 to 5.3.

Second Meetings with Statutory Consultees

5.15 Two further meetings were held with Statutory Consultees on the 28th January 2002 in Bristol. The attendees at each meeting are given in Table 5-4. The meetings were intended to discuss the options to be taken forward to the assessment stage and to gather initial feedback from attendees.

Table 5-4 Attendees at the Second Consultation Meeting
Session 1 Session 2
  • English Heritage
  • Countryside Agency
  • English Nature
  • Environment Agency
  • Cheltenham Borough Council
  • Gloucester County Council
  • Cotswold District Council
  • Cheltenham Borough Council
  • National Trust

5.16 The results of these meetings with environmental statutory consultees were in summary:

Option 1

5.17 The Environment Agency (EA) questioned why a short term option was being considered. Concerns over the impact of the scheme on designated sites and loss of vegetation at Air Balloon were highlighted. Despite these concerns it was agreed that the option should be considered for further assessment.

Option 2

5.18 Concerns were expressed over the potential impacts on designated sites and the loss of vegetation at Crickley Hill. Visual impacts of the grade separated junction, its proximity to Crickley Hill and its affect on the context of the designated sites on Crickley Hill were also highlighted. However, in principle, consultees accepted that the scheme had some merit and agreed that it could be considered for further assessment.

Option 3

5.19 The EA voiced serious concerns over the need for de-watering during construction and post construction and on the potential for pollution of the aquifer. These were confirmed at a subsequent meeting with the EA in February 2002. They did accept, however, that pollution control equipment would be incorporated in the scheme (as with all options considered). They were concerned over effects on groundwater, springs and the River Churn. They stated that significant assessment work would be required if this option were to be taken forward. English Nature voiced concern over possible impacts on two SSSIs. Other concerns were the visual impact of the portal and link roads. Despite these concerns it was agreed that this option should be considered for further assessment.

5.20 The results of the second meeting with local authorities in summary were:

Option 1

5.21 Concerns over the short-term nature of this option. Issues relating to archaeology, ecology, landscape and impact on the context of National Trust land were particularly highlighted.

Option 2

5.22 Concerns over landscape impact, archaeology and the two additional lanes down Crickley Hill, loss of vegetation and impacts on the setting of Crickley Hill (Archaeology, National Trust and Ecology) were highlighted. Access to Cold Slad Lane and Grove Farm was also a concern.

Option 3

5.23 This option resulted in significant discussion and many concerns were raised. Cotswold District Council thought this to be the best option. Concerns over safety/accidents within the tunnel were also highlighted.

5.24 It was agreed that all three options should be considered for further assessment.


5 option development

6 options considered further

6 OPTIONS CONSIDERED FURTHER

6.1 Three main options have been taken forward for assessment. Two alternatives for the Air Balloon Junction have also been considered. These are discussed below.

Option 1

6.2 Option 1 is shown in schematic preliminary form in Figure 5.1. It should be noted that this is an indicative drawing for information only. Detailed design has not been undertaken at this stage. The scheme comprises the following elements:

Option 2

6.3 Option 2 includes all elements as listed above, differing only at Air Balloon Roundabout and on Crickley Hill, as described below.

6.4 At the Air Balloon roundabout, the current arrangement would be replaced with a new grade separated junction and a new roundabout further up the A436 at the Leckhampton road junction. This is shown in Figure 5.2.

6.5 On Crickley Hill an additional two downhill lanes would be provided on a separated alignment that would run parallel to the existing road but at lower level. The existing three lanes would be retained for eastbound ("up") traffic with one being used as a climbing lane (see Figure 5.2). There would be loss of vegetation on the southern side of the existing road.

Option 3

6.6 Option 3 comprises a twin bore tunnel with two lanes in each bore. Each bore would be D shaped with a maximum diameter of 14m. The bores would have linking tunnels every 100m. The tunnel would be in the order of 2.8km long and be at a gradient of approximately 4.5%. The tunnel would be mechanically ventilated and no ventilation shafts would be required.

6.7 Cowley roundabout would be removed and excavation of a cutting through the hill immediately to the north followed by a short section of road on embankment opposite the Golden Heart Public House would be provided. A control building would be required at this end of the quarry and it is possible that it could be located behind screening in the former Birdlip quarry potentially using the existing gate to the quarry for access.

6.8 The southern tunnel portal would be located in the northern slope of the hillside in the dry valley opposite the Golden Heart.

6.9 A small (500m3) reservoir for fire fighting purposes at the southern portal and possibly a small pump-house would be required. These could be located either in the landscaping within the valley opposite the Golden Heart, or within the Birdlip Quarry.

6.10 The northern portal would be close to the A417 at the bottom of Crickley Hill near Crickleigh Farm. Both portals could be faced with limestone from the excavation of the tunnel. A control building would be required. Two link roads would provide access to Crickley Hill and the A436 from the end of the Brockworth Bypass. Opportunities for landscaping and re-use of material excavated from the tunnel, would be considered to reduce impacts in the AONB.

6.11 This option is shown in Figure 5.3.


6 options considered further

7 engineering comparison of options

7 ENGINEERING COMPARISON OF OPTIONS

7.1 This chapter considers the engineering issues associated with each of the three options being considered. Because of the importance of geotechnical and groundwater issues for this scheme, this section is divided into two sections: Geotechnical Engineering and Highway Engineering. However, all assessments carried out to-date should be treated as preliminary and require more extensive investigation and analysis to refine the points made.

Geotechnical Engineering

Options 1 and 2 - Crickley Hill

7.2 Above the escarpment route options would be constructed over the limestones of the Lower Inferior Oolites, which are likely to provide relatively simple construction conditions, although some minor solution features may be present. It should also be noted that the Geotechnical Feedback Report for Birdlip Bypass (Gloucester County Council, 1989) indicates that toward the edge of the escarpment the limestone becomes cambered and a series of deep fissures (sub-parallel to the escarpment) were encountered during construction of the present route south of the Air Balloon Roundabout.

7.3 Where options cross the unstable escarpment slopes, it is considered that a combination of cutting, particularly within side-sloping ground, and embankment would be required to provide suitable vertical alignments. Special stabilisation and drainage measures would also be required for highway construction. Although not prohibiting construction within the escarpment slopes, the presence of this significant thickness of unstable ground on the slopes below the escarpment would provide considerable engineering and construction difficulties.

7.4 The slopes below the escarpment consist of a complex mass of landslipped material, masked by a mantle of soliflucted Head deposits. In general, the fines content of the colluvial deposits increases westwards towards the Vale of Gloucester. Three main types of colluvial deposit have been identified as forming the slopes and comprise the following:

7.5 Where the ground comprises silts and sands, slope angles of up to about 20o (in most groundwater conditions) may be achieved. However, where silty clays are present, i.e. below, say, Cold Slad, a slope of approximately 10o (typical of much of the middle and lower slopes) is considered to be only marginally stable, assuming the presence of relic shear zones. Therefore, it is considered that even minor changes to the groundwater regime or disturbance could reactivate failure, which could affect a significant area up slope.

7.6 Stability analysis carried out as part of this study suggests that, within the colluvium, unsupported cut slopes should not exceed approximately 2m. Where cut slopes are required in excess of 2m, a retaining measure would be needed.

7.7 In general, groundwater conditions are not well understood, but Hutchinson (1991) has identified some areas of artesian and near-artesian groundwater. In addition, during the construction of the present alignment in the 1960's an area of the slope below Crickley Hill is recorded to have failed due to a significant groundwater issue. This section was stabilised with the use of deep counterfort drainage.

7.8 The extent of colluvial (landslipped) materials within the study area can be seen from the constraint mapping to generally cover the area from the top of the escarpment to the floor of the Vale of Gloucester. These slopes are likely to comprise soliflucted and landslipped material, comprising a mixture of the overlying and underlying strata, with a number of randomly located springs, indicating the complex nature of the hydrogeological regime. They comprise unstable ground on which special stabilisation and drainage measures would be required for highway construction. However, various options are available for the proposed highway improvements below the escarpment. Each option would affect the marginally stable valley slopes to a varying degree. At this stage and in light of the uncertainties regarding depth of existing potential failure surfaces, groundwater pressures and soil properties, the amount of stabilisation works required would be dependant upon the route alignment adopted.

0 to 400 metres from the Balloon roundabout

7.9 Improvement options over the initial 400 metres from Air Balloon Roundabout, are likely to involve on-line widening on one or both sides of the existing carriageway within the existing highway boundary. However, Option 2 includes some off-line construction in the vicinity of the existing roundabout together. Existing cut slopes in this area would appear to be stable and, therefore, slopes at the existing gradient (approximately 30º) should be assumed at this stage, where these can be accommodated. Where options require slopes to be steeper than approximately 30º retaining measures, probably in the form of walls, would be required.

7.10 No significant engineering constraints are anticipated over the initial section and it is proposed that widening at existing slope angles to both the north and south could be achievable. Similarly, where new or deeper cuttings are proposed, these could be achieved at the angles of the existing slopes.

7.11 Where required, widening of the shallow cut slopes on both the northern and southern sides of the existing carriageway could be achieved by cutting at similar slope angles and maximising the use of the relatively wide verges, particularly to the north of the existing carriageway. Where there is a need to create new cuttings similar slope angles to those already adopted within the limestone should be employed. Further investigation would be required where steepening of the existing slopes is required or where new or deeper cuttings are proposed. To achieve widening within existing land-take, low retaining structures may be required to support cut slopes and minimise land take.

7.12 The northern slope, below Crickley Hill, is observed to be within the Oolitic Limestone Group. Improvement options are constrained by these slopes and would require construction of an anchored retaining wall (probably by top-down construction) together with deep drainage measure to stabilise the slope.

7.13 With the slopes above the northern boundary of the A417 being designated as SSSIs, together with them being nationally important rock exposures, widening options to the south side of the existing route are considered the only ones viable. 400 metres from the Air Balloon Roundabout to Grove Farm Access and Cold Slad

7.14 Below approximately 400m from the roundabout the ground drops away steeply on the southern side of the existing carriageway. The carriageway is, currently, partly supported by an embankment at about 18º supported by a minor retaining measure. To the north, the cut slopes comprise 30º to 40º separated by wide benches within the Oolitc Limestone Group.

7.15 The road improvement options over this section are again constrained to the north by the slopes below the Crickley Hill escarpment. Widening options of these slopes would require cutting back of a substantial part of this rock slope at a similar angle. This is not considered possible due to the adjacent SSSI, National Nature Reserve and National Trust land and, therefore, construction of an anchored wall (by top-down construction) and deep drainage would be required to stabilise the slope. However, founding such a wall on potentially unstable ground below the limestone, i.e. from approximately 450m west of the Air Balloon Roundabout, may require substantial piles to support the wall and to stabilise the slope. Stabilisation methods, such as piled dowels and drainage techniques, may also be required in combination with the retaining measures in order to provide a stable solution.

7.16 Off-line widening to the south could be achieved over this section of the route by the use of earthworks and retaining walls between carriageways. The use of a split-level carriageway in Option 2 may be considered, thus, reducing the volume of imported fill and, hence, the potential destabilising effect of filling on these slopes. However, careful consideration is required to the founding of any retaining measures used between the carriageways within the potentially unstable ground, particularly where the carriageways start to converge toward Air Balloon Roundabout (it is recognised that the retained height would decrease over this transition). This option would potentially involve the demolition of the former Cotswold Restaurant, although has the advantage of having little impact on the existing road during construction as well as not impacting upon the SSSI to the north. As a result of the recorded artesian groundwater pressures in this area, it is likely that a provision of a drainage blanket would be required beneath any earthworks to ensure dissipation is achieved.

7.17 Provision of a piled carriageway may be considered should land-take issues constrain the route of this section.

7.18 Further culverting of the stream would be required in Option 2. Agreement from the Environment Agency would be required to culvert or divert the stream below the embankment.

7.19 The Crickley Hill escarpment, again, constrains the improvement options over this section. Widening to the northern side of the existing carriageway may also cause considerable disruption to the traffic during the construction phase and may require single lane traffic flow during some phases of the works.

7.20 Widening to the south could be achieved over this section of the route by the use of earthworks and retaining walls between carriageways. Embankment widening of up to 6m high together with the use of a split-level carriageway may be considered. This potentially involves the demolition of the former Cotswold Restaurant. This option has the advantage of having little impact on the existing road during construction as well as not impacting upon the SSSI.

Cold Slad / Grove Farm Access Road to Dog Lane

7.21 The ground is likely to comprise a mixture of the colluvium types described in previous sections, but it is likely to be dominated by clays and silts on these lower slopes. The ground and groundwater conditions are likely to be highly variable over this section of the route and, as a result of previous ground movements, a detailed investigation would be required to provide sufficient data to design earthworks and stabilisation / retention measures.

7.22 The improvement options over this section are again constrained to the north by the slopes below the Crickley Hill escarpment. However, as a result of the wide verge, minor widening to the north may be achieved without disturbing the existing slopes.

7.23 Widening to the south could be achieved over this section of the route by the use of earthworks and possibly retaining walls between carriageways. The use of a split-level carriageway may be considered, thus reducing the volume of imported fill and, hence, reducing potential destabilising effects of filling on these slopes. Further culverting or realignment of the stream would be required should a southern widening option be taken forward. As a result of the recorded artesian groundwater pressures in this area, it is likely that a provision of a drainage blanket (together with other slope drainage measures) would be required beneath any earthworks to ensure dissipation is achieved.

7.24 Widening works to the south would encounter deep soft organic deposits within the valley floor. Construction of embankments over these materials are likely to lead to considerable, potentially uneven, settlement and may interfere with the existing groundwater regime as a result of the stream re-alignment/culverting. These settlements may affect the existing carriageway, particularly where the toes of existing fill slopes are close to the valley centre. Likely differential settlement between the existing and new carriageways may be significant and careful consideration of these would be required in design of the widening options.

7.25 The anticipated engineering problems associated with widening to the south, while still significant, are considered to be less than those associated with widening to the north of the existing alignment.

7.26 The road improvement options over this section are again constrained to the north by the slopes below the Crickley Hill escarpment. However, minor widening to the north, within the existing wide verge, may be achieved without disturbing the existing slopes.

7.27 Widening to the south could be achieved over this section of the route by the use of earthworks and retaining walls between carriageways. The use of a split-level carriageway may be considered.

7.28 Significant widening works to the northern side of the existing carriageway may also cause considerable disruption to the traffic during the construction phase and may require single lane traffic flow during some phases of the works.

7.29 The anticipated engineering problems associated with widening to the south, while still significant are considered to be less than those associated with widening to the north of the existing alignment and, therefore, widening to the south is considered more appropriate.

Dog Lane to the Brockworth Bypass.

7.30 The ground is likely to comprise a mixture of the colluvium types described previously, although clays and silts will dominate. The groundwater conditions are likely to remain highly variable over this section of the route as a result of the highly variable ground conditions and a detailed investigation would be required to provide sufficient data for geotechnical design.

7.31 Widening to the north of the existing carriageway is constrained by the marginally stable shallow angle slopes below Crickley Hill. However, minor widening may be achieved by a series of retaining measures, particularly below the section of Dog Lane that runs close to the existing alignment (1200 to 1420 metres from the Air Balloon Roundabout).

7.32 Widening could be achieved to the south of the existing carriageway by the construction of further earthworks. Again a split-level carriageway may be considered. Comments made previously regarding the provision of adequate drainage measures and differential settlement of the carriageway would apply here.

7.33 Agreement from the Environment Agency would be required to culvert or divert the stream below the embankment.

Option 3 - Tunnel

7.34 The tunnel option has been assumed to consist of two road tunnels, connected at approximately 100 m intervals by cross passages (DMRB:BD 78/99). Both tunnels would have twin lanes. At approximately 2.8 km, the tunnel is long by UK standards, but is modest compared with many existing road tunnels outside of the UK.

7.35 The tunnels are considered to be within the practical limit for longitudinal ventilation (less than 5 km) and a ventilation shaft/shafts is not considered, at this stage, to be necessary.

Secondly, the Cotswolds Escarpment at this location is quite steep and is recorded as being unstable. Therefore, ventilation shafts would be difficult to accommodate on the lower parts of the tunnel. Lastly, ventilation shafts would cause additional visual impact within the AONB.

7.36 The cross passages should not be used to ventilate the road tunnels in the event of a fire in the road tunnels. Each tunnel would require its own separate ventilation system. Some ventilation control equipment may be located in the cross passages.

7.37 It is assumed that the tunnels would have an excavated diameter of 14 m, similar to the Round Hill Tunnel at Folkestone. Some allowance could also be made for additional verge/walkway width, if required, on the slow lane side, as with the Southwick Hill Tunnel at Brighton. The excavation height would be about 10m. The cross passages are assumed to have an excavated diameter of about 4m and a length of less than 25m.

7.38 It is assumed that the tunnel bores have been assessed on the basis that no side ducts would be required to supply ventilation air and as such are giving bore diameters as follows:

7.39 The road tunnels would be designed to the requirements of BD78 (DMRB:BD78/99). However, the recent series of fires within road and rail tunnels in Europe has resulted in new recommendations from the UN. There is also new guidance as to the requirements for tunnel ventilation and smoke control due from the EEC. This guidance is yet to be published and, when released, could seriously affect the way that tunnel ventilation systems are designed. While it would be inconsistent with the requirements of BD78, consideration should be given to a local variation in the distance between adjacent cross passages.

Summary of Geology within the Tunnel Corridor

7.40 The published geological mapping suggests that the south-east portal would be excavated in landslip material and Fullers Earth. The tunnels would probably pass through the Fullers Earth at shallow depth and then encounter in sequence the Upper, Middle and Lower Inferior Oolite, Cotteswold Sands, Upper Lias, Marlstone Rock, Middle Lias and possibly the Lower Lias. The lower, 850 to 900 m and the north-west portal itself would be driven through a major landslip. Some landslip material and surface deposits (drift, scree and head) may also be encountered at both portals. Some weak organic material may also be present at levels within and below the lower sections of the slips. The limestones are generally more permeable than the underlying Lias and the contact is assumed to form the spring line.

7.41 Towards ground surface, the Oolitic limestones is recorded as becoming thin/very thinly bedded and having numerous vertical fissures. The upper 0.5 to 1.5 m is fractured. Bands of silty/sandy clay and rare, thin marl also occur. The fissures are not usually continuous from bed to bed. Some fissures were found in surface excavations to have an open width of 20 to 300 mm and an estimated depth of over 17 m. The open width tends to decrease with depth. The excavation of surface trenches with a backactor resulted in considerable overbreak. Cutting slopes 1:1 to 1:2 and rock embankment slopes of 1:1.5 are reported. The Upper and Lower Inferior Oolites are limestones. The Middle Inferior Oolite is a limestone with clay and sand.

7.42 As described in Section 3, the slopes below the Cotswold Escarpment and those located near the proposed south-eastern portal comprise landslipped material. Two forms of landslip may be encountered by the tunnel:

Construction Issues

7.43 The portals are some 3 km apart and the roads between them are not considered suitable for continual movement of construction plant. Other than for an initial section of clays, the upper 1/3 of the tunnel would be excavated in hard limestone rock and the lower 2/3 section of the tunnel would be excavated in clays and silts and, probably, landslipped material. The methods of excavation and support, and the plant to be used for these two tunnel sections are likely to be different. Each portal/tunnel section should therefore be considered as being in effect two separate sites.

Brockworth Portal

7.44 The tunnels are likely to encounter landslip material (consisting of gravel to boulder sized pieces of limestone within unstratified clays, silts and fine sands) at the Western portal and probably in the first 870 m of the tunnel drives. There are several private properties and public roads built on the landslip material. Several streams and springs are also present. It should be assumed that the landslips are in a marginal state of equilibrium and the portal should be designed such that the least possible ground deformation occurs during construction.

7.45 It is likely that the two portals would be constructed in a similar way to the UK portals of the Channel Tunnel, which were excavated through a glacial landslip on the side of Castle Hill (Harris, Hart, Varley and Warren 1995) and the Serre la Voute Road Tunnel, Turin. The slips at Castle Hill were assumed in the design to be only marginally stabile. Following a major site investigation; which included detailed orientated core logging, instrument installation (surface survey points, inclinometers and piezometers) and an exploration tunnel, substantial toe weighting was placed over the toe of the slips to increase their apparent stability. The portals to the three tunnels at Castle Hill were constructed within a box with end walls and side-walls, formed of bored piles. Tension piles were also installed through the basal slip plane below the subsequent floor of the box to prevent heave due to the excavation of the ground within the box. The roof slab was cast on top of the piles at ground surface. The ground within the box was then excavated in benches from the top down; the excavated material being deposited above and around the box to limit the net change in ground load on the slip planes caused by the excavation. The tunnel eyes were made through the piles in the end walls, while the tunnels were driven towards the portal from the opposite side of the hill. Drainage galleries were subsequently installed to further increase the long term stability of the slipped mass. The lining of the running tunnels was built in 5 m long, reinforced sections through the slips. These allow minor movements along the slip to continue in the long term, without damage to the lining.

7.46 There is little information on the location and current stability of the landslipped materials below Crickley Hill. The slipped mass at Crickley Hill appears to contain more soft, non-cohesive material, flowing water and groundwater standing (although this is very variably) within 1 m of ground surface. The material would appear to be more sensitive to shallow excavation than those encountered at Castle Hill and it would, probably, be necessary to lower the water table by pump wells prior to portal excavation. Moving the portal further to the north-west could put the portal in the Lower Lias, outside of the slipped materials, but would also increase the length of the tunnel within the water-bearing silts of the Middle Lias.

7.47 Although a site investigation would be required, slip planes within clay materials are very difficult to locate and to evaluate and, therefore, it should be assumed that the slopes have only a marginal stability and require additional long term instrumentation and stabilisation measures. The topography, scenic location and diversity of the slips would probably not allow wide scale toe weighting. However, deep drainage may be a viable solution. In addition, it may be possible to install gravity drainage in the Lias Clays, without significantly affecting the overlying springs seen within the landslipped material. Where the slope drainage does effect the surface springs, it may be possible to use the drainage to re-supply water to the streams and to any properties currently abstracting from well or spring sources.

Cowley Portal

7.48 The geological mapping suggests that the southern portal and first 100 m of the tunnels would be excavated in a shallow landslip, probably formed of weathered Fullers Earth. The slip may only effect the portal and first few metres of the tunnel, but its apparent direction of movement (approximately 45º to the proposed axis of the tunnel) complicates the design and it would be preferable either to move the portal or to remove the slip material in an open cut.

7.49 It is probable that the portal would need to be excavated by first constructing a bored pile, or other structural head wall, to secure the ground at the tunnel face and then excavating the portal in open cut with protected (probably shotcreted and drained) side walls. Excavation with normal surface excavation plant would probably be adequate until the limestone is reached. A single cutting pick and a de-mountable impact hammer should also be provided to assist in the excavation of any rock bands and the underlying limestone until a full face of secure limestone is established.

7.50 The low intersection angle between the Oolitc Limestone and the over-lying Fullers Earth would probably result in some blasting as the limestone rises in the lower face. Continuous lattice arch, shotcrete and rock dowel support are likely to be required and some spilling; particularly close to ground surface, is likely. The Fullers Earth is probably an aquaclude and the dip of the underlying, permeable, limestone may result in initial sub-artesian water inflows from the limestone. Forward and downward probing would be necessary as the contact is approached in order to assess likely water ingresses.

Pilot Tunnel

7.51 Most of the tunnel alignment would encounter the Upper Lias clay and the Inferior Oolite limestone, but some Middle Lias, a weakly cemented silt, may also be encountered towards the lower end of the tunnel; particularly if the portal is located out of the landslip, within the Lower Lias. The ground pressure (from approximately 100 m of overburden and 50 m of groundwater) is likely to be very much greater than the mass strength of the ground and squeezing conditions can be expected in the Middle Lias. Prior drainage through a pilot tunnel would increase face stability during the excavation of the larger road tunnels; although drainage could also result in ground shearing and this would require careful consideration during the detailed design. It is considered at this stage that a pilot tunnel should be excavated in advance of the excavation of the portal and the breakout of the road tunnels from the portal. While this tunnel could be excavated with a roadheader and supported with lattice arches and shotcrete in the Lias, the consequences of a face collapse in this critical area close to the main basal slip plane are considered to be too high for this method of excavation and, therefore, it is recommended that the tunnel is excavated with a road header mounted in a shield and supported with a one-pass, bolted, segmental concrete tunnel lining. Ports in the lining should be provided to permit drainage holes to be drilled towards the road tunnels and cross passages. Filters may be required to prevent the silt from washing into the pilot tunnel.

7.52 The north-west portals would take some time to establish and it is recommended that the pilot tunnel should be driven from the base of an adjacent shaft while the portal box and stabilisation work are being undertaken. The shaft should be lined with concrete segments and the ground/lining annulus grouted to restrict ground movement. The stability of the shaft floor during excavation (possible boiling) needs to be considered in the detailed design. The shaft would be replaced by a portal as part of the permanent drainage of the slope.

Oolitic Limestone

7.53 Excavating down grade would result in the flow of any groundwater infiltration, rain or construction water towards the face, from where it would need to be pumped out to permit face excavation. This is not considered to be a critical factor; at least for the initial section within the Oolitic Limestone. Excavation up grade would result in gravity drainage away from the face towards the Brockworth Portal and is, therefore, preferred for the softer Lias deposits.

7.54 Groundwater is anticipated in the oolitic limestone and is likely to flow laterally along the sub-horizontal discontinuities; particularly above low permeability beds such as marl bands, and vertically along the open fissures. Clay beds and clay filled fissures (possibly including faults) are likely to act as aquacludes, resulting in perched water. The Upper Lias can be considered to be an aquaclude, such that the groundwater within the overlying limestone is likely to impose an additional pore water pressure within the Lias and landslides. Excavation of the pilot tunnel is likely to reduce the pore water pressure within the Lias prior to the excavation of the larger road tunnels, although as a result of relative difference in permeability, drainage of the Lias may only result in a marginal benefit.

7.55 The bedding is sub-horizontal (+/-50 dip towards the south-east, Hutchinson, 1991). Frank Graham's 1989 Crickley Hill Tunnel Study, includes a geological section; which shows the strata to have an apparent dip of about 20 towards the south in the plane of the tunnel section, which agrees with the published geological mapping and data presented in Gloucestershire County Council's 1989 Geotechnical Feedback Report. The bedding would, therefore, dip towards the face where the tunnel is excavated down slope from the south-east towards the north-west. This has a positive benefit; in that a forward probe hole would be more likely to intersect any beds of water bearing or soft strata before they appear in the tunnel face. Such strata would first be encountered in the lower part of the face, rather than in the roof and would be more likely to drain before intersecting the roof. There is a risk in excavating the face in the up slope direction that any water bearing strata or beds acting as aquacludes to water bearing strata above, could be undetected by the probe holes and approach the roof at the face above the section of tunnel previously excavated. A face collapse could, therefore, run back for some distance (given the low strata/tunnel intersection angle) from the face.

7.56 The Brockworth Portal and landslip additional stabilisation works are likely to take longer to construct than are the Cowley Portal and tunnel excavation within the Fullers Earth. The Oolite limestones are also likely to be easier to excavate and support than the Lias deposits. Excavation of the two road tunnels at the upper end is, therefore, likely to be well advanced before excavation of the road tunnels at the lower end begins.

7.57 The limestone is probably too strong for excavation with roadheaders. Impact hammers could be used, if the bedding and joint planes are close enough for the hammer to breakout the intervening blocks. The most likely method of excavation would be by 'drill and fire'. The sub- horizontal bedding in the limestone is likely to result in a flat, stepped longitudinal roof profile. The sub-vertical jointing is likely to control the lateral profile; blocks falling from the shoulders and walls. A stepped, square profile is likely to result. Tensioned rock bolt support with local mesh is anticipated. Lattice arches and shotcrete are not generally suitable for these conditions but would be required to some extent to support broken, faulted and clay/marl ground. Arches are designed to deform under load and this can result in long term problems (for example, Lewes Road Tunnel in chalk rock). Some face grouting may be required to seal any fissures giving large water inflows, but this is unlikely to be a major problem given the proximity of the tunnel to the escarpment edge. A D-shaped tunnel profile with a flat invert is recommended.

7.58 As the Crickley Hill Tunnels would be permanently lined with a concrete lining, it is reasonable to reduce the support to that suitable for the temporary condition. Any instability would be dominated by block falls and therefore the rock bolts and arches need to be maintained, whereas the amount of shotcrete can be reduced. Under normal conditions it is anticipated that the upper heading can be advanced in 3 m rounds, the bench in 3 m+ rounds to suit the contractor's operational requirements. Most of the support in the heading would be provided by 3.5 m long (4.5 m long for the three-carriageway tunnel), tensioned rock bolts at 2 m centres. Some, 100 mm thick, mesh-reinforced shotcrete would be required where the ground is locally fractured. Elsewhere, open mesh, supported by rock bolts, would be sufficient. Some roof support; consistent with safety would be required close to the face. The rock bolts should be retensioned and the remaining support installed within 20 m of the face. Little, if any support would be required in the bench walls.

7.59 Slightly longer rock bolts of 4 to 5 m at 1.5 m centres, 100 mm thick, mesh-reinforced shotcrete and shorter advance lengths of 1.5 m may be required in the heading in the areas of more fractured ground. Some colliery arches would also be required. Most of the support would need to be installed at the face and should be completed within 10 m of the face. The arch legs would need to be extended after bench excavation. Advance lengths of 3.0 m+ would still be possible in the bench.

The Upper and Lower Lias

7.60 As with the Fullers earth, the Upper and Lower Lias clays could be excavated with either a roadheader or with surface excavators (Volvo EC series or similar). A demountable impact hammer and single cutting tooth should be provided to excavate any rock bands. Some explosives may be required to excavate hard bands of rock intersecting the invert and roof. The upper heading would probably need to be excavated in two sections with a temporary invert arch. Bands of sand would require foreprobing and possibly glassfibre face dowel support. A curved invert arch would be required. The detailed design should consider the excavation sequence and support requirements in detail.

7.61 Instrumentation in the pilot tunnel should be used to verify the excavation and support design for the larger road tunnels. Instrumentation in the road tunnels should be used to verify the design of the lining.

7.62 Taking the example of Castle Hill, it is initially considered likely that the tunnels would require 200 mm of fibre or mesh-reinforced shotcrete, lattice arches at 1.5 m centres, 4 m long rock dowels at 1.5 m centres and 4 m long spiles across the roof at 0.5 m centres. The face should be protected between each advance with shotcrete. The heading would require a temporary invert arch. The invert arch would require to be closed to prevent continued tunnel deformation.

The Middle Lias and Cotteswold Sand

7.63 The weakly cemented, water-bearing silts of the Middle Lias and Cotteswold Sand are likely to present the most challenging tunnel excavation problems of all of the ground types on the project. Partial face excavation, continuous face support and overlapping roof support are probably required. The intersection lengths are probably too short to make a Perforex solution viable, unless this system is used to excavate all of the none-hard rock sections.

7.64 These are both weakly cemented, probably water-bearing, sandy silts. Some strong, massive, medium to thickly bedded limestone bands may also be present. The in situ condition of these silts needs to be determined by a site investigation. Even a small amount of cohesion would have a positive effect on face stability and hence in the speed of construction and amount of support needed.

7.65 Assuming that the unweathered material at tunnel depth is encountered as a weakly cemented silt/sand, it is probable that the heading would need to be advanced as a partial face with a central buttress, under the cover of an umbrella of grouted or jet grouted pipe spiles. The heading would require an arch of at least 200 mm thick, mesh-reinforced shotcrete, supported by 'elephants feet' or jet grouted foundations. A temporary invert arch may be necessary. The face would need to be supported with shotcrete between each excavation cycle. Lattice arches and rock dowels would be required at 1 m centres.

7.66 The bench would need to be excavated in tandem with the heading to allow early closure of the invert.

Cross Passages

7.67 Cross passages would be excavated within each of the rock types. Wherever possible, the distance between adjacent cross passages should be reviewed where this requires their excavation within the Middle Lias or Cotteswold Sand. Where it is unavoidable, a method of excavation and support similar to that for the road tunnels is required.

7.68 Those cross passages eyes within limestone rock are likely to be excavated concurrent to the adjacent road tunnel bench. The subsequent rounds could be drilled while the bench (at some point remote from the cross passage being extended) is being mucked. The method of support would be similar to that for the road tunnels. Arches should be located such that the adjacent cross passage can be excavated without removing the arch.

7.69 Cross passages within the Upper and Lower Lias, and the Fullers Earth would probably be excavated by hand tools, unless the contractor has a small roadheader available.

Tunnel Ventilation Options

7.70 The general parameters for ventilation design would be taken from Department of Transport Existing Standards pending the receipt of revised requirements from the EEC, which are understood to be issued imminently.

7.71 There are three options for the ventilation of the tunnel, these are transverse ventilation, semi-transverse ventilation and longitudinal ventilation. The systems differ as follows:

Transverse Ventilation

7.72 Transverse ventilation comprises a supply and extract system with graded inlet and outlet jets over the tunnel length which supplies and extracts air over the whole length of the tunnel. This system requires both supply and extract ducts over the length of the tunnel bores. As a result of the length of the tunnel and the required ventilation air volume, vertical ventilation ducts would be required at points along the line of the tunnel. The spacing of the vertical supply and extract shafts is governed by their possible locations and a requirement to limit the dimensions of the bore. Therefore, if shafts are provided at the lower level and at the upper level the necessary duct capacity can be quartered providing that a uniform spacing is feasible. This would mean that with the proposed tunnel alignment one of the shafts would need to penetrate the landslip area at the lower end of the bores. The location of the other shaft at the upper end is not so critical as the ground is more solid.

7.73 The shafts would contain input and extract fans separate to each tunnel bore. Generally the airflow would be side, inlet and crown outlet so that fresh air would be introduced at low level and exhausted at high level. This system tends to ensure that combustion products in the case of a fire are induced upwards and are exhausted to leave the lower occupied areas of the tunnel clear of smoke and products of combustion.

Semi-Transverse Ventilation

7.74 In this option the air generally enters through the tunnel portals and is uniformly exhausted along the length of the tunnel. Similarly to fully transverse systems ventilation shafts would be required, but these would be extracted only with air being extracted at high level. Generally this system would maintain clear air at low level in the case of fire, though in this case the airflow would be from the portals to the point of extract. This system may require reversible fans to control smoke in the case of fire.

Longitudinal Ventilation

7.75 In this option air is moved unidirectionally, generally in the same direction as the traffic flow, by jet fans mounted at high level in the bores.

7.76 This system requires no shafts but the airflow is generally in the same direction as the traffic flow. In case of fire, this would tend to move smoke and products of combustion away from the source in the case of a vehicle fire. On the assumption that traffic would stop and that the traffic in front of the fire would exit the tunnel, this should provide reasonable smoke clearance. Sufficient redundancy is required to be provided to cover failure and maintenance plus the possibility of a fire affecting the fan operation. Generally such fans are capable of operating in temperatures of up to 300°C and can be reversible to permit the tunnel to be operated in either direction under normal operation. However, bearing in mind the slope of the tunnel, a fire in the downgrade bore could permit smoke to travel against the direction of traffic flow.

7.77 As a result of the geological instability of overlying rock and the environmental sensitivity of the area, options requiring ventilation shafts would not be taken forward.

Alternative Option

7.78 There is the possibility for a compound arrangement of semi-transverse ventilation with jet fan assistance. In this option the jet fans may be used to create opposing airflow in the case of fire and hence confine the smoke to a prescribed area of the tunnel where it may be extracted by the exhaust system. The disadvantage of this option is that ventilation shafts are still required, but potentially more effective smoke control may be maintained particularly in the downgrade tunnel.

Fire Fighting

7.79 Consideration would need to be given for the provision of fire hose reels and fire hydrants over the length of the tunnel bore. The availability of a water supply bearing in mind the level differential between lower and upper portals and the extent of the local distribution may possibly require the installation of a reservoir and pump near the southern portal.

7.80 The local water reticulation at the upper area (Cowley Portal) may be adequate.

7.81 Hydrant and hose reel spacing in each tunnel bore would need to ensure that there is overlap in prospective hose lengths so that more than one hydrant or hose reel may be applied to a fire and to allow for the fact that facilities may be in accessible due to the fire. The provision of hand held equipment need also be considered.

Drainage

7.82 Internal roadway drainage would need to be considered within the overall design and section of the tunnel. The provision of gullies at the tunnel entrance and exit would reduce the quantity of water carried into the tunnel leaving only driven rain through the portals and such as may be carried in be vehicles. However, drainage would be required in the tunnel bores to allow for the disposal of fire fighting water, foam etc. As the tunnel is used by vehicles, fuel and oil entrapment would be required prior to discharge.

Engineering Risk Evaluation

7.83 An engineering risk evaluation has been undertaken and is presented in Appendix C.

Highway Engineering

7.84 A detailed description of the three option taken forward for assessment is provided in Chapter 6. However, for ease of reference, brief descriptions of the Options are included below.

7.85 A comparison of the construction issues and general engineering issues associated with the different options is included in Table 7-1 and Table 7-2.

7.86 The options have only been developed to the stage where outline design elements of the proposed routes have been identified. The detailed design of the scheme has not been undertaken and therefore the comparison of engineering issues is based on the preliminary information currently available.

7.87 As part of the examination of the potential options, consultation has taken place with the local Police and Fire Services to gain their views. Further details are provided in Chapter 11. However, the key issues raised by each are summarised below.

7.88 Gloucester Constabulary have formally expressed a preference for a conventional 'open air' highway layout. This is because of the large financial and operational burden to the service of a tunnel. They also expressed an opinion that the length of the tunnel would see many motorists choosing to use the alternative 'open air' routes through fear of the unknown.

7.89 Gloucestershire Fire Service have also formally expressed support for a surface option. They have expressed concerns regarding public safety and the risk to fire-fighters as well as the potential cost of providing dedicated services at the tunnel.

7.90 As part of the assessment of the Options, a preliminary safety review was carried out on Option 1, the at grade option with traffic signals, because of the safety issues associated with traffic signals on steep gradients. The review identified a number of safety issues that are particular to this Option.

7.91 The downhill gradient, of generally 5% or 6% on Birdlip Hill would encourage high approach speeds to the signals, particularly as they would need to be visible from some distance. This would give rise to overshoot and rear shunt type accidents. The traffic bound for the A436 would need to move out to the offside lane to go 'straight ahead', which is likely to give rise to further rear shunt type accidents.

7.92 The link road between Birdlip Hill and Crickley Hill would have a downhill gradient of about 9% leading to the left-hand bend to join the existing Crickley Hill. The gradient, although in character, would be well below desirable standards. The bend would also be well below desirable standards, particularly so after the steep downhill gradient and could lead to loss of vehicle control and HGV's overturning.

7.93 The traffic signals would require a speed limit, preferably even lower than the current 60 mph, however compliance is likely to poor and enforcement unlikely. In itself a lower speed limit is unlikely to reduce the problems to any great extent, as even 30mph on the link road could be excessive for some vehicles.

7.94 The combination of geometric features present with Option 1 is likely to result in a hazardous road layout. This cannot be overcome by careful detailed design.


7 engineering comparison of options

8 economic comparison of options

8 ECONOMIC COMPARISON OF OPTIONS

8.1 Economic appraisals of Options 1, 2 and 3 have been undertaken to evaluate the comparative economic value of constructing the different layouts taken forward for assessment.

8.2 The appraisals have been undertaken for the standard 30 year assessment period, using the cost-benefit analysis computer program COBA in accordance with Volume 13 of the Design Manual for Roads and Bridges (DMRB), Economic Assessment of Road Schemes. A predicted scheme opening year of 2010 has been used and therefore the final appraisal year is 2039.

8.3 The objective of an economic evaluation is to assess the value for money of the scheme by comparing those elements of scheme benefits that can be expressed in monetary values against the total cost of having the scheme in place. The benefits of a scheme that are included in an economic assessment are time savings to travellers, savings in accident costs and vehicle operating cost savings.

8.4 The scheme costs, which form the basis for the calculation of the value of Present Value of Cost (PVC) includes works, land, statutory utilities and future preparation and supervision. The scheme costs are discounted to 1998 to provide a comparison with the Present Value of Benefits (PVB) which are also discounted to 1998.

8.5 The currently estimated scheme costs (without applying COBA discount values to obtain the PVC in 1998 prices) are approximately £40 Million for Option 1, £55 Million for Option 2 and £200 Million for Option 3. The Option 3 estimated cost of £200M includes the tunnelling operations, disposal of spoil and the associated roadworks at either end of the tunnel. As the options have only been developed to the stage where outline design elements of the routes have been identified, the detailed design has not been undertaken and therefore the cost estimates are based on the preliminary information currently available. Land costs, future design fees and site supervision costs are excluded from the above costs.

8.6 The difference between the PVB and PVC is known as the Net Present Value (NPV). A positive NPV indicates that a scheme is economically worthwhile. A further measure of a scheme's economic worth can be obtained by dividing the PVB by the PVC to obtain the Benefit to Cost ratio (BCR). The BCR thus gives the benefits per unit cost. A BCR of more than 1.0 indicates a scheme that is economically worthwhile.

8.7 An economic appraisal for each option has been calculated using both high and low traffic growth forecasts. A summary of the economic results obtained, for both high and low growth scenarios, is included in Table 8-1 below.

Table 8-1 Economic Comparisons of Options 1, 2 and 3
  SCHEME OPTION
Option 1 Option 2 Option 3
Low Growth High Growth Low Growth High Growth Low Growth High Growth
Time Benefits (£M) 58.3 75.5 101.9 138.6 113.0 148.9
Vehicle Operating Benefits (£M) 0.4 0.8 1.4 1.9 7.3 8.7
Accident Benefits (£M) 5.6 6.7 10.8 12.4 7.5 8.7
Total Benefits (£M) 64.2 83.0 114.1 152.9 127.8 166.3
Cost (£M) 26.9 26.9 36.9 36.9 144.3 144.3
Benefit / Cost Ratio 2.39 3.09 3.09 4.14 0.88 1.15
Net Present Value (£M)
(Benefit minus Cost)		 37.4 56.1 77.1 116.0 -16.6 21.9
Notes Costs and Benefits are in 1998 prices
Costs and Benefits are in multiples of a Million Pounds Figures based on COBA

Discussion of Economic Assessment

8.8 The results indicate that, with the exception of the low growth scenario for Option 3, the schemes would represent value for money with positive NPV's.

8.9 The primary economic benefits for each of the options are from the time savings resulting from the removal of congestion along the route. Each of the options would also result in accident benefits due to the predicted reduction in accidents.

8.10 Option 1 would be the least expensive option but has the least benefits, primarily due to the low time saving benefits because of the traffic signals at the Air Balloon roundabout.

8.11 Option 2 represents the best value for money as it has the highest NPV's and BCR's. The NPV's are more than twice those for Option 1.

8.12 Option 3 has the highest total benefits but, due to the costs of the tunnelling operations, is also the most expensive option and therefore has the lowest NPV's and BCR's. Due to the high cost of the tunnelling operations the low growth scenario for Option 3 would not be economically viable. Although the tunnel route is considerably shorter than Option 1 and 2, this would only benefit approximately 60% of the traffic as traffic using the A436 would continue to use the existing route.

8.13 To reflect an overall benefit from the low growth and high growth economic scenarios, a weighted average of benefits (60% of low growth benefits plus 40% of high growth benefits) can be used. The weighted average of benefits results in a BCR of 2.67 for Option 1, 3.51 for Option 2 and 0.99 for Option 3. Using the weighted average of benefits Option 3 would have a poor return because the benefits are less than the costs, and it would therefore be difficult to justify a case in economic terms.

8.14 Overall, Option 2 is considered to represent better value for money.

Estimated Tunnel Costs

8.15 A large and complex project such as this is likely to produce a variation in the bid prices. Assumptions on the chosen method of construction have been made here, but other methods are possible which could alter the programme and the bid price. For example, the contractor may price the hard rock section assuming a computer controlled, three four-boom drilling rig to advance the full tunnel face without a heading. The Laerdal Road Tunnel in Norway for example adopted 500 kg of explosive and 100 drill holes per 5 m deep round, giving 500 m3 of spoil and an advance rate of 60 to 70 m/week. Similarly a Perforex type system could be used to excavate the Lias clays and silts, landslip material and Fullers Earth, again as a full face without the need for a heading and bench.

8.16 To produce a reasonable estimate of the tunnel construction costs it would be necessary to estimate the labour, engineering, equipment and material quantities and to make some allowance for site facilities, office, overhead, profit, contingency and inflation. This could be done by adapting the rates given in Spon's, Wessex, Laxton, etc to current inflation prices. At this early stage, it is probably sufficient to make a less detailed assessment based on contractor's advice. To this end both Skanska and Amec were asked what in their opinion would be the likely construction costs of the project. Skanska suggested a sum of £100 to £120 million for the construction of the twin, 2 and 3 lane tunnels, exclusive of the finishing works (for the excavation and lining, but not for the lighting, road surface, ventilation etc). With contingency, they suggested £150 million. By comparison, the 4.5 km Dublin Port Tunnel (TBM) was estimated to cost £204 million, equating to approximately £136 million for the Cowley - Brockworth tunnel at current prices. The A3 Hindhead, twin tube, twin carriageway 1.5 km tunnel was estimated at ~£75 million (1995), which is equivalent to £150 million for Crickley Hill (£193.5 million at current prices). Amec suggested a rate of between £12,000 and £13,000/lin.m for the excavation, support and lining; based on their current estimate for the Stonehenge Tunnel. This is equivalent to a much lower figure of ~£75 million for the Cowley - Brockworth tunnel if two, twin carriageway tunnels are selected (approximately £94 million if one, three lane carriageway is included). Stonehenge is a relatively straightforward tunnel in relatively good ground and this would be reflected in their estimate for Cowley - Brockworth.

8.17 The only comprehensive analysis of recent road tunnel construction costs that we are aware of was undertaken by STUVA in Germany. They made detailed cost analyses for both cut and cover, and bored road tunnels of various standard profiles, construction methods, ground types and lengths in Germany (described in English in ref. 13, reproduced as a guideline in ref. 15). A comparison could be made using the data presented by STUVA, but this would reflect the German market and some allowance would also need to be made with contemporary (1996) and current UK prices. There is also no English language version of this report.

8.18 Considering German road tunnels as an example; four twin bore, twin carriageway road tunnels were opened on the A71 German Federal Motorway in 2001/2002. The Rennsteig (7.8 km) and Alte Borg (0.865 km) Tunnels are in a combination of hard lava flows, siltstone and clay. The Hochwald (1.06 km) and Berg Bock (2.74 km) Tunnels are in granite. The tubes are 25 m apart and connected by cross passages at 300 m intervals. The final lining is 300 mm of unreinforced concrete. The cost of these four tunnels averaged 22,000 DM/linear metre/tube. This is equivalent to about £7,000/lin.m/tube, or approximately £42 million for the Cowley - Brockworth Tunnel. This is much less than suggested by Skanska and Amec. The excavation and support of the two running tunnels through Castle Hill averaged £3,900/lin.m/tube (1992). The lining similarly averaged 4,000/lin.m/tube. The excavation and support (concrete segments) of the 1.5 m diameter pilot tunnel cost £1,100/lin.m. This suggests an equivalent value of about £48.5 million for the excavation and lining of Crickley Hill at 1992 prices, £68 million at current prices, which is similar to the price suggested by Amec.

8.19 Recent information developed by the Highways Agency gives a bored cost of £40 million per kilometre for twin bore two lane tunnels. This would give a cost of £112 million for the Cowley -Brockworth tunnel.

8.20 Given the wide range of possible construction prices, it is suggested that a price of £100 million be taken for the excavation and lining at this initial stage. To this would be added road construction through the tunnel, access roads and links, M&E, signage and lighting, and construction of the control buildings and reservoir. With contingency, a figure of £150 million should be adopted. This excludes waste disposal costs which would increase the overall cost to approximately £200 million.

Cost Comparisons with Other Tunnels

8.21 The Pen y clip Tunnel on the A55 in North Wales is a single bore tunnel of 930 m in length, 9.95 m width and 7.1 m height. The heading broke through after 33 months and the tunnel was completed in 48 months. The tunnel was excavated by drill and fire, with 130 holes per face, pulling an average 3.5 m advance. The initial and final thirds of the tunnel were in highly fractured rock, requiring steel arch support. The central third was less broken and was supported with steel mesh-reinforced shotcrete and rock bolts.

8.22 The Trattenbach Tunnel in Austria is a twin lane, single tube road tunnel of 419 m in length, about 85m2 face area and 2.5% gradient. The tunnel was excavated by NATM using a heading and bench. A 250 mm thick concrete lining with a membrane was installed. The project included some 750 m of slope stabilisation. The rock types are metamorphic sandstones and phyllites. Constructed took 24 months (1988 to 1990) at a cost of 261 million Aus.sch. This is roughly equivalent to £255 million at current value for a twin tube tunnel of the length of Crickley Hill.

8.23 Zell am See Tunnel in Austria is a twin lane, single tube road tunnel of 5109 m length and about 80 m2 face area. This project also has a 35 m long ventilation cavern, 480 m deep ventilation shaft, 652 m ventilation tunnel, 682 m of interchange tunnels and 375 m emergency exit tunnel. There is also a 4.7 km, 3.2 m diameter pilot tunnel, built by TBM. Also in metamorphic sandstones and phyllites, the project was constructed over 42 months (1992 to 1996) by NATM at a cost of 1200 million Aus.sch. This is roughly equivalent to £79 million for Crickley Hill at current prices.

8.24 Karawanken expressway tunnel on the Austria/Yugoslavia border is a twin tube, twin carriageway tunnel of 7,864 m and a face area of 86 to 100 m2. The tunnel has a gradient of 0 to 1.35% and was constructed by NATM through hard and soft rock. The tunnel ventilation is divided into two, about 3.5 km long sections for semi-transversal ventilation at each side. The central 1.2 km has only longitudinal ventilation. There are ventilation stations at each portal, but no ventilation shafts. The project was constructed between 1986 and 1991 at a cost about 2,600 million Aus.sch. This is roughly equivalent to £68 million for Crickley Hill at current prices.

8.25 Four, twin tube, three lane motorway tunnels were built along the Adana Toprakkale Gaziantep motorway in Turkey. These tunnels (Tasoluk Tunnel is 361 m, Ayran Tunnel is 600 m, Kizlac Tunnel is 520 m and the Aslanli Tunnel is 1360 m) were built through sedimentary and metamorphic rock by NATM and have face areas of 126 to 152 m2. The tunnels are lined with 0.4 m concrete. The tunnels were constructed at a cost of $150 million between 1990 and 1996 (equivalent to $189 million at current prices).

8.26 A twin tube, three lane tunnel of 3.24 km and 126 to 152 m2face area was built through sedimentary and metamorphic rocks by NATM along the Anatolian Motorway in Turkey. The tunnel has a 0.4 m thick concrete lining and longitudinal ventilation. The tunnel was built between 1992 and 1996 at a cost of $270 million. This is roughly equivalent to $340 million at current prices.


8 economic comparison of options

9 environmental appraisal of options

9 ENVIRONMENTAL APPRAISAL OF OPTIONS

9.1 The assessment methods used are as described in the "Guidance on the Methodology for Multi-Modal Studies" (GOMMMS) published by the DETR (now the DTLR). Further guidance on the application of GOMMMS on highway schemes has been taken from the bridging document "Applying the Multi-modal New Approach to Appraisal to Highway Schemes" published in March 2001.

9.2 At this stage in the scheme design, there is a limited amount of detail available on each of the options. The assessment that has been undertaken is thus equivalent to a DMRB Stage1 assessment.

9.3 GOMMMS considers five main objectives, namely:

9.4 Within each of these areas, a number of sub-objectives are assessed according to specific assessment and significance criteria, each having its own worksheet detailing the results. The assessment scores are then combined into an Appraisal Summary Table (AST) for each option. When reading the assessment results, ASTs and worksheets it should be borne in mind that the GOMMMS methodology assumes that all appropriate mitigation measures will be incorporated. This includes the Highways Agency own best practice guidelines as well as other recognised best practice measures such as pollution control technology, landscaping, good construction site management, environmental enhancements and sustainable drainage systems. The following sections describe the results of the assessments under the Environment Objective. This covers the following topics:

9.5 The corresponding ASTs are provided in Appendix D. A summary table comparing all the options is provided at the end of this chapter (Table 9-7).

OPTION 1 - Dualling of A417, signalised junction at the Air Balloon, and an additional lane down Crickley Hill

9.6 A plan showing the indicative road layout and alignment is provided in Figure 5.1.

OBJECTIVE : ENVIRONMENT

Noise

9.7 The change in alignment for the A417 to the north of Cowley Roundabout would move the carriageway closer to one dwelling (with a resulting potential increase in noise levels) but further away from 4 other dwellings (with a resulting potential decrease in noise levels).

9.8 The new slip road at the Air Balloon Roundabout would move traffic closer to one dwelling, with a resulting potential increase in noise levels.

9.9 The additional downhill lane for Crickley Hill could be largely accommodated within the existing highway boundary. However, the 27 dwellings in the vicinity of Crickley Hill would be affected by increased noise levels due to an increase in average traffic speeds on Crickley Hill. Mitigation measures, such as fences, could be designed to reduce noise impacts.

9.10 Other noise sensitive locations (e.g. Birdlip County Primary School and Crickley Hill Country Park) would be likely to remain relatively unaffected. However, the Gloucestershire Way, Cotswold Way and other footpaths might experience an increase in noise levels due to the widened carriageways between the Air Balloon and Cowley Roundabouts and increased traffic flow. Alternative quieter road surfacing could be considered to reduce noise at source.

9.11 A comparison of the number of properties that would be likely to experience a net benefit / disbenefit under Option 1 is shown in Table 9-1.

9.12 Movement of the road away from the Golden Heart public house, but closer to Stockwell Farm would result in changes in noise impacts. Possible noise mitigation measures would reduce overall impact at Nettleton Bottom and Stockwell.

Table 9-1 Number of properties likely to experience a change in noise lev