Breaking News
Quick Links
Map Based Road Projects Finder - Feedback
We are currently piloting a new map based tool to help you find road projects. Please let us know what you think
The Project Control Framework
This framework sets out how we, together with the Department for Transport, manage and deliver major road improvement projects.
A417 Cowley to Brockworth Bypass Improvement
13 CONCLUSIONS
13.1 A process of option development and assessment has been undertaken for the remaining section of single carriageway road on the A417 between Cowley roundabout and the Brockworth bypass.
13.2 This has included technical reviews and discussions with statutory environmental consultees, non-statutory bodies, local interest groups, Parish Councils and others. From this process three possible improvement options have been identified. These are:
- Option 1 - Short bypass at Nettleton Bottom, on-line asymmetric widening of the current Birdlip bypass, new grade separated junction at Birdlip, redevelopment of the roundabout at Air Balloon with the addition of traffic signals, and re-modelling of lanes on Crickley Hill to provide an additional lane down, mainly within the existing highway boundary. Remodelled accesses to properties along Crickley Hill would also have to be undertaken.
- Option 2 - Short bypass of Nettleton Bottom, on-line asymmetric widening of Birdlip Bypass, new grade separated junction at Birdlip, new grade separated junction at Air Balloon, construction of additional two lanes down Crickley Hill. These would be provided in an adjacent carriageway separated from the existing road by grade. Remodelled accesses to properties at Cold Slad and along Crickley Hill would be provided.
- Option 3 - Tunnel Option. A short off-line bypass at Nettleton Bottom to the southern (Cowley) tunnel portal, a 2.8 km long 14m diameter twin bore, dual carriageway tunnel which emerges at a western (Brockworth) portal close to the existing A417 near Crickleigh Farm, and link roads to the existing route. The tunnel would require two control buildings one to each portal together with link roads to the existing A417 which would be retained. A small (500 m3) reservoir (and possibly a pump house) would also be required near the Cowley (southern) portal for fire fighting purposes. The existing A417 would be retained to provide connections with the A436 and Birdlip.
13.3 All statutory and non-statutory bodies consulted agreed that doing nothing is not viable and that the current road needs improvement. There has also been general agreement that the current situation has had an adverse impact on the local economy.
13.4 The Study Area contains:
- a wide range of environmental constraints resulting from the long history of habitation and land use practices. In particular, landscape, archaeological and ecological constraints occur throughout the study area;
- the Cotswold AONB. This and local landscapes are recognised for their visual, aesthetic, historic and tranquillity value. In particular, the escarpment slope, which crosses the north-western part of the study area, is recognised for its landscape value;
- four sites of national ecological value and a number of sites of regional importance. In addition, the presence of nationally rare, protected and increasingly scarce species in the study area has been noted;
- over 400 listed archaeological sites, of which 6 are scheduled monuments and 31 are listed buildings. The investigations so far undertaken have identified significant volumes of archaeology and pointed to the fact that other archaeology probably lies below areas not surveyed to date;
- a Major Aquifer, which lies in the limestones on the plateau top. This feeds many of the springs and rivers in the region and is a major source of drinking water. The majority of the area falls into a groundwater abstraction protection zone. In addition, a minor aquifer lies in strata below the slopes of Crickley Hill;
13.5 All three options are buildable. Option 3 would cause the least disruption during construction as the majority of the work would be undertaken offline. Of the two surface options, Option 2 would cause less disruption to vehicle flows up Crickley Hill. However, remodelling of the Air Balloon roundabout under both Options 1 and 2 would cause major disruption to vehicle flows during construction.
13.6 Retention of carriageways on Crickley Hill in Options 1 and 2 would mean that the existing steep gradients (in the order of 10%) would remain. The tunnel would have an average gradient of 4.4%.
13.7 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 is not considered viable. Departures from standards would be required on Option 2 but safety would not be compromised. Option 3 would not require departure from standards.
13.8 Temporary contractor camps would be required for all three options, although these would be 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 more than 3 years (depending on hydraulic conditions encountered).
13.9 It is anticipated that only Option 2 would have a severe impact on properties.
13.10 Both the fire service and police have concerns over Option 3.
13.11 Waste material from excavation work would be generated by all three options, with Options 1 and 2 generating in excess of 300,000 m3 and Option 3 over 1.1 Mm3. Under Options 1 and 2, a large proportion of the material could be re-used within the scheme for landscaping, fill and/or embankments. For Option 3, re-use of waste material within the scheme would be very limited and restricted to the areas around the tunnel portals. The majority would need to be disposed of off-site. It is possible that re-use options may be available e.g. for quarry restoration at the Cotswold Water Park, although quantities would be small because of the restrictions placed on material suitability by the Environment Agency. This issue would have to be considered in more detail in the detailed design phase.
13.12 The preliminary economic assessment undertaken has identified that all options provide positive Net Present Values (NPV's) on the high growth scenarios. In addition, Options 1 and 2 shown positive NPV's for low growth scenarios. Under the low growth scenario, Option 3, the tunnel option, exhibits a negative NPV and under this scenario it would not be viable.
13.13 All three options contribute timesavings and accident reductions. With Option 1 the accident reductions are suppressed because they do not reflect the safety concerns on certain aspects of the layout.
13.14 Option 1 would be the least expensive option at approximately £40 million, but has the least benefits and would only represent a short to medium term improvement for congestion relief. 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 has the highest benefits but is also the most expensive at approximately £200 million.
13.15 Geotechnical and hydrogeological issues are important and affect all three options. Particular issues include the underlying Major Aquifer and the presence of landslipped material along the escarpment slopes and valley sides. Detailed geotechnical site investigations and groundwater modelling would likely be required in the next stage of the project.
13.16 The in-situ soils and rocks over this section of the study area are masked by a mantle of landslipped and soliflucted materials derived from the underlying lithologies.
13.17 The hydrogeology of the route is complex and would be dominated by the variations in natural strata together with the history of slope failures along the escarpment slope. A number of springs and areas of artesian or near-artesian groundwater pressure are recorded within the area reflecting the impact of both topography and the previous landslipping.
13.18 For on-line widening, existing slope angles should be employed, wherever possible.
13.19 Toward the eastern end of the route, the presence of the Crickley Hill SSSI restricts the widening options to the north of the existing carriageway. To maintain existing slopes benching details would require the removal of a large part of the slope. Widening is, therefore, likely to be restricted to an anchored wall over the lower parts of the slope.
13.20 Environmental and geotechnical constraints limit what can be achieved on Crickley Hill under Options 1 and 2. Widening to the south of the existing carriageway, using a series of retaining measures, over the section beneath Crickley Hill SSSI would alleviate the need to cut into the toe of the slope to the north of the carriageway. The use of a split-level carriageway in Option 2 would reduce loading on slopes and, therefore, reduce any destabilising effects of the widening works. 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, a drainage blanket (together with other slope drainage measures) would be required beneath any earthworks to ensure dissipation is achieved.
13.21 The slopes below the escarpment have been demonstrated to be only marginally stable. Preliminary analysis would suggest that maximum cut slopes of 1 in 3 are required for slopes less that 2 metres high and flatter slopes are required for cut slopes greater than 2 metres high. A reinforced soil slope or use of soil nailing may provide a suitable solution for cut slopes greater than, say 2.5 metres. However, a detailed ground investigation would be required in order to check that the cut would not reactivate historic failure of a slope.
13.22 It is anticipated that, in their present condition, the Crickley Hill colluvial deposits would be unsuitable for re-use as structural engineering fill without some form of improvement. However, where fill materials derived from the colluvial deposits are to be used, maximum side slopes of 1 in 3 should be assumed for preliminary design purposes for embankments greater than 2m high. Using imported fill materials embankment side slopes of 1 in 2 would appear suitable up to a maximum height of 2 metres. For embankment slopes greater that 2 metres in height, slope reinforcement would be required. However, this is dependent upon the founding material and would require a detailed ground investigation. Partial removal and replacement of materials together with deep drainage measures may provide a more stable design. The use of lightweight fill, such as PFA or even polystyrene, may be considered to increase embankment heights. However, the environmental implications of using PFA should be considered at preliminary design stage together with implications of using polystyrene adjacent to the stream.
13.23 It is likely that cut slopes would require drainage in the form of deep toe or counterfort drains and embankments may require drainage blankets together with reworking of shear zones. The use of strengthened basal zones to embankments may also be considered.
13.24 Environment Agency approval would be required for the culverting of the stream where the off-line options are carried on embankment across the valley. A low bridge / viaduct may need to be considered depending upon the Environment Agency's response, together with consideration of the most cost effective engineering solution in order to ensure adequate performance of the earthwork.
13.25 Various improvement options are available for the section of the route below the escarpment, although these are constrained by existing slopes close to the Air Balloon Roundabout. Each option would affect the valley slopes to a varying degree by cutting and/or filling. At this stage and in light of the uncertainties regarding depth of potential failure surfaces, groundwater pressures and soil properties, further investigation would be required to geotechnically assess the feasibility of each option.
13.26 Towards the base of the valley it would be advantageous for any route to use low embankment wherever possible in this 'near toe' situation avoiding cutting into previously unstable ground. However, for widening works on the higher valley slopes, embankments may have a less beneficial effect and may possibly be destabilising. Careful assessment of proposed horizontal and vertical alignments would, therefore, be necessary to optimise the performance of new earthworks.
13.27 An environmental appraisal of the three options has been undertaken using the GOMMMS methodologies.
13.28 In terms of landscape, Option 1 would have a moderate adverse impact on the AONB and landscape character, whilst both Options 2 and 3 have large adverse impacts, the latter due to the link roads and portals.
13.29 Cultural and archaeological resources within the study area are very important. Options 1 and 3 are predicted to have slight adverse impacts whilst Option 2 is predicted to have a moderate adverse impact, mainly due to its affect on the context of Crickley Hill.
13.30 The two surface options largely avoid nationally and locally designated ecological 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 high level of risk, a very serious adverse impact has been predicted for Option 3.
13.31 Water is a major issue in the study area and has had a major impact on previous schemes in the study area. Options 1 and 2 are such that mitigation measures could be added so that the long term impact on the ground and surface water regime would be slight adverse. However, considerably more doubt exists about the potential impact of the tunnel and the Environment Agency have serious concerns. The tunnel option, therefore, has a large adverse impact score.
13.32 Options 1 and 3 are anticipated to slightly improve journey ambience whilst loss of facilities in Option 2 lead to a neutral impact prediction. In terms of physical fitness, Options 1 and 2 are scored as slight beneficial whilst Option 3 is scored as large beneficial.
13.33 In terms of local air quality, Options 1and 2 lead to increases in localised air pollution. Under Option 3 overall local air quality would improve, although the pollutants would be concentrated at the tunnel portals. Both Options 1 and 2 also lead to an increase in the release of greenhouse gasses, whilst the tunnel option leads to a decrease in greenhouse gas emissions.
13.34 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 properties would experience an increase in noise and 37 a decrease.
13.35 A summary of the overall scores for each option is shown in Table 13-1.
