Highways Agency - Safe roads, Reliable journeys, Informed travellers

Related pages

• 1. introduction
• 2. scheme proposals
• 3. engineering assessment
• 4. environmental assessment
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 4. environmental assessment (continued)
• 5. traffic and economic assessment

3. engineering assessment

3. Engineering Assessment

3.1 Introduction

This chapter presents a detailed engineering assessment of the Alternative Proposal in terms of highway alignment, junction layouts, existing and new structures, geotechnics and contaminated land, earthworks, public utilities and compliance with Standards. Significant engineering difficulties associated with the proposal are also identified. Since the CHUMMS Strategy is identical to the Orange Route Option with Junction Strategy B, in terms of engineering, reference should be made to Section 4.4 in SAR Part I.

3.2 Alternative Proposal

3.2.1 Mainline Alignment

The alignment details of the Alternative Proposal are shown on drawing numbers 5021044/DR/HW/230 to 239 and 240 to 241. As shown on the drawings the proposed dual 2 lane all purpose (D2AP) A14 leaves the existing A14 corridor to the west of Brampton Hut Junction and heads south, adjacent to the west of the A1 before crossing it approximately mid-way between Brampton and Buckden.

The A14 continues eastwards as a D2AP carriageway, passing to the south of the Buckden South Landfill Site and crossing the River Great Ouse and its floodplain and the East Coast Mainline railway on a multi-span viaduct. It then travels to the south of the Wood Green Animal Shelter and in doing so crosses a number of local roads, which are listed below:

• Offord Road
• Silver Street
• The A1198
• Mere Way
• The B1040
• Hilton Road
• Conington Road

The route then passes approximately 1.5km to the north of the village of Hilton before continuing eastwards to the south of Fenstanton and the Conington Landfill Site prior to rejoining the existing A14 at the site of the Fen Drayton Interchange. At this location the A14 gains a lane and continues as a Dual 3 Lane All Purpose Road east to Fen Ditton, identical to the CHUMMS Strategy. This is described in more detail in Section 3.2.2 below.

With the exception of a 9 metre deep cutting approximately 2km in length, the Alternative Proposal generally follows the local topography. Where localised floodplains are crossed the alignment has been lifted approximately 2 to 3 metres, depending on the flood levels provided by the Environment Agency.

Both the horizontal and vertical geometry of this section has been designed to a standard above that of the desirable minimum. There are however relaxations and Departures from Standard required in order to realign the local road network in those areas affected by the route. These are discussed in more detail in Section 3.2.2.

The alignment of the new section of the Alternative Proposal described above runs along the same route and has the same vertical profile as the CHUMMS Strategy. The main difference between the two options is centred on the utilisation of the existing A14 trunk road and the replacement of the Huntingdon Railway Viaduct. As discussed briefly in Section 1.2, the Alternative Proposal involves replacing the Huntingdon Viaduct, which has a residual life of less than 10 years, and maintaining the existing A14 carriageway as a D2AP carrying traffic from the north to the east, and vice versa. The new section of the A14 will carry traffic between the east and the west as well as to and from the south. By keeping the existing A14 operational as a strategic traffic route the new section of the A14 will carry approximately two-thirds of the traffic flows of the CHUMMS Strategy. Therefore, between the A1 and Fen Drayton the Alternative Proposal has been designed as a D2AP standard road.

In order to improve safety on the existing A14, a review of the direct farm accesses onto the A14 carriageway has been undertaken. Localised accommodation tracks adjacent to the existing road have been incorporated into the scheme between Huntingdon and Fen Drayton Interchange. These accommodation tracks link those properties that currently have direct access to the existing A14 and divert them to the nearest main junction where acceleration and deceleration lanes assist in providing a safer merge and diverge movement onto the A14.

As a result of the mainline horizontal and vertical geometry remaining unchanged from that of the CHUMMS Strategy, the realignment of local roads affected by the route remain the same. The significant difference, in terms of sideroads, between the options is at the proposed A1 Brampton and Fen Drayton Interchanges, which are discussed in Section 3.2.2 below.

3.2.2 Junctions

A1 BRAMPTON INTERCHANGE

The proposed interchange arrangement, shown on drawing number 5021044/DR/HW/278, involves a restricted movement option that links the existing A1 Southbound carriageway with the proposed A14 Eastbound carriageway, and the proposed A14 Westbound carriageway with the existing A1 Northbound and Southbound carriageways. The proposed A14 Westbound to A1 Northbound Link crosses over the existing A1 and passes under the proposed A14 before tying into the A1 Northbound carriageway at the existing Brampton-Grafham Road structure.

The vertical profile of the A14 Westbound to A1 Northbound Link requires sub-standard vertical geometry in order for it to pass under Brampton Road and over the A1 before passing beneath the proposed A14. This is discussed in more detail in Section 3.1.3.

The alignment of these links remains the same as for the CHUMMS Strategy, however the number of lanes on some of the links and the geometry of the merges and diverges on all links have been modified. This modification is a result of the reduction in traffic that will use this section of the A14 and Brampton Interchange since the existing A14 will be carrying approximately two-thirds of the traffic volume.

The A14 Westbound/A1 Northbound and A1 Southbound/A14 Eastbound links have been reduced in cross section from a two-lane to single-lane carriageway. The A14 Westbound/A1 Southbound Link remains as a single-lane carriageway. Simple taper merge and diverge layouts tie the links into the main carriageways of the A1 and A14. By modifying the merge and diverge on the A1 northbound and southbound carriageways respectively the A1 weaving issue generated by the CHUMMS Strategy is removed and there is no longer a requirement to widen the A1 between Brampton Interchange and the existing Brampton Hut Junction. The desirable minimum distance of 1km, as stated in the Design Manual for Roads and Bridges (DMRB) TD22/92 is achieved between the successive junction slip roads.

On joining the proposed A14, the A1 Southbound to A14 Eastbound Link ties into the A14 carriageway as a taper merge. The need for a lane gain merge on the eastbound carriageway, as well as a lane drop diverge on the westbound carriageway, as required for the CHUMMS Strategy is removed due to the reduction in traffic flows using the proposed road.

FEN DRAYTON INTERCHANGE

The general layout of the Fen Drayton Interchange (Refer to drawing number 5021044/DR/HW/279) involves a two-tier, restricted movement interchange linking the proposed A14 with the existing A14. The movements that can be made on the interchange are as follows:

• Proposed A14 Eastbound to proposed LAR Eastbound
• Existing A14 Eastbound to proposed A14 Eastbound
• Proposed A14 Westbound to existing A14 Westbound
• Proposed LAR Westbound to proposed A14 Westbound.

Westbound. All other movements, for example proposed A14 Eastbound to existing A14 Westbound, are made using the Trinity Foot Local Access Road Junction situated approximately 1 km east of the Fen Drayton Interchange.

Interchange. At the location of the Fen Drayton Interchange, the proposed A14 is on an embankment of between 2 and 3 metres so that the road sits above the maximum flood level of a floodplain in the area. This results in the vertical profile having to be designed so that it crosses over the A14, which in turn produces a design that requires large amounts of fill material to construct embankments up to 10 metres high in places.

The main difference between Fen Drayton Interchange in the CHUMMS Strategy and the Alternative Proposal is the link design and how they tie into the proposed A14. As this is where the two A14 routes bifurcate in the Alternative Proposal the combined traffic flows require that the proposed A14 to the east of the interchange be a D3AP standard road. Therefore, in order to make the transition between D2AP and D3AP standard road the eastbound merge and westbound diverge take the form of a lane drop and lane gain respectively, in accordance with TD 22/92 of the DMRB. In comparison, the CHUMMS Strategy sees the lane gain and lane drop occurring at the A1 Brampton Interchange and therefore D3AP is maintained through Fen Drayton Interchange with simple taper merges and diverges connecting the links to the mainline.

All interchange links have been designed without the need for Departures from Standard although some relaxations have been required in order to minimise structure spans and reduce land take. Where these relaxations have been applied to the geometry of the links, verge widening and parapet and abutment setbacks have been applied to ensure that stopping sight distances are achieved.

GODMANCHESTER EMERGENCY ACCESS JUNCTION

Although not shown in detail on the drawings, consideration has been made to the provision of a junction where the A1198 crosses over the proposed mainline south of Godmanchester. This junction would be for the use of emergency vehicles in account of the fact that access onto the proposed D2AP A14 is limited to the A1 Brampton and Fen Drayton interchanges over a distance of approximately 13km. It would also allow queued traffic to leave the A14 in the event of an accident.

It is considered necessary that this additional junction be included in the Alternative Proposal due to the risk of the 2 lane carriageway of the D2AP standard road becoming impassable due to a road traffic accident or incident. A junction midway between the two interchanges will allow emergency access to and from the main carriageway without the need for excessive distances to be travelled.

The junction itself would comprise four gated slip roads linking the realigned A1198 with the D2AP carriageway of the proposed A14. Only a simple T-junction would be required to connect the slip roads to the A1198, as movement to and from these slips would be by emergency vehicle only.

3.2.3 Structures

EXISTING STRUCTURES

Existing structures along the scheme have been discussed in Section 2.5 of SAR Part I. Table 3.2 of Appendix B lists those structures affected by the Alternative Proposal and makes recommendations as to their treatment under this project.

NEW STRUCTURES

The number of new structures on the Alternative Proposal is the same as the CHUMMS Strategy (refer to Appendix C in SAR Part I, Volume I, Part B). However, for the Alternative, the narrower A14 Westbound/A1 Northbound Link results in a reduction in deck width of 3.0m for Underbridge No. 6.04 and a reduction in span of 3.0m for Underbridge No. 5.03 (Refer to drawing number 5021044/DR/HW/278 in Volume II Part A).

A small cost saving could be achieved by reducing the overbridge spans quoted in the SAR Part I to allow for the reduced carriageway widths associated with the Alternative Proposal. However, any future widening would then require the bridges to be demolished and rebuilt, the cost of which would be much greater than any initial saving. The overbridge spans assumed for the Alternative Proposal are therefore the same as those quoted for the CHUMMS strategy.

RIVER GREAT OUSE VIADUCT

VIADUCT A possible arrangement of the viaduct is shown on drawing number 5021044/DR/BR/026 in Volume II, Part B.

The width of the central reservation over the structure is shown as 2.5m including hard strips, as permitted by clause 4.8 of TD27/96 for long underbridges. At the west end, the structure will have to be widened over a length of approximately 100m to allow for sight line requirements.

HUNTINGDON RAILWAY VIADUCT

Brief details of the existing viaduct and retaining walls are given in Table 3.2 in Appendix B.

The deck, and possibly the piers, of the viaduct will need to be replaced. This can be undertaken once the new A14 to the south of Huntingdon has been opened, allowing the existing A14 through Huntingdon to be closed whilst the viaduct is demolished and rebuilt.

3.2.4 Relaxations and Departures From Standards

Standards As a result of various constraints along the scheme corridor it has been necessary when designing the scheme to drop below desirable minimum standards in certain locations, as permitted by the Highway Agency's Design Manual for Roads and Bridges (DMRB).

As discussed in Section 3.2.1 the vertical profile of the A14 Westbound to A1 Northbound Link has a number of constraints that were required to be taken account of in the design. The combination of these constraints results in a vertical crest curve that is one step below desirable minimum. A one step relaxation of a crest curve automatically produces a one step relaxation in the vertical stopping sight distance, as the radius of the curve becomes too tight to be able to achieve the desirable sight envelope. In accordance with the DMRB the relaxation in the vertical element together with the relaxation in the vertical stopping sight distance requires a Departure from Standard, which has been approved by the Highways Agency. Drawing numbers 5021044/DR/HW/614 and 615 provide more details on this.

The Alternative Proposal mainline has been designed within standards without the need for any relaxations or Departures from Standard. Relaxations were however required in the design of Fen Drayton Interchange as well as in the realignment of some of the existing roads in order that they could be taken across the proposed A14 with minimal land take. One Departure from Standard was required on Conington Road side road resulting from a substandard curve in close proximity to a junction.

Details of these relaxations and Departures from Standard are given on drawing numbers 5021044/DR/HW/610 and 611.

3.2.5 Earthworks

The earthworks between Ellington and the A1 are very similar for both the CHUMMS Strategy and the Alternative Proposal. The difference occurs at Brampton Interchange where the cross-sections of two of the links have been reduced for the Alternative option and along the A1 where widening is no longer required.

From the A1 Brampton Interchange to the A1198 the main earthworks required are the approach embankments to the River Great Ouse Crossing, which are up to 13 metres high in places and the cutting through the ridge to the south of Godmanchester, approximately 7 to 8 metres deep. There is also a pronounced cutting between Offord Road and Silver Street which is approximately 2 km in length and up to 11 metres deep.

From the A1198 to the point where the route ties into the existing A14 at Fen Drayton, the proposed vertical profile generally follows the surrounding topography, requiring only minor cuttings and low embankments. These low embankments have been designed into the scheme to ensure that the route runs above the flood levels of the floodplains that are located along the A14 corridor. Fen Drayton Interchange is situated on a floodplain and therefore requires embankments of up to 10 metres in order to lift the link roads over the A14.

From the A1198 to the point where the route ties into the existing A14 at Fen Drayton, the proposed vertical profile generally follows the surrounding topography, requiring only minor cuttings and low embankments. These low embankments have been designed into the scheme to ensure that the route runs above the flood levels of the floodplains that are located along the A14 corridor. Fen Drayton Interchange is situated on a floodplain and therefore requires embankments of up to 10 metres in order to lift the link roads over the A14.

For both strategies the junctions and side roads form the majority of the fill requirement. This is because all the side roads cross above the A14. For the more major side roads, the adoption of desirable crest curves, required in order to avoid Departures from Standard, have meant long lengths of high embankment.

Drawing numbers 5021044/DR/HW/235 to 239 provides details of the earthworks required along the scheme and Table 2.1 in Appendix A of Volume I, Part B provides an indication of the cut/fill ratio for the two strategies within this section of the scheme.

3.2.6 Contaminated Land

Contaminated land issues along the Alternative Proposal are common to those of the CHUMMS Strategy. Therefore reference should be made to Section 4.4 of SAR Part I

3.2.7 Public Utilities

The Public Utilities' (SU) apparatus that is affected by the Alternative Proposal is very similar to that of the CHUMMS Strategy. The main difference between the two strategies is on the A1 where BT cabling is located in the existing southbound verge. Approximately 500m of this is affected by the A1 widening as part of the CHUMMS Strategy but avoided in the Alternative Proposal since widening of the A1 is not required.