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A419 COMMONHEAD JUNCTION IMPROVEMENT
5. ENGINEERING ASSESSMENT
Engineering Standards
The preliminary design for this scheme has been completed in accordance with the following standards:
- TD9/93 'Highway Link Design'
- TD22/92 'Layout of Grade Separated Junctions'
Option A (Overpass)
5.1 This option would entail re-aligning the existing A419 such that it crosses approximately 7.0m above the existing junction (see Figure 5.1). The proposed carriageway layout would be as the existing i.e. dual 2 lane carriageway. It would involve raising the existing embankments and the construction of two underbridges to take the A419 over the existing junction. There would also be the requirement to make provision for the slip road merge/diverge lanes.
5.2 This option together with earthworks of 1 in 2 side slopes would fit within the existing central reserve. The route could therefore be constructed with minimal effect on the existing A419 traffic, thereby reducing delays during the construction period.
5.3 It is proposed that the side road (Day House Lane, to the south east of Commonhead junction) egress from the A419 is closed for safety reasons because it would be located in the slip road merge area.
Option B (Underpass)
5.4 The horizontal alignment and carriageway layout of Option B (see Figure 5.2) would be the same as for Option A. However Option B would entail re-aligning the vertical alignment of the existing A419 such that it passes approximately 6.5m below the existing junction. This would involve the construction of the proposed alignment in cutting with two proposed overbridges above the re-aligned A419 to accommodate the roundabout. In addition there would be the requirement to make provision for the slip road merge/diverge lanes.
5.5 The presence of a high water table would result in structural design considerations, which would increase construction costs. A structure approximately 350m in length and anchored by tension piles would be required to ensure a satisfactory waterproof construction. An alternative method dealing with the high groundwater levels would be permanent de-watering by pumping, but the volume of water expected would not make this option feasible.
5.6 There is an added risk that future groundwater levels during the anticipated life of the structure may be even higher than those recorded to date, due to both seasonal and long term climatic effects. At the moment, water levels would be around a maximum of 2.0 to 2.5m above road level. As expected, if groundwater levels rise further by around 2m, then the length of waterproof structure required would increase from approximately 350m to around 500m. Construction would be further complicated by the need for temporary de-watering over a substantial length of the cutting.
5.7 The Environment Agency has commented on Option B to say that their Water Resources function would prefer to see a sealed cutting option as opposed to long term de-watering which would not be environmentally sustainable. In addition, they highlight the fact that soakaways would be inappropriate for the drainage in this area because of the presence of a high groundwater level and the likelihood of groundwater pollution. There would also be an increased risk of flooding in adjacent catchment areas as a result of the anticipated quantities of water that would require to be discharged from the cutting.
5.8 Ground conditions comprise predominantly Gault Clay, and preliminary design work suggests that the earthworks cutting slopes would need to be between 1 vertical in 3 horizontal and 1 vertical in 5 horizontal. With these slopes, the scheme would not fit within the existing central reserve.
5.9 Therefore, structural retaining walls would be required to contain the construction works within the existing central reserve and prevent encroachment into the existing A419 carriageways. Without retaining walls, the A419 would need re-aligning which would cause severe disruption to traffic during the construction of the works.
5.10 It is proposed that the side road (Day House Lane, to the south east of Commonhead junction) egress from the A419 is closed for safety reasons because it would be located in the slip road merge area.
Option C (Throughabout)
5.11 The Throughabout junction would involve re-construction of the approaches to the existing roundabout and the introduction of a new road link for the A419 through the centre island of the roundabout with associated traffic signals (see Figure 5.3). The proposed alternative would consist of a standard throughabout, with three approach lanes, together with the associated filter lanes.
Drainage
5.12 Whilst the outline drainage proposals have not yet been finalised for all three options, drainage direct to ground is not likely to be an option due to the predominance of clay just below road level. It is more likely that road drainage would pass into the nearest stream via use of balancing ponds.
Public Utilities
5.13 Information from service providers relating to significant plant that may be located on site are detailed in Table 5.1.
5.14 Swindon Borough Council's Environmental Health Office was contacted particularly with regard to pollution incident records and the presence of any overground or underground fuel storage facilities. Confirmation was subsequently received that no records of any pollution incidents or underground storage tanks are held by Swindon Borough Council.
| Service Provider | Details of Significant Plant and Location |
|---|---|
| British Telecom | The Swindon Exchange is shown crossing Commonhead junction in a north west - south east direction. A spur is also shown running in a northerly direction along the centreline of the central reserve to the north of the roundabout. BT plant is also shown as follows:-
|
| NTL, Comtel & Cable and Wireless | NTL have indicated that they have plant crossing the A419 immediately to the north of Commonhead junction between two manholes. NTL also have plant in the verge of the northbound carriageway. Comtel and Cable & Wireless also have plant crossing the A419 just to the north of the NTL plant. |
| Water (Clean and Foul) | There are surface water sewers crossing the A419 at the south and north end of the lake which is situated to the north west of the roundabout. There is a trunk foul sewer in the verge of the northbound carriageway of the A419. There are water pipes in the southbound carriageway verge to the south of Commonhead junction. |
| Electricity | There does not appear to be any high or low voltage cables located within, or crossing, the site area. There are however cables supplying streetlights in all verges in the immediate vicinity of Commonhead junction. |
| Transco | Transco have confirmed that they do not have any plant that is affected by the proposed works. |
| Fisher German | Fisher German have confirmed that there are no Esso/Mainline pipelines in the area of the proposed works. |
The following services would be potentially affected by the proposed schemes:
- British Telecom - existing underground plant located in the southern sector of Commonhead junction.
- NTL - existing fibre optic cable crossing the A419 to the north of Commonhead junction.
- Thames Water - existing plant crossings to the north and south of Commonhead junction.
Assessment of Anticipated Departure from Standard
5.15 For Option A (Overpass), it has been determined that for the vertical curvature to comply totally with Departmental standards and to be able to maintain the scheme within existing land take, there would be a requirement for the provision of either retaining walls or the realignment of the existing A419 carriageways. To avoid this, a Departure from Standard has been applied for and approved.
5.16 The application of the Departure would not compromise driver safety because full visibility would only be unavailable over a short section. In order to mitigate this loss of visibility, warning signs advising motorists that there would be traffic merging onto the carriageway ahead would be proposed on the approaches to the slip road merging areas.
Summary
5.17 On engineering grounds, the impact of groundwater for construction of an underpass would be severe. It is not certain that waterproofing a structure in the underpass would be successful and groundwater pumping measures, as well as pumping to remove rainfall, would also be required. Pumping groundwater alone is not a sustainable operation.
5.18 An overpass solution does not have these high risks and can be constructed within the highway land currently available. The cost differential between the two is about £6M. At this stage the underpass solution would not seem a viable option.
