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Meaningful relationship

Full-time geotechnical presence on site and a partnership atmosphere are vital ingredients in the success of the A2/M2 widening scheme in Kent.

Maunsell senior geotechnical engineer Mark Dawson and WS Atkins geotechnical engineer Matthew Naylor say the partnering philosophy on site is reducing delays and helping the smooth running of the A2/M2 widening project near Rochester in Kent.

Dawson and Naylor, respectively representing the Highways Agency (HA) and contractor CSM (a joint venture of Costain Civil Engineering, Skanska Civil Engineering International and John Mowlem & Company), work jointly to solve geotechnical problems as they occur on site.

'This is a good thing because it speeds up the response time, ' says Dawson, adding that the design change review process is also greatly improved because new designs can be checked and approved on site.

Their involvement is critical because the widening project involves major earthworks, including using 140,000m 3of chalk excavated from the Channel Tunnel Rail Link (CTRL) North Downs Tunnel nearby to build a massive embankment for the new road.

The £124M A2/M2 contract is the UK's largest road project under construction. It involves widening a 17km stretch of road between Cobham junction on the A2 and junction 4 of the M2 near Gillingham, typically from dual two-lane to dual four-lane carriageways. This will mainly be achieved by building a new four-lane road for Londonbound traffic alongside the existing A2/M2 which will be modified to form the coast-bound carriageway.

The London-bound carriageway will cross the River Medway on a new four-lane bridge being built between the existing road crossing, which is being upgraded by Edmund Nuttall, and the CTRL bridge (see panel story).

Work also includes major improvements to junctions 2 and 3 of the M2 and is due to finish at the end of 2002.

CSM was awarded the contract in November 1999.

Maunsell is the HA agent and WS Atkins is CSM's designer.

The design and build scheme aims to improve access to the Medway ports and to allow the road to deal with higher traffic flow in the area, partly because of increased trade with Europe. A key aspect of the scheme is the Channel Tunnel Rail Link which is being built alongside this section of the A2/M2, explains Maunsell's employer's site representative Ron Martin.

'Because CTRL is right next to the job, there are many interfaces making it impossible to build one without the other, ' he says adding that the A2/M2 widening went through as part of the CTRL Bill.

This has led to some interesting collaborations between the two projects, including sharing of site investigation data.The overwater investigation for both bridges across the Medway was carried out by Soil Mechanics for CTRL. In return, the HA and Maunsell undertook investigations for both projects where they shared the same footprint and land ownership.

Modifications to junction 2 of the M2 formed part of both schemes, initially because of uncertainty over whether either would be built. 'Each contract had to be able to build it, in case one scheme did not go ahead, ' Martin says. In fact, there were three designs for the junction one without the widened road, one without CTRL and one with both. In the end, work was carried out as part of the CTRL project.

Because unforeseen ground conditions are not in the contract 'all ground risk is with the contractor, ' explains Martin the geotechnical aspects of the project are of prime importance.These include construction of embankments and the formation of cut slopes, involving extensive use of soil nailing and geosynthetics, for the new road.

Dawson says: '1.6M. m 3of material is being placed, mainly Class 3 chalk and Class 2 clay with flints.'

Some of the major geohazards on the contract are chalk solution features.These occur in large numbers along the alignment, causing problems in both the cut slopes and beneath the pavement. In one area there were 30 over 100m, Naylor confirms.

'The main problem was not that they were there, but where they were, ' he says.'The question was, what was the infill and how we could improve it?'

The solution features beneath the new road are between 3m and 5m deep, although some have been found up to 30m deep, he says.They are typically filled with clayey sand with SPT N values of between 4 and 10.

Dawson explains that the original approach was to dig out the filled solution features and fill them with concrete but instead a more systematic (and ultimately more economical) approach was adopted.When a solution feature is encountered, CSM site engineers follow a set procedure, recording and where necessary investigating the density of the infill by dynamic probing.The results are given to WS Atkins which decides the treatment.

Typically, a solution feature under the carriageway is first sealed with a plastic membrane to prevent collapse through water ingress and, if the infill is particularly weak, it is strengthened with an increased thickness of capping reinforced with a layer of geotextile if necessary.

While work is proceeding well, Martin says last year's wet summer and the extremely wet winter have made earthworks difficult, with some delays while fill material was allowed to dry out.

Careful handling and compaction control was therefore needed when using Middle Chalk from the North Downs tunnel as Class 3 fill.This was used to build a major embankment that will support the London-bound carriageway as it approaches the Medway bridge. Using the Chalk saved 20,000 lorry movements. However it was of a quality that would not normally be used for a highway project, Dawson explains. The CTRL chalk fill was therefore placed to an end product specification of 8% air voids. Insitu testing was carried out to ensure this was achieved.

Next to junction 3, unforeseen ground conditions were encountered, in the form of an area of uncompacted fill containing voids and low levels of methane, Dawson says.

Additional site investigation was carried out using window sampling, gas spikes and dynamic probing. Following liaison with the Environment Agency, an environmental risk assessment by WS Atkins found that although the contamination was above the chalk aquifer, it could be treated insitu, avoiding the need to excavate and replace the material.

The solution was to use geogrid basal reinforcement beneath the embankment to span any voids and to spread settlement.A gas blanket was put down to allow methane gas to be vented off into a nearby trench and impermeable liner installed to prevent surface water ingress.

'Much of the project is controlled by environmental considerations, ' Dawson says.'For instance, all earthworks are required to have 1 in 2.5 slopes wherever possible.This is to allow extensive landscaping works, which aim to produce a green finish without chalk scars.

'Where landscaping is difficult because of landtake, soil baskets supported by soil nails are being used to allow grass to grow on steep slopes.'

He adds that ecological considerations included planting of two new woodlands.

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