Bachy Soletanche’s slurry wall solution holds water at Alderley Edge By-Pass project
Bachy Soletanche Limited is working on the A34 Alderley Edge by-pass project to provide assistance on water table challenges found at the Welsh Row section on behalf of main contractor, Birse Civils. It is Bachy Soletanche’s experience in the construction of slurry cut-off walls – a system rarely used in Britain on road construction – that is benefiting this delicate section of the project and the sub-contractor will be on site until the end of September.
The A34 bypass around Alderley Edge in Cheshire is a major ongoing road project. It has to cross existing infrastructure, in particular the main Manchester to London railway line and due to environmental impacts the bypass will pass under the railway line. The road level will be in a cutting with the road box formation being 6m below the ground level at the Welsh Row section.
The location however poses several challenges for Birse Civils, as the ground to the north and south of the railway has a very shallow water table, ranging from approximately 1m to 3m below ground level. In this particular area and in a wet period, the water table can actually rise to just below the ground surface.
The major issue is that cutting through this area would draw down the high water table, which could have an affect on the structural integrity of the railway track via the risk of increased settlement. What’s more, if the water table was altered in any way it could affect the surrounding area by way of drying out natural wet areas.
One of the biggest challenges facing the designer was how to overcome the problem and create an impermeable barrier, preventing water seepage to maintain the natural water table levels. Several options were explored including sheet piling or piled wall techniques, but the most cost effective solution to this problem was to create a slurry cut-off wall.
A slurry wall is a non-structural barrier that is constructed underground to impede groundwater flow and is often used on land reclamation projects. The construction of a slurry cut-off wall requires the excavation of a narrow trench which is kept full of an engineered fluid or ‘slurry’. The slurry exerts pressure against the trench walls and acts as shoring to prevent collapse.
Bachy Soletanche, along with Birse Civils, Cheshire East Council and geologists carefully planned the detail of the wall. The best way forward was to construct the impermeable barrier with a 800mm wide trench, a length of 1km and varying depths up to a maximum of 20m – ultimately forming a large rectangle.
Due to its expertise in slurry wall construction, Bachy Soletanche is constructing the wall using a cement-bentonite slurry wall technique. For this method, cement is added to the bentonite water slurry just before it is installed into the trench. This mix not only prevents collapse but also forms the permanent cut-off wall.
Previous ground strata sampling found that to the north of the railway line a trench of approximately 14m, going down to 20m along the south side of the track, needed to be dug. This means Bachy Soletanche can reach the impermeable layer of mudstone. In fact excavation keys approximately 1m into the low permeability stratum to ensure minimal leakage under the final wall.
Due to the shallow excavations at the north side of the railway, a 360-degree excavator is used, working to a 14m depth. For the deepest sections of the slurry cut-off wall – to the south of the railway line – Bachy Soletanche used a crawler crane fitted with a clamshell grab to dig out the trenches. This was brought in from Spain especially for the project.
The road will be constructed within the impermeable barrier through the cutting under the railway line and rising out of the other side, so the road will ride over the impermeable barrier on the exit of the cut. The process of forming the slurry wall will provide a 100 per cent seepage-free rectangle. The natural rainwater that falls within the cut-off wall section will be drained by the highway drainage system. Once complete, the new road would provide a bypass to the west of Alderley Edge and Nether Alderley villages in Cheshire and will be 5.4km long.
Bachy Soletanche Project Manager, Rob Howarth concluded:
“It is pleasing that Bachy Soletanche’s expertise in the slurry cut-off wall technique has proved essential to the succession of this project. It’s also an interesting and challenging project from laying over 1km of 90mm alkethene water supply pipe at the start of the works up to our recent completion.”