Preparation is the key for Bachy Soletanche at Must Farm
Leading geotechnical specialist, Bachy Soletanche Limited (BSL) is currently constructing an 1825metre slurry cut-off wall at Must Farm in Cambridgeshire – preparing the site for future quarrying of Oxford Clay and any present sands and gravels. In a contract worth around £800,000, BSL will oversee all environmental works as the project’s main contractor.
The Must Farm site, near Whittlesey, Peterborough is to be developed by Hanson Building Products as a quarry to extract brick clay, sand and gravel, and these will be transferred to its Kings Dyke and Saxon Brick factories. However, before quarrying works can begin, migration measures have to be carried out by BSL to prevent the ingress of groundwater from Kings Dyke to within the Must Farm boundary, which in turn will protect the groundwater levels of the Nene Washes.
Barrie Arkwright, Contract Manager at Bachy Soletanche commented:
“Must Farm has several links to groundwater networks, such as an existing flooded quarry located in the Western part of the site and also the Kings Dyke. As a result, Hanson Building Products required the construction of a slurry cut-off wall to stop the water from the Kings Dyke migrating through the sands and gravels – whilst preventing the external groundwater levels from lowering – as the site is quarried.”
The Kings Dyke tributaries also cross the site in five places, as Barrie Arkwright explained:
“We had to create a temporary causeway across each of the five ditches by laying casings lengthways into the water. This created a makeshift bridge for the plant equipment to drive over, and for the excavator to stand on when excavating the slurry trench.”
In order to create the slurry cut-off wall, a trench has to be installed using two excavators that work simultaneously. As these continue along the wall, the slurry, which is a carefully selected mix of bentonite, cementitious materials and water, is pumped into the trench utilising two mixer plants. Barrie Arkwright continued:
“Our supply of slurry had to remain constant due to the vast 1.8kilometre distance between the trench and the slurry plant. This required the two mixers to have enough pressure to pump the slurry through approximately 2000 metres of pipeline in order to reach the trench. Up to 450m3 of slurry can be mixed and pumped daily to produce approximately 50 linear metres of wall.”
“Once the trench is excavated and filled with the slurry mix, it will set to provide a low permeable barrier.” He added.
The land at Must Farm is occupied by tenant farmers and as the farmers will return to crop the land after BSL completes the migration measures – further preparation works were required to protect the farming land around the site. Barrie Arkwright explained:
“As the farmers will continue to crop around the Must Farm site, a 25metre strip of vegetation and topsoil needed to be stripped down to a depth of 300 millimetres. This would stop any of the slurry arisings contaminating the topsoil and allow them to continue farming once the slurry wall is capped with clay and the topsoil replaced. The Oxford clay used to cap the wall is quarried from the old Must Farm quarry on the site. As the wall runs right through the steep banks of the now depleted lake the steep banks have had to be built up to create a level platform to enable excavation of the wall once the Oxford Clay has been quarried.”
“The excavated soil will remain on site and then replaced when the slurry wall is finally complete.” He added.
The slurry wall is to be constructed along sections of the southern and western areas of the quarry boundary and will stretch to a total length of 1825 metres. The longer section is 1600metres, and the shorter section, on the far side of the flooded quarry is 225 metres. The depth of the wall varies between 6metres and 10.5metres deep and has to be installed 1.5 metres into the Oxford Clay to ensure it is watertight.
When the slurry cut-off wall is completed at the end of June, the area around the Must Farm site will then be dewatered enabling excavation of the upper superficial deposits.