This contractor were appointed by their client to install a new deep sewer at their Build-to-Rent scheme in Wandsworth, London. The contractor has been carrying out the demolition, contamination remediation, earthworks and groundworks since 2019 and these civils works are the final elements of their commission.
Due to the proximity of the proposed sewer access chamber to the site boundary and adjacent infrastructure assets, conventional open cut, sheet or trenching methods were seen as high risk in terms of vibration and ground movement. A novel approach was required to minimise construction risk. They proposed a Caisson Shaft methodology for the installation of the access chambers. This method was proposed on the basis that it avoided the requirement for traditional open cut, sheet pile or trench box methods with a subsequent reduction of potential vibration or subsidence risk. The proposal was quickly approved by the Client and adjacent infrastructure operator who commented on the multiple benefits of the proposed method over traditional deep manhole construction methods.
The methodology allows for the manhole shafts to be installed using a ‘jack-down’ method of pushing the manhole rings from the top, using hydraulic jacks mounted into a concrete collar whilst simultaneously removing soils from within the manhole. This approach avoided digging out the shaft area and surrounding soils and the associated risk of ground settlement and potential subsidence. The method also resulted in a much reduced volume of excavated soil and a similar reduction in import of granular backfill.
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