Innovative Caisson Shaft Construction for Deep Sewer at Homebase Wandsworth

7 June 2021

Cognition Land and Water were appointed to install a new deep sewer at their Build-to-Rent scheme in Wandsworth, London.  Cognition has been carrying out the demolition, contamination remediation, earthworks and groundworks at both the Homebase site and the adjacent B&Q site for Legal & General Investment Management since 2019 and these civils works are the final elements of our commission. 

Due to the proximity of the proposed sewer access chamber to Network Rail 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 the risk to the rail infrastructure.  Any works within the 5m Network Rail easement zone required legal agreements and were subject to Network Rail oversight.  Working with civil engineering specialists Sub-Tech, Cognition proposed a Caisson Shaft methodology for the installation of the access chambers.  This method was proposed by Cognition on the basis that it avoided the requirement for traditional open cut with a subsequent reduction of potential vibration or subsidence risk.  The proposal was quickly approved by Network Rail 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.  This all fits with Cognition’s mantra of minimising waste soils from construction sites and maximising reuse of soils wherever possible. 

Apart from the quicker Network Rail approval process, the method adopted at Wandsworth offers significant construction programme benefits as access shafts can be installed more quickly than conventional methods.  In this instance, the manholes were constructed in half the allocated programme time. 

In addition, so impressed were the Considerate Constructors Scheme with the methodology during a recent site audit, that they have submitted our Construction Innovation Report to their CCS Review Group in support of the innovation.

If you would like more information on the work we are doing at our Wandsworth sites or our innovative approach to construction problem solving, please contact David Skinner on david.skinner@cognitionltd.com

Standard method of manhole construction

Caisson Shaft jacks installed at ground surface to push the manhole rings down into the surrounding soils, whilst material is removed from within the rings using a ‘clam-shell’ excavator attachment. As the caisson shaft progresses downwards, additional c

The concrete base was cast with a sump to allow groundwater control whilst the connection to the sewer is made.