Royal Woolwich Arsenal – Crossrail Box Sewer Diversion

Summary

In a unique arrangement Berkeley are collaborating with Crossrail Ltd (CRL) to deliver the Royal Arsenal Woolwich Station Box. In order to construct the Box initial ground investigation was required as well as site clearance and multiple utility service diversions. The most complex of the utility diversions was the diversion of 325m of diameter live private sewer serving the Woolwich Arsenal estate. In February 2011 Berkeley instructed Waterman to undertake a feasibility assessment and propose a permanent design solution.
Waterman’s design anticipated trenchless methodologies would need to be utilised to minimise settlement of the pipeline when the Crossrail tunnels are bored beneath the realigned sewer in 2013 as disturbances in the ground may increase the ground settlements and consequently the deflections in the pipe. Detailed analysis of ground settlements as a result of the Crossrail Box were undertaken along with the resultant pipe deflections and the ability of the pipework used to accommodate the anticipated movements. Due to the stage of tunnel design, the ground settlement from the tunnelling operation was unknown and as such in the future further analysis will be required by Crossrail of the pipe’s ability to accommodate settlements from the tunnelling operation. The revised alignment included 9 new manholes and micro-tunnels varying in diameter from 225 mm to 1200 mm.
In preparation for tunnelling a number of neighbouring buildings required strengthening to prevent any damage if subsidence was to occur. Remote 3D monitoring with robotic total stations by Soldata was established to monitor the buildings and existing services prior to tunnelling and during the construction of the Station Box. This provided cycles of readings every 15 minutes providing assurance that the buildings were not suffering any structural damage.
Following successful tender, Joseph Gallagher Ltd was awarded the contract to undertake the sewer diversion. Their experience in tunnel boring in testing ground conditions proved essential to the successful completion of the works in line with the constrained project programme. The tunnels were bored through the shallow aquifer at depths of between 4.4 m and 8.4 m below ground level. Gallagher commissioned a bespoke tunnel boring machine (TBM) & pipe jacking rig from Iseki. The TBM was equipped with ground water pressure sensors. The TBM utilised a slurry pumping system to remove the arisings from the tunnel face and to maintain a positive pressure against the ground water, negating the need for any dewatering.
The completed foul and combined sewers needed to meet stringent adoptable standards requiring design approvals and regular inspections by Thames Water during construction. Temporary over-pumping of significant flow rates was required during the diversion works and will also be required during the Crossrail tunnelling operation in case any significant settlement occurs. Crossrail’s tunnelling team, Hochtief Murphy JV, is examining temporary support options to prevent any significant settlement of the diverted pipeline.