Water & Wastewater Treatment

PROJECT FOCUS Crossrail TBM sewer pass – closest ever A unique strengthening project has protected a major London sewer, sited just 500mm from two Crossrail tunnel boring machines. UKDN Waterflow's technical director, Mark Lusher, explains how it was achieved P rotecting the strategically important Ham sewer in London's east end from one of Crossrail's 7.1m-diameter tunnelling boring machines (TBMs) earlier this year led to a unique civil engineering project. The 500mm sewer pass by one of two large-diameter TBMs is believed to be the closest ever and led to an innovative response from an alliance of partners. The complex 20-day project (with associated works spread over a number of months) saw a 50m, continuous, duplex stainless steel structural lining jacked into place within the existing 2.06m-diameter sewer and precision-welded to standards normally associated with very high-pressure pipelines. UKDN Waterflow worked with the Dragados Sisk, a joint venture between Spanish contractor Dragados and Irish construction company John Sisk & Son (DSJV), and Thames Water on behalf of Crossrail on this project. Engineers Fairhurst provided consultancy services. The project first started in 2011 when UKDN Waterflow was asked to carry out the initial survey of the sewers as part of Crossrail preparatory work. It was identified that the TBMs would pass close beneath the Ham Sewer and the Wick Sewer in the area alongside the northern approach to the Blackwall Tunnel. The coverage and complexity of the Ham Sewer is of strategic importance to the east of London – 2M residents and businesses rely on its continual flow. Engineers working on behalf of Crossrail searched for examples of protection works for this type of issue and found that, of 25 projects worldwide, none came anywhere near such limited parameters. Design This was by far the closest pass by a tunnel to such a major and sensitive structure. Various ideas were considered and rejected, including using a huge UV-cured GRP liner. In the end, precision surveys of the sewer using laser-burst technology by UKDN Waterflow were passed to engineers Fairhurst, who helped turn UKDN's design into a workable technical solution. Ground movements along the sewer had been previously assessed based upon empirical methods and this had led to the conclusion that there was a high likelihood that movements would damage the masonry of the sewer wall. To design the steel lining and to determine the effects of lining stiffness on ground movements, longitudinal deformations were modelled under a 2D finite element analysis. Ground movements were initially calculated assuming a zero tunnel stiffness with a 1% tunnelling volume loss. Ground movements were then recalculated with the stiffness of the steel lining input as a beam strip. These results were then used to proportion the predicted empirical ground movements to determine the effects of the continuous steel lining on overall settlements. Modelling Once the settlement had been assessed, the lining was modelled as a beam element with a deflected shape to match predicted deformation. The induced bending moment was used to calculate bending stresses within the lining. Circumferential bending was also considered using predicted settlements at the crown and invert of the sewer and modelling this as an imposed distortion to the tunnel lining. Global buckling of the lining shell was considered, assuming that the lining is mostly supported by the surrounding ground and that any loss of support will be local only. The effects of local buckling was also assessed by considering the critical theoretical stress at which buckling of the liner would occur. The design for the 50m long continuous 12 Water & Wastewater Treatment January 2014 "Engineers working on behalf of Crossrail searched for examples of protection works for this type of issue and found that none came anywhere near such limited parameters" structural steel lining was subject to a full independent Category 3 design check and acceptance by Crossrail and Thames Water experts. To satisfy Thames Water's concerns on the durability of the lining, it was decided to construct the lining in stainless steel plate. To resist the stresses induced by predicted deformations a high-strength duplex stainless steel, flown in from Italy and the US, was used.

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