Abstract
Abstract As part of the Three Valleys Water Scheme a 6 km long 2.54 m internal diameter tunnel was driven from Iver Treatment Works (Bucks) to Wraysbury Reservoir. The general geology of the area consisted of 2–3 m of river terrace gravels overlying London Clay and Woolwich and Reading Beds. During investigations carried out in advance of the tunnel face anomalous ground conditions were detected. A deep gravel filled scour hollow was located which cut down 30 m into the London Clay onto the proposed tunnel line. In addition the underlying clay and water bearing sands (under artesian pressure) of the Woolwich and Reading Beds had been folded upwards. Extensive ground treatment was carried out consisting of an initial programme of claquage and gel grouting followed by ground freezing. Measurements of tunnel and lining movements were carried out initially in the wedge block line section and through the frozen section. Measurements indicated that ground treatment had been successful with negligible ground movement towards the tunnel face and with little lining deformation.
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