The Moorhouse development, the Crossrail shaft and the proposed adjacent running tunnels are shown schematically in Figure 1; out of view of Figure 1 is a proposed deep Crossrail station cofferdam box immediately beyond Moorhouse. The cofferdam box, tunnels and cross-passages to the shaft will be built as and when Crossrail is constructed. Detailed information on the design basis for the shaft can be found in Morrison et al.,1 but it can be summarised as follows.
(a) The shaft has a total length of 37·5 m through the London Clay and Lambeth Group strata.
(b) The internal diameter of the shaft is 8.2m; it was sized to accommodate the secondary lining and future Crossrail equipment.
(c) Primary shaft segment thickness is 0·35 m for the full depth of the shaft. Each ring comprises ten ordinary segments and two tapered joint segments.
(d) ‘Special’ segments were fabricated for the rings through which secondary dewatering of the Upper Lambeth stratum may have been necessary by means of inclined wells constructed from within the shaft.
(e) A concrete grade C45/55 mix (class 2 sulphate resistance) with 30 kg/m3 Dramix steel fibre reinforcement was used for segment construction.
(f) Low-friction bearings (taped and sealed polytetrafluraethylene (P1FE)) were used on every fourth circumferential joint to allow for relatively easy shear distortion of the shaft during the main phase of Crossrail construction (tunnels and cofferdam box). These P1FE bearings act to limit bending stresses in the shaft that would result in a substandard design factor of safety. All bearings (standard or P1FE) were 4 mm thick, thereby avoiding the need for special segments or gaskets.
Figure 1: Schematic view of Moorhouse and Crossrail shaft and running tunnels (not to scale).