Design and construction of a deep shaft for Crossrail

Abstract

An 8·2m diameter, 40m deep shaft for Crossrail is being constructed below the 10m deep basement of the Moorhouse development near Moorgate in the City of London. The depth of the shaft is such that it will penetrate through stiff London Clay and will be founded at the bottom of the Lambeth Group. The shaft is being constructed after the Moorhouse structure has been completed and the design of the Moorhouse foundations places tight constraints on acceptable ground movements due to construction of the shaft. Furthermore, the shaft needs to be designed to accommodate future ground movements associated with construction of Crossrail. The paper describes the complex relationship between the foundations of Moorhouse, the draught relief shaft and the future Crossrail assets. The optimised design includes extensive slip coating and base grouting of the Moorhouse piles, a complex temporary works dewatering system around the shaft and the option to carry out additional dewatering from within the shaft during construction. Control of ground movements through the Lambeth Group was perceived to be a particular problem in relationship to destressing the ground around the Moorhouse piles. To prevent longterm settlement of these piles, provision was made for radial grouting to “restress” the ground should the need arise.

Surface settlements arising from elliptical shaft excavation in clay

“…In an urban environment where there are limitations on available space, noise and vibrations, shafts are usually constructed by underpinning (e.g. Morrison et al, 2004). Underpinning is a construction technique which incrementally excavates the shaft followed by installation of a pre-cast concrete segmental lining and grouting behind the annulus (Allenby & Kilburn, 2015).…”

Construction of a deep shaft for Crossrail

“…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 (2004) and are summarised as follows:…”

“…Details of the finite element analyses associated with the shaft construction are given in Morrison et al (2004) and the calculated and measured deformations at inclinometer IG1 are shown in Figure 10 for four intermediate dig stages and the final conditions with the base of the shaft constructed. These show the general trend of increasing ground movement with depth in keeping with the larger stress release at depth compared to shallow positions.…”