Publication Abstract

Proceeding of the 2018 DFI-EFFC International Conference on Deep Foundations and Ground Improvement, Rome, Italy, (DFI, EFFC)

Delivering Added Value via Advanced Ground Investigations for Deep Shaft Design in Urban Areas
Anthony S. O'Brien, Global Practice Leader-Geotechnics and Hock L. Liew

For the Crossrail project the Moorgate shaft, at depth of 42m, was the deepest shaft on the project and was also located within one of the most confined areas, with surface and subsurface structure located with a few metres of the shaft perimeter. The start of the shaft construction was delayed due to difficulties in removal of old foundation piles that had previously occupied the site. A major constraint was limiting the movement of an adjacent existing tunnel, which meant that the original design required a series of closely spaced props close to the base of the excavation. The paper describes the practical application of an advanced soil stress-strain model, which directly uses field geophysics measures of shear modulus at very small strain (Go). The advanced soil model also facilitates stiffness anisotropy to be considered, based on field or laboratory geophysical tests. The shaft construction programme was shortened by 14 weeks (which meant that the Crossrail TBM drive was not delayed) by implementing the advanced soil model, within a 3D numerical model, together with a carefully controlled verification process (which included detailed field observations of actual shaft deformation). The modelling predictions were sufficiently accurate to reassure external stakeholders that internal propping could be significantly reduced, without causing adverse movements of adjacent buildings or tunnels. The paper outlines the ground investigations and the way in which the advanced soil model input can be measured directly.

 article #2945; publication #1040 (ROME-2018)