Back Analysis of Observed Measurements for Optimised SCL Tunnel Design

Summary

The Crossrail Farringdon Station is a deep level station with two platform tunnels, 300 metres long at a depth of approximately 30 metres below street level. The running tunnels are primarily TBM bored with the platform tunnels enlarged using sprayed concrete lining (SCL) tunnelling method. The station is located in challenging ground conditions due to the presence of several faults and the heterogeneity of the Lambeth Group, which call for a variety of temporary works contingency measures and in-tunnel depressurisation scheme. Back analyses for the already constructed cross passage CP1 were performed using finite element (FE) analysis in order to calibrate them against observed deformations, deriving valuable information for future tunnel design. In order to achieve this, the accuracy of assumptions made in the initial design stages had to be checked. Such parameters as ground relaxation factor for construction advance, at rest earth pressure coefficient and soil stiffness are back-calculated and compared with the initial values. The outcome of this work was refined FE modelling parameters based on measured deformation results, demonstrating a greater accuracy for predicting both in-tunnel and surface deformations and thus allowing for a better understanding of the tunnel lining and ground behaviour to be anticipated in advance of construction.