Delivering Added Value via Advanced Ground Investigations for Deep Shaft Design in Urban Areas-Paper

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The Crossrail Moorgate shaft was a 42 m deep excavation in a highly constrained urban site, with nearby buildings and London Underground tunnels—one just 4 m from the shaft. Limiting ground movement was essential, but the original support design included closely spaced temporary props that would slow construction.<br /><br />The project used advanced ground investigations to measure soil stiffness across different strain levels. Tests included seismic geophysics, self-boring pressuremeter testing and instrumented laboratory triaxial tests. Results highlighted significant stiffness non-linearity and anisotropy: horizontal small-strain stiffness was typically 1.5–2 times vertical stiffness. Laboratory measurements were also affected by sample disturbance, particularly in deeper and more variable strata.<br /><br />To interpret the data, the team applied the A* framework, which combines directly measured small-strain shear modulus, undrained shear strength and failure strain to derive non-linear stiffness curves. These parameters were incorporated into a three-dimensional FLAC3D model. A staged verification process compared predictions with measured shaft movements as excavation progressed. The model was refined to represent horizontal stiffness, producing predictions that closely matched observed wall deflections.<br /><br />Confidence in the predictions allowed the project to omit temporary props across nearly 12 m of excavation and reduce construction stages. Monitoring confirmed that movements of the adjacent tunnels were acceptably small. The revised approach shortened the shaft programme by 14 weeks, preventing a delay to the Crossrail tunnel-boring machine drive.<br /><br />The case demonstrates how advanced ground characterization, appropriate modelling and systematic verification can improve movement predictions, reduce temporary works and accelerate deep excavation construction in sensitive urban environments.

Keywords

Crossrail

Moorgate shaft

deep excavation

urban construction

ground movement

soil stiffness

A* framework

FLAC3D modeling

temporary props

tunnel monitoring