ABSTRACT
The Nine Elms station sub-structure is a key component of the Northern Line Extension scheme. The sub-structure comprises a 24 m deep, 142 m long and 29 metre wide excavation formed with secant pile walls. Excavated through superficial deposits underlain by the London Clay formation, construction adopted a top down sequence with three levels of propping. The basis of design for the primary works (excavation of the substructure and construction of the base slab) was for completion within a twelve month period.
Modifications to the overall project delivery programme increased the construction period for the primary works from 12 months to 19 months, compromising the original basis of design and indicating that the as built structural capacity of the secant walls would be exceeded by in the order of 15%. The way forward was through implementation of the Observational Method by progressive modification. Controls on the excavation sequence and contingency actions, including the use of structural blinding struts formed part of the Observational Method (termed Verification Process for this application).
As the works progressed monitoring data and calibration with a 3D numerical model was used at successive excavation stages to make forward predictions of soil-structure interactions. The outcome of the Verification Process and modelling predictions was the permitted elimination of all additional foreseen temporary support measures. This paper describes the excavation of the box, the numerical analysis to support the excavation sequence, the monitoring and application of the Observational Method by progressive modification.
