Design optimisation and challenges with the deepest shaft on HS2

This paper describes the design development of the deepest ventilation shaft along the C1 alignment - Chalfont St Peter Ventilation Shaft. The developed design solution was a 17.8m internal diameter, 65m deep shaft constructed within Chalk strata using 1200mm thick, 78m deep diaphragm walls. The Tunnel Boring Machines (TBMs) forming the twin running tunnels are positioned at 25m centre to centre spacing and directly intersecting the shaft walls to form the ventilation openings and service access routes into each bore. Specific technical challenges associated with the design are presented in this paper together with details of how advanced soil-structure interaction modelling using PLAXIS3D^[7] finite element program has been utilised to optimise the design and construction of the shaft.

The detailed modelling of the stage construction and TBM advancement provided a clear insight of the load path and the arching/relaxation effects of the surrounding ground during different stages of the construction. This allowed reliable optimisation of the diaphragm walls and the collar structure design around the shaft openings without any temporary propping requirements inside confined work environment. The design forces for the critical elements of the shaft structure were directly extracted from the PLAXIS3D model to inform the detailed structural design.