Ground heave in deep cuttings: evolution from generic to specific models to enable refinement of a risk management strategy

The route of the UK’s high-speed rail network traverses the hilly terrain of the south midlands of England on a low, flat, straight alignment. Creating this alignment requires the formation of deep excavations through hills and ridges formed from the local geology which includes argillaceous (clay) strata. The process of excavation removes significant weight from the strata below the base of the excavation, and it is expected that this ground will rebound. Observation of similar effects elsewhere, particularly in basement excavations in London Clay, indicate that this ‘rebound’ can take many months to occur and can be of magnitudes far greater than the movement tolerances set for the High Speed Two (HS2) track formation. This paper presents the evolution of a numerical modelling technique for the assessment of long-term heave in the base of deep, open, infrastructure cuttings. The key drivers and impacts of the phenomenon are discussed and illustrated with case studies. The development and calibration of soil models for geological materials anticipated on the HS2 route are presented. The application of the modelling is related directly to the engineering challenges on HS2 Phase One with examples of both conventional and unconventional mitigation options.