Comparison of earth pressure balance and slurry tunnel boring machines used for tunnelling in chalk

Earth pressure balance (EPB) and slurry tunnel boring machines (TBM) have both been utilised for construction within the Chalk in the UK, however, there has been a general trend recently in preferential selection of slurry TBMs. This presents greater logistical considerations in terms of handling and treatment of spoil in comparison to an EPB TBM. In this study, case histories of tunnel construction in chalk in the UK have been reviewed and the advantages/disadvantages of each machine type in relation to various geologies, ground properties and operational aspects evaluated. This review suggests that slurry TBMs are appropriate where the Chalk is present at outcrop and has been subject to near-surface weathering processes or is overlain by superficial deposits or a thin cover of Palaeogene deposits, where the permeability of the Chalk is high and where the tunnel is below the water table. Where the Chalk is present at subcrop and has not been subject to near-surface weathering process (so is relatively unweathered e.g. typical CIRIA Grade A1/A2) or is overlain by thick Palaeogene deposits and where the permeability is moderate or low and the tunnel is either above or below the water table, an EPB TBM may be suitable subject to an appropriate maintenance schedule. This suggests that slurry TBMs should not automatically be considered the most appropriate technique for tunnelling in chalk, and selection and optimisation of TBMs for future tunnelling projects in the Chalk should be made by consideration of the full ground model.