Permeation grouting and excavation at Victoria station, London

Paid area link 6 (Pal6) underpass, a crucial tunnel connection of London Underground's Victoria station upgrade project, was engineered within notoriously difficult sub-artesian granular River Terrace Deposits (RTD) immediately below the Victorian-age District and Circle line masonry tunnel, a structure highly sensitive to ground displacements. Due to the proximity of the overlying operational railway, the consequences of ground movement were of very high risk. Historic tunnel construction in the heterogeneous RTD had limited success with permeation grouting, and face losses and instabilities occurred. For the case of Pal6, the railway tunnel overhead magnified the difficulties. Permeation grouting still remained the most feasible stabilising option, but only with significant improvements in both grouting technique and grout types. A low-risk tunnelled solution, using a bespoke and sophisticated construction approach, enabled safe continuous excavation within the severely restricted structural constraints. Successful tunnelling demanded: complete sequence recovery of the RTD for accurate soil characterisation for design of the grouting system; caution with permeation grouting; appropriate grout selection to promote permeation without claquage (uncontrolled fracturing by grout with its consequent irregular distribution), thus precisely targeting discrete zones; uniform treatment; and observational control by monitoring real-time ground movements.