Dr Toby Roberts FRENG WJ GROUNDWATER LTD Depressurisation strategies in the Lambeth Group

Mid-Lambeth Hiatus River Terrace Deposits Terrace Gravels Upper Aquifer Thames Group London Clay Aquiclude Harwich Formation Intermediate Aquifer Cohesive + granular laminations to channels Lambeth Group Upper Mottled Beds Laminated Beds Lower Shelly Beds Lower Mottled Beds Lower Aquifer Upnor Thanet Sand Formation Thanet Sand Bullhead Beds Chalk Mid-Lambeth Hiatus + occ sand channels London basin typical soil profile

Laminated Beds: Stiff Clay Upper Mottled Beds Laminated Beds: Stiff Clay Lower Mottled Beds: Stiff Clay Channel Sands MLH Lambeth Group/ Intermediate Aquifer Depth: 5 to 60 m to top Thickness: 15 to 20 m Predominantly cohesive Granular horizons range from laminations to sand channels: width: 2 to 300 m length: ? correlation with lost Rivers Thickness: mms to 12 m Reduced Oxidised

Crossrail: Stepney Grn Caverns 100 m long 40 m apart 8 to 17 m wide EB WB Caverns 100 m long 40 m apart 8 to 17 m wide 62 installations

Non-hydrostatic Recharge required to sustain profile

Boundary Conditions: Source of recharge? Seepage from London Clay Very low permeability Leakage via fissures or scour features Scour features relatively rare, little evidence Seepage via ungrouted wells or boreholes Plenty of them but not much evidence Horizontal flow from areas with higher groundwater Significant distances involved

Wellpoint pumps Access constrained in an advancing SCL cavern

Crossrail: Stepney Grn 45 Surface ejectors EB WB 105 tunnel wellpoints Crossrail: Stepney Grn

Crossrail: East London Stepney Grn Cross passage CP6 Works impacted by Lambeth Group

Cross Passage CP6 LW9 approximately 10 m away Lambeth soils above MLH cohesive Sump extends below MLH: Ejector dewatering required

Sump ejector system

As built: Combined probe holes and wells

Surprise!

Sand channel axis As built: Plan View

Technique Site Notes Passive relief wells Whitechapel Shafts, Limmo Pressure relief Surface deepwells Valence Gardens Extensive channel sands Surface ejectors Stepney Grn, Cross passages Tunnel wellpoints LIV/WHI, Cross passages Overbleed Tunnel ejectors VD4, Cross passage sumps >5 m drawdown Under drainage Some cross passages Proven Lower Aquifer link Crossrail dewatering: Techniques used to target Lambeth Group

Lambeth Group: Permeability range CIRIA: C515 Lambeth Group: Permeability range

Dewatering scheme drivers: Lithology and Geometry Excess pore pressure? Lithology: Laminations Layer Sand Channel Unknown? Elevation: Pressure relief Dewatering Overbleed Access: Constraints Opportunities Channel sands Laminated beds VD4 Dewatering scheme drivers: Lithology and Geometry

Dr Toby Roberts FRENG WJ GROUNDWATER LTD Dewatering strategies In the Lambeth Group Crossrail Liverpool Stn: VD4 inclined SCL tunnel: tunnel ejector system