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Dr Toby Roberts FRENG WJ GROUNDWATER LTD Depressurisation
strategies in the Lambeth Group
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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
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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
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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
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Non-hydrostatic Recharge required to sustain profile
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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
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Wellpoint pumps Access constrained in an advancing SCL cavern
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Crossrail: Stepney Grn
45 Surface ejectors EB WB 105 tunnel wellpoints Crossrail: Stepney Grn
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Crossrail: East London
Stepney Grn Cross passage CP6 Works impacted by Lambeth Group
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Cross Passage CP6 LW9 approximately 10 m away Lambeth soils above MLH cohesive Sump extends below MLH: Ejector dewatering required
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Sump ejector system
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As built: Combined probe holes and wells
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Surprise!
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Sand channel axis As built: Plan View
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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
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Lambeth Group: Permeability range
CIRIA: C515 Lambeth Group: Permeability range
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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
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Dr Toby Roberts FRENG WJ GROUNDWATER LTD Dewatering strategies In the Lambeth Group Crossrail Liverpool Stn: VD4 inclined SCL tunnel: tunnel ejector system
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