Property of WJ Groundwater

Transcription

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

2 River Terrace Deposits Terrace Gravels Upper Aquifer Thames Group Lambeth Group Thanet Sand Forma=on Chalk London Clay Harwich Forma:on Upper MoDled Beds Laminated Beds Lower Shelly Beds Lower MoDled Beds Upnor Thanet Sand Bullhead Beds Chalk London basin typical soil profile Aquiclude Intermediate Aquifer Cohesive + granular lamina:ons to channels Mid- Lambeth Hiatus + occ sand channels Lower Aquifer

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

4 62 installa=ons Crossrail: Stepney Grn EB WB Caverns 100 m long 40 m apart 8 to 17 m wide

5 Non- hydrosta:c Recharge required to sustain profile

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

7 Access constrained in an advancing SCL cavern Wellpoint pumps

8 45 Surface ejectors EB WB 105 tunnel wellpoints Crossrail: Stepney Grn

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

10 mtd LW MLH Made Ground River Terrace Deposits A3 London Clay A2 UMB LTB LMB UF Lambeth Group Thanet Sand Cross Passage CP6 LW9 approximately 10 m away Lambeth soils above MLH cohesive Sump extends below MLH: Ejector dewatering required East Bound West Bound 71.21

11 Sump ejector system

12 mtd MLH 60 LW9 WU2/4 WU1/3 PR4/9 PR1 EB WB PR6 WP2 UMB WP3 PR3/ LTB LMB Lambeth Group NW1/2 NW6 NW3/5 NW8/11 NW9 EJ3 NW NW4 UF EJ2 P1 Thanet Sand As built: Combined probe holes and wells NW10 NW19 NW18 WP7/14 EJ4 EJ5 NW16 EJ1/6 NW13/14 NW15 PR10 NW17 PR5 WP8/13 WP19/22 WP18/23

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14 Pr op er ty of J W nd ou Gr w at er

15 Pr op er ty of J W nd ou Gr w at er

16 LEVEL OF CONCRETE mtd LW9 UMB LTB LMB UF Thanet Sand EB UPPER MOTTLED BEDS mtd mtd mtd WB LONDON CLAY SAND CHANNEL LAMINATED BEDS LOWER MOTTLEED BEDS UPNOR FORMATION

17 As built: Plan View East Bound NW20 LW9 TCBs NW3 NW2 NW1 EJ2 EJ3 P1 NW11 NW10 EJ1 NW12 PR1 NW17 PR5 NW18 NW19 WP5 Scale 0 WP6 WP2 WP17 NW5 NW6 NW8 NW4 NW9 EJ6 EJ4 EJ5 PR6 PR10 NW16 WP18 WU2 WP3 EJ8 NW15 NW7 NW13 NW14 WP14A WP15A WP14 WP16 WP15 PR3 WU2B WP12 PR9 WP13 WP28 WP11 WU1 WP26 WP10 WP25 WU4 WP27 WP24 WP24a PR4 EJ7 WU3 WP9 WP7 WP8 WP22 WP23 WP30 WP29 WP21 WP20 WP19 West Bound 5m PR8 Key: Sump ejectors Downwards probes/wellponts Upwards probes/wellpoints not installed

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19 Technique Site Notes Passive relief wells Whitechapel ShaYs, Limmo Pressure relief Surface deepwells Valence Gardens Extensive channel sands Surface ejectors Stepney Grn, Cross passages Extensive channel sands 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

20 Lambeth Group CIRIA: C515 Lambeth Group: Permeability range

21 Excess pore pressure? Lithology: Lamina=ons Layer Sand Channel Unknown? Channel sands Laminated beds Eleva=on: Pressure relief Dewatering Overbleed Access: Constraints Opportuni:es Dewatering scheme drivers: Lithology and Geometry

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