Dewatering strategies for tunnels, shafts and excavations
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These guys are installing a wellpoint out of a tunnel, I’ll be explaining why and how later on.
Groundwater has a habit of seeping through the best theories.Professor Michele JamiołkowskiUniversity of Turin, Italy
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…so there’s no theory in this presentation just practical solutions to real situations. Clean water ingress to a tunnel can be a pain but instability, uncontrolled ingress and associated ground loss is highly dangerous.
Permeability range for active dewateringCIRIA: C750
ŁódźPoland
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…from CIRIA C750. Plot of drawdown and permeability which are the main drivers for selection of a dewatering technique. Range dictated by several factors including pumping constraints - wellpoints, geotechnics – vacuum needed <10-5, tricky <10-7 and economics – excessive flow.
Excavation 20 m depthPlan size 780 m by 50/130 m60 m thick sand aquiferPermeability 1.8 x 10-4 m/s
Łódź Station Poland
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New mainline railway station being built underground with tunnel connections. The ultimate in homogeneous isotropic conditions.
49 wells design flow 600 l/s
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A classic perimeter well arrangement comprising 49 wells with a design flow of 600 l/s.
Top-down excavation
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Top down, ground flow cast as a top prop followed by mining below. Wells installed internally despite our advice that there is no cut-off benefit in a deep uniform aquifer. Switched to external wells as soon as data proved our numerical model results correct.
Łódź, Poland: Water level and flow data
General dig
Max dig
15 m
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Useful drawdown / flow data set showing 15 m drawdown with peak flow 450 l/s reducing to 300 l/s after 3 or 4 months. Pumping on subsequent sections commenced part way through which was why flow dropped below 300 l/s
Crossrail: East LondonStepney GrnCaverns
Limmo shaft
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Crossrail east to west railway which is tunnelled through central London. Dewatering mainly on the eastern section which reached through the London Clay. I have chosen some examples from 34 separate dewatering schemes to illustrate a range of strategies.
Upper aquifer
Intermediate aquifer
Lower aquifer
Cros
srai
l
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Typical central London soil and groundwater profile. Note aquifers and impact of over abstraction. Crossrail tunnels in the London Clay and below
Terrace Gravels
Lambeth Group
Thanet Sand
Chalk
London strata permeability ranges
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London soils cover almost the full permeability range for dewatering plus all techniques providing for an interesting test bed.
LimmoShafts
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Second case study is The Limmo drive shaft, the larger shaft to the left. Close to the Thames as you can see. Water trasnport used for tunnel spoil disposal.
Main Shaft: - TBM drives
-Ventilation shaft-30m i.d. 44.3m deep
-diaphragm wall to 55 m
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100
80
60
40
20
London Clay
Lambeth Group sands
Upnor Formation/Thanet Sand
Chalk
MainShaft
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Depressurization required in the Intermediate Aquifer and Lower Aquifer Upper aquifer cut-off Intermediate cut off by D-wall: internal relief only Lower: avoid uplift failure. Required drawdown variable depending on the stage of the excavation. Max (25m) for base slab. Some reduced drawdown required during 3 year tunnelling period with no drawdown on completion of vent shaft.
Excavation100
80
60
40
20
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Proposed scheme for shaft excavation and base slab based on pumping test data 7 external Chalk wells + 6 external Thanet wells. 3 internal Lambeth Group passive wells + further 6 Thanet Sand.
Excavation + base slab TBM drives
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Green chalk, red Thanet Sand GWL to left hand scale mTD Blue lines flow to right hand scale: Dark blue chalk, light blue Thanet Sand Initially the Thanet Sand follows the chalk, note additional drawdown when Thanet Sand wells pumping Once excavation and base slab complete chalk decommissioned, then just Thanet passive at greatly reduced flow.
100
80
60
40
20
Base slab complete
External wells off
Internal passive relief only
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Base slab was installed: required drawdown reduced: external wells decommissioned: internal passive relief only Big reduction in abstraction flow as chalk pumping now ceased.
Stepney Green: SCL Cavern
17 m
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Large caverns in London Clay with Lambeth Group sands at invert at 1 to 2 bar pressure.
Access constrained inan advancing SCL cavern
Wellpointpumps
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GI proved a continuous sand horizon across the site. Surface installations preferred as works in an advancing SCL tunnel can be very congested. Note some in-tunnel wellpoint pumping which I will explain.
Crossrail: Stepney Grn
EBWB
45 Surface ejectors
105 tunnelwellpoints
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Surface scheme comprised 45 vertical and inclined ejector wells with wellpoints in the deeper west bound tunnel. Note pore pressure targets for east and west bound caverns.
Fall
71
72
KIN
G J
OH
N S
TREE
T
L Twr
GAR
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STR
EET
Farm
L Twr
0 5 10 15 20 25mScale
W1W2W3W4
W5W6W7W8
W9
W10
W11(location to be confirmed)
W12
W13
W14W15
W16W17
W20
W21
W22
W19
W18
W23W24W25W26
W27
W28
W29
W30
W31
W32
W33
W34
W35
W36
W37
W38
W39
W41
W43
W44
W45
W40
W42
20°
20°
20°
20°
10°
30°
20°
10°
20°10°
10°
10°
30°
20°
10°
20°
10°
30°
EB ? PM
EB ? CW
WB ? PM
WB ? CW
EB
WB
Pumpingstation
Discharge point
150mm Supply &return headermains
150mm Supply &return headermains
Pumpingstation
To discharge
Site
SCL caverns
0 25m
Inclined wells reach beyondsite boundary
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SCL caverns Constructed from d-wall shaft Site area does not cover extent of caverns Inclined wells used to achieve a perimeter ring around the structures, particularly the deeper west bound cavern
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Installation arrangements for an ejector well inclined at 30 deg from vertical. Right hand photo shows a set of operating inclined ejector well heads.
Surface schemes?• Congested urban area• Intermittent/thin sand channels/horizons• Short screen length in target stratum• Poor flexibility to respond to feedback
Go underground!
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This shows a drilling set-up at a planned intervention because there was a known sand horizon ahead in this case. Planned interventions used to avoid wells being destroyed by the advancing heading.
Augur probe drilling
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Augur drilling for investigation to prove clay or where conditions uncertain. This is a typical set up for upward augur drilling. Note SGI rings to be removed for cross passage access.
CP5 PILOT TUNNEL - GEOLOGICAL LONG SECTION SKETCH
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Augur returns logged to build up geological profile. The best geological sections were prepared immediately by hand by the onsite engineering geologists. In this case the pilot tunnel has been constructed in Clay (shown in brown) but the probe holes show that the enlargement will encounter water bearing sand (shown in yellow).
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Drilling our of a tunnel into sand at 1 bar plus pressure requires careful planning. Essential to always be able to seal the bore rapidly under any circumstances. The strategies used included; grouted inserts, fixed stuffing box with shutoff gland, lost bit drilling for well installation. Sometimes necessary to install short sacrificial wellpoints to reduce the pressures to allow redrilling with longer well screens
Enlarge ~11m ΦPilot ~6m Φ
SCL
Photo by Dr A. Stärk
LondonClay
Channel Sands
Liverpool St+
Whitechapel
Platform tunnels
1 bar +excess head
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At Liverpool St and Whitechapel stations platform caverns both involved pilot tunnel excavations in London Clay. This was followed by enlargement which extended in to the Lambeth Group with potential for intermittent water bearing channel sands with 1 to 1.5 bar excess head.
Wellpoints
Dewatered sand channel
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SCL tunnel with sand channel successfully dewatered using wellpoints. Note wellpoints often destroyed by the advancing SCL tunnel: need compact, efficient, easily maintained and adaptable pumping plant.
High flowElectric piston pumps
Low flowDMVexpumps
Wellpoints pumps
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Wellpoint pumping equipment: high efficient electric piston pumps - for higher flows; Dmvex vacuum pumps for lower flows normally used for remediation. Low power consumption ideally suited for use in a tunnel environment.
FarringdonTBM enlargement
Sand channel at axis
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At Farringdon we had the luxury of a TBM pilot tunnel but with a sand channel at axis. The TBM tunnel was ‘blind’ so we had to pass the pumping main through he advancing face. Occasionally necessary to install replacement wellpoints as the enlargement advanced. Sometimes able to drill wells or connections from adjacent tunnels to facilitate dewatering or pump connections.
60
BH LW9
67.12
71.2171.21
70
80
Thanet Sand
UF
LMB
LTB
mTD
EB WB
Lam
beth
Gro
up
UMB
MLH
Combined probe holes + wellpoints
Sump ejector wells
Cross passage 6
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On particular challenge was at CP6: the cross passage was in the upper Lambeth Group with a borehole just 10 m showing cohesive soils at tunnel horizon. The sump reached to the Lower Aquifer requiring an ejector well scheme. The probe drilling required around the cross passage to prove the clay. Risk of a sand channel identified tool box contingency plan.
Compact ejector pumping stationsump ejectors
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Compact tunnel ejector pumping station incorporates tank, pumps and controls
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Drilling upwards encountered wet sand instead of expected clay. Switched to cased lost bit drilling and wellpoint installation.
TCBs
Scale0
LW9
East Bound
West Bound
Key: Sump ejectors Downwards probes/wellponts Upwards probes/wellpoints not installed
5m
CP6 as built: Plan View
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Plan view of the as built well array with sump ejectors plus downwards and upwards installations identified. Installations started as auger bore to get logging data and switched to cased lost bit drilling and wellpoint installation when wet sand encountered. As the pressure reduces installation becomes easier, bore control measures can be relaxed and logger data improves. Local sand channel identified at crown down to invert and fully dewatered to allow cross passage completion highest flow from centre of channel.
These case studies represent a range of strategy options for dewatering well installations ranging from: Vitally important to understand the constraints and the opportunities when developing these strategies.
