Design of Temporary Excavation and Lateral Support …1 Asia HKIE Technical Talk – ELS Design for...

Asia HKIE Technical Talk – ELS Design for SWSPS – 18 June 2008

Design of Temporary Excavation

and Lateral Support System

for the DSD Sheung Wan

Stormwater Pumping Station

By Eric Li - GCG (Asia)

Presented to the HKIE Geotechnical Division

18th June 2008

Sheung Wan Stormwater Pumping Station (SWSPS)

Project Set Up

� DSD Contract DC/2006/10

� Contract Sum : HK$ 78 million (approx)

� Designer : Drainage Services Department

(with GEO as in-house geotechnical consultant)

� Site Supervision : DSD Drainage Project Division

(with the engagement of an IGE)

� Contractor : China National Chemical Engineering Group

� ELS Designer : Geotechnical Consultant Group (Asia) Ltd.

Pile Cap Layout Plan

~ -5.9mPD

~ -7.1mPD

Retaining Wall

Pile Cap / Base Slab

Typical North-South Section

~ +4.0mPD

~ -5.9mPD~ -7.1mPD

Selection of Retaining Wall Type

Wall Types Considered:

• Diaphragm Wall

• Bored Pile Wall

• Pipe Pile Wall

• Sheet Pile Wall

• No wall (open cut)

Design Objectives:

• Approximate max. 10m deep excavation

• Effective control of settlement due to excavation and groundwater drawdown

Reclamation and Seawall Dredging Plan

Approx. Location of Site

Typical Cross Section

Vertical Seawall (Indicative) Approx. Extent of SWSPS

Victoria Harbour

Pre-Contract Borehole Location Plan

Pre-Contract Ground investigation

• 2 series in 2002 & 2006 (32 holes)• Common sampling, field and laboratory tests• In situ vane & pressuremeter tests

North-South Geological Section

Sand Fill

Rock Fill

Marine Deposits

Alluvium

East-West Geological Section (away from Seawall)

Marine Deposits

Alluvium

Existing Salt Water PS

Isopach of Soft Marine Clays

Seawall Cope Line

Small sink hole at sea wall

5m3 sink hole

15m3 sink hole

-0.300

-0.200

-0.100

Settlement near Seawall-0.300

-0.200

-0.100

‘SS’ Markers on Seawall

‘GS’ Markers on Ground Adjacent to Seawall

Design Considerations

� Reclamation Site

� Sensitive Structures nearby, i.e. WSD Pumping Station

� High water table, i.e. 1.2m below existing ground level

(+3.0mPD)

� Presence of thick layer of variable FILL

& soft MARINE DEPOSITS

Design Methodology

� Drained and Undrained Conditions

� Lateral Stability

� Global Slope Stability

� Base Stability

� Hydraulic Stability

� Dewatering

� Ground Deformation / Settlement

� Structural Capacities

Plot of SPT-N against Elevation

Plot of Undrained Shear Strength (Cu) against Elevation

Adopted Shoring Layout Plan

FSP IV Sheet Pile Walings & Struts(3 levels)

Longitudinal Cross Section further from Seawall

Longitudinal Cross Section nearest Seawall

Transverse Cross Section

Elevation of Wall D(Showing Grouting Ground Improvement)

Penetration (T.A.M.) Grout Jet Grout

Sheetpile (not shown for clarity)

Marine DepositAlluvium

EastWestCulvert

Grout A

Grout B

Finite Element Analysis

(East-West Section)

Wall Deflection (East-West Section)

East WallWest Wall

δmax = 60mm

δmax = 115mm

Ground Settlement (East-West Section)

West WallEast Wall

δmax = 18mm

δmax = 82mm

Wall Deflection (North-South Section)

South Wall North Wall

δmax = 102mm

δmax = 88mm

Ground Settlement (North-South Section)

North WallSouth Wall

δmax = 53mm

δmax = 75mm

Exaggeration scale: 30

Tilt = 1/650 Seawall

Fill (Sand)

Fill (Rock)

Seawater

Deformation of Seawall

Slope Stability Analysis

(South Wall)

Instrumentation Layout Plan

Principal Instrumentation :

- Ground settlement markers (SS, GS, SA)

- Inclinometers (IN, INX)

- Standpipe Piezometer (SP, PZ, PP)

- Building Tilt Marker

East Wall

West Wall

North Wall

Outline Construction Programme for Key Activities

Settlement against Time

-0.350

-0.300

-0.250

-0.200

-0.150

-0.100

-0.050

01-Jan-07

01-Jun-07

01-Jul-07

01-Jan-08

Settlement (m

SS3 SS2 SS1 SS4

-0.600

-0.500

-0.400

-0.300

-0.200

-0.100

01-Jan-07

01-Feb-07

01-Jun-07

01-Jul-07

01-Jan-08

Settlement (m

GS23 GS24 GS20

Ground Settlement Time Plot behind North Wall (Seawall)

‘SS’ Markers on Seawall

‘GS’ Markers on Ground Adjacent to Seawall

ExcavationH-Piling, Sheetpiling, Ground Improvement Grouting

Pile Cap

-0.500

-0.450

-0.400

-0.350

-0.300

-0.250

-0.200

-0.150

-0.100

-0.050

GS31 GS34 GS28

Ground Settlement Time Plot behind East Wall

Excavation

H-Piling, Sheetpiling, Ground Improvement Grouting

Pile Cap

Ground Deformation behind East Wall(due to excavation only)

GS28 (6.2m) GS31 (9.6m) GS34 (15.6m)

Inclinometer IN2 (1m)

Predicted δδδδmax = 115mm

-0.070

-0.060

-0.050

-0.040

-0.030

-0.020

-0.010

SS13 SS9 SS10 SS14

Ground Settlement Time Plot behind West Wall

0 5 10 15Displacement (mm)

)"+" toward excavation

Ground Deformation behind West Wall(due to excavation only)

Inclinometer IN5 (1m)

Jet grouting

• Jet grout cuts insitu soft clay and mixes that with injected cement

• Disturbed clay is displaced to surface in slurry form

• Jet nozzle is typically 1.4mm to 3mm dia.

• Applicable to more cohesive soils where T.A.M. grouting may not work

Typical Set up of Jet Grouting

(Double Tube Method)

Temporary casing for drilling through fill.Withdrawn prior to jetting. (optional)

Jet Grout Zone

Soil Overburden

PVC tubing to assist spoil return. (optional)

Ref. : CNCEC

1. Set up rig 2. Drill to toe level 3. Test jetting

6. Jet grout completed. Withdraw grout string

5. Simultaneously withdrawal and rotation of grout string to next level4. Commence Jetting

Sequence of Jet Grouting Ref. : CNCEC

Jet grout parameters

• Jet grout parameters adopted at SWSPS

Target grouted dia. = 1500 – 2000 mm

Grout pressure = 300 – 400 bars

Air pressure = 8 - 12 bars

Withdraw rate = 13 to 15 min/m

Rotation rate = 6 – 7 rev/step

• Target strength & stiffnessUCS = 600 kPa

Elastic modulus = 150 MPa

Jet Grout Core Sample

Hole S2Hole S5

Jet Grout Core Sample

(UCS: typically 7-10 MPa, sometimes >40 MPa)

T.A.M. Grout Core Sample

Sheetpiling (by push-in method)

• Suitable in area where vibration is undesirable

• With limited penetration depth

• Difficult to overcome obstruction

• Tend to be more costly

Ref : Giken Seisakusho Co. Ltd

Sheetpiling by push-in method

At SWSPS

• Giken rig with special tool to overcome obstruction (up to 1m)

• Maximum 24m long sheet pile

• Limited to Type IV sheetpile

Sheetpiling by push-in method

Augering to assist penetration

Drilling through Obstructions

Cutter Head for Rock Obstructions

Alignment out-of-line when encountering obstructions

Groundwater leaking throughretaining wall

Excavation in progress

Disposal by barge

Installing waling and strut

Excavation completed

Concreting in progress

Concreting in progress (3 concrete pumps & 13 concrete trucks)

Foundation Completed

園林景觀工程設計散石徑散石徑散石徑散石徑／／／／太極區太極區太極區太極區散石徑散石徑散石徑散石徑／／／／太極區太極區太極區太極區

遮雨蓬

長椅

小型廣場小型廣場小型廣場小型廣場小型廣場小型廣場小型廣場小型廣場

方形花園方形花園方形花園方形花園方形花園方形花園方形花園方形花園

觀賞園林觀賞園林觀賞園林觀賞園林觀賞園林觀賞園林觀賞園林觀賞園林

海濱長廊

海濱長廊海濱長廊海濱長廊海濱長廊海濱長廊海濱長廊海濱長廊海濱長廊

綠化天台綠化天台綠化天台綠化天台綠化天台綠化天台綠化天台綠化天台

散石徑

裝飾性街燈Ref. : DSD

Before Commencement After Completion

Ref. : DSD

Summary

Issues to be considered in ELS design

• Site history

• Site characterization

• Design parameters (drained or undrained)

• Design objective and constraints

• Retaining wall installation effects

• Economy and construction programme

Acknowledgement

• Drainage Services Department

• China National Chemical Engineering Corp

• Hyder Consulting Limited

Thank You !

Design of Temporary Excavation and Lateral Support …1 Asia HKIE Technical Talk – ELS Design for...

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