Agreement No. CE 30/2014 (DS) RELOCATION OF SHATIN … · Permanent Shotcrete + Rock Bolts Temporary Shotcrete + Rock Bolts Permanent Plain/Reinforced Concrete Lining Design Approach

Agreement No. CE 30/2014 (DS)

RELOCATION OF SHATIN SEWAGE TREATMENT

WORKS TO CAVERNS:

Caverns and Sewage Treatment Works –

Investigation, Design and Construction

Rock Reinforcement Approach for Tunnelling

30 May 2019 (Thursday)

Guy Bridges

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Innotech Forum on Geotechnology

1. History of Cavern Development in HK

2. Recent Cavern Studies in HK

3. Overseas Examples of Treatment Plants in Caverns

4. Relocation of Sha Tin Sewage Treatment Works to Caverns

5. Cavern Design

6. Rock Reinforcement Approach

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2009: Western Service Reservoir (HKU)1985: MTR Station (Tai Koo)

History of Cavern Development in HK

1985

1990

1995

2000

2005

2010

2015

20201980

1984: Western District Aqueduct

1985: MTR Station (Sai Wan Ho)

1995: Stanley Sewage Treatment Works

1997: Explosives Depot (Kau Shat Wan)

1997: Island West Transfer Station

2010: Explosives Depot (MTR WIL)

2014: MTR Station (HKU)

2015: MTR Station (Sai Ying Pun)

2016: MTR Station (Admiralty)

MTR Station (Lei Tung)

MTR Station (Ho Man Tin)

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History of Cavern Development in HK

MTR Station (Sai Ying Pun)• Completion Year: 2015• Category: Transportation• Span: 22.8m• Rock Type: Granite

MTR Station (Tai Koo)• Completion Year: 1985• Category: Transportation• Span: 24.2m• Rock Type: Granite

MTR Station (Ho Man Tin)• Completion Year: 2016• Category: Transportation• Span: 22m• Rock Type: Granite

MTR Station (Lei Tung)• Completion Year: 2016• Category: Transportation• Span: 19m• Rock Type: Tuff

MTR Station (Sai Wan Ho)• Completion Year: 1985• Category: Transportation• Span: 24.2m• Rock Type: Granite

MTR Station (HKU)• Completion Year: 2014• Category: Transportation• Span: 22.4m• Rock Type: Granite

Western Service Reservoir• Completion Year: 2009• Category: Water• Span: 17.6m• Rock Type: Tuff with

sedimentary bed

Island West Transfer Station• Completion Year: 1997• Category: Waste• Span: 27m• Rock Type: Tuff

Explosives Depot (MTR WIL)• Completion Year: 2010• Category: Dangerous Goods• Span: 5.5m• Rock Type: Tuff/granite

interface

Relocation of Sha Tin SewageTreatment Works• Category: Water• Max. Span: 32m• Rock Type: Granite

MTR Station (Admiralty)• Completion Year: 2016• Category: Transportation• Span: 24.3m• Rock Type: Granite

Explosives Depot (Kau Shat Wan)• Completion Year: 1997• Category: Dangerous Goods• Span: 13m• Rock Type: Granite intruded by

feldsparphyric rhyolite

Legend:● Caverns developed in 1980s

● Caverns developed in 1990s

● Caverns developed after 2000

Stanley Sewage Treatment Works• Completion Year: 1995• Category: Water• Span: 15m• Rock Type: Granite

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Recent Cavern Studies in HK

• 1990s A Study of the Potential Use of Underground Space (1990)

Cavern Project Studies (1991)

Cavern Area Studies (1994)

• 2010s Enhanced Use of Underground Space in Hong Kong (2011)

Enhancing Land Supply Strategy - Reclamation outside Victoria Harbour and Rock Cavern Development (2011)

Long Term Strategy for Cavern Development (2012)

Relocation of Sha Tin Sewage Treatment Works to Caverns – Feasibility Study (2013)

Cavern Master Plan for Hong Kong

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Overseas Examples of Treatment Plants in Caverns

Viikinmäki Wastewater Treatment Plant

• Location: Helsinki, Finland• Completion Year: 1994 (Expanded in 2003)• Span: 17-19m (height: 10-15m)• Rock: mostly over 10m cover of migmatite• Support: grouted rebar bolts & shotcrete

Käppala Wastewater Treatment Plant

• Location: Stockholm, Sweden• Completion Year: 1969 (Expanded in 1990s)• Rock: approx. 150m cover of igneous rock

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Overseas Examples of Treatment Plants in Caverns

Location: Helsinki, Finland

Under Construction

7 No. 20m span caverns

10m wide rock pillars

17 permanent shafts

4 No. 20m Dia. Digesters

Depth to caverns 50 to 60m

870,000m3 cavern excavation

800,000m3 tunnel excavation

Outfall tunnel ‘many km long’

Sewage inlet 30m below, is pumped

up into the caverns

Blominmäki Wastewater Treatment Plant

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Existing STSTW

Nui Po Shan

Ma On Shan

Proposed STSTW in Caverns

Relocation of STSTW to Caverns

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Relocation of STSTW to Caverns

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Cavern Orientation

at 11o to North

• Geological Plan

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Relocation of STSTW to Caverns

624 m Rock Pillar

Ventilation Adit

Main Access Tunnel

Secondary

Access Tunnel

Branch Driveway

32 m Main Cavern Complex

Effluent

Tunnel

Ventilation Shaft• Isometric View and Cross Sections

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Effluent

Emergency

Bypass

Influent

6mm Bar Screens/

Aerated Grit Channels

Primary Sedimentation

with Plate Settlers

Bioreactors (MBBR)

DAF and UV

Sludge Treatment

Sewage Flow Direction

Flowmeter Chamber

Bypass Flow Direction

Electrical

Relocation of STSTW to Caverns

• General Layout of Sewage Treatment Works

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• Cast concrete lining has recently been adopted for permanent support

Cavern Design

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Rock Reinforcement Approach Cast-in-situ Concrete Lining

General Arrangement

Support Elements

Permanent Shotcrete + Rock BoltsTemporary Shotcrete + Rock Bolts

Permanent Plain/Reinforced Concrete Lining

Design Approach

Rock as structural materials to self-support by rock bolt reinforcement

Concrete and rebar as structural materials to support all loads

Design Load Field StressesAn array of load combinations (overburden, imposed, water, grout pressure, E&M etc.)

Structural Analysis

FEM and DEM Bedded-beam Structural Model

Design Checking

Individual failure modes and Numerical Modelling

Shear stress, M-N diagram etc. with partial factors according to structural code

Cavern Design

Hoop StressRock Arch

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• Conventionally, “Rock Support” is adopted and cast-in-situ concrete lining is used to sustain all possible loadings

• In Hong Kong, strong igneous rock has a compressive strength typically greater than concrete

• Rock is a structural material to self-support itself by utilizing the “arching effect”. It is perfectly capable to support the ground above the excavation through a theoretical arch

• For STSTW, the concept is switched to “Rock Reinforcement”. Permanent rock bolts are considered as reinforcement and permanent shotcrete supports rock wedges between bolts

• The inherent strength of rock mass is utilized by applying confining pressure via rock bolts

Rock Reinforcement Approach

Lateral

Earth

Pressure

Lateral

Earth

Pressure

Vertical Design Pressure

Rock Support

(Rock is considered as loading)

Rock Reinforcement

(Rock is a part of solution NOT problem)

• The thrust capacity is therefore increased and the rock arch formed around the tunnel is capable of providing the required force to stabilise the opening

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Rock Reinforcement Approach

• Design Procedures1. Empirical Derive initial support from the NGI Q-system

Rock Support Chart (i.e. shotcrete thickness,rock bolts spacing and length)

Establish Rock Mass Parameters for GeneralizedHoek-Brown Criterion

2. Analytical Check the adequacy of support using Rock

Reinforcement Approach, and amend asnecessary

3. Numerical Verify the design by numerical analyses and

confirm the support requirement

Q-system

Rock Mass Parameters

Rock Reinforcement

Approach

Numerical Modelling

Checking of Support Capacity

1

2

3

4

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Rock Reinforcement Approach

• Rock Bolts Systematic bolting to reinforce the overall stability Spot bolting to secure individual loosened blocks Typical diameter: 20 to 32 mm Typical length of 2 to 6 m Common type: Fully grouted, (temporary) expansion shell at end Design life 100 years Double corrosion protection – Galvanized with epoxy coating

Bischoff and Smart (1977)

Lang (1961) and later re-modelled by Hoek (2007)

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*Adhesive Failure Flexural Failure *Direct Shear Failure

Punching Shear Failure Compressive Failure Tensile Failure

Rock Reinforcement Approach

• Shotcrete/Sprayed Concrete Thin layer (75 to 200 mm) along the uneven excavated profile Does not act as an arch, and does not support loads via compression

or bending Failure modes of shotcrete in the RRA are very hypothetical Compression or tension cannot develop within the shotcrete Six Potential Failure Modes:

Figure 6.23 from GEOGUIDE 4 (2018)

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• Verification of Design by Numerical Analyses• Can model a continuum with material properties suitable for the

rockmass, or discontinuum with joints

• Need to model different excavation sequences as they can give different results

Rock Reinforcement Approach

Discontinuum ModelContinuum Model

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• Details of Waterproofing Elements Cast-in-situ Lining – Sheet Waterproofing Membrane

Sprayed Concrete – Drainage Strips

Rock Reinforcement Approach

Tunneltalk, Sep 2008

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• Design Groundwater Pressure Prescriptive groundwater pressure could only act locally

through rock fissure as a point load on the sprayed concrete lining

Groundwater pressure would generally be supported by the surrounding rockmass

Special attention is needed in highly fractured zones

• Sequential Installation of Rock Support Initial ground movement occurs before any support can be

installed Rockbolts will take the majority of the loading Numerical models typically have ‘perfect’ excavation profiles,

such that a shotcrete liner could act as an arch in compression – in reality this will not occur

Rock Reinforcement Approach

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THANK YOURelocation of Sha Tin Sewage Treatment Works to Caverns