RELOCATION OF SHATIN SEWAGE TREATMENT WORKS TO CAVERNS: Caverns and - - PowerPoint PPT Presentation

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 2020 1980

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 Sewage Treatment 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

6 24 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 Bolts Temporary 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 Stresses An 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 Stress Rock 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 Procedures
• 1. 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 Generalized

Hoek-Brown Criterion

• 2. Analytical
• Check the adequacy of support using Rock

Reinforcement Approach, and amend as necessary

• 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
• r 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 Model Continuum 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 YOU

Relocation of Sha Tin Sewage Treatment Works to Caverns