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
2
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
3
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)
4
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
5
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
6
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
7
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
8
Existing STSTW
Nui Po Shan
Ma On Shan
Proposed STSTW in Caverns
Relocation of STSTW to Caverns
8
9
Relocation of STSTW to Caverns
1
12
23
3
Cavern Orientation
at 11o to North
• Geological Plan
10
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
11
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
12
• Cast concrete lining has recently been adopted for permanent support
Cavern Design
13
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
14
• 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
15
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
16
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)
17
*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)
18
• 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
19
• Details of Waterproofing Elements Cast-in-situ Lining – Sheet Waterproofing Membrane
Sprayed Concrete – Drainage Strips
Rock Reinforcement Approach
Tunneltalk, Sep 2008
20
• 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
21
THANK YOURelocation of Sha Tin Sewage Treatment Works to Caverns