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Underground Space Development in Singapore Rocks ZHAO Jian Professor of Rock Mechanics and Tunnelling, EPFL Tan Swan Beng Endowed Visiting Professor, NTU Underground Space in Singapore Rocks Singapore Geology and bedrocks Potential Cavern Development in Singapore Rocks Some Existing Cavern Development Studies Technology and Innovation Challenges Nanyang Centre of Underground Space (NCUS) PTRC and NCUS Workshop on Underground Space and Rock Cavern Development in Singapore, NTU, 17 January 2012 College of Engineering School of Civil and Environmental Engineering Nanyang Centre of Underground Space Singapore Geology and Bedrocks Main Geological Formations Igneous rocks (north and central-north): Bukit Timah granite, Gombak norite Sedimentary rocks (west and south-west): Jurong Formation Quaternary deposits (east): Old Alluvium Recent deposits (throughout the island): Kallang Formation Singapore Geology and Bedrocks Singapore Geology and Bedrocks Age of the Main Geological Formations Jurong Tuas Bt Timah Bt Gombak Changi Kallang Punggol Bukit Timah Granite: Triassic (250 million years) Jurong Sedimentary Formation: Jurassic (230 million years) Old Alluvium: Quaternary (5 million years) Simplified Singapore Geology Section Bukit Timah Granite Jurong Formation Gombak Norite Old Alluvium Sajahat Formation Kallang Formation Kallang Formation: Recent (< 2 million years) Older Rocks: Gombak Norite, Sajahat Formation (oldest) 1 Singapore Geology and Bedrocks Singapore Geology and Bedrocks Bukit Timah Granite Varying granite, through adamelite to granodiorite, acidic igneous. Main minerals are quartz (30%), feldspar (60-65%), biotite and hornblende. Medium to coarse grains, usually light grey, also pinkish Bukit Timah Granite underlies about one-third of the Singapore Island and the whole of Pulau Ubin. Bukit Timah granite is considered as the base bedrock of the Singapore Island, i.e., underlies below the Jurong Formation and the Old Alluvium. Typical Bukit Timah granite and jointing Singapore Geology and Bedrocks Singapore Geology and Bedrocks Bukit Timah Granite Bukit Timah Granite Weathering is extensive, mainly decomposition. Depth varying between a few to 80 m. Undulating bedrock surface with sharp change from residual soil to granite. Sometimes large boulders. The granite rock mass is mostly of good and above quality, but varies from locations. The rock mass has 4 to 5 joint sets, dominant one is sub-vertical, with NNW-SSE strike. Groundwater flow is only likely in fractured zones and faults. The fresh granite intact rock has average UCS 180 MPa, highest being over 300 MPa. Weathered materials has much lower strength. High in situ horizontal stress (about 3-4 times vertical stress) in NNE-SSW direction. Singapore Geology and Bedrocks Singapore Geology and Bedrocks Jurong Sedimentary Formation Jurong Formation covers west and southwest Singapore Island, and southern islets. Jurong formation was formed during Triassic and lower to middle Jurassic. The formation consists of various types of mudstone, shale, siltstone, sandstone, conglomerates and limestone, with low degree of metamorphism. Jurong Formation Sedimentary Rocks 2 Singapore Geology and Bedrocks Singapore Geology and Bedrocks Jurong Sedimentary Formation Many layers are thin (to a few meters). Weak layers and strong layers are often sandwished. The Jurong Formation has been intensely folded. The strike of folds is NWW-SEE. Horizontal stress in SSW-NNE is 4-6 times the vertical stress in some strong rocks. Folding and high horizontal stress is related to regional tectonic movement. Pandan limestone at Pandan, Pasir Panjang, Tuas, Seraya, etc. In some locations, the limestone is partially metamorphised to marble. Singapore Geology and Bedrocks Jurong Formation Rocks Singapore Geology and Bedrocks Jurong Sedimentary Formation Old Alluvium Formation Most of the rocks of the Jurong Formation are of weak. Rock mass quality if general below good, most fair to poor, due to intensive fracturing and low strength. East Singapore Island, is underlain by the extension of the Bukit Timah granite. The thickness of these Quaternary deposits varies from a few ten meters to more than 200 meters. Rock type and quality can vary rapidly, due to folded rock layers. Old Alluvium is formed by the sediments brought down by the rivers in the region during the Pleistocene time. Relatively high permeability due to fractures. Semi-consolidated/lithofied sand and fine gravel with silt and clay lenses. Singapore Geology and Bedrocks Singapore Geology and Bedrocks Gombak Norite Noritic and gabbroic rocks. Coarse-grained and plagioclaserich with varying amounts of clino- and ortho- pyroxene minerals in intergranular texture. Engineering properties similar to the Bukit Timah granite, high strength and modulus. Sajahat Formation Appears at the northeast Singapore, Punggol, Pulau Tekong. It consists of well lithified quartzite, quartz sandstone, and argillite. Formed during the lower Paleozoic and is the oldest rock formation in Singapore. Old Alluvium 3 Singapore Geology and Bedrocks General Hydrogeology of Singapore Cavern Development in Singapore Rocks Examples of Rock Caverns Near surface groundwater: Replenished by constant rainfall, usually stable and at a few metres below ground level. Subsurface groundwater: Determined by the permeability and storage capacity of the rock masses at depth. Granite: Groundwater in residual soil, permeability increase with depth, very high permeability at soil-rock interface. Little flow in rock masses except in some faults. Sedimentary: Very high permeability at soil-rock interface. Likely flow in highly fractured rock masses, difference in layers and rock types. Cavern Development in Singapore Rocks Examples of Rock Caverns Cavern Development in Singapore Rocks Key Factors for Site Selection Rock mass quality Geological characteristics Topographic features Potential Sites Rocks Granite – the whole granite and norite at various locations and depths Carbonate rocks – e.g., Pandan Sandstone and siltstone – e.g., NTU, Labrador, Sentosa, Mount Faber, Kent Ridge, Southern Islands, Jurong Island. Cavern Development in Singapore Rocks Cavern Development in Singapore Rocks Caverns in Bukit Timah Granite Caverns in Pandan Limestone Extremely good rock quality, low permeability, groundwater flow restricted to some major joints. Uniform in lithology and extensive thickness. Caverns with very large span. Good rock mass quality from 50 m below limestone surface. Deep caverns in eastern Singapore below OA. Oil and gas storage caverns can be built. Good medium for LPG storage caverns. 4 Cavern Development in Singapore Rocks Cavern Development in Singapore Rocks Bedrock for Cavern Development Caverns in Jurong Sandstones Good to very good rock mass quality usually from 50 m below surface. Caverns of 20 m span are technically feasible. General – car parks, offices, laboratories, libraries, and recreational facilities. Industrial – hydrocarbon storage, warehouses, factory and workshop. Military – storage for ammunitions, radar and air control, coastal artillery, naval and ship service base. Granite/Norite are ideal for large caverns. Jurong Formation sandstones and limestone are suitable for caverns. Some Cavern Development Studies Granite bedrock below OA is suitable for deep caverns. Some Cavern Development Studies Earlier Studies and Construction (1990-2005) Caverns in Bukit Timah Granite 1990-1994 General cavern constructablity in Buki Timah granite and Jurong Formation (NTU-PWD), Underground Science City and Jurong Rock Caverns (NTU-JTC), various preliminary studies. UAF construction (DSTA). Underground Ammunition Storage (1999-2002) Recent Studies and Construction (2005-date) JRC construction (JTC). USC further study, warehouse, power station, incineration plant, ash-fill, waste water treatment, desalination plant, LNG storage etc. Caverns in Jurong Formation 1995-1998 USC at Kent Ridge (1997-2000) JRC in Jurong Island (2001-) Some Cavern Development Studies Room-andpillar caverns Underground Science Centre at Mount Faber ExhibitionWalkway tunnels Exit to mid-hill walkways Surface Building Section shown in next slide Underground Science City at Kent Ridge 5 Some Cavern Development Studies Caverns Below the Jurong Hill for the Bird Park extension Some Cavern Development Studies Some Cavern Development Studies Construction of the Jurong Rock Caverns for oil and gas storage, following earlier studies by JTC-NTU Team Warehouse cavern complex, under study by MND-JTC Some Cavern Development Studies Some Cavern Development Studies Largest Rock Cavern in Singapore Granite Gjøvic Cavern (92x62x25m) 62 m Proposed Largest Cavern (120x80x30m) 80 m Incineration plant underground, under study by MND-JTC 6 Some Cavern Development Studies Largest Rock Cavern in Singapore Granite Bukit Timah granite: High strength (UCS > 180 MPa) Favourable jointing (sub-vertical) High horizontal stress Low permeability The very good quality rock provides good medium for large cavern. Some Cavern Development Studies Some Cavern Development Studies Comparisons of the Proposed Cavern and the Gjøvik Cavern Gjøvik Cavern Proposed Cavern Cavern dimensions 92x62x25 m 120x80x30 m Excavated volume 114,100 m3 250,000 m3 Floor area Maximum seating capacity Rock mass quality (Q-value) 9,600 m2 5,000 above 10,000 1~12 6~100 15~50 m 50~60 m 0.5~1.8 MPa 2.7~5.4 MPa Limited Limited Rock cover Horizontal rock stress 5,700 m2 Groundwater Technology and Innovation Challenges Technology Innovation The large cavern should be located in the granite formation and with easy surface access, e.g., Daily Farm, Upper Bukit Timah, Rifle Range. There are many challenges in engineering, planning, environment and sustainability, and they are interdisciplinary. As this will be the world largest cavern, it will be a tourist attraction and a showcase for underground space utilisation and technology in Singapore. Technology and Innovation Challenges Cavern Technology A cavern is a large opening excavated in underground rock masses that are fractured and discontinuous and varying in properties. Cavern construction involves: (i) knowledge of the subsurface rocks, (ii) optimisation of cavern construction, and (iii) coping with environment. Environment engineering, sustainability Civil engineering, construction technology Architecture, urban planning, mobility Interdisciplinary nature of underground space technology Safety and risk, protective technology Information technology, system engineering Geology, earthquake engineering The cavern can be used for a multi-purpose hall for functions including sports, entertainment, exhibition and congress, and mass activities, for more than 10,000 people. It can be used as a defence shelter in wartime. Geothermal energy, resources engineering Largest Rock Cavern in Singapore Granite Law, economics, sociology, engineering design Technology and Innovation Challenges Knowledge of the Subsurface Rocks Technologies to assess rock mass quality and strength, and to detect discontinuities, and water and gas. Model to predict the behaviour and response of rock mass during and after construction. 7 Technology and Innovation Challenges Optimisation of Cavern Construction Coping with Environment Method of excavate and support caverns in adverse rock mass. Improve excavation using machine and explosive, minimise blasting damage, and optimise excavation sequence. Method of construction cavern in urban area. New construction method for various environment constraints. Optimise cavern dimensions and shape with ground conditions. Methods to refurbish caverns. Minimise support by utilise rock’s self-support capability. Technology and Innovation Challenges A caverns is built for specific usage. Knowledge associated cavern application involves the response and long-term stability of rocks under various conditions of cavern operation, including extreme temperature and stress conditions, fire, explosion, earthquake and natural hazards. • Conceptualizing, planning and undertaking feasibility studies for large scale deep underground space utilization in Singapore in coordination with national agencies. Frozen Zone • Leading technology development and innovation for underground space development at the national and international scene. LNG -180°C • Establishing a broad-based education and research platform in the area of underground space technology and sustainable development. Nanyang Centre of Underground Space Construction technology for large scale urban underground development Nanyang Centre of Underground Space Nanyang Centre for Underground Space (NCUS) is to provide sustainable technology solutions for Singapore’s underground space creation by: Knowledge associated with Applications NCUS will conduct R&D on (i) creating multilayered underground space, integrating aboveand under-ground urban system, to offer the best technology solutions of developing and utilising physical underground space for a sustainable and global city. Technology and Innovation Challenges Integrating above- and under-ground spaces to create a linked space system Nanyang Centre of Underground Space Space Integration Planning and optimising underground space with geology Underground Science City integrates Science Parks 1 and 2 Science Park 1 Safety against natural and man-made hazards, earthquakes and tsunamis Comfort and appearing of underground space and human factors Challenges in research and development to create underground space Land ownership, subsurface space pricing, publicprivate partnership Coupling underground space and resource/energy utilisations Kent Ridge Green and zeroenergy concepts and sustainability of underground space Kent Ridge Science Park 2 USC “Main Concourse” connecting Science Parks 1&2 8 Nanyang Centre of Underground Space Strategic Development Underground water reservoir in rock caverns to increase the reservoir capacity and to improve water security and safety. Nanyang Centre of Underground Space Innovation is not just on construction technology, but also on architecture and planning, to cope with the economic and social needs. Underground space is to achieve a better living quality and environment in Singapore. 9 General Aspects of Rock Tunnel and Cavern Engineering Jian ZHAO Professor of Rock Mechanics and Tunnelling, EPFL Tan Swan Beng Endowed Visiting Professor, NTU Rock Tunnel and Cavern Engineering Introduction to Rock Tunnelling Engineering Rock Mechanics Rock Excavation and Support Design and Construction of Caverns PTRC and NCUS Workshop on Underground Space and Rock Cavern Development in Singapore, NTU, 17 January 2012 College of Engineering School of Civil and Environmental Engineering Nanyang Centre of Underground Space Introduction to Rock Tunnelling Introduction to Rock Tunnelling Rock Tunnelling Rock Tunnelling Methods Rock tunnelling is an engineering process to construct a permanent and safe opening (tunnel, cavern, shaft) in rock for specific utilisations. Excavation: Rock tunnels are generally excavated by 2 main methods: (a) drill-and-blast, and (b) tunnel boring machine. Excavations can also be done by roadheader and other excavation machines. Rock tunnelling involves: Support: Rock tunnels are generally supported by rock bolts, sprayed concrete, cast-in-situ concrete, or concrete segments, and in some cases, steel sets, wire mesh, and other means. (a) excavation of the tunnel, and, (b) support of the tunnel. Introduction to Rock Tunnelling Introduction to Rock Tunnelling TBM excavation is a continuous process. TBM Tunnelling Figure by AlpTransit Gotthard Drill-and Blast Tunnelling Figure by AlpTransit Gotthard Drill-and-blast is a cyclic process. • Drilling • Charging • Blasting • Ventilation • Scaling • Mucking • Bolting • Shotcreting Figure by AtlasCopco 1 Introduction to Rock Tunnelling Introduction to Rock Tunnelling Key Principles of Rock Tunnelling Governing Rock Properties and Rock Mechanics Rocks are generally hard/strong materials, to be broken dynamically by blasting and impact loading. Rock properties influences all aspects of rock tunnelling: excavation, support, and use of excavated materials. In poor and weak rocks, the rock mass may be unstable and therefore need temporary support. Most rocks are stronger than concrete. Rock tunnel stability can be achieved by utilising the surrounding rock mass to be self-supported, i.e., the surrounding rock mass is reinforced to be a supporting structure. Engineering Rock Mechanics Rock mechanics form the basis of rock tunnel engineering, particular rock support. Engineering Rock Mechanics Rock of Tunnelling Scale Tunnels are at least a few metres in diameter and up to a few ten kilometres in length. (Largest span 62, longest length 57 km). Rock to be engineered at a tunnelling site is therefore a large mass of rock at the site. It is represented by the in situ rock mass, consists of intact rock blocks and all types of discontinuities (joints, faults etc). Rock mass = Rock materials + Rock discontinuities Engineering Rock Mechanics Rock Mass Quality and Classifications Rock mass can be of good or poor qualities, and are assessed by rock mass classifications (Q and RMR). Engineering Rock Mechanics Rock mass classification provides the basis of rock support design, and engineering parameters. Rock mass classifications consider several rock mass parameters, e.g., RQD, rock material strength, joint set and spacing, joint surface condition, groundwater, and in situ stress. Q = (RQD/Jn) (Jr/Ja) (Jw/SRF) RMR = Rstrength + RRQD + RJS + RJC + RGW Figure by Barton et al. 1992 2 Engineering Rock Mechanics Engineering Rock Mechanics Rock Mass Strength Rock mass strength can be approximately expressed by the Mohr-Coulomb (linear) criterion, or better by the Hoek-Brown (non-linear) criterion. Rock mass strength is governed by the degree of fracturing and joint strength. Hoek-Brown Strength Criterion 1 Using the GSI, Hoek-Brown equation can estimate rock mass strength based on rock type, rock material strength, rock mass structure, and joint surface condition. c 1 = 3 + (mb 3 ci + s ci2)a t 3 Engineering Rock Mechanics Rock mass parameters is available by this approach. Engineering Rock Mechanics Rock Mass Deformability Rock Discontinuities Rock mass deformation modulus can be obtained approximately from rock mass quality (Q and RMR). Rock mass failure, particularly in hard rock tunnelling, is governed by the existing rock joints and discontinuities. Figure after Bieniawski 1978, Serafim and Pereira 1983 Rock Excavation and Support Figure after Hoek 1997 Projection graphic tools and discontinuous numerical modelling can be used for the analysis. Rock Excavation and Support Basic Rock Tunnel Excavation Approaches Common Rock Excavation Methods Rock are hard materials to be removed during tunnelling. At presented, they are excavated primarily by using explosive or using powerful excavation machines. Other means are also being explored. Drill-and blast (full face, heading and benching) – medium to very hard rocks Full face excavation with face reinforcement – poor/weak rocks Sequential excavation and invert closing (NATM) – poor/weak rocks Partial face machines and roadheader – soft to medium rocks Full face tunnel boring machine (TBM) – poor, soft to hard rocks Rocks need to be broken into suitable sizes to be transported from tunnel face to outside. 3 Rock Excavation and Support Tunnel Boring Machine Cutting the rock full face by pushing and rotating the cutterhead, equipped with roller cutters. Rock Excavation and Support Rock properties, e.g., material strength, brittleness and abrasivity, and joint spacing and orientation, have great impact on TBM progress. Rock Excavation and Support Rock is fragmented by the roller cutters. Rock Excavation and Support Other Mechanised Methods Cutting rocks with excavation machines for partial face, e.g., roadheader. TBMs encounter problems in high fractured and blocky rock masses, and mixed faces. Rock Excavation and Support Rock Excavation and Support Drill-and-Blast Excavation of Soft/Poor Rocks Drilling charge holes advancing into rocks and using explosives to blast the rocks. Excavating small sections and quickly closing of invert. Figures by AtlasCopco 4 Rock Excavation and Support Rock Excavation and Support Rock Properties related to Rock Excavation Excavation Selection and Rock Properties Rock cuttablity/drillability: rock material strength, abrasivity. TBM – Low to high strength, high groundwater possible. Less flexible with changing geology, problem for squeezing, spalling and rock burst. Rock fragmentation: rock strength. Drill-and-blast – Variable geology, medium to high strength. Possible for full face and headingbenching. Problem with groundwater inflow. Others: groundwater (and permeability), deformation (squeezing and swelling), stress (rock burst and spalling), rock type (for reuse). Roadheader – As D&B, low to medium strength. Sequential excavation – Only for poor rock mass. Rock Excavation and Support Rock Excavation and Support Basic Approaches in Rock Support Design Rock Support based on Rock Mass Classifications (a) Rock is used as a structural material, i.e., rock reinforcement instead of rock support. Design of support and reinforcement for hard rocks are primarily based on rock mass classifications (Q or RMR) prior to construction. (b) Design is based primarily on precedents, i.e. empirical methods. (a) Temporally reinforcement is applied immediately after excavation. It often serves also as permanent reinforcement. (c) Design is related to and optimised on rock mechanics and construction methods. (d) Numerical methods can be used to predict problem areas and to extrapolate experience (b) Further permanent reinforcement is applied later, as required by rock mass classification. (e) Monitoring used to verify the design. (c) Monitoring is often done to verify design. Design of Rock Support Rock Excavation and Support Support for Soft/Poor Rocks 3 4 1 Q = 1.33, tunnel span 20 m Support design for poor rock is based on the interaction between the displacement of rock mass surrounding the tunnel and the load mobilised from the support material, RockSupport Interaction. Pressure required to limit displacement, P displacement 2 pressure Displacement, 3 5 Rock Excavation and Support Rock Excavation and Support Use of Numerical Methods Deformation accelerates, additional support installed, stabilisation achieved. (a) Numerical methods can be continuum (FEM) and discontinuum (DEM) based. (b) FEM are often used to obtained ground deformation characteristics. DEM is more specifically for stability for jointed rock mass. (c) Numerical models are also used to extrapolate and to check the empirical designs, and to back calculate. Rock Excavation and Support Rock Excavation and Support Selection of Support Design Method Rock mass classification – poor to good rock masses, best suited for fair to excellent rock masses. Ground response and observation – generally best suited for poor rock masses. FEM modelling on sequential excavation and support in poor rock. DEM modelling on stability and support for cavern in hard rock. Rock Excavation and Support Rock Excavation and Support Basic Rock Support Elements Reinforcement elements: bolts, cables, sprayed concrete, fibre reinforced spray concrete. Support elements: steel sets, cast-in concrete, segmental lining. Other elements: waterproof and drainage Plastic membrane Shotcrete surface drainage layer Concrete lining Rock Bolts and Cables Frictional End-anchored Grouted Rock mechanics Expansion shell anchor bolt Swellex Stress and deformation of rock mass, rock-bolt interaction. 6 Rock Excavation and Support Rock Excavation and Support Sprayed Concrete Wet concrete Steel fibre reinforced Other fibre reinforced Steel Sets Rock mechanics Cement penetration and rock blocks locking, improved rock mass behaviour. Segmental Lining Cast-in Concrete Rock Excavation and Support Selection of Rock Support Techniques Fair to good rock mass – bolts, sprayed concrete Poor rock mass – steel set, sprayed concrete, castin concrete Squeezing rock – yielding steel sets, sprayed concrete, cast-in concrete Design and Construction of Caverns Design and Construction of Caverns A rock cavern is a manengineered cave, for a specific application. There are over 20,000 caverns built around the world, for a variety of applications, ranging from storage of oil and gas to sport and concert halls. Design and Construction of Caverns Suitable Geology Basis of Design Rock caverns are generally unsupport openings. They are generally constructed in competent rock masses so the rock masses can be self-supported. a) The rock is used as a structural material. Most caverns are constructed in granitic and crystalline rocks. Limestone and strong clastic sedimentary rocks are also possible hosts. c) The design is related to construction procedures. b) Geotechnical design is based primarily on precedents, i.e. empirical methods. d) The design is optimised on the basis of rock mechanics, construction methods and usage, etc. e) Numerical methods can be used to predict problem areas and to extrapolate experience. f) Monitoring used to verify the design. 7 Design and Construction of Caverns Design and Construction of Caverns Design Sequence (i) Design Sequence (ii) a) Identification of the geometrical and physical requirements for the cavern. f) Determination of optimal location, orientation, lay-out and geometry for the cavern or cavern system based on the above factors. b) Identification of areas with geology suitable for cavern construction. c) Evaluation of the topography in relation to the geometrical requirements. d) Location of suitable access to the underground facility. g) Optimisation of the design with respect to cavern use and construction methods, which may include modification of the cavern use. h) Evaluation of rock support measures. e) Evaluation of geological and hydrogeological data. Design and Construction of Caverns Design and Construction of Caverns Design Consideration on Location and Orientation Minimum Rock Cover (i) a) Adequate rock cover. The rock cover should be sufficient so that the roof and walls will be self-supporting. The minimum rock cover is determined from many factors: b) Avoid weakness zones. c) Cross weakness zones in the shortest possible distance. d) Avoid adverse orientation relative to major joint sets. e) Make favourable use of groundwater pressures. f) Avoid rock with abnormally low stresses, or with very high stresses. a) the quality of the geological information and the rock properties, b) thickness of superficial deposits and depth of weathering, c) the cavern span and, d) cost implications. Design and Construction of Caverns Minimum Rock Cover (ii) As a general rule, the minimum cover of strong rock should be not less than half the cavern span. In general, reduced cover increases the amount and cost of ground investigation and rock support work and this cost must be offset by advantages in adopting reduced cover. Reduced rock cover is normally limited to small areas, such as the section of cavern closest to the portal. Design and Construction of Caverns Weakness Zones Weakness zones can be formed by weak rocks, faults and deeply weathering, with thickness from a few centimetres to several hundred metres. In dealing with weakness zones, a) weakness zones must be identified, b) if possible, avoid weakness zone, c) minimise excavation in weakness zone, d) consider the orientation of the weakness zones. 8 Design and Construction of Caverns Design and Construction of Caverns Joints The orientation of joints with respect to the axis of the excavation influence the stability of a cavern. The orientation of joints influence the amount of overbreak. Groundwater The location of the groundwater surface and predictions of changes created by the underground openings can be important considerations in determining the elevation of a cavern scheme. Optimization of excavation direction with respect to joint orientation can be achieved. E.g., the longitudinal axis of the cavern is ideally oriented normal to the line of intersection of the two dominant joint sets. a) Groundwater inflow can be problem for excavation. Design and Construction of Caverns b) Most cavern applications need dry environment. c) Water curtains are used to confine the oil and gas in caverns. Design and Construction of Caverns In Situ Stresses Cavern Layout and Shape (i) In situ stresses influence the stability of excavations. The design of cavern geometry and layout of a system of caverns is normally based on: a) In generally, increased stresses give increased stability. b) Excessive high in situ stresses influence can cause strength failure of cavern. a) Requirements given by the cavern usage. c) Stresses in hard rocks are normally anisotropic, can influence cavern stability. c) Geometry of the opening, i.e. the total height and arch shape, influences the cost of excavation and support. b) Costing of excavation and support operations. d) For high stresses, cavern section shape can be optimised. Design and Construction of Caverns Design and Construction of Caverns Cavern Layout and Shape (ii) The main parameters defining cavern layout and geometry are the cavern size and shape and the spacing between caverns. They are primarily based on empirical guidelines from previous experiences. Design of Cavern Shape (General) Rock mass is discontinuous of low tensile strength. The design of shape is to evenly distribute the compressive stresses in the surrounding rock mass: Large span caverns, caverns in difficult ground conditions and multi-cavern schemes are commonly subjected to stability and stress distribution analyses using various methods. b) Avoid intruding corners; a) Use an arched roof; c) Optimise cross-section sizes to the lowest combined excavation and support costs; d) Optimise cross-section shape to the best stress distribution. 9 Design and Construction of Caverns Design and Construction of Caverns Design of Cavern Shape (Roof) Roof arch in generally is design to have height:span of 1:5, and, a) The roof shape is not commonly altered to suit particular geological structures; Design of Cavern Shape (Wall) Cavern walls are normally vertical. Wall stability is a function of wall height, the in situ stresses and the orientation and properties of the principal joint sets. b) Height may be reduced if the dominant joints have a shallow dip; a) The flat wall surface has no arching action and high walls tend to be unstable; c) Usually height will not be increased as economically not justified; b) Major joints and seams can dominate wall stability and can affect the chosen wall height; d) Increasing the roof arch height only if the space under the arch for ducts and services is needed. c) The cost and scale of stabilising measures can increase substantially with wall height; Design and Construction of Caverns Design and Construction of Caverns Design of Cavern Shape (Wall) Design of Cavern Shape (Stress) d) Joints with shallow dip favour wall stability as the dominating vertical stresses in the walls increase joint friction; Anisotropic and high stresses may have to be taken into account in cavern design. e) Steeply dipping joints with strikes parallel to the wall reduce stability as the horizontal stresses on the joints are small. a) For exceptionally high stresses, the shape of the crosssection should be optimised; b) Optimisation of shape can be analysed based on stress condition; c) There are cases that cavern cross-section reshaped due to anisotropic high stresses. Design and Construction of Caverns Design and Construction of Caverns Design of Cavern Horizontal Spacing (Pillar Width) Pillar width depends primarily on the rock quality, the discontinuity orientation, the cavern spans and heights. Design of Cavern Vertical Spacing (i) Vertical separation in generally should not be less than span or height. It depends on the rock quality, the orientation of the discontinuities, the cavern dimensions, and in situ stresses. a) Pillar widths are normally equal to 0.5–1.0 full cavern span or height, whichever is the greater; b) Pillar widths are normally determined on the basis of judgement and simple analysis, e.g., possible sliding on unfavourable joints; c) Narrow pillars may be necessary because of site availability and other factors. a) It generally requires detail analysis and modelling; b) It should consider overbreak and loosening of rock in both upper and lower caverns; c) It should consider the risk of outfall of rock may cause stability of upper cavern; 10 Design and Construction of Caverns Design and Construction of Caverns Design of Cavern Vertical Spacing (ii) d) It should consider the cost for blast and support; e) The stability of the separating rock may be improved by pre-grouting and bolting from either the upper or lower cavern; f) Excavation of the upper caverns before the lower caverns is recommended. This avoids the risk of damage to the roof support installed in the lower cavern by vibrations from the heavy charges used in the bottom of the upper caverns. Design and Construction of Caverns Basis of Cavern Support Design a) The rock is used as a structural material, i.e., primarily reinforcement b) Support design is based rock mass quality and precedents, i.e. empirical methods c) Numerical methods can be used to predict problem areas and to extrapolate experience d) Monitoring used to verify the design Design and Construction of Caverns Cavern Support Design Approach Cavern Support Design Preliminary design of rock support may be based on rock classifications (Q or RMR), to provide the most suitable types of support for the various rock classes that have been identified. Roof: Use Q-support design chart directly. Wall: For Q > 10, Temporally reinforcement (often bolts and shotcrete in hard rock tunnelling) applied immediately after excavation can also serve as permanent reinforcement. Qwall = 5 Q For 0.1 < Q < 10, Qwall = 2.5 Q For Q < 0.1, Qwall = Q Further permanent reinforcement is applied (bolts and shotcrete) later. Design of Rock Support Design and Construction of Caverns Cavern Support Design 2 3 Roof 4 Wall 1 Q = 1.33, tunnel span 20 m, wall 8 m Temporary support: use the following adjustment, Increase ESR to 1.5 ESR or Increase Q to 5 Q (applicable to roof and wall). Maximum unsupported span = 2 ESR Q0.4 Example: Q = 10, ESR = 1, maximum unsupported span = 5 m 3 11 Design and Construction of Caverns Construction Method Rock caverns are generally excavated by drill-andblast method, and supported by bolts and shotcrete. Cavern excavation is usually done by: • face blasting with horizontal drillholes for tunnelling or top heading excavation, • benching with horizontal drillholes, or Design and Construction of Caverns A good rock tunnelling practice can be achieved by: Good knowledge of rock properties through appropriate site investigation; Good rock mechanics analysis, including using physical and numerical modelling, to anticipate the response of rock mass during and after construction; Good engineering practice supported by monitoring and risk control exercises. • benching with vertical drillholes. 12