Underground Space Development in Singapore Rocks

1

Underground Space Development in Singapore Rocks

ZHAO Jian Professor of Rock Mechanics and Tunnelling, EPFL Tan Swan Beng Endowed Visiting Professor, NTU

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

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)

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

Bt Timah

Bt Gombak

Changi Jurong

Tuas Kallang Punggol

Bukit Timah Granite

Gombak Norite

Sajahat Formation

Jurong Formation

Old Alluvium

Kallang Formation

Simplified Singapore Geology Section

Singapore Geology and Bedrocks Singapore Geology and Bedrocks

Age of the Main Geological Formations

Bukit Timah Granite: Triassic (250 million years)

Jurong Sedimentary Formation: Jurassic (230 million years)

Old Alluvium: Quaternary (5 million years)

Kallang Formation: Recent (< 2 million years)

Older Rocks: Gombak Norite, Sajahat Formation (oldest)

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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



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 fresh granite intact rock has average UCS 180 MPa, highest being over 300 MPa. Weathered materials has much lower strength.


Bukit Timah Granite

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.

High in situ horizontal stress (about 3-4 times vertical stress) in NNE-SSW direction.


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


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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.

Jurong Formation Rocks




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.

Rock type and quality can vary rapidly, due to folded rock layers.

Relatively high permeability due to fractures.


Old Alluvium Formation

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.

Old Alluvium is formed by the sediments brought down by the rivers in the region during the Pleistocene time.

Semi-consolidated/lithofied sand and fine gravel with silt and clay lenses.

Old Alluvium

Singapore Geology and Bedrocks Singapore Geology and Bedrocks

Gombak Norite

Noritic and gabbroic rocks. Coarse-grained and plagioclase-rich 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.

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General Hydrogeology of Singapore

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


Examples of Rock Caverns 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.


Caverns in Bukit Timah Granite

Extremely good rock quality, low permeability, groundwater flow restricted to some major joints.

Caverns with very large span.

Deep caverns in eastern Singapore below OA.


Caverns in Pandan Limestone

Uniform in lithology and extensive thickness.

Good rock mass quality from 50 m below limestone surface.

Oil and gas storage caverns can be built.

Good medium for LPG storage caverns.


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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.

Cavern Development in Singapore Rocks Cavern Development in Singapore Rocks

Bedrock for Cavern Development

Granite/Norite are ideal for

large caverns.

Jurong Formation sandstones and limestone are

suitable for caverns.

Granite bedrock below OA is suitable

for deep caverns.

Earlier Studies and Construction (1990-2005)

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).

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.

Some Cavern Development Studies

Caverns in Bukit Timah Granite 1990-1994

Caverns in Jurong Formation 1995-1998

USC at Kent Ridge (1997-2000)

Underground Ammunition Storage (1999-2002)

JRC in Jurong Island (2001-)


Underground Science City at Kent Ridge


Section shown in next slide

Room-and-pillar caverns

Exhibition-Walkway tunnels

Exit to mid-hill walkways

Surface Building

Underground Science Centre at Mount Faber

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Caverns Below the Jurong Hill for the Bird Park extension



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


Incineration plant underground, under study by MND-JTC

Proposed Largest Cavern (120x80x30m)

Gjvic Cavern (92x62x25m)

62 m

80 m


Largest Rock Cavern in Singapore Granite

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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.


Comparisons of the Proposed Cavern and the Gjvik Cavern

Gjvik Cavern Proposed Cavern

Cavern dimensions 92x62x25 m 120x80x30 m

Excavated volume 114,100 m3 250,000 m3

Floor area 5,700 m2 9,600 m2

Maximum seating capacity 5,000 above 10,000

Rock mass quality (Q-value) 1~12 6~100

Rock cover 15~50 m 50~60 m

Horizontal rock stress 0.5~1.8 MPa 2.7~5.4 MPa

Groundwater Limited Limited



The large cavern should be located in the granite formation and with easy surface access, e.g., Daily Farm, Upper Bukit Timah, Rifle Range.

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.

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 Innovation

There are many challenges in engineering, planning, environment and sustainability, and they are interdisciplinary.


Interdisciplinary nature of underground space technology

Civil engineering, construction technology

Environment engineering, sustainability

Architecture,urban planning,

mobility

Geo

ther

mal

ene

rgy,

re

sour

ces

engi

neer

ing G

eology, earthquake engineering

Safety and risk, protective technology

Law, economics, sociology,

engineering design

Information technology, system

engineering

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.


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.


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Optimisation of Cavern Construction

Improve excavation using machine and explosive, minimise blasting damage, and optimise excavation sequence.

Optimise cavern dimensions and shape with ground conditions.

Minimise support by utilise rocks self-support capability.


Coping with Environment

Method of excavate and support caverns in adverse rock mass.

Method of construction cavern in urban area.

New construction method for various environment constraints.

Methods to refurbish caverns.


Knowledge associated with Applications

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.


LNG

-180C

Frozen Zone


Nanyang Centre for Underground Space (NCUS) is to provide sustainable technology solutions for Singapores underground space creation by:

Conceptualizing, planning and undertaking feasibility studies for large scale deep underground space utilization in Singapore in coordination with national agencies.

Leading technology development and innovation for underground space development at the national and international scene.

Establishing a broad-based education and research platform in the area of underground space technology and sustainable development.


Planning and optimising

underground space with

geology

Safety against natural and man-made

hazards, earthquakes and

tsunamis

Comfort and appearing of underground

space and human factors

Coupling underground

space and resource/energy

utilisations

Green and zero-energy concepts

and sustainability of

underground space

Land ownership, subsurface space pricing, public-

private partnership

Construction technology for

large scale urban underground development

Integrating above- and

under-ground spaces to create

a linked space system

Challenges in research and

development to create

underground space

NCUS will conduct R&D on (i) creating multilayered underground space, integrating above- and under-ground urban system, to offer the best technology solutions of developing and utilising physical underground space for a sustainable and global city.

Space Integration

Underground Science City integrates Science Parks 1 and 2

USC Main Concourse connecting Science Parks 1&2

Science Park 1

Science Park 2

Kent Ridge

Kent Ridge


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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 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.

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General Aspects of Rock Tunnel and Cavern Engineering

Jian ZHAO Professor of Rock Mechanics and Tunnelling, EPFL Tan Swan Beng Endowed Visiting Professor, NTU

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


Rock Tunnel and Cavern Engineering

Introduction to Rock Tunnelling

Engineering Rock Mechanics

Rock Excavation and Support

Design and Construction of Caverns

Rock Tunnelling

Rock tunnelling is an engineering process to construct a permanent and safe opening (tunnel, cavern, shaft) in rock for specific utilisations.

Rock tunnelling involves:

(a) excavation of the tunnel, and,

(b) support of the tunnel.

Introduction to Rock Tunnelling Introduction to Rock Tunnelling

Rock Tunnelling Methods

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.

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.

Drill-and Blast Tunnelling

TBM Tunnelling


Figure by AlpTransit Gotthard

Figure by AlpTransit Gotthard

TBM excavation is a continuous process.

Drill-and-blast is a cyclic process.

Drilling

Charging

Blasting

Ventilation

Scaling

Mucking

Bolting

Shotcreting


Figure by AtlasCopco

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Key Principles of Rock Tunnelling

Rocks are generally hard/strong materials, to be broken dynamically by blasting and impact loading.

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.


Governing Rock Properties and Rock Mechanics

Rock properties influences all aspects of rock tunnelling: excavation, support, and use of excavated materials.

Rock mechanics form the basis of rock tunnel engineering, particular rock support.


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


Rock Mass Quality and Classifications

Rock mass can be of good or poor qualities, and are assessed by rock mass classifications (Q and RMR).

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


Rock mass classification provides the basis of rock support design, and engineering parameters.

Figure by Barton et al. 1992

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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.

1

3

c

t


Hoek-Brown Strength Criterion

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.

1 = 3 + (mb 3 ci + s ci2)a

Rock mass parameters is available by this approach.

Figure after Hoek 1997


Rock Mass Deformability

Rock mass deformation modulus can be obtained approximately from rock mass quality (Q and RMR).

Figure after Bieniawski 1978, Serafim and Pereira 1983


Rock Discontinuities

Rock mass failure, particularly in hard rock tunnelling, is governed by the existing rock joints and discontinuities.

Projection graphic tools and discontinuous numerical modelling can be used for the analysis.


Basic Rock Tunnel Excavation Approaches

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.

Rocks need to be broken into suitable sizes to be transported from tunnel face to outside.

Common Rock Excavation Methods

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


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Tunnel Boring Machine

Cutting the rock full face by pushing and rotating the cutterhead, equipped with roller cutters.


Rock is fragmented by the roller cutters.


Rock properties, e.g., material strength, brittleness and abrasivity, and joint spacing and orientation, have great impact on TBM progress.

TBMs encounter problems in high fractured and blocky rock masses, and mixed faces.


Other Mechanised Methods

Cutting rocks with excavation machines for partial face, e.g., roadheader.


Drill-and-Blast

Drilling charge holes advancing into rocks and using explosives to blast the rocks.

Figures by AtlasCopco


Excavation of Soft/Poor Rocks

Excavating small sections and quickly closing of invert.


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Rock Properties related to Rock Excavation

Rock cuttablity/drillability: rock material strength, abrasivity.

Rock fragmentation: rock strength.

Others: groundwater (and permeability), deformation (squeezing and swelling), stress (rock burst and spalling), rock type (for reuse).


Excavation Selection and Rock Properties

TBM Low to high strength, high groundwater possible. Less flexible with changing geology, problem for squeezing, spalling and rock burst.

Drill-and-blast Variable geology, medium to high strength. Possible for full face and heading-benching. Problem with groundwater inflow.

Roadheader As D&B, low to medium strength.

Sequential excavation Only for poor rock mass.


Basic Approaches in Rock Support Design

(a) Rock is used as a structural material, i.e., rock reinforcement instead of rock support.

(b) Design is based primarily on precedents, i.e. empirical methods.

(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

(e) Monitoring used to verify the design.


Rock Support based on Rock Mass Classifications

Design of support and reinforcement for hard rocks are primarily based on rock mass classifications (Q or RMR) prior to construction.

(a) Temporally reinforcement is applied immediately after excavation. It often serves also as permanent reinforcement.

(b) Further permanent reinforcement is applied later, as required by rock mass classification.

(c) Monitoring is often done to verify design.


Design of Rock Support

1

2

3

4

Q = 1.33, tunnel span 20 m

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Support for Soft/Poor Rocks

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, Rock-Support Interaction.

displacement

pressure

Displacement,

Pre

ssure

required t

o lim

it d

ispla

cem

ent,

P


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Deformation accelerates,

additional support installed,

stabilisation achieved.


Use of Numerical Methods

(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.


DEM modelling on stability and support for cavern in hard rock.

FEM modelling on sequential excavation and support in poor rock.


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.


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

drainage

layer

Concrete

lining

Plastic

membrane

Shotcrete

surface


Rock Bolts and Cables

Frictional

End-anchored

Grouted

Rock mechanics

Stress and deformation of rock mass, rock-bolt interaction.

Expansion shell anchor bolt

Swellex


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Sprayed Concrete

Wet concrete

Steel fibre reinforced

Other fibre reinforced

Rock mechanics

Cement penetration and rock blocks locking, improved rock mass behaviour.


Cast-in Concrete

Steel Sets

Segmental Lining


Selection of Rock Support Techniques

Fair to good rock mass bolts, sprayed concrete

Poor rock mass steel set, sprayed concrete, cast-in concrete

Squeezing rock yielding steel sets, sprayed concrete, cast-in concrete


A rock cavern is a man-engineered 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.


Suitable Geology

Rock caverns are generally unsupport openings. They are generally constructed in competent rock masses so the rock masses can be self-supported.

Most caverns are constructed in granitic and crystalline rocks. Limestone and strong clastic sedimentary rocks are also possible hosts.


Basis of Design

a) The rock is used as a structural material.

b) Geotechnical design is based primarily on precedents, i.e. empirical methods.

c) The design is related to construction procedures.

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.


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Design Sequence (i)

a) Identification of the geometrical and physical requirements for the cavern.

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.

e) Evaluation of geological and hydrogeological data.


Design Sequence (ii)

f) Determination of optimal location, orientation, lay-out and geometry for the cavern or cavern system based on the above factors.

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.


Design Consideration on Location and Orientation

a) Adequate rock cover.

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.


Minimum Rock Cover (i)

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:

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.


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.


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.


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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.

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.


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.

a) Groundwater inflow can be problem for excavation.

b) Most cavern applications need dry environment.

c) Water curtains are used to confine the oil and gas in caverns.


In Situ Stresses

In situ stresses influence the stability of excavations.

a) In generally, increased stresses give increased stability.

b) Excessive high in situ stresses influence can cause strength failure of cavern.

c) Stresses in hard rocks are normally anisotropic, can influence cavern stability.

d) For high stresses, cavern section shape can be optimised.


Cavern Layout and Shape (i)

The design of cavern geometry and layout of a system of caverns is normally based on:

a) Requirements given by the cavern usage.

b) Costing of excavation and support operations.

c) Geometry of the opening, i.e. the total height and arch shape, influences the cost of excavation and support.


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.

Large span caverns, caverns in difficult ground conditions and multi-cavern schemes are commonly subjected to stability and stress distribution analyses using various methods.


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:

a) Use an arched roof;

b) Avoid intruding corners;

c) Optimise cross-section sizes to the lowest combined excavation and support costs;

d) Optimise cross-section shape to the best stress distribution.


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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;

b) Height may be reduced if the dominant joints have a shallow dip;

c) Usually height will not be increased as economically not justified;

d) Increasing the roof arch height only if the space under the arch for ducts and services is needed.


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.

a) The flat wall surface has no arching action and high walls tend to be unstable;

b) Major joints and seams can dominate wall stability and can affect the chosen wall height;

c) The cost and scale of stabilising measures can increase substantially with wall height;


Design of Cavern Shape (Wall)

d) Joints with shallow dip favour wall stability as the dominating vertical stresses in the walls increase joint friction;

e) Steeply dipping joints with strikes parallel to the wall reduce stability as the horizontal stresses on the joints are small.


Design of Cavern Shape (Stress)

Anisotropic and high stresses may have to be taken into account in cavern design.

a) For exceptionally high stresses, the shape of the cross-section 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 of Cavern Horizontal Spacing (Pillar Width)

Pillar width depends primarily on the rock quality, the discontinuity orientation, the cavern spans and heights.

a) Pillar widths are normally equal to 0.51.0 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.


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) 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;


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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.


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


Cavern Support Design Approach

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.

Temporally reinforcement (often bolts and shotcrete in hard rock tunnelling) applied immediately after excavation can also serve as permanent reinforcement.

Further permanent reinforcement is applied (bolts and shotcrete) later.


Cavern Support Design

Roof: Use Q-support design chart directly.

Wall: For Q > 10, Qwall = 5 Q

For 0.1 < Q < 10, Qwall = 2.5 Q

For Q < 0.1, Qwall = Q


Design of Rock Support

1

2

3

4

Q = 1.33, tunnel span 20 m, wall 8 m

3

Wall

Roof

Cavern Support Design

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


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Construction Method

Rock caverns are generally excavated by drill-and-blast 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

benching with vertical drillholes.


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.


Underground Space Development in Singapore Rocks

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