Download Underground Space and Rock Cavern Development in Singapore

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.
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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;
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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
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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.
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