tunnelling and underground construction technology · 2015-04-20 · 2015.04.10 2 tunnelling and...
TRANSCRIPT
2015.04.10 1
Tunnelling and Underground
Construction Technology
Course Lectures
Part 3.1 – Temporary Support
Dr Ákos TÓTH
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Construction Technology
Support Methods
Basic Idea of Support
1. Since the deformation of the ground is connected with the
deformation of the lining, it follows that the load acting upon the
lining depends on its own deformation: We must consider the
interaction between support and rock mass.
2. The questions is not « which is the pressure acting upon the lining »,
but rather « which is the relation between pressure and
deformation »
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In sufficiently strong rock (Scandinavia) permanent lining is not provided
for, as the temporary support is enough for the long-term.
However, usually conventionally driven tunnels obtain a permanent lining
of cast concrete in addition to the temporary lining of shotcrete-rock bolts-
steel girders, etc.
Some prevailing idea is that the loads exerted on the initial lining are
initially reduced due to arching but then slowly increase. It is also believed
that the primary lining decays with time (shotcrete weathering, rock bolts
rusting, etc.), so that an inner lining of cast concrete becomes necessary.
On some occasions, the final lining accomplishes a functional goal
(aerodynamics, ventilation, lightening, etc.)
In the case of segmental lining, there is usually no inner lining of cast
concrete: The temporary and final support coincide.
Support Methods
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This kind of support warrants the stability of the underground opening
through the process of its excavation
Different methods of temporary support exist:
• Shotcrete
• Reinforced shotcrete (i. e. with fibres)
• Steel meshes
• Rock-bolts and dowels
• Timbering
• Steel girders
Temporary Support Methods
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• Shotcrete is a sprayed concrete with very similar characteristics as
those of conventional concrete, but with a lower E and smaller size of
aggregates
• It can be used to seal freshly uncovered surfaces, support of cavities,
strengthening of the rock mass and creation of a supporting shell.
• Up to the age of 28 days the stiffness and strength of shotcrete
develop approx. As they do with cast concrete. Afterwards, the
strength increases due to post-hydration.
Shotcrete
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DRY METHOD
Image from Atlas Copco
Shotcrete
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DRY METHOD
Advantages Disadvantages
• Lower total investment
• Smaller machines ok for limited
space
• Suitable for manual applications
(airborne aggregate in the hose
and light nozzle)
• Ok for varying ground conditions
(particularly when there’s water)
• Mix can be conveyed over a long
distance
• Lower W/C ratio = higher strength
• Good quality at a low cost
• High rebound rate:
− 15-40% from vertical walls
− 20-50% from overhead
− ~50% steel fibres
• Rebound rate is sensitive to
changes in aggregate
• Rebound pockets
• Performance reduced by wet sand
• Less homogenous quality
• dust
Shotcrete
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WET METHOD
Image from Atlas Copco
Shotcrete
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Advantages
• Reduced dust production
• Reduced rebound
• Reduced scatter of concrete properties
• Higher capacity
WET METHOD
Shotcrete
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Influence of angle and distance to rebound
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Image from Atlas Copco
Shotcrete
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Conditions for a good shotcrete
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Shotcrete
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• Dry System (most of all)
• The operator skill is essential
• The operator can change the
water to ciment ratio: dangerous
MANUAL
Shotcrete
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MECHANISED – MANUAL OPERATED
Shotcrete
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MECHANISED – SHOTCRETING BOOM
Image from Atlas Copco
Shotcrete
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MECHANISED – SHOTCRETING BOOM
Shotcrete
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Prepare the sprayed surface
– Clean out dust and dirt with high water pressure or compressed air
adhesion will be improved
– Keep the surface wet but not too much in order not to change the W/C
ratio
Spraying techniques
– Spray from down up
– Distance 1.2-2 m
– Thickness 30 mm on the roof and 50 mm on the walls
– First layer less air to keep the rebound rate low
– First layer is done essentially for smoothing the surface and make
easier the creation of additional layers
Shotcrete
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• Adding steel fibres increases the tensile strength, ductility
and stiffness of shotcrete
• It also improves the distribution of cracks and the capacity
to handle bending loads
• The length of the fibres should not exceed 2/3 of the
minimum hose diameter
• Usual sizes are 45-50 mm length, 0.8-1.0 mm diameter
• The steel content should be > 30 kg/m3
• Water/Cement ratio should not exceed 0.5
Reinforced Shotcrete
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Advantages
•Homogeneous distribution of the fibre reinforcement in the
sprayed concrete
•Great improvement of the ductility
•Higher tensile strength in bending
•Greater safety due to the higher post-cracking strength
•Improved adhesive strength
•Reduced early shrinkage behaviour
•Increased fire resistance (PP)
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Reinforced Shotcrete
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Image from Atlas Copco
FIBRES
Reinforced Shotcrete
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Image from Atlas Copco
ADVANTAGES
Reinforced Shotcrete
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Image from Atlas Copco
SHOTCRETE CONSUMPTION
Reinforced Shotcrete
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• Steel meshes can be manually mounted
and should not be too heavy.
• Normally, they are >100 mm of mesh
size, diameter<10 mm and need a
concrete cover > 2 cm
• Mesh installation is labour intensive and
hazardous, as the personnel is exposed
to rock falls.
• Nowadays, it tends to be less used in
favour of fiber-reinforced shotcrete
• Very good when the ground is very poor
and the adhesion of the shotcrete to the
rock is not good enough
Steel Mesh
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The main objective is the improvement of the rock properties in terms of
stiffness and strength
Difference between Dowels and Rock bolts:
Dowels: They depend upon movement in the rock to activate the
reinforcing action, so they do not strengthen the rock mass from the
moment of their installation. So, in order to be effective, they must be
installed before the rock mass has experienced large deformations.
They normally inhibit the relative slip of two adjacent rock blocks.
Rock bolts: From the very moment of their installation they
strengthen the rock mass. They are always tensioned and compress
the rock mass.
Rock Bolts and Dowels
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REINFORCING ACTIONS
Rock Bolts and Dowels
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REINFORCING ACTIONS
Rock Bolts and Dowels
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APPLICATIONS
Rock Bolts and Dowels
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MECHANICALLY ANCHORED ROCK BOLTS
Rock Bolts
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• These bolts are anchored by means of a wedge force at the tip end of the
rock bolt
• They can be used when the rock is sufficiently strong (UCS > 100 MPa)
and when it is not very jointed
• They run the risk of getting loosen with shocks and vibrations
• They do not need any grouting. However, for durability reasons, if the
bolts are to be used as permanent support grout should be injected
• Tensioning is important to ensure that all of the components are in contact
and that a positive force is applied to the rock. When just « safety » bolts,
the tension is not critical, but when required to carry big loads, they must
be tensioned at 70% of their capacity
MECHANICALLY ANCHORED ROCK BOLTS
Rock Bolts
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RESIN ANCHORED ROCK BOLTS
Rock Bolts
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• When mechanically anchored bolts are not suitable, resin anchored bolts might be
used. They work in soft rocks, jointed rocks and when shocks are to happen
(blasts)
• Resin is made up of two component cartridges containing a resin and a catalyst in
separate compartments. When the cartridges are « broken » the mix sets within a
few minutes
• If they are just for temporary support, a fast setting resin cartridge at the tip is
enough. For permanent support, slow setting cartridges are set behind for
protection
• The procedure is: – Installation of fast and slow setting resin cartridges
– insertion of steel rod
– spinning of rod
– fast setting of end cartridges
– pre-stressing
– slow setting of rear cartridges and fixation of rock bolt
RESIN ANCHORED ROCK BOLTS
Rock Bolts
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Dangers
– Process of mixing the resin is quite delicate, the operator must be
experienced
– When set in very broken rock, the resin might seep into the
surrounding rock, leaving voids: Better option the use of cement grout
in this case
– In argillaceous rocks the clay might cause slipping of the resin
cartridges during rotation, without correct mixing
– There is some uncertainty about the long-term corrosion protection of
resin with aggressive groundwater. So, for permanent support
measures, it might be better the use of cement grouts instead of resin
RESIN ANCHORED ROCK BOLTS
Rock Bolts
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• They consist on a corrugated steel bar or cable that is fixed with grout
within the rock mass
• The procedure of installation is: A thick grout is pumped into the hole by
inserting the grout tube to the end and slowly withdrawing the tube - the
dowel is then inserted - a face plate and nut are fitted onto the end
• They are cheap to install
• When flexibility with impact and abrasion resistance is needed, the bar
can be changed by a cable
• They can be used as temporary and permanent support, and the risk of
corrosion is minimised
GROUT ANCHORED DOWELS
Dowels
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FRICTION «SPLIT-SET» STABILISERS
Dowels
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• They consist of a slotted high strength steel tube and a face plate
• It is installed by pushing it into a slightly undersized hole and the radial
spring force generated provides the frictional anchorage along the entire
lenght of the hole
• The correct diameter of the hole is of capital importance
• Quick and simple to install: Very used in mining
• Useful for rockburst environment (ductile)
• The main problem is corrosion, so they are just suitable for temporary
support
FRICTION «SPLIT-SET» STABILISERS
Dowels
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FRICTION «SWELLEX»
Image from Atlas Copco
Dowels
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FRICTION «SWELLEX»
Image from Atlas Copco
Dowels
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• Developed and marketed by Atlas Copco
• No pushing force is needed during insertion and the dowel is activated by injection
of high pressure water (30 MPa) which inflates the tube into intimate contact with
the walls of the borehole
• They are sold in two sizes: Swellex (25.5 mm) and SuperSwellex (36 mm)
• They are very ductile, most of all in its version Mn-enriched, and so very well
adapted to rockburst, large convergences and plastic deformations
• They can be set very quickly. They are very suitable for mechanized installation
(rock bolters)
• Although they are dowels, they can be slightly pre-stressed at 20 KN
• They present, as well as the split-set system, the problem of corrosion, that might
make them not suitable for permanent support. However, corrosion-resistant
versions of Swellex have been created by the manufacturer
FRICTION «SWELLEX»
Dowels
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SELF DRILLING ANCHORS SDA
Image from Atlas Copco
Dowels
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SELF DRILLING ANCHORS SDA
Image from Atlas Copco
Dowels
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• Grouted cables can be used when a very
flexible solution is needed
• As the cable pulls out of the grout, the cable
causes dilation – a confining pressure – friction
or shear resistance
• They are very used in large excavations
(caverns) and rock slope stabilisation
• Special care must be taken with the quality and
correct injection of grout
CABLES
Dowels
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• They are used mainly for face stabilisation, as they can be easily
broken in the ulterior excavation step
• They are made in fibre-glass. Once put in place, they are embedded
in grout
• They are also used for stabilisation of pilot tunnels that are to be
widened by TBM
FIBRE-GLASS DOWELS
Image from Atlas Copco
Dowels
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Rock Bolts and Dowels
Image from Atlas Copco
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Drilling with conventional boomer rigs and ulterior setting of the rock
bolts from a platform
SEMI-AUTOMATIC
Rock Bolts and Dowels
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Drilling and rock bolt setting is done automatically with special machines
FULLY AUTOMATIC
Rock Bolts and Dowels
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Individual bolts are placed to stabilise isolated blocks
SPOT BOLTING
Rock Bolts and Dowels
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Systematic installation of a more or less regular array of rock bolts
PATTERN BOLTING
Rock Bolts and Dowels
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• Checking depth and diameter of holes
• Correct cleaning of drillholes
• Appropriate grouting
• Tensioning tests, non destructive
• Destructive pull out tests
Rock Bolts and Dowels
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• In the early days of tunnelling, timbering was the only means for
temporary support
• Nowadays it is mainly used for the support of small and / or irregular
cavities
• Wood is easy to handle and indicates imminent collapse by cracking
• However, the discontinuous contact with the rock is problematic
Timbering
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• Support arches are composed of segments of rolled steel
profiles or lattice girders
• The arch segments are placed and mounted together with fixed
or compliant joints
• The contact with the adjacent rock is achieved with wooden
wedges or with bagged packing. They can also be mounted
with rock bolts
• Normally the arches are covered with shotcrete
• They can also serve to mount forepoling spiles in longitudinal
direction
Steel Ribs