Tunnelling and Underground Construction Technologyse.sze.hu/images/ngm_se108_1/Tunnels_2015-04-10_Toth_1-Temporar… · Tunnelling and Underground Construction Technology o • They

2015.04.10 1

Tunnelling and Underground

Construction Technology

Course Lectures

Part 3.1 – Temporary Support

Dr Ákos TÓTH

2015.04.10 2Tunnelling and Underground


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 »



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



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



• 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



Shotcrete



o

Image from Atlas Copco

Shotcrete



o


Shotcrete



o

DRY METHOD


Shotcrete



o

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



o

WET METHOD


Shotcrete



Advantages

• Reduced dust production

• Reduced rebound

• Reduced scatter of concrete properties

• Higher capacity

WET METHOD

Shotcrete



COMPARISON

Shotcrete



Influence of angle and distance to rebound

o


Shotcrete



Conditions for a good shotcrete

o

Shotcrete



o

• Dry System (most of all)

• The operator skill is essential

• The operator can change the

water to ciment ratio: dangerous

MANUAL

Shotcrete



MECHANISED – MANUAL OPERATED

Shotcrete



MECHANISED – SHOTCRETING BOOM


Shotcrete



MECHANISED – SHOTCRETING BOOM

Shotcrete



MECHANISED – TBM ROBOT

Shotcrete



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



o

• 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



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)

o




o


FIBRES





ADVANTAGES




o


SHOTCRETE CONSUMPTION




o

• 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



o

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



o

REINFORCING ACTIONS




o

REINFORCING ACTIONS




o

APPLICATIONS




o

MECHANICALLY ANCHORED ROCK BOLTS

Rock Bolts



o

• 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



o

RESIN ANCHORED ROCK BOLTS

Rock Bolts



o

• 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


Rock Bolts



o

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


Rock Bolts



o

GROUT ANCHORED DOWELS

Dowels



• 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



o

FRICTION «SPLIT-SET» STABILISERS

Dowels



o

• 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



o

FRICTION «SWELLEX»

Dowels



o



Dowels



o



Dowels



o

• 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


Dowels



o

SELF DRILLING ANCHORS SDA


Dowels



o

SELF DRILLING ANCHORS SDA


Dowels



o

CABLES

Dowels



o

• 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



o

• 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


Dowels



o





o

Drilling with conventional boomer rigs and ulterior setting of the rock

bolts from a platform

SEMI-AUTOMATIC




o

Drilling and rock bolt setting is done automatically with special machines

FULLY AUTOMATIC




o

Individual bolts are placed to stabilise isolated blocks

SPOT BOLTING




o

Systematic installation of a more or less regular array of rock bolts

PATTERN BOLTING




o

• Checking depth and diameter of holes

• Correct cleaning of drillholes

• Appropriate grouting

• Tensioning tests, non destructive

• Destructive pull out tests




o

• 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



o

• 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



o

Steel Ribs



o

Steel Ribs

Tunnelling and Underground Construction Technologyse.sze.hu/images/ngm_se108_1/Tunnels_2015-04-10_Toth_1-Temporar… · Tunnelling and Underground Construction Technology o • They

Documents