DIFFERENT TYPES OF ROCK SUPPORTS

INTRODUCTIONRock support : The procedures and materials

used to improve the stability and maintain the load bearing capacity of rock near to the boundaries of an underground excavation.

Primary objective : To mobilize and conserve the inherent strength of the rock mass so that it becomes self-supporting.

Rock support combines the effects of reinforcement, by such elements as dowels, tensioned rock bolts and cables, and support, with shotcrete, mesh and steel sets.

Rock reinforcement or active support :The supporting elements are an integral part of the reinforced rock mass. Ex : A pattern of un-tensioned grouted dowels.

Rock support or passive support : These supporting members are external to the rock and respond to inward movement of the rock surrounding the excavation. Steel sets are an example of passive support.

CLASSIFICATION OF ROCK SUPPORT SYSTEMS

TYPES OF ROCK SUPPORT SYSTEMSRock boltsShotcreteGroutingSteel ribsConcrete lining

Note : Rock bolts when used in a pattern they act as a permanent support.

temporary

permanent

TUNNEL LINING

Permanent lining is required in most tunnels, always in soft ground and frequently in rock. The purpose of a lining is partly structural, to contain and support the ground and control inflow of water, as well as to provide an internal finishing suitable for the equipment of the tunnel.

FUNCTIONSThe final tunnel lining has to sustain a large number of influences. The main ones are:

Stemming from the ground:Ground pressuresLandslides, earth subsidenceEarthquakesWater pressuresChemicals actions of aggressive subsoil

components

Resulting from constructionconstruction activities such as dead weight in

fresh stateshrinking annular gap grouting, roof grouting transport activities for ready-made parts

(segments, ready-made pipes) jacking forces, back-up loads.

Through utilizationinfluences of temperature on the air or from

sewagechemical attacks from gases, sewage, thawing salttraffic influencestransportation of rubble or stones in the case of

water pressure tunnelsfire in the case of transport tunnels.

TYPES OF LININGPrimary lining :To sustain the loads and deformations that the

ground may induce during the tunnel's working.Control of water egress (exit) & ingress

(entrance).

Secondary lining :Smooth bore profiles for their intended useErosion and corrosion protection for the

primary liningFurther waterproofing

The principal materials and construction methods for permanent lining of bored tunnels are: in-situ concrete sprayed concrete (shotcrete)segments in prefab concrete or cast-iron.

INSTALLATIONThe permanent tunnel lining has to be

dimensioned both in static and constructional terms to cope with these influences.

Shallow tunnels are often lined by dropping concrete down holes drilled from the surface while the greater depth of most rock tunnels requires concreting entirely within the tunnel.

The invert is generally concreted first, followed by the arch where forms must be left in place from 14 to 18 hours for the concrete to gain necessary strength.

Voids at the crown are minimized by keeping the discharge pipe buried in fresh concrete. The final operation consists of contact grouting, in which a sand-cement grout is injected to fill any voids and to establish full contact between lining and ground.

The method usually produces progress in the range of 40 to 120 feet per day.

APPLICATIONSExcavation stabilization in tunnelingSealing works in underground constructionTunnel and underground chamber liningSlope stabilisationStabilisation in mine and galleryProtective lining construction

ADVANTAGES

Compressed air conditions in the tunnel Better visibility More attractive appearance Simpler installation

WIRE MESH

Wire mesh is used to support small pieces of loose rock and broken rock from falling.

Two types of wire mesh currently used in underground operations are chainlink meshweld mesh.

ADVANTAGESMesh is very economical and quickly installed.It is easy to attach to roof reinforcement with

extra faceplates and nuts.It is easily repaired.

Mesh cannot carry excessive load of broken rock without failure.

It is easily damaged by flyrock from nearby blasts.

DISADVANTAGES

APPLICATIONSMesh is useful for supporting small pieces of

broken rock. Weldmesh is used traditionally as

reinforcement for shotcrete.

STRAPSThis system is used extensively in mining and

civil engineering applications to hold slabby ground between rock bolts or to prevent slabs from loosening.

The strap is formed to follow the rock surface and held with regular rock reinforcement or specially installed pins.

Straps should always be installed across weakness planes.

ADVANTAGESStraps are easy to install with rock bolts.They are very effective in stratified, slabby

ground.

Straps cannot be used to control ravelling in loose, blocky ground if the block size is small.

DISADVANTAGES

APPLICATIONSSteel straps are used to give roof control in

ground where rockbolts or dowels alone cannot hold intermediate and relatively thin slabs of rock.

ROCK BOLTSSteel bolts are frequently set in holes drilled into

the rock to assist in supporting the entire roof or individual rock slabs that tend to fall into a tunnel.

MECHANISM-Rock bolts maintain the stability of an opening by suspending the dead weight of a slab from the rock above by providing a normal stress on the rock surface to clamp discontinuities together and develop beam action by preventing key blocks becoming loosened so that the strength and integrity of the rock mass is maintained.

Underground mines use two principal types of rock reinforcement:Tensioned mechanically anchored rock bolts.Un-tensioned grouted or friction anchored

dowels.

MECHANICALLY ANCHORED TENSIONED ROCKBOLT

ADVANTAGESBolt can be tensioned immediately after

installation using an impact wrench, torque wrench or hydraulic jack, and grouted at a later stage when short term movements have ceased.

Provides very reliable anchorage in good rock.

High bolt loads can be achieved.DISADVANTAGESCorrect installation requires skilled workmen

and close supervision.Grout tubes are often damaged during

installation.

SHOTCRETEShotcrete is the generic name for cement,

sand and fine aggregate concretes which are applied pneumatically and compacted dynamically under high velocity.

The difficulty of access and unusual loading conditions are some of the problems which are peculiar to underground mining and which require new and innovative applications of shotcrete technology.

TYPES OF SHOTCRETE

Dry mix shotcreteWet mix shotcreteSteel fibre reinforced micro silica shotcreteMesh reinforced shotcrete

DRY MIX SHOTCRETE

WET MIX SHOTCRETE

COMPARISON OF DRY AND WET MIX SHOTCRETE

Dry mix system tends to be more widely used in mining, because of inaccessibility for large transit mix trucks and because it generally uses smaller and more compact equipment.

The wet mix system is ideal for high production applications, where a deep shaft or long tunnel is being driven and where access allows the application equipment and delivery trucks to operate on a more or less continuous basis.

STEEL FIBRE REINFORCED MICRO SILICA SHOTCRETE

Additives :cementitious admixture - silica fume steel fibre reinforcement

Silica fume or micro silica is a by-product of the ferro silicon metal industry and is an extremely fine pozzolana.

Pozzolans are ce- mentitious materials which react with the calcium hydroxide produced during cement hydration.

Silica fume, added in quantities of 8 to 13% by weight of cement, can allow shotcrete to achieve compressive strengths which are double or triple the value of plain shotcrete mixes.

Result - an extremely strong, impermeable and durable shotcrete.

Added benefits:reduced reboundimproved flexural strengthimproved bond with the rock massthe ability to place layers of up to 200 mm thick

in a single pass because of the shotcrete's 'stickiness‘.

Steel fibre reinforcement – a replacement of wire mesh.

Reinforcement – increases ductility.

Tests showed that the addition of steel fibres to silica fume shotcrete enhances both the compressive and flexural strength of the hardened shotcrete by up to 20%.

The peak strength of these slabs increased by approximately 85% and 185% for 1.0 and 1.5 volume % of fibres, respectively.

MESH REINFORCED SHOTCRETEIn very poor quality, loose rock masses,

where adhesion of the shotcrete to the rock surface is poor, the mesh provides a significant amount of reinforcement, even without shotcrete.

Plain shotcrete is applied later to provide additional support and to protect the mesh against corrosion.

COMPARISON OF FIBRE AND MESH REINFORCED SHORTCRETE

Same strength , but mesh superior in bending.

In small mines, quality control is a problem with fibre reinforced concrete because of non-uniform distribution of fibres.

ADVANTAGESPrompt application before loosening starts to

reduce the strength of the rock mass.

Increased safety while application.

Shotcreting can be developed into a single-support system that is strengthened progressively as needed for conversion into the final support.

Application to any elevations because sprayed concrete adheres immediately and bears its own weight.

Can be applied on uneven substrates

Good adhesion to the substrate.

Totally flexible configuration of the layer thickness on site.

Rapid load-bearing skin can be achieved without forms (shuttering) or long waiting times.

DISADVANTAGESShotcrete's quick-setting properties have

been achieved by the injection of high-alkaline additives at the spraying nozzle. The resulting concrete is highly porous, and lacks strength.

Caustic dust from the additives can cause skin and lung problems, and represents a health hazard to construction workers.