Shotcrete in Mining

MASTERBUILDERSADMIXTURES

Shotcrete (Sprayed Concrete) for Mining Applications

by

A.J.S. (Sam) SpearingDirector International Mining

Underground Construction GroupMaster Builders Technologies

A Division of Degussa Construction Chemicals

Telephone: +1-216-8397213Telefax: +1-216-8398801E mail: sam.spearing@mbt.com

January 2002

INDEX

1. An introduction to shotcrete1.1 Definitions1.2 History1.3 Shotcrete as a support element

2. Shotcrete raw materials2.1 Binders2.2 Aggregates2.3Water2.4 Admixtures2.5 Fibres

3. Application processes3.1 Dry process3.2 Wet process3.3 A comparison between the processes3.4 Process selection guidelines

4. Shotcrete design4.1 Placed strength4.2 Time4.3 Shotcrete components and performance

5. Shotcrete mining equipment5.1 Shotcrete spray equipment5.2 Shotcrete dosing pumps and systems5.3 Shotcrete lines and nozzles

6. Shotcrete service needs6.1 Power6.2 Compressed air6.3 Water

7. Logistics7.1 Slick line design and operation

8. Shotcrete application8.1 Substrate preparation8.2 Shotcrete spraying8.3 Application safety

9. Training

10. Quality control and testing10.1 Quality control10.2 Field testing10.3 Laboratory testing10.4 Quality assurance planning

11. Costs11.1 Material costs11.2 Equipment costs

11.3 Material and equipment transportation costs11.4 Labor costs11.5 Application inefficiency costs11.6 Time related costs11.7 Other costs

12. Specification and design procedures for shotcrete support12.1 Observational methods12.2 The Q system12.3 The modified Q system12.4 The ground characteristic curve method

13. Mix design examples and case studies13.1 Standard dry mix13.2 Standard wet mix13.3 High early strength with high single pass13.4 Fiber reinforced shotcrete for slick-lines13.5 Fiber reinforced shotcrete for slick-lines in sinking shafts13.6 A mix to replace bolts and screen13.7 Shotcrete for kimberlite (blue rock)13.8 Shotcrete for water inflow areas13.9 Wet shotcrete for economical reasons

14. The future14.1.Future advances in shotcrete equipment14.2.Possible improvements in synthetic fibers14.3 Future admixture advances

15. References

16. MBT Shotcrete Admixture Products

17. Useful contacts American Concrete Institute American Shotcrete Association American Society for Testing and Materials Australian Shotcrete Society British Standards Institute Deutsches Institut fuer Normung EFNARC International Centre for Geomechanics International Tunneling Association

1. An introduction to shotcrete (sprayed concrete)

1.1 Definitions

Shotcrete according to the American Concrete Institute (ACI) is defined aspneumatically applied mortar or concrete, projected at high velocity.

Sprayed concrete according to the European Federation of Producers andApplicators of Specialist products for Structures (EFNARC) is a mixture ofcement, aggregate and water projected pneumatically from a nozzle into place toproduce a dense homogeneous mass. Sprayed concrete normally incorporatesadmixtures and may also include additions of (steel or synthetic) fibers or acombination of these.

Sprayed concrete and shotcrete refer to the same material. The trend especiallyin Europe is to refer to the product as sprayed concrete.

The principles applied to standard concrete technology are no different inshotcrete (hence the growing preference to calling shotcrete sprayed concrete).This means that shotcrete should be designed to achieve a technicallyacceptable and a cost effective mixture. This implies that the following technicalaspects must be considered:

the material components and overall composition the application conditions (including access and the availability of services) the application mode (the dry or wet process) logistic constraints (mainly as it influences material handling) health and safety requirements

The true final (placed) costs should also always be considered, and this seldomoccurs, at present, in the mining industry. The key costs are:

material costs (an easy cost to establish) logistic costs equipment capital and operating costs labor costs (including transportation to site) application efficiencies (wastage especially rebound) time related costs (the influence on the overall mining cycle time)

Shotcrete is applied by two different processes (methods), defined below (afterEFNARC) and explained in detail in Chapter 3:

Wet process is a technique in which cement, aggregate and water are batchedand mixed together prior to being fed into a purpose-made machine andconveyed through a pipeline to a nozzle where the mixture is pneumatically and

continuously projected into place. The mixture normally incorporates admixturesand may also include additions or fibers or a combination of these.

Dry process is a technique in which cement and aggregate are batched, mixedand fed into a purpose-made machine wherein the mixture is pressurized,metered into a compressed air stream and conveyed through hoses or pipes to anozzle where water is introduced as a spray to wet the mixture which is thenprojected continuously into place. The mixture may also incorporate admixturesor additions fibers or a combination of these.

1.2 History

The following time line indicates the development of the support technology:

1907 - The first machine to spray concrete was developed in Pennsylvania,USA by Carl Akeley for use in construction.

1910 - The machine and process were introduced to the public at large at theCement Show in Madison Square Gardens in New York, USA.

1915 - This concept was later improved by the Cement Gun Company, thatlater became the Allentown Gun Company. Carl Akeley also registered theterm gunite for his sprayed mortar mix and this term is still sometimes used.

1920s - Gunite was used in the USA to fireproof mine drifts (supported bytimber sets).

1930s - The term shotcrete was introduced by the American RailwayEngineering Association.

1940s - Coarse aggregate was introduced into the sprayed concrete mixes.

1954 - Anton Brunner, an engineer from Salzburg in Austria, replaced heavysteel and timber support with shotcrete in a new diversion tunnel, insqueezing ground, at the Runserau Power Plant

1955 - The wet shotcrete process was introduced.

1957 - Little major equipment development took place until in 1957 when therotary drum was perfected by Meynadier (now MEYCO Equipment) and Aliva.These machines (still in use today for dry shotcreting) have a feed hopperabove a rotor with chambers that intersect feed openings allowing somematerial to enter under gravity. A compressed air stream forces this materialinto the discharge pipeline and keeps it in suspension until the shotcretenozzle. This development was probably as a result of the use of shotcrete inAustria as a civil tunnel support in the 1950s.

Around 1965 - Gunite was already used in some Boliden Mines in Sweden.This involved the dry process and in some applications the machine was onsurface and the material applied some 300m below surface (at GarpenbergMine).

1968 - It appears that the first recorded mining shotcrete application forsupport was at the Hecla Mining Company in the USA.

1975 - The Norwegians were the first to realize the benefits of silica fume(microsilica).

1977 - The Norwegians introduce steel fibers for the first time as areplacement for mesh/screen.

1994 - The Underground Construction Group of MBT are the first to introducenon caustic alkali free accelerators.

1999 - MEYCO start field testing the first shotcrete equipment capable to beoperated in a fully robotic mode (with no operator necessary).

1.3 Shotcrete (sprayed concrete) as a support element

Shotcrete in mining is a very effective support element.

For shotcrete to be the productive and efficient support that it can and should be,all aspects must be considered. Too often in the mining industry, technicallyinferior or inappropriate support systems are used, due to convenience,ignorance, resistance to change and/or expediency.

The Table below indicates the type of performance that could be expected fromshotcrete underground 28 days after placement:

Performance parameter Performance range (at 28 days)Compressive cube strength (MPa) 30 to 80Flexural strength (MPa) 3 to 8Bond strength to competent hard-rock (MPa) 0.5 to 1.5

Shotcrete is typically used for the following applications in mines:

As a temporary support. As a permanent excavation support element (main application). As the only permanent support (bolt and screen/mesh replacement). For roadways (major roadways such as declines, ramps or loading/discharge

areas).

Pillar reinforcement or even replacement. Backfill retaining wall construction. As part of ventilation seals. As part of an orepass lining support and wear resistant system. As a permanent final lining in vertical shafts.

Shotcrete offers is a passive support on application (by definition). The slightestdeformation however generates a significant resistance because it is stiff and hasa high Youngs Modulus. In addition it has a micro-reinforcing effect bypenetrating and/or bridging micro-cracks in the rock. This is in contrast to thereally passive supports such as mesh/screen and most arch systems.

In typical mining applications, the sprayed shotcrete lining is thinner than used intypical civil projects. A common thickness range in mining is between 50 and 100mm, and hence the shotcrete cannot be considered a structural arch. It offers agood rock reinforcement (support) however mainly because it:

Reduces unraveling. Limits air slack (rock weathering due to moisture and air). Offers lateral confinement to the rock surface. Fills fractures in the rock.

For optimum performance, where bolts are also used, the bolts should have largebase plates (washers) fastened and tensioned to the bolts, against the shotcrete.This causes the shotcrete and bolts to act as a single unit, providing a bettersupport system.

Poorly designed and/or applied shotcrete can be hazardous and costly. The mainreason for poor shotcreting is:

Ignorance !

This can result from

A poorly designed mix. Unmatched and unsuited equipment and shotcrete infrastructure (mainly

supply logistics). Inadequate services. Ineffective quality control. False economy.

1910 - The first shotcrete machine made

1918 Tunnel in Europe using Guniting

Early use of gunite in construction in America

Shotcreting underground (date unknown).

1957 Rotary drum dry shotcrete machine from MEYCO

2. Shotcrete (sprayed concrete) raw materials

The material constituents of the shotcrete are important in order to achieve adesired target placed performance at the lowest cost. The materials wouldinclude:

2.1 Binders

2.1.1 Cement binders

Cement is the bonding material (glue) that holds a cementitious materialtogether. For most shotcreting applications, Portland cement is used. Thiscement was invented by an English bricklayer Joseph Aspdin in 1824. Thename was derived from the set materials colour and texture that resembled alocal limestone called Portland stone. The cement is produced from mainly amixture of klinker and gypsum. The klinker is typically produced in a rotary kilnfrom lime, silica, alumina and ferric oxide.

Other types of cement that are commonly used in shotcrete include:

Sulphate resisting cement, that typically has a lower tricalcium aluminatecontent than Portland cement

High alumina cement (HAC) that is produced by fusing (melting) a mix ofbauxite and limestone together. HAC is therefore not a Portland cementderivative, and is often used in refractory applications where it can betroweled in place or shotcreted.

For basic applications a cement content by mass of 20% is typical.

2.1.2 Cement extenders (supplementary materials)

Cement extenders are commonly used in shotcrete mainly for a cost reduction.

Fly ash is the most commonly used and is obtained as a waste product fromcoal-fired power plants. The fly ash for use in shotcrete is mainly supplied as ablend with the Portland cement already in bags or bulk.

Ground granulated blast-furnace slag (GGBS) is another cement extender thatcan be used. GGBS is a glassy and granular material that is usually produced asa by-product of iron production. The molten slag is rapidly quenched and thenfinely ground.

Fumed silica, silica fume or microsilica as it can be called is also a cementextender but is dealt with under the Admixture Section due to its very uniqueand desirable properties.

Cement extenders are commonly used to replace about 30% of the cement used(up to as high as 50%), but generally the rate of strength development isreduced, and this is frequently undesirable in mining (due mainly to safetyconsiderations). The same applies to sulfate resistant cements and a betteralternative is the use of Portland cement and micro silica that can gain strengthrapidly (without an accelerator overdose).

2.2 Aggregates

As in concrete, aggregates are used to provide dimensional stability by providinga rigid skeletal structure, reduce the void space to be filled with the cementitiouspaste and hence reduce the cost. The main parameters to consider are:

Grading and the maximum size Particle shape and density The aggregate type especially the presence of reactive chemicals and

minerals (for possible alkali-acid reaction) The compressive strength Moisture content

When applying the shotcrete by the dry process, the aggregates can be selectedsuch that the voids are minimized, as is the common approach when usingconcrete. As a general guide, the use of +16 mm material is to be avoided andthe modern trend is to use a maximum size of 10 mm, due to rebound and wearconsiderations.

Figure 1 gives the aggregate envelope for shotcrete recommended by EFNARC.

It is possible to use gradings outside the envelope, but the performance will bereduced, or the material cost increased as additional admixtures would need tobe used.

A typical dry process base mix by weight would consist of:

20 to 25 % of cementitious binder 20 to 15 % of coarse aggregate 55 to 65 % of sand (natural and wash sand is preferred)

Where the dry process is used, the moisture content of the aggregates should bea maximum of 6 %.

The mix design needs to be modified when using the wet process because thepumpability is a major issue, and generally more fines are needed to reduce thechance of line blockages, and make pumping easier (at a lower pressure).

FIGURE 1RECOMMENDED AGGREGATE GRADATION ZONE FOR SHOTCRETE

COMPARISON BETWEEN THE LOAD BEARING CAPACITY OFPLAIN, MESH AND STEEL FIBRE REINFORCED SHOTCRETE

5 0403020Deformation in mm

Load in kN

1000

510

1520

2530

Plain shotcrete

Fibre reinforced shotcreteFibre reinforced shotcrete (at a higher dose)Wire mesh reinforcement

(after Vandewalle)F i g u r e 2

COMPARISON BETWEEN THE LOAD BEARING CAPACITY OFPLAIN, MESH AND STEEL FIBRE REINFORCED SHOTCRETE

5 0403020Deformation in mm

Load in kN

1000

510

1520

2530

Plain shotcrete

Fibre reinforced shotcreteFibre reinforced shotcrete (at a higher dose)Wire mesh reinforcement

(after Vandewalle)F i g u r e 2

12

26

50

72

90

100

90

73

55

37

22

11

4

100

80

60

40

20

00.125

(after EFNARC)

0.25 0.5 1 2

ISO sieve (mm)

Mat

eria

l pas

s in

g (%

by

wei

ght)

4 8 16

100

2.3 Water

The water quality can be important and should be free from oils and not heavilyacidic. Basically if the water is potable (i.e. drinkable), it is suitable for shotcrete.

2.4 Admixtures

The American Concrete Institute in the ACI Manual of Concrete Practice (1999edition) defines an admixture as:

A material other than water, aggregates, hydraulic cement, and fiberreinforcement, used as an ingredient of concrete or mortar, and added to thebatch immediately before or during its mixing.

Many different types of admixtures can be beneficial in shotcrete, depending onthe specific application and requirements. Most admixtures can be used in thewet process only (discussed later), but generally only a set accelerator (usually inpowdered form) is used when dry shotcreting. In order to achieve the quantifiablebenefits from admixture usage, it is critical that the dosing equipment is reliableand appropriate.

2.4.1. Accelerators

The use of an accelerator is essential in mining applications. Accelerators ensurethat the shotcrete:

Develops a bond with the rock as soon as possible. Generates internal strength quickly. Is able to be placed in thick single passes (if needed).

Shotcrete accelerators generally fall into the following categories:

Silicates (e.g. water glass or sodium silicate) Sodium or potassium aluminates Alkali free accelerators

Silicates are not really true accelerators as they only create a gelling effect ratherthan a rapid early strength gain. Accelerators also tend to reduce the finalstrength of the shotcrete. This is partly because a slower rate of strength gainresults in a finer and a more dense crystalline growth that creates a stronger finalproduct.

In a typical dry mix, a powdered accelerator addition of between 2 and 5% basedon the weight of the total cementitious addition would be reasonable. Accelerator

dose rates are normally expressed as a percentage of the total cementitiouscontent.

There is confusion in the mining industry over the definition of an alkali freeaccelerator. To understand this, the difference between alkalinity and alkalicontent must be noted.

Alkalinity refers to a basic liquid with a pH between 7 and 14. Alkali content refers to the presence of alkali cations, and a liquid may

contain alkali cations but have a neutral pH.

To be a true alkali free accelerator according to CEN definitions (ComiteEuropeen de Normalisation) an accelerator must contain

Improved durability (more resistant to freeze/thaw cycles and improvedsulphate attack resistance).

Improved bonding to substrates. Higher strengths (compressive and flexural) Reduced rebound Improved flow in the delivery hose (in the wet process). Reduced wear in the pump and nozzle (in the wet process). Improved mix cohesiveness. Thicker single pass applications

A typical dosage would be 5 to 10% by weight of the cementitious binder.

2.4.4. Curing agents/concrete improvers

There is an incorrect perception that an underground environment provides goodcuring conditions for shotcrete. This is incorrect because the ventilation tends tocause premature drying of the shotcrete surface resulting in:

Poor hydration causing a weaker final product Reduced substrate bonding if the shotcrete layer is relatively thin (less than

75 mm/3 inches) Significant shrinkage cracking

Solutions to this include:

Regular wetting of the placed shotcrete. The application of an external curing agent (such as a spray applied wax) The inclusion in the mix (wet process only) of a concrete improver that has

the potential to improve curing and increase the bond with the substrate.

Regular wetting of the placed product is often impractical and too timeconsuming. The application of an external curer involves a second (albeit simple)operation and makes it difficult to apply a further layer of shotcrete at a later time(for whatever reason) unless the coating is removed.

As with all concrete construction, moist curing conditions are important for 7days. With accelerated shotcrete, good curing is important basically from afterspraying.

2.4.5. Consistency controllers

Under certain wet shotcrete applications, the use of a consistency control systemcan help. The first component is added into the mix (before pumping) to keep theopen time and improve pumpability. The second component is added at thenozzle to stiffen the mix and improve the strength development rate.

2.4.6. Hydration controllers

The useful life of a wet shotcrete mix can be a limiting factor in undergroundapplications due to logistic considerations. A typical batch of untreated shotcreteshould be discarded as waste after between about 1.0 and 2.0 hours (dependingon the ambient temperature). Conventional retarders can extend this to about4 hours, but hydration controllers can effectively put the mix to sleep for up to72 hours. This technology is of great advantage in many underground wetshotcreting applications, because it helps resolve logistic problems.

Hydration controllers function in 2 ways:

By acting as an effective dispersant thus keeping hydrating particles apart. By forming a barrier around all the cementitious particles, thus stopping the

hydration process altogether (unlike conventional retarders).

This effect is over-ridden when shotcreting, by adding an effective accelerator,and the hydration controller has no adverse effect on the rate of strength gainand the ultimate strength, provided that adequate accelerator is added.

If a hydration controller is used, a minimum flow (to BS 1881, Part 105) of 50 cmshould be used.

2.4.7. Pumping aids

Pumping aids usually also act as water reducers in shotcrete. They improve thepumpability of harsh mixes, frequently caused by poorly graded aggregates. Airentraining agents are a common pumping aid.

2.5 Fibers

The application of fibers in construction dates back centuries to the use of horsehair, jute, sisal and cotton.

Concrete is by nature a brittle product and is weak in tension. In shotcreting,reinforcement can be provided by the use of screen/mesh or fibers. Fibers haveobvious advantages over screen including:

Fibers are more evenly distributed throughout the shotcrete. Mesh is difficult and labor intensive to apply. It can also represent a safety

hazard if manually installed. The fiber reinforced shotcrete effectively lines the tunnel periphery, and

additional shotcrete is needed when screen is used, to fill the depressions (asthe mesh is usually fixed at the high points on the tunnel periphery).

Mesh can increase shotcrete rebound significantly (due mainly to screenvibration during spraying).

Figure 2 is a comparison between the performance of unreinforced shotcrete andshotcrete reinforced using screen or fibers.

Where movement of the rockmass is expected, shotcrete is frequently appliedover mesh/screen or fibers are introduced into the base shotcrete mix.

The trend is away from mesh/screen reinforced shotcrete to fiber reinforcedshotcrete in mining because:

Durability is improved. Ductility (toughness) is increased. Impact resistance is increased. Surface cracking is reduced (not a major concern on mines). Rebound is reduced. Compaction is improved (see Figure 3). Application productivity and safety is improved (major benefit). Final placed costs are reduced. Logistics is simplified.

Typical fiber reinforcement materials include:

Polyolefin fiber of which polypropylene is the most common (mono-filament orfibrilated)

Carbon fiber Glass fiber Drawn steel wire Slit sheet Milled steel pieces Melt extract pieces

The most commonly used types in shotcrete are the drawn wire andpolypropylene fibers.

The most important parameters for fibers are:

The aspect ratio (overall ratio of the fiber length to its diameter). The tensile strength. The shape.

An ideal fiber should have the following:

A length such that it can overlap and bridge at least 2 of the largest aggregateparticles used in the mix (typically a length between 25 and 40 mm).

A high aspect ratio (i.e. thin).

FIGURE 2COMPARISON BETWEEN THE LOAD BEARING CAPACITY

OF PLAIN, MESH AND STEEL FIBRE REINFORCED SHOTCRETE

50403020

Deformation in mm

Lo

ad in

kN

1000

5

10

15

20

25

30

Plain shotcrete

Fibre reinforced shotcrete

Fibre reinforced shotcrete

(at a higher dose)

Wire mesh reinforcement

(after Vandewalle)

FIGURE 3THE ADVANTAGES OF STEEL FIBRE REINFORCED SHOTCRETE

OVER MESH REINFORCED SHOTCRETE

MESH REINFORCED SHOTCRETE

Plain shotcrete

Potential voids or poorercompacted shotcrete

Welded wire mesh

Mesh pinned to rock

Cover to mesh

Rock

Maintains contact andbonds with the rockover the entire surface

STEEL FIBRE REINFORCED SHOTCRETE

Steel fibrereinforced shotcrete

Rock

(after Vandewalle)

A high tensile strength A shape that results in a good anchor particularly at the fiber ends in the

shotcrete.

The use of steel fibers and particularly synthetic fibers in dry shotcrete is notgenerally recommended due to the high fiber loss found in the rebound(significantly more fiber loss than the overall measured rebound and in the +50%range).

Where steel fiber is used a dosage rate of between 30 and 50 kg/m (0.4 to 0.6% by volume) is generally used.

The potential use of polypropylene fibers in shotcrete for mining applications hasincreased dramatically recently with the development of high performancepolymer fibers. The high tensile strength and crimped shape have resulted inperformances very similar to that obtained with steel. Dosage rates of betweenabout 7.0 and 13.5 kg/m (0.75 to 1.5 % by volume) are typical.

Figure 4 shows the wide variety of fiber performance that can be obtained usingdifferent fibers. Typically high performance steel fibers have higher residual loadsimmediately after failure but less load at high deformations than highperformance synthetic fibers.

The key to the cost effective use of fibers is to consider the:

Cost/J of energy

In the future, more use will be made of polymer fibers, possibly blended withsteel fibers, in high performance mining shotcrete applications.

FIGURE 4ROUND PANEL TEST RESULTS WITH DIFFERENT FIBRES

35

30

20

25

15

5

10

0 5 10 15 20Deflection in mm

25 30 35 400

Lo

ad in

kN

Plastic fibre A

Plastic fibre B

Plastic fibre C

Plastic fibre D

Steel fibre X

Steel fibre Y

Examples of Shotcrete Fibres

DD fibre from Synthetic Industries

Collated Dramix from Bekaert

HPP fibre from Synthetic Industries

Xorex fibre from Synthetic Industries

Barchip fibres from Hagihara

STRUX fibres from Grace

Forta fibre

GSF fibre from Grace

3. Application processes

There are two application processes in shotcreting and the selection depends onthe specific application and the site conditions.

3.1 The dry process

In this application, the particulate material is conveyed (pneumatically) in abasically dry state from the pump to the nozzle, where the water is added.A typical section through a dry machine is given on Figure 5.

The nozzleman is the key to the successful application because he controls thevital water addition. Too much water causes the shotcrete to sag away from therock and reduces the strength, and too little water causes higher rebound andcan also lead to a strength reduction.

The moisture content of the mix (prior to water addition at the nozzle) should bebetween 2 and 5% to minimize dust production at the pump. More than 5% watercan cause blockages in the line. A pre-dampener can be used to reduce the dustat the machine and is relatively successful although the unit is large.

In a typical dry application, the water to cement ratio should be in the 0.40 to 0.45range.

3.2 The wet process

Until the last decade, the dry process was the most common method ofapplication, but the wet process is rapidly gaining popularity in mining, in line withthe general move in mines to mechanize underground operations for safety andproductivity reasons.

In the wet process, the entire mix (including the total water) is fed into a hopperand then pumped to the nozzle. The mix needs to be fluid enough in order to bepumped and therefore the introduction of water reducers into the mix is generallyessential in order to maximize the strength gain and reduce the overall costs.

The two main pump arrangements for wet machines are shown on Figure 6.

The nozzle design in wet shotcrete is important because compressed air isadded to produce the necessary spray velocity, and generally accelerators arealso added to improve the early strength gain.

A water cement ratio around 0.40 to 0.45 is typical.

FIGURE 5THE ROTOR PRINCIPLE OF A TYPICAL DRY-SPRAYING MACHINE

Feeding hopper

Pressurized air

Rotor Rotor

Outlet

FIGURE 6WET SHOTCRETE PUMP TYPES

WORM PUMP

PISTON PUMP(With S tube)

200 O 125 O

A D

S

(after Vandewalle)

3.3 A comparison between the processes

The Table below compares the main differences between the dry and wetprocesses.

Wet process Dry process

Little dust (about 10% of dry) Considerable dustLow maintenance cost High maintenance costHigh capital cost Low capital costLow rebound(typically about 5 to10%)

High rebound(usually 25 to 40%)

Moderate to high placementrate(between 4.0 and 25m/hr)

Low to moderate placementrate(up to about 6.0m/hr)

Low transport distance(up to about 200m)

High transport distance

Moderate to high placedquality

Moderate placed quality

Low operator sensitivity High operator sensitivityBetter suited to highapplication volumes

Better suited to lowapplication volumes andstop/start operations

3.4 Process selection guidelines

The selection of the most appropriate application process should be site specificand based on total cost, performance and monthly consumption. As a generalguideline, if a mine places more than 3000m/year, wet shotcrete should beconsidered (unless the spray areas are numerous and access between them isdifficult.

There can be no doubt that the overwhelming trend in mining internationally, istowards wet shotcreting, due mainly to performance, productivity and overall costconsiderations. This trend has mainly occurred because of the improvement inthe equipment and the development of high range water reducers and hydrationcontrol admixtures. These developments have resolved most of the logisticsissues on the mines associated with shotcrete transportation from the batchingplant to the spray equipment.

There is also no truth in the commonly held view that dry shotcreting is the bestoption, where the overall worker skills level is low (i.e. in developing and thirdworld countries). Wet shotcrete has proved itself a cost effective method all overthe world, if the placement volume is high and the performance requirement high.

The main considerations when selecting the most appropriate shotcreteapplication process for a specific application are:

The overall volume needed for the application and the time available to sprayit.

Logistic considerations (can bulk bags be handled, or is wet shotcreteavailable via a pipeline for example).

The performance required (fiber usage for example is only a reasonablesolution in the wet process).

Overall cost considerations. The location of each spray site and the requirements. If fibers are needed for most applications, then the wet process is the obvious

choice due to the high fiber losses in the rebound if the dry process is used.

4. Shotcrete (sprayed concrete) design

The final strength of shotcrete, like concrete, is mainly dependant on the water tocement ratio and the air content after placement.

Much is available concerning the design of shotcrete, but simple aspects arefrequently overlooked initially, and these can create major losses, costly delaysand final sprayed linings that do not meet the desired performance requirements.

Shotcrete design must include more than creating a laboratory mix that meetsthe strength gain requirements, with locally available raw materials (such ascement, sand, stone and water) in adequate supply. Whilst this is important,other equally vital aspects must not be ignored:

the fact that the strength must be achieved on the rock (not in the lab) the shotcrete must bond adequately to the rock time available for spraying a given volume the placed cost of the mix

4.1 Placed strength

The strength gain (and general performance) of the shotcrete needs to bereliably achieved as sprayed. This means that the mix must be pumpable, bondwell to the substrate (with the minimum of rebound), build up desirable thicknessin few passes and usually gain strength rapidly.

This generally implies the need for a cohesive mix, with an initial high slump andfinally a low slump on placement. Such needs would not have been evidentduring lab trials. Achieving such requirements is governed by the use ofadmixtures and additives:

micro-silica for cohesion, rebound reduction and durability superplasticizers for slump and water to cement ratio control (in the wet mix

process) accelerators for early strength development and high single pass application

thickness (e.g.: with alkali free accelerators overhead single pass thickness ofbetween 30 and 50 cm are possible)

concrete improvers (internal curing admixtures) to achieve long term strength,better bond, less cracks and improved durability (in the wet mix process)

4.2 Time

Time is often a scarce commodity in mining, and when shotcreting, it presentstwo main problems:

Supplying adequate volumes of material to the shotcrete site for spraying. Actually spraying the area in the time available to fit in with the mining cycle.

Adequate material can be obtained by many means, but the most exciting is thetransport of the material as a slurry down a pipe.

Untreated shotcrete has an open (useful) time of only 1 to 2 hours, and hence itis often essential to use a hydration control admixture to increase the open timeto that needed for the safe operation and application of the shotcrete, oftenaround 10 hours or even more.

The time to actually spray the area is not a problem, if the shotcrete machine canbe adequately supplied with material. With dry mix machines the volumes placedcan be up to 8 m/hr, and with wet shotcrete machines, up to 25 m/hr can besprayed.

Shotcrete components and performance

The following Table (modified from Munn, 1997 by Garshol, 2002) gives thetypical material ranges and properties that can be expected from normal dry andwet process shotcrete:

Parameter Dry process Wet process

Strength range (MPa) 20 60 20 80Slump (mm) N/A 50 250Maximum aggregate size (mm) 10 10Maximum coarse aggregate (%) 25 40Cement content (kg/m) 300 450 350 500Water to cement ratio 0.40 0.45 0.36 0.50Typical rebound without micosilica (%) 25 50 10 15Typical rebound with microsilica (%) 20 40 5 10

5. Shotcrete mining equipment

Shotcrete equipment must be well matched to the specific applicationunderground. High quality equipment is needed to produce a high qualitysprayed product on the rock. Shotcrete is safety related and all aspects must bewell considered, implemented and checked.

5.1 Shotcrete spray equipment

The spray capacity of shotcrete equipment has traditionally been given inm/hour of water in most literature. The throughput of shotcrete is usually about20% less than this figure.

The requirements for shotcrete equipment in mining are usually different fromthose in underground civil applications because in mines:

Space is frequently a constraint. A shotcrete machine will need to travel more (between development ends)

and on poorer surfaces. Logistics are difficult. Quantities needed per blast are much less (smaller cross sectional area and

smaller thickness applied).

The choice of equipment depends on the following:

The placement rate needed during a shift (to minimize the mining cycle ifnecessary).

The specific excavation size and the general mining tunnel dimensions. The quantity of shotcrete to be placed per month. The number and location of areas needing shotcrete. The ease of access between the different areas requiring shotcreting. The overall time that shotcrete will be needed (i.e. for on-going routine

support work, or only a specific project area in a mine).

It is important to note that in many applications where the advance rate of anexcavation needs to be high, the utilization of the shotcrete equipment tends tobe low, but the overall utilization of the mechanized equipment suite (drill rig,LHD unit etc.) is increased.

The Table overleaf outlines the equipment specifically available for mining fromMEYCO Equipment, based in Switzerland. Other equipment is available but ismore suited to shotcrete repair work or major underground civil projects and istherefore not given.

Machine Process Shotcrete sprayvolume (m/hr)

Conveying distance(horizontal/vertical in m)

Drive

MEYCO Piccola Dry

5.3 Shotcrete lines and nozzles

The nozzle design is important as it effects:

The compaction of the sprayed shotcrete (the higher the better). The bond of the shotcrete to the rock (the higher the better). The rebound during spraying (the lower the better). The consistency of the mix when dry spraying.

The accelerator needs to be well mixed into the shotcrete for optimum results.The hardware to help achieve this is therefore important.

In the dry process, the water ring and assembly is critical to ensure thoroughwetting of the mix. MEYCO have a system that also splits the water feed to thenozzle to help the wetting process.

The air ring and general housing design for the wet mix is important for goodpropulsion onto the rock and accelerator introduction and mixing.

Safety chains holding flexible hoses are essential and should be judiciouslyplaced.

Full write-ups on this equipment can be obtained from the following referencesources:

MEYCO Equipment Tel: +41-52-2440700Hegmattenstrasse 24 Fax: +41-52-24407078404 WinterthurSwitzerland

The MEYCO web site is http://www.meyco-equipment.ch

Wet shotcreting equipment

The MEYCO Rambo for handspraying

Variations of the Rambo, including apan mixer for converting a pre-bagged shotcrete product into a wetmix

The MEYCO Mamba

The MEYCO Cobra with on-boardcompressor.

A MEYCO boom with the Logicarobotic system fitted (refer toChapter 14)

Close-up of the laser scanner (partof the Logica system)

6. Shotcrete (sprayed concrete) service needs

For a successful shotcrete application the following is needed:

adequate mix design for spraying and performance an adequate material supply at the machine matched equipment and infrastructure for the application trained crews correct preparation prior to spraying adequate services (e.g.: power, air and water) correct application technique appropriate quality control and remedial actions

6.1 Power

A reliable and well earthed electrical power supply at the correct voltage isneeded.

6.2 Compressed air

A well maintained supply of compressed air is needed with adequate pressureand volume that depends on:

The particular equipment specification. The condition of the equipment. On site operating conditions. The hose length and diameter.

As a guideline, the typical air requirements (the most critical service) are asfollows:

For dry shotcreting about 5 m/hr, the air consumption is about 15 m/minuteat a pressure between 3 and 6 bars.

For wet shotcreting about 15 m/hr, the air consumption is about 12 m/minuteat a pressure of about 6 bars.

Mine compressed air pressure (if available) is frequently too low and hence thelarger mining shotcrete rigs are often fitted with compressors.

6.3 Water

As mentioned earlier, the water quality can be important and should be free fromoil and not heavily acidic. Potable (drinkable) water is suitable for shotcreting.

7. Logistics

Getting the shotcrete to the equipment in adequate quantities and when requiredis the key to the successful introduction of shotcrete in mines. It is also frequentlyoverlooked in the design and planning stages leading to bottlenecks andinefficiencies later on.

There are many ways used to get shotcrete to the site including:

Vertically down a slick-line and into some form of pressure dissipater beforedischarging it into a concrete pump, or some form of agitator vehicle(transmixer).

Down a cased borehole, then as above. Down a decline in concrete trucks. In dry product bulk bags (containers) then if wet mix, to an underground

batching plant then into agitator vehicles. Small bags (dry mix only).

The choice of transportation method for the shotcrete material depends mainlyon:

The mine infrastructure. The material handling system. The location of the working places needing shotcrete and the demand per

shift. The daily shotcrete requirement.

7.1 Slick line design and operation

A slick line is the only shotcrete material transportation method that needs to beelaborated on.

The following is a guideline for a slick line:

A 150 mm diameter pipe appears suitable, although a smaller diameter pipe(say 100 mm) may be more appropriate in order to reduce the velocity.

If the vertical drop is high, allowance should be made in the pipeline forexpansion due mainly to the high temperature generated by the shotcretefalling in the pipe.

The wear rate is high due to the free fall velocities, and this potential problemis further exacerbated if the pipe column is not installed vertically. Generally itis advantageous to have a second pipeline installed as a back-up.

If the mix is optimally designed, segregation is not a problem except at thestart of a pour. Before the first batch is dropped down the line, the columnmust be lubricated with water and then possibly a cement and water paste.

Unless this is done, the fines will adhere to the sides of the pipe and only theaggregate will fall to the bottom.

Some form of pressure dissipater is needed at the bottom of the pipe tocontrol the exit of the shotcrete from the pipe. This is generally achievedusing a kettle of some suitable design. This has a sacrificial wear plate ofsolid steel, tungsten or ceramic as shown in the examples in Figure 7. Thekettle can also perform the function of a re-mixer, in case any minorsegregation has occurred.

A spare kettle should always be kept as a replacement if needed.

FIGURE 7SHOTCRETE PRESSURE DISSIPATORS

Pipe flanges

Sacrificialwear block

Dead ends

(i) (ii) (iii)

8. Shotcrete (sprayed concrete) application

8.1 Substrate preparation

Substrate preparation in a key element in the successful application of shotcrete.The substrate should be free from loose materials, dust and films (such as oils).This can generally be achieved by using a combined water and compressed airjet. Cleaning should start at the high point (the excavation roof, back orhangingwall) and work down systematically to the low point (excavation floor orfootwall).

Adhesion onto weak structured materials (such as shales and mudstones) isfrequently poor, and should be considered when designing an appropriatesupport system.

Spraying onto a surface that can vibrate (such as screen/mesh) can causeproblems such as poor placed density (and even voids) as well as increasedrebound.

8.2 Shotcrete spraying

Good application techniques are the key to ensuring compliance with targetspecifications. The following should be undertaken whenever possible:

Caution must be taken not to incorporate rebound lying on the tunnel flooragainst the sidewall, into the shotcrete applied to the sidewall, as this impactsthe in situ shotcrete strength very negatively. To avoid this, it is commonpractice to start shotcreting on the sidewalls (ribs) and move upwards to thetunnel roof. Rebound should always be discarded and never considered forre-use.

During spraying, it is beneficial to rather fill cracks instead of bridging them.This has a beneficial effect on the overall rock mass stability by tending tostabilize the rock.

Larger voids or cavities should be carefully filled first before shotcreting thesurrounding area.

8.3 Application safety

Particular attention should be given to operator eye protection, mainly associatedwith rebound. Preferably goggles that seal around the eyes should be worn. Alloperators should wear at least dust masks, but the problem becomes severe inthe case of the dry process.

Skin and eye protection becomes critical if caustic accelerators are used. Thisbecomes less of a concern if alkali free accelerators are used, but the cement isstill alkali and can cause minor irritation. Long durable gloves should be worn atall times (except by the operator if using a remote control).

Lighting should be adequate not only from a safety point of view, but also toimprove the properties of the final sprayed shotcrete, and this is frequentlyoverlooked (unless rigs are used).

9. Training

The nozzleman is the key to successfully placed shotcrete whether it is by thedry or wet process; or manual or robotic placement.

An aspect that is frequently overlooked, is that if hand held spraying is to besuccessful, the effective volume sprayed should be limited to between 4 and 8m/hr. Shotcrete nozzle-men should also be rotated regularly during a shift, asthe work is arduous. Should higher volumes be required, the use of a spraymanipulator (robot) is essential, and offers other benefits such as higher placedquality, especially overhead.

The correct shotcrete application technique is shown on Figure 8. Figure 9 showsthe correct hand spraying method needed for spraying at different sidewallheights.

Various training courses are available internationally for nozzlemen including:

ACI Nozzleman Certification for hand spraying (ACI 506.3R-91). CUC Certification for robotic spraying planned for possible introduction late

2002.

Most of the training courses are in two main parts:

Shotcrete mix design theory and testing. Shotcrete practical application.

Both aspects are very important.

Further information on training and related matters can be taken from thefollowing sources:

ACI web site at www.aci-int.org CUC web site at www.icguc.com

FIGURE 8CORRECT SHOTCRETE APPLICATION

Extremerebound

Highrebound

Lowrebound

MANIPULATING THE NOZZLE TO PRODUCETHE BEST SHOTCRETE

SHOTCRETING ANGLE

FIGURE 9CORRECT HAND SPRAYING POSITIONS

10. Quality control and testing

Poorly applied shotcrete can create a more hazardous condition than noshotcrete as it can give a false sense of security.

There is also no benefit to be derived from having a good quality managementsystem in place, unless there is a feasible and safe contingency action plan ifmeasured limits are not met in the shotcreting process.

10.1 Quality control

Quality control is an important and generally overlooked, parameter needed toensure safe, consistent and cost effective support. Quality control tests shouldbe:

reliable meaningful timeous simple relatively inexpensive

The main targets checked in any shotcrete quality program should be associatedwith design compliance (bond and strength) and sprayed design thickness. It ishowever totally unacceptable to have a quality control system in place, but fail totake adequate actions, if non-compliance is identified.

The absolute minimum parameters that should be checked regularly during aspraying shift are:

mix design (including water content) services (e.g. air volume and pressure) strength (e.g. by using sprayed panels) thickness (can use pins into the rock, that need to be covered during spraying)

It should be noted that depending on the compressive strength test shape used,different results are obtained with the same mix, tested at the same time. Thereis however a relationship between the more common shapes tested (the cube,the cylinder and the cylindrical drill core). EFNARC gives the relationship in theTable overleaf:

Height to diameter ratioOf the drilled core

Equivalent cube strengthfactor

Equivalent cylinderFactor

2.00 1.15 1.001.75 1.12 0.971.50 1.10 0.951.25 1.07 0.931.10 1.03 0.891.00 1.00 0.870.75 0.88 0.76

10.2 Field testing of shotcrete

This commonly can consist of:

Slump tests (BS 1881: Part 102) Hilti gun MEYCO penetration needle Schmidt hammer tests Drilled cores (minimum diameter of 50 mm) Thickness marker tests Pull-out tests Bond/adhesion strength tests (EN 1542) Sprayed pre-fabricated panels (BS1881: Part 120)

The thickness marker tests are important operational tests, during and just afterspraying.

The operation of the Hilti Gun is shown in the photographic sequence overleaf.

10.3 Laboratory testing of shotcrete

Laboratory test methods for shotcreting are really only useful for comparativepurposes as none of the common methods take account of the adhesion to thesubstrate (rockmass), and this is the critical performance parameter.

For small scale testing of the shotcretes workability, the flow method (spreadtable) is useful and relatively quick. This method is cover in BS 1881, Part 105,and for certain high performance superplasticizers, it gives more meaningfulresults than the slump test.

Shotcrete in many mining applications frequently performs under post failureconditions (if fiber reinforced) and thus lab test methods that utilise lowdeformations are considered of less relevance in mining (such as the ASTMC1018 beam test).

Hilti Gun early shotcrete strength testing

The Hilti gun method is suitable forshotcrete strengths between 2 and15 MPa.

Different lengths of nails are usedfor different strength shotcrete.

The nail is shot into the freshshotcrete.

The shotcrete layer should bearound 15cm thick if possible.

Caution should be used as the gunis dangerous if not used correctly.

A nail shot into the shotcrete to adepth depending on the strength ofthe shotcrete and the embeddedlength is recorded (using theshoulder on the nail as a reference).

To obtain a more reliable result atleast 8 bolts should be used at eachtime period where the strength isneeded.

A nut is screwed fully onto the nailas shown.

The test nut on the nail.

The pull tester is placed securelyover the nut.

A direct pull-out load is recorded onthe dial that can be converted into acompressive strength.

The equipment should be kept ingood condition, stored in a safeplace and regularly serviced.

The strength of the shotcrete is then calculated using the depth of the specificnail into the shotcrete, the pull out force and a correction factor supplied by Hilti.

The following are however considered appropriate for most shotcrete miningapplications:

The round determinate panel test. The European Federation of Producers and Applicators of Specialist Products

for Structures (EFNARC) panel test.

10.3.1 The round determinate panel test

The round determinate panel developed by E.S. Bernhard (1998) is rapidlygaining acceptance as a suitable method for testing different shotcrete mixperformance for mining applications in particular.

A photographic sequence showing the preparation and testing of the round panelspecimens is show overleaf.

Details on this method can be found in the following references:

Bernhard, E.S. 1998, Measurements of post-cracking performance in fibrereinforced shotcrete. Australian Shotcrete Conference 1998 Sydney,Australia October 8 and 9 1998.

Bernhard, E.S. 1998, The behaviour of round steel fibre reinforced concretepanels under point loads. Engineering Report CE8, Dept. of Civil andEnvironmental Engineering, University of Western Sydney Australia 1998.

Bernhard, E.S., Pircher, M. 2000, The influence of geometry on performanceof round determinate panels made with fibre reinforced concrete. EngineeringReport CE8, Dept. of Civil and Environmental Engineering, University ofWestern Sydney Australia January 2000.

10.3.2 The EFNARC panel test

This test is also widely accepted particularly in the underground constructionindustry in Europe. With this test a point load is applied to:

A 600 by 600 mm shotcrete panel at least 100 mm thick for hand sprayapplication tests.

A 1000 by 1000 mm shotcrete panel for robotic application.

The failure mode is generally flexural, but sometimes punching can occur.

The normal testing age is at 7 and 28 days. The center point load is appliedtypically over a 100 by 100 mm surface.

Round determinant panel testing

Considerable equipment andservices are needed to shoot theround panels and cleaning theequipment and hoses is importantbetween different mixes.

At least 3 panels are used per testmix.

A standard shotcrete mix must beused each test for fiber comparisonstudies. In this case acceleratorsare not necessary.

For comparative mixes the resultsfor poured (properly compacted witha vibrator) and shot panels aresimilar.

The advantage of spraying howeveris it indicates whether the mix withthe specific fibre is easy to spray.

For actual mix design tests, the fullmix including accelerator must beused.

The finish is important (especiallyensuring a near uniform thickness).

Panel condition before finalfinishing.

Curing the round panels isimportant.

They should be kept damp andmoved as little as possible for atleast a full day.

Recording the average diameter ofthe round panel is important.

An average thickness for the panelis critical (and the differences in themeasurements should be as smallas possible).

The round panel in the frame priorto testing in a press.

The load and deformation data isautomatically recorded so that theenergy etc. can be calculated afterthe test.

Close-up of one of the failure cracksduring a test using a synthetic(plastic) fibre.

Examination after the test canreveal whether the fibres tended topull out or snap, and how wellditributed through the mix theywere.

The normal and desired mode offailure is a 3 pointed star.

Occasionally only a single crack isformed. This can be due to using apoor fiber, too low a dose rate orfrom a poorly sprayed panel.

EFNARC specifies 3 types of shotcrete toughness (energy absorption) based onthe panel test at a 25 mm deflection:

Class a 500J Class b 700J Class c 1000J

Bernhard (1999) has established an approximate linear correlation between theEFNARC panel tests and the round determinate panel test as follows:

1000J at 25mm deflection in an EFNARC test = 400J at 40mm deflection in around panel test

Details on this method can be found in the following references:

EFNARCAssociation House235 Ash RoadAldershot,Hampshire, GU12 4DD,UK

Tel: +44-1252-342072Fax: +44-1252-333901

www.efnarc.org (publications can be downloaded at no cost).

10.4 Quality assurance planning

Successful shotcreting requirements a comprehensive quality management plan,that checks all aspects of the process and takes effective and appropriate actionimmediately any problem is identified.

The Table overleaf could be the basis of a quality assurance plan for shotcrete:

Main parameter Test Parameter Comments

Pre-spraying Composition Pre-determined based ontesting

Grading Sieve analysisStock levels Buffer stock essentialStorage conditions Keep dry

Equipment condition Preventive maintenanceand daily inspections

Services Must be checked each shiftSafety Key at all times

During and after spraying Substrate condition Must be preparedAccelerator level Set based on conditionsEarly strengthThickness A critical issueRebound Mainly a cost issueVisual Skilled operators make

good use of this.

Safety Always paramountDays after spraying Shotcrete performance Sound for bond etc.

Support effectiveness Monitoring

11. Costs

Shotcrete placed costs are seldom if ever accurately estimated. The same isevident for most underground support systems.

The following are some of the more significant cost elements:

Material costs Equipment costs Material and equipment transportation costs Labor costs Application inefficiency costs Time related costs Other costs

11.1 Material costs

The material cost is generally relatively easy to establish, and is higher for wetmixes.

Material costs can be judiciously reduced by careful raw material selection,blending and admixture usage (e.g. water reducers).

11.2 Equipment costs

The capital cost for the shotcrete and ancillary equipment is also easy todetermine, and varies significantly depending on the capacity and on theapplication process. Wet machines are always significantly more costly than drymachines.

The maintenance cost of the equipment is generally overlooked, but can besubstantial. Dry equipment maintenance per m sprayed is between 2 and 4times more than with wet equipment. In Canada and the USA, maintenance (andreplacement) costs for the dry machine, hoses and nozzle are typically around$14.00/m sprayed.

Depending on the tax laws in a specific country, capital equipment is written off inbetween 3 to 10 years usually. Assuming a volume of shotcrete to be placedeach year, a depreciation figure can be readily estimated.

11.3 Material and equipment transportation costs

This important cost is generally ignored or avoided for any support system, but itcan be very significant and should be considered. Mines that take the time toinvestigate this aspect are generally able to justify the installation of a shaft

pipeline with pumps or agitator-cars (transmixers) for the horizontaltransportation. This assumes that the monthly volumes needed are regular andrelatively large.

11.4 Labor costs

The true labor cost involved with the entire process needs to be considered.When comparing shotcrete against other supports, any rehabilitation costsassociated typically with any support system should also be estimated. Forexample, the useful life of unprotected mesh/screen is typically between 3 to 5years (depending on the conditions).

11.5 Application inefficiency costs

The rebound must be considered when shotcreting, and is not too difficult toestimate. Quality wet shotcreting should be 15% or less (10% or less if appliedwith a manipulator), and quality dry shotcreting should be 25 to 40%. This mustbe accounted for in any shotcrete costing exercise.

11.6 Time related costs

These need to be considered if the mining cycle is critical for a specificexcavation development. The effects of lost blasts, due to the support installationtaking too long is very significant in many cases. This aspect is frequentlyignored, but considering this aspect usually more than justifies the capitalexpenditure of a large self contained robotic spray rig for many mining projects.

11.7 Other costs

These include the cost of:

Services Down time on overall mining production and development

An adequately detailed sprayed concrete costing spreadsheet is available fromMr. Knut Garshol at kgarshol@mbt.com.

12. Specification and design procedures for shotcrete (sprayedconcrete) support

The fact that shotcrete (especially fiber reinforced) is a very effective supportliner in mines is well accepted, based mainly on experience.

The design of the performance (including thickness) requirements for shotcreteunder given rock engineering conditions in mines is still relatively poorlyunderstood. Detailed geotechnical examination in all the developing tunnels andexcavations is not feasible or justified. Frequently on mines therefore, thereasonable assumption is made that if a support system has worked in the pastunder similar conditions that it will work again. This method however tends toresult in a worst case design that is seldom the most cost effective. There arehowever methods that can be used if needed.

Methods that can be used include:

Observational Methods (mainly NATM). The Q System developed by Barton (1974). The Q system modified for toughness by Grant et al (2001). The ground characteristic curve method by Speers and Spearing (1996).

12.1 Observational methods

Rock masses are very complex systems and are further complicated in miningbecause the stress tensor typically changes during the operational life of mostexcavations. Predicting future rock mass responses is therefore difficult at best.

Observational Methods involve field observations and monitoring to makepredictions on rock mass stability and the effectiveness of any support. The mostcritical parameter measured is closure (convergence).

The most common and widely accepted system is the New Austrian TunnelingMethod (NATM), used mainly in the underground construction industry. It wasdeveloped in the 1960s with the aim of stabilizing a tunnel in the most costeffective manner, therefore making the most of the rocks self supporting ability.The name was chosen by the main developer of the system Prof. L.V.Rabcewicz.. Rabcewicz and Muller used the sprayed concrete based supportsystem in 1964 in the Schwaikheim Tunnel.

The first NATM application in the USA was designed for the Pittsburgh streetcartransportation system by the Urban Mass Transportation Administration (UMTA)in 1983 (Kirwin, 1985). This method was selected over a more conventionalapproach due to the potential cost savings.

This is generally an iterative process involving the following repeating steps:

1. Gather relevant on site rock and stability data.2. Analyze the data using any method considered suitable and determine a rock

stability or classification.3. Design an appropriate shape for the excavation profile taking account of the

final function of the excavation and the local conditions as dictated by the rockmass properties and stress regime (tensor).

4. Develop a support system that usually involves bolting and shotcreting.5. Install the support in the developing excavation, after each round (if blasted)

with the aim to limit rock movement (unraveling).6. Monitor the rock stability with the installed support (with the application of

shotcrete it is sometimes difficult to identify local geological features and rocktypes etc.). This always involves closure measurements but could alsoinclude extensometer measurements and physical load measurements in theanchors using load cells.

7. Start at step 1 again and update the information and redesign/alter thesupport if necessary.

12.2 The Q system

This system is a refinement of the Rock Quality Designation (RQD) developed byDeere et al (1969) and takes account of the presence and properties of joint setsand the state of stress. The use of this system for shotcrete design is flawedbecause it does not include a toughness requirement.

12.3 The modified Q system to include toughness

The Q system was modified by Grant et al (2001) to incorporate the shotcretetoughness performance levels (TPL) developed by Morgan (1990). The EFNARCpanel toughness (J) is specified for each ground condition identified using the Qsystem as follows:

Rock class Description EFNARC paneltoughness (J)

F Extremely poor +1400E Very poor 1000 1400D Poor 700 1000C Fair 500 700B Good < 500A Very good 0

Grant et al suggest a rock class of G where the ground movement is expected tobe so large than even fiber reinforced shotcrete will be inadequate. They definethis when the ground movement is expected to exceed 0.05 times the boltspacing. Under this condition they recommend the following be considered:

Steel sets or reinforced shotcrete ribs (as per the Norwegian ConcreteAssociation Publication No.7 of 1993).

Conventional mesh reinforcement. Secondary linings.

12.4 The ground characteristic curve method

This method was developed by Speers and Spearing (1996) and is useful in highdeformation applications. The method utilizes the 2 or 3 dimensional FLACprogram (ITASCA, 1992).

It uses the Mohr-Coulomb criteria for brittle failure, and although it is known thatrock failure can seldom be classified by a single criterion (Stacey and De Jongh 1977), it is a useful design tool that is especially powerful qualitatively.

In essence the numerical model is used to derive a relationship between thepressure on a tunnel periphery generated by the support and the correspondingrock displacement. This relationship is called the ground reaction curve, theground characteristic curve or the required support line (Brady and Brown 1985).

Input parameters for the support (such as cable bolts, grouted bolts, mesh andshotcrete) can be obtained from the published literature and the manual for thecomputer code.

Full write-ups on these methods can be obtained from the following references:

Rabcewicz, L.V. The New Austrian Tunneling Method. Water Power November/December 1964 and January 1965.

Barton, N., Lien, R., Lunde, J. 1974, Engineering classification of rockmasses for the design of tunnel support. J.S.Afr.Inst.Min.Metall. vol 74, no. 8.pp. 312-320 South Africa 1974.

Grant, N.B., Ratcliffe, R., Papworth, F. 2001, Design guidelines for the use ofSFRS in ground support. Shotcrete: Engineering Developments Hobart,Australia 2 to 4 April 2001.

Speers, C.R., Spearing, A.J.S. 1996, The design of tunnel support in deephard-rock mines under quasistatic conditions. J.S.Afr.Inst.Min.Metall. vol 96,no. 2. pp. 47-54 South Africa March/April 1996.

13. Mix design examples and case studies

The case studies cover the applications listed below and are given as a guideonly. Any shotcrete mix should first be lab tested to ensure that it meets theperformance criteria set for the specific application, and then field tested. Resultswill vary dramatically depending on the raw materials sources and no supportshould be used (including shotcrete) without the approval of a suitably qualifiedand responsible person (e.g. a geotechnical or rock engineer).

Case studies will deal with the following applications:

Standard dry mix Standard wet shotcrete mix High early strength mix with high single pass build Fiber reinforced shotcrete mixes for slick lines especially in sinking shafts A mix to replace bolts and screen/mesh Shotcrete for kimberlite (blue) rock Shotcrete for water inflow areas Wet shotcrete for economical reasons

Not all the case studies are from mines, but the applications are typical.

13.1 Standard dry mix

The water to cement ratio is typically between 0.40 and 0.45. To better wet themix and reduce dust formation at the nozzle, the water ring can be placedbetween 1 and 3m back from the nozzle.

A standard typical mix could be per m:

- Portland cement 350 kg- -8 mm aggregate 350 kg- Fine sand 1400 kg

An accelerator dose of between 3 and 4% is typically used. This can be added asa powder or a liquid. A water to cement ratio of 0.45 to 0.50 is typical.

13.2 Standard wet shotcrete mix

There really is no standard mix design, but a basic mix could be:

- Portland cement 425 kg- -8 mm aggregate 300 kg- Fine sand 1400 kg- Water reducer 4 kg

An accelerator dose between 4 and 6 % is typically used. A water to cement ratioaround 0.40 to 0.45 is typical.

Clearly this mix can be readily modified at a nominal cost to become moreeffective as illustrated in the other examples below.

13.3 High early strength mix with high single pass build

Where high early strength and thick single pass layers of shotcrete are required,the following mix could be a guideline per m:

- Portland cement 520 kg- Microsilica 25 kg- Aggregate (0 to 8 mm) 1600 kg- Water reducers 6.5 to 7.5 kg- Internal curer 5 kg- Hydration controller 2 kg- Accelerator 8 %- Steel fiber (25 mm) 50 kg- Water:cement ratio 0.45- Average thickness 20 cm

The above mix was used at North Cape Tunnel in Norway (Melbye, 2001) andproduced the following results:

compressive strength of > 2 MPa at I hour > 7 MPa at 7 hours > 30 MPa at 1 day > 40 MPa at 28 day

rebound of around 5 % average single pass thickness of about 250mm.

It should be noted that the poor rock quality and the arctic weather conditionslead to the relatively high cement content.

13.4 Fiber reinforced shotcrete mixes for slick lines especially in sinkingshafts

Fundamental aspects need to be considered when designing support systems foruse during shaft sinking, and these include (Spearing and Nel 1999):

Space and access is always a major constraint. Dust can be a concern, especially due to the limited space and ventilation

availability. Hoisting capacity is limited.

Time is a very precious commodity, because the costs associated with delaysare huge as the time to access the orebody is directly effected.

Humidity and temperature are generally high.

The use of shotcrete as support can be highly beneficial under such constraints,because:

The shotcrete machine can be placed out of the way on one of the platformsof the sinking stage, or lowered when needed onto the shaft bottom.

Dust can be virtually eliminated by using the wet application process. The shotcrete mix can be transported via a pipe range in the shaft, making it

independent of hoisting. Shotcrete can be applied very rapidly. The effects of temperature and humidity can be negated.

The design of the shotcrete mix is critical, and must be considered carefully.

The shotcrete mix should contain much the same constituents as any othershotcrete, but more care is needed, as indicated below:

For standard shotcreting, typical cement contents of around 400 kg/m can beused, but in shaft sinking, more than 400 kg/m is recommended mainly tolimit segregation during transportation.

Micro-silica is frequently used in shotcrete, but is considered very necessaryfor shaft sinking, for its ability to improve the cohesiveness of the mix (in theslick-line). Typical additions between 30 and 50 kg/m should be considered.

Hydration controllers are frequently used in shotcreting operations, mainly toextend the useful life of the mix prior to spraying. In shaft sinking it is vital, toreduce the effect of a concrete pipe blockage in the shaft.

Accelerators are commonly used, and when shotcreting in shaft sinkingoperations they are important to gain early support. Alkali free acceleratorsare becoming more frequently used in shafts mainly because they cause noproblems to people working in the vicinity, and cause less final strengthreduction.

Effective water reducers are needed to help the mix flow in the pipe, and stillgive a high rate of early strength gain, by having a low water to cement ratio(less than 0.4).

For permanent shotcreting during shaft sinking and related operations, theuse of fibre reinforced (wet) shotcrete (FRS) is highly desirable, because therock tends to move considerably during the useful life of the shaft and nearbyinfrastructure, and the potential for rockburst damage is often a consideration.

13.4.1 Vaal River Operations 11 Shaft Moab Khotsong (South Africa)

A typical mix per m used for sinking down to 7500 feet (2300 m) below surface,using a batch plant on surface at the 11 Shaft of Vaal River Operations in SouthAfrica (Buckley, 1998) was:

- Portland cement 450 kg- Microsilica 40 kg- -8mm stone 1400 kg- Sand 320 kg- Water reducer 10.7 kg- Internal curer 5.0 kg- Hydration controller 2.8 kg- Accelerator 5 % (on total binder content)- Steel fiber 50 kg

The above mix was batched on surface and sent vertically underground in anominal 6 inch (150 mm) diameter pipe.

13.4.2 Western Deep Levels South Mine (South Africa)

In order to access gold ore reserves in excess of 4000m below surface, areplacement ventilation shaft was sunk between 84 and 109 levels (betweenabout 2580m and 3300m below surface) as shown on Figure 10. A novel raiseboring method, utilizing the V Mole (see Figure 11) was used in thedevelopment of the 7.0m diameter shaft, by a South African contractor - RUC.

Sidewall support during the final sinking, consisted of split sets and SFRSinstalled concurrently with the advance. The shotcrete was installed using amanipulator, attached on a steel beam that was fitted around the sinking stage.

The shotcrete was mixed at a batching plant located on 84 level, and was fed viaa 150mm diameter pipe to a 1.0m re-mixing tank that was located on the seconddeck of the stage.

The shotcrete design mix was as follows per m:

- Portland cement 480kg- Micro-silica 48kg- Total aggregate 1525kg- Steel fiber 40kg- Water reducer 5.5kg- Internal curer 5kg-.Hydration controller used part of the time- Alkali free accelerator up to 5%- Water 207litres

FIGURE 10SOUTH SHAFT DEEPENING PROJECT

MainShaft SS

11 Level

14 Level

19 Level

108/99 Level

Ventersdorp Contact Reef

Carbon Leader Reef

109 Level

113 Level

116 Level120 Level

123 Level126 Level

130 Level

132 Level

Shaft bottom-4117mbd

SS1 SS2

SSV Shaft

Car

bo

n Le

ader

Ree

f bo

und

ary

Ven

ters

dor

p C

onta

ct R

eef b

oun

dar

y

FIGURE 11SECTION THROUGH THE SHAFT

50 ton winchsupport structure

Kibble

Stage

Drill rig

V mole

Mixer and pump

Robotic arm andshotcrete spray nozzle

The mine personnel estimated that the total rebound was about 5%, althoughphysical measurements could obviously not be made.

The shaft development that passed through the Ventersdorp lava sequence, withan unconfined compressive strength (UCS) of over 300MPa, proceeded asplanned. In the weaker Witwatersrand quartzites, with a UCS of about 200MPa,excessive sidewall slabbing (dog-earing) occurred behind the cutter head, beforethe shotcrete could be applied. This caused some delays, and it was concludedby the project team on the mine, that the SFRS should have been applied evensooner (right up to the face).

13.4.3 Alp Transit Tunnel Sedrun, Switzerland

The Alp Transit tunnel system for cars and trucks is aimed to acceleratetransportation through Switzerland, and reduce pollution. This will be achieved bythe use of railway transport carrying motor vehicles through the Swiss Alps.

The Gotthard Tunnel will be used mainly to transport trucks, on rail cars at highspeed (about 140km/hr). The Sedrun section of this project consists of an accesstunnel to a shaft that is being sunk some 800m, with a diameter of 7.75m, asshown on Figure 12. Shaft Sinkers is part of the consortium involved with thispreliminary phase. The shaft will be used to provide additional starting locationsfor the main tunnels, of about 57km length. Once these tunnels are complete, theshaft and access tunnel will be used to provide services and ventilation to themain tunnels below.

The shotcrete is batched on surface and taken to the shaft platform via a 150mmdiameter steel pipeline, fitted with a kettle (energy dissipater) at the bottom. Acustom designed kibble (bucket) with a 4.5m capacity is used to transport theshotcrete material from the kettle to the shaft bottom (about 40m vertically). Theuse of this kibble frees the hoisting system for other operations, whilst theshotcrete is being sprayed. The shotcrete is then applied using a small airpowered wet mix pump (Allentown AP10), as a temporary support, before thefinal lining is cast.

The mix design is as follows per m:

- Portland cement 450kg- Micro-silica 40kg- -8mm aggregate 670kg- Sand 1080kg- Water reducer 5.9kg- Accelerator 4 to 8%

The water to cement ratio was 0.47.

FIGURE 12DETAIL OF SHAFT AND EQUIPMENT AT SEDRUN

SEDRUN

Temporaryaccommodation Pipeline for

cementtransport

Access tunnelaprox. 990m

Silos

Shaft bottom

Concreteplant

Gotthard basetunnel

Headgear

Ventilationshaft

Ventilationstructure

Hoisting cage

Passengerlift

The above mix produced the following results:

compressive strength of > 3 MPa at 4 hour > 11 MPa at 12 hours > 27 MPa at 1 day > 40 MPa at 28 day

rebound of around 8 % thickness of about 100 to 150 mm.

13.5 A mix to replace bolts and mesh INCOs Stobie Mine - Canada

A mix that has been successfully used to replace bolting and screening in driftsat INCOS Stobie Mine in Sudbury, Canada (OHearn and Buksa, 1997) is asfollows per m:

- Portland cement 400 kg- Microsilica 40 kg- Coarse aggregate 350 kg- Fine aggregate 1275 kg- Water reducer 4.0 to 5.0 kg- Steel fiber (30 mm length) 50 kg- Water:cement ratio 0.40 to 0.45- Average thickness 65 mm

The above mix produced the following results:

Slump of 65 mm (150 mm is recommended generally) Bond strength of >0.18 MPa at 8 hours

>0.45 MPa at 1 day> 1.2 MPa at 28 days

Compressive strength of > 9 MPa > 40 MPa > 44 MPa

13.6 Shotcrete for Kimberlite (blue) rock

Kimberlite pipes are frequently associated with diamonds.

Kimberlite (a variety of mica and peridotite) is hygroscopic and tends to pull waterfrom a shotcrete mix. This causes the shotcrete to kimberlite interface surface toweaken to such an extent that the shotcrete bonds very poorly (if at all) to therock. Some form of seal is therefore needed to stop weathering from occurring inlonger term excavations.

Where shotcrete has been needed in the past, the general approach has been toapply a thin membrane (liner) first, and then spray the shotcrete onto themembrane. This works technically but is costly and involves a two stageoperation.

Dry shotcrete has been found to work, but according to Storrie (2001), wetshotcrete has advantages such as higher productivity, less rebound and lessdust.

For rock engineering considerations, the shotcrete needs a 28 day strength of 45MPa and an EFNARC energy absorption of 750 J.

Based on successful underground trials, the following wet shotcrete mix (Storrie,2001) was found to be successful:

- Cement 500 kg- Microsilica 40 kg- Sand 1558 kg- Water reducer 5.4 l- Plastic fiber 8 kg- Accelerator 5%- Water:cement ratio 0.43

13.7 Shotcrete for water inflow areas

Clearly shotcrete cannot be placed over substantial water outflows, but with thecorrect mix design and especially the correct accelerator and dose rate,shotcrete can be sprayed over surprisingly high water outflows (if the pressure islow). The ideal solution is always water sealing (injection) if required, or at least awell considered water management plan.

The following is an example from a rail tunnel Blisadona Tunnel in Austria(Melbye et al 2001). There was general heavy water ingress and therefore thesprayed concrete had to gain strength rapidly, in order not to fall off the rock.

The mix design and results were as follows per m of material:

- PZ375 cement 420 kg- Total aggregate 1750 kg- Water reducer 0.7 %- Hydration controller 0.4 % (for 7 hours open time)- Alkali free accelerator 7.5% (average)- Water:cement ratio < 0.5

Compressive strength:

6 minutes (Hilti gun) > 0.3 MPa10 minutes (Hilti gun) > 0.4 MPa30 minutes (Hilti gun) > 0.6 MPa12 hours (Hilti gun) > 8.0 MPa7 days (cores) 25.0 MPa28 days (cores) 31.0 MPa

13.8 Wet shotcrete for economical reasons

The support system used on any mine must first protect people but also must becost effective. The total installed cost of support (including any longer termpossible rehabilitation costs) must always be considered. The economics of a dryprocess versus a wet process should always be the main deciding factor.

Telfer Gold Mine, part of Newcrest Mining Ltd in Australia, compared theeconomics of both wet and dry shotcreting for their application (Cepuritus, 1996).The wet shotcrete process (produced on site) proved to be about 10% cheaperthan the dry process (transported to site in bulk), mainly due to transportationcost savings as the mine is very remote (and other considerations such asrebound reduction).

- Portland cement 425 kg- Microsilica 40 kg- Aggregate (-7 mm) 500 kg- Coarse sand 1000 kg- Dune sand 250 kg- Water reducer 4 liters- Stabilizer 3 liters- Water:cement ratio 0.49- Average thickness +50 mm

More detailed information on shotcrete (in English, German and Spanish) can beobtained from the following sources:

www.ugc.mbt.com

Americas Tel: +1-216-8397500MB Inc. Fax: +1-216-839880123700 Chagrin Boulevard,Cleveland,OHIO 44122, USA

Asia/Pacific Tel: +65-8607305MBT Singapore Fax: +65-863095133 Tuas Avenue 11,Singapore 639090

Australia Tel: +61-2-96244200MBT Australia Fax: +62-2-9624768111 Stanton Road,AUS Seven Hills,NSW 2147, Australia

Europe Tel: +41-1-4382211MBT Schweiz Fax: +41-1-4382246Vulkanstrasse 110,8048 Zurich, Switzerland

Japan Tel: +81-3-35828814NMB Fax: +81-3-35833800

16-26, Roppongi 3-chome Minato-ku Tokyo 106-0032, Japan

Latin America Tel: +1-305-66742395941 SW 47th Street, Fax: +1-305-6670793Miami,Florida 33155-6028, USA

South Africa Tel: +27-11-7541343MBT Mining and Tunneling Fax: +27-11-754110511 Pullinger Street,Westonaria 1780, South Africa

14. The future

The future for shotcreting in mines is positive and is likely to grow significantly.This is not only because of the on-going advances in particularly wet shotcrete,both from an equipment and product point of view, but also safety concernsassociated with the installation of mesh/screen (the most common alternative).

These advances will include:

Shotcrete equipment automation. Advances in the performance of synthetic fibers. Advances in admixture performance.

14.1 The future of shotcrete equipment

Mines typically apply shotcrete at between 50mm to 100mm thick. This is incontrast to most underground civil applications were the typical thickness wouldexceed 100mm.

A development (Tschumi, 1998) that should have a most significant impact onshotcrete in the future, and quality control in particular, is the current on-goingdevelopment of a semi-automatic (or automatic) spray manipulator (robot) thatcan:

i) Measure the tunnel profile using a laser scanner.

ii) Shotcrete to a desired thickness basically automatically, keeping the sprayangle and distance at the ideal settings (thus minimizing rebound).

iii) Check that the design thickness has been achieved, re-spraying any areasthat are under-sprayed.

The first prototype has been evaluated by INCO at its test mine in Canada(Runciman et al 2001). These tests clearly showed the potential for such asystem. The thickness variation in fully automatic mode was found to be only7.5mm and this is a significant achievement.

This development will permit support systems that mainly rely on fiber reinforcedshotcrete to be designed and implemented in underground mines. The safetyaspects of this are obvious because it should help resolve the most difficultquality control issue: that of achieving the designed placed shotcrete thickness.Less obvious are the potential cost benefits of the equipment such as:

Reduced rebound (achieved by maintaining the correct nozzle to substratedistance and angle at all times).

Reduced maintenance costs and increased equipment life due to the moreprecise and less erratic control of the manipulator under computer control.

Less operator training needs (but more computer literate maintenanceneeds).

The rebound in well controlled manipulator applied wet shotcrete is typicallybetween 10 and 15%. The rebound using the Logica has been measured atbetween 5 and 10%, so a reduction of 5% rebound is possible.

The complete Logica control system costs less than $110k, and hence can payfor itself just with the rebound reduction after only 11000 m sprayed. This is asimplistic approach and assumes a cost of fiber reinforced shotcrete to beUS$200/m. This calculation assumes that the use of a conventional manipulator(boom) mounted shotcrete rig has already been justified and only the Logicatechnology needs to be justified as an add-on.

In the near future, mining booms will be routinely available with and without theLogica technology, but the future will clearly include Logica.

Full write-ups on these methods and equipment can be obtained from thefollowing reference sources:

Tschumi,O. 1998, State of the art of the latest generation concrete sprayingrobot. 100th CIM Conference Montreal, Canada.

Runciman, N., Rispin, M., Newson, G. 2001, Tele-operated shotcretespraying with the MEYCO Robojet Logica. Shotcrete: EngineeringDevelopments Hobart, Australia 2 to 4 April 2001.

The MEYCO web site is http://www.meyco-equipment.ch

14.2 Possible improvements in synthetic fibers

Improvements in the handling and performance of synthetic (plastic) fibers willcontinue, and the trend will be away from steel fibers. This is because:

In mining, shotcrete frequently operates post first crack, and with the steel,corrosion of the fibers can be an issue.

The plastic fibers are not a minor hazard that can cause minor cuts topersonnel after placement (as the steel fibers do that stick out from theshotcrete).

The plastic fibers seem able to perform such that the shotcrete can deformmore before being totally ineffective.

An issue that still needs to be fully resolved is that associated with the density ofthe synthetic fibers. They tend to float and hence can cause problems with sumppumps etc.

14.3 Future admixture advances

Many mine operators are calling for even faster early strength development sothat they can re-enter a freshly sprayed end safely but quickly. At present manyoperators want at least a compressive early strength of 2 MPa after 2 hours, buteven higher strengths would be desirable. The bond early strength is actually theimportant parameter rather than the compressive strength but this is not specifiedat all at present, probably due to practical field testing issues. Future acceleratorswill tend to develop strength even faster and still have only a little effect on thefinal strength. Alkali free accelerators will be the only approved shotcreteaccelerator due to the issues of health and safety.

15. R