▲121The Journal of The South African Institute of Mining and Metallurgy MAY/JUNE 1998

Development end shotcreting

Shotcreting procedureSix crews of five persons each were initiallyemployed for development end shotcreting.The basic procedure consisted of applying a

25 mm layer of shotcrete to the newly-exposedhanging- and side-walls of the excavation,then applying a second 25 mm layer to thearea that had been supported the previous day.Where necessary, areas where fallout or blastdamage had occurred were also resprayed. Theshotcrete was supplied in 25 kilogram bags asa sand mixture with a separate small bagcontaining cement, which had to be mixedtogether before application. As there wasinsufficient space in the development ends forscrew feeder/mixers, the two ingredients weresimply mixed on the footwall using shovelsbefore being tipped into the shotcrete machinehopper.

Problems and solutions

The major problems and subsequent solutionsto date include:

➤ Training the work force to applyshotcrete and operate the shotcretemachines. Most of the workers had comefrom a production environment, with noprevious experience of shotcreting.There were no training facilities forshotcreting and very little training time

Application of shotcrete at Oryx Mineby D.P. Venter and L.J. Gardner*

Synopsis

When Oryx Mine down-scaled in 1994, focus was placed ondevelopment, with the primary aim being the exposure and provingof ore reserves. In order to achieve this aim, development ends hadto be advanced to reef at the maximum possible rate. This in turncalled for support methods which could keep up with the advancingfaces, taking up minimum space and time for installation, whileproviding on the face areal coverage and effective long-termsupport. In the majority of the development ends which werelocated in fairly competent ground, the grouted rod developmentsupport proved to be adequate until such time as secondary supportin the form of meshing and lacing could be installed.

The ends developing towards the eastern boundary of the mine,especially in the ‘C’ block, encountered extremely adverse groundconditions due to the presence of smectite zones within the footwallquartzites. Smectites are a group of montmorillonite clay materials,mainly composed of hydrous aluminosilicates. They occur asminute platy crystals in the rock mass and are especially notable fortheir ability to absorb liquids. Generally, the zones are found inbetween the bedding planes of the host rock, and may vary consid-erably in thickness over short distances.

Wherever the smectites were exposed by the developmentblasting, the rockwalls weathered as the smectites absorbed waterand the condition of the tunnel rockwalls deteriorated rapidly.When the smectites were left exposed and allowed to weather, itbecame almost impossible to install secondary permanent supportsuch as meshing and lacing at a later stage. To minimise theexposure time and subsequent deterioration of the smectite zones, itwas recommended that all development ends encounteringsmectites be shotcreted as soon as possible after blasting.

In developing a shotcreting support methodology, both adrycreting and wetcreting system were investigated. The situationat the time ruled out the use of a wetcreting system on the basis ofthe extensive and costly infrastructure required, as well as itslimited manoeuverability. Instead, it was decided to opt for asmaller scale but more manoeuverable drycreting system. Thesystem utilized a number of portable drycrete machines with pre-bagged cement, which could support several development ends inclose proximity. Initial trials using normal 30 MPa shotcrete on 19level proved successful and the system was implemented.

* Oryx Mine, P.O. Box 55068, Eerstemyn 9446.© The South African Institute of Mining and

Metallurgy, 1998. SA ISSN 0038–223X/3.00 +0.00. This paper was first published at the school:Shotcrete and its application, Feb. 1998.

Figure 1—Typical shotcrete application indevelopment end

Application of shotcrete at Oryx Mine

available. The solution was to conduct on the jobtraining at the actual work site underground. Theentire gang was trained at the same time, with theactual training process relying heavily on closesupervision by the miner and shiftboss. As can beimagined, the learning curve was a steep one and thefirst few months were extremely hectic. Small,senseless accidents occurred regularly, which led tolarge numbers of special instructions being issued.These special instructions were in time incorporatedinto a proper training manual and training programmefor shotcrete workers.

➤ Shotcrete machine reliability and the time and cost ofrepairs by the supplier. The shotcrete machines did notcope well with the harsh conditions that wereencountered underground and their initial breakdownrate was extremely high. The problem was furthercompounded by the supplier being located inGermiston, which meant that the machines weresometimes off the mine for weeks at a time. In order tokeep all the shotcrete gangs supplied, we had to resortto purchasing several additional spare machines.The high cost of having the machines serviced andrepaired by the supplier, combined with the downtimeinvolved, forced us to attempt the repairs ourselves. Atfirst we sent the machines to the mine workshops, butwe soon found that they were being shifted aside tomake place for larger jobs. This meant that they wereoften unavailable for longer periods than if we had sentthem to the supplier.We eventually took one of the support shiftbosses whowas mechanically inclined out from underground andsent him to the supplier to learn how to repair andmaintain the shotcrete machines. On his return to themine, he formed a shotcrete machine repair gang andthey took over the repair and maintenance of themachines. This has been extremely successful,resulting in increased reliability and much less downtime on the machines. The repair gang have also madea number of modifications to the machines, furtherincreasing their robustness and reliability.

➤ Material supply problems. The layout of the shaftsystem, comprising a main and sub-shaft, meantlonger shaft times and delays in getting material to theworking face. Material cars were also not allocated tospecific sections or levels, and the production sectionsgot first pick of the cars. The solution was to allocateonly side tippers to the shotcrete section. Not only didthis solve the allocation problems, it also allowed thebagged shotcrete mix to be off-loaded faster thannormal material cars, thus allowing more time to bespent actually shotcreting and ensuring a faster turnaround time in getting the cars back to the station andup to surface for re-loading.

➤ Creation of dust by the machines. In the confines ofdevelopment ends with forced ventilation, the dustcreated by the machines often resulted in low levels ofvisibility and difficulty in breathing. This in turnaffected the quality of the shotcrete application.Numerous attempts were made at dust suppression,with limited success. These measures included varying

the moisture content of the shotcrete mix, changing theshotcrete mixes, using dust suppressing additives,different rotor shapes and various types andcompositions of gaskets. Most cases of excessive dust were, however, tracedback to poor maintenance of the machinesunderground. Despite having been trained how to dobasic repairs and maintenance underground, it seemedthat the gangs simply were not interested in doing anymore than the bare minimum. Disciplinary action couldnot be taken as it was impossible to pinpoint who theculprit was. The only effective way found to ensurecorrect maintenance of the machines was the creationof a special bonus, paid to a gang only if their machinehad spent a certain time underground without needingany major repair. This scheme has proved so successfulthat some of the machines have spent more than a yearunderground to date.

➤ Segregation of material due to the use of excessivelylong hoses. In the drive for production, very fewcubbies were laid out along the tunnels and those thatwere blasted were generally used for storage. Properpipe and track equipping was also sometimes left farbehind the advancing faces. The shotcrete machinesand the shotcrete material, which were stored incubbies, were at times 150 metres behind theadvancing faces. As the shotcrete mix had to beconveyed in the hose over long distances, segregationwas almost inevitable and the result was a poor qualityapplication. While the basic remedy was to ensure that cubbieswere blasted regularly and kept open for the shotcretemachines and material, a ‘quick fix’ solution was tomount the machines on flat cars. These were storedwell behind the working face and, when needed,pushed into the face together with the side tipperscontaining the shotcrete mix. At one stage the processwas refined to the point where the shotcrete was mixedin the back area in a side tipper. When the flat car withthe shotcrete machine and the side tippers were pushedinto the face for use, a funnel was hooked onto thefront end of the side tipper and the mix poured directlydown this funnel into the shotcrete machine hopper.

➤ Poor adhesion of the shotcrete to the rockwalls. Sometime after the development end shotcreting had begun,it was noticed that in numerous places the shotcretewas falling off the rockwalls. No signs of stress-induced rockwall slabbing were observed in theseareas, but closer investigation revealed excessive duston the underside of the fallen shotcrete slabs. A checkon the work in progress also showed that the rockwallswere not being washed down before shotcreting began,obviously preventing proper adhesion of the shotcrete.A change in the work procedure to ensure the washingdown of the rockwalls before shotcreting begins andstrict supervision has virtually eliminated this problem.Although it may sound as if the shotcrete supportsystem is nothing but a series of ongoing problems,most of the items mentioned above were merelyteething problems and were quickly remedied. Atpresent, Oryx Mine supports approximately 5000

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122 MAY/JUNE 1998 The Journal of The South African Institute of Mining and Metallurgy

square metres of haulages per month using shotcretewith minimal problems other than those associatedwith day to day production.

Special applications

The success with the drycreting system in development endsformed the backbone of Oryx Mine’s interest in shotcrete as asupport medium. This in turn lead to our subsequentinvolvement with the SRK Shotcrete working group andexperimentation with new developments in shotcretetechnology.

Although the applications used on Oryx Mine differ fromthe basic aims of the SRK working group, the application ofthe technology used and results obtained by the workinggroup proved to be worthwhile.

Mesh reinforced shotcrete

During a routine visit in 1995 members of the RockEngineering department discovered that several areas in themain return airways were scaling excessively. If the scalingwere to continue, it would result in ventilation restrictionsand, in the long term, the loss of the return airways. Further

investigation revealed that although the areas in questionwere being affected by weathering rather than stress-inducedfailure, they would have to be supported.

The high temperatures and humidity levels in the returnairways effectively negated the use of meshing and lacing asa support medium, unless it was shotcreted to preventcorrosion. The services had been stripped out of the returnairways and re-equipping them to be able to drill holes forinstalling meshing and lacing would have been extremelycostly. As some success had previously been achieved in thediamond mines with mesh reinforced shotcrete, it wasdecided to attempt the use of this support system in thereturn airways.

The basic procedure in this case was to apply a 25 mmlayer of shotcrete to the rockwalls and leave this until dry.Diamond mesh would then be stapled to the shotcrete using aspring loaded Hilti gun. Finally the mesh would be sealed bya second 25 mm thick layer of shotcrete, ensuring noexposure of the mesh and thus no corrosion.

Although the mesh reinforced shotcrete appeared to befairly effective as a support system, it was extremely timeconsuming to apply, as it required triple the amount of workof normal shotcrete. Uneven areas in particular proved to bea problem as the mesh would not lie flat against the firstlayer of shotcrete and excessive shotcrete had to be used for

Application of shotcrete at Oryx Mine

▲123The Journal of The South African Institute of Mining and Metallurgy MAY/JUNE 1998

Figure 2—Example of poor shotcrete adhesion to the rockwalls

Figure 3—Area in return airway supported with mesh reinforcedshotcrete

Figure 5—Area where corrosion of steel fibres has resulted in shotcretebreaking up

Figure 4—Example of steel fibre reinforced shotcrete - note steel fibresprotruding from applied shotcrete

Application of shotcrete at Oryx Mine

the second layer. The concept of fibre reinforced shotcretewas thus greeted with much anticipation.

Steel fibre reinforced shotcrete

The first fibre reinforcing used at Oryx Mine was steel fibre.An extremely jointed major dyke was intersected by footwalldrives in the northern block. There were two major joint setsin the dyke, both of which exhibited low cohesion levelsacross the joint surfaces. The size of the blocks created bythe joint sets varied between 100 and 600 mm and thefallout rate was high, resulting in extremely poor and blockyground conditions. Blocks simply fell out around thestandard 1.0 m x 1.0 m pattern grouted development supportand it was decided to shotcrete the rockwalls to give a greaterareal coverage.

In an effort to increase the strength and deformationcapability of the shotcrete, steel fibres were used as re-inforcement. Major problems were encountered in thisapplication, as listed.

➤ The low corrosion resistance of the steel fibres meantthat they could not be premixed into the baggedshotcrete mix. Instead, like the cement, they wereplaced in a separate small plastic bag inside the main25 kilogram bag, the intention being that the labourersmixing the shotcrete break open the two smaller bagsand add the fibres, together with the cement, into themain mix, ensuring even distribution by correctmixing. Initial attempts to add the steel fibres to therest of the mix on top of the shotcrete machine hopperresulted in numerous small wounds on the mixer’shands as the sharp Dramix fibres easily punctured therubber gloves worn by the mixer. The gang wassubsequently instructed to mix the ingredients togetheron the footwall and then tip the mixture into theshotcrete machine hopper.

➤ The Dramix fibres tended to cling together, so thatdespite the gang’s attempts to ensure an even mix,they regularly formed an obstruction in either themachine or the hose. All work would then be stoppeduntil the blockage had been found and removed.Enlargement of the size of the delivery port and thedelivery hose partly solved the problem, but any bendor kink in the hose quickly blocked up.

➤ The wear rate on the shotcrete machine and hoses wasmuch higher than normal. In the space of two months,the entire machine was replaced twice. Gaskets werereplaced daily and hoses lasted no more than twoweeks. Bends or kinks in the hoses, where higherfriction occurred, wore through in less than a shift.

The combination of these factors was a sequence ofdelays, which were frustrating not only to the shotcrete gangbut also to the production crew who had to wait for the endto be shotcreted before they could begin work in the face. Itwas soon discovered that the gang were not including thesteel fibres into the shotcrete mix, or were including onlyenough not to block the machine. Despite disciplinary actionbeing taken against the gang, the situation did not improveand the project was stopped.

Subsequent visits to the project sites revealed anabnormally high corrosion rate of the steel fibres. Theexpansion of the fibres caused by the corrosion, resulted inthe shotcrete cracking up in places. In other areas the fibreshad corroded through and were no longer contributing to theintegrity of the support system. This phenomenon was alsoobserved in another area where steel fibres had been used asa reinforcement above the settler dams. It is thereforerecommended that future users of steel fibre reinforcedshotcrete bear this in mind when shotcreting areas with highmoisture and humidity levels.

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124 MAY/JUNE 1998 The Journal of The South African Institute of Mining and Metallurgy

Figure 6—Shaft headgear bin chamber supported by shotcretereinforced with polypropylene fibres

Figure 7—Shaft waiting place supported using polypropylene fibrereinforced shotcrete

Figure 8—Section of applied polypropylene fibre reinforced shotcrete

Polypropylene fibre reinforced shotcrete

Following the disappointing results obtained with the steelfibres and the successes obtained by the SRK working groupwith monofilament polypropylene fibres, these wereinvestigated as shotcrete reinforcement. Tests were done onthe mine with the standard shotcrete machines using needlefibres and more flexible multifilament fibres. Better distrib-utions were obtained with the needle fibres, provided thefibre length was restricted to no more than 60% of thedelivery hose diameter.

Shortly after the tests had begun, management requestedthat the loading bin chamber in the sub-vertical shaft besupported, as it was showing signs of deterioration. The sizeand shape of the chamber ruled out the possibility of usingmeshing and lacing, so the choice of support fell to shotcrete.Because of the size and complexity of the job, it was decidedto hire a larger shotcrete machine from one of the membersof the SRK shotcrete working group, MASH Engineering.This machine had been designed for use with polypropylenefibres and incorporated a number of unique features,including a built-in pre-mixer/feeder. It had previously beenused for testing purposes by the working group. MASHEngineering also supplied the pre-bagged shotcrete mix.

Both the machine and the polypropylene fibre reinforcedshotcrete performed very well under the circumstances. Therewere no major breakdowns, rebound from the specificshotcrete mix was lower than that of normal shotcrete andvirtually no dust was created during the shotcreting process.

Delays in available working time meant that the job took

longer than expected and the machine had to be returned. Asecond scheduled job, the support of a large waiting area,could not be completed. As there were still a number of bagsof MASH Engineering shotcrete mix left over we decided totry them with the standard shotcrete machines, rather thanwaste them. To our surprise the combination workedperfectly and we were able to finish the job using thepolypropylene fibre reinforced shotcrete.

Future application

Our aim for the future is increasing the use of shotcrete as asupport system in place of meshing and lacing. Examples ofthis include:

➤ The support of excavations with high moisture andhumidity levels, such as return airways, using fibre,rather than mesh as reinforcing.

➤ The support of crosscuts to reef. If we are able todestress a crosscut quickly enough to limitdeformations to a minimum, then a support systemconsisting of development grouted rod support andfibre reinforced shotcrete should prove to be adequate.

➤ The support of large chambers in cases whereminimum stress changes are anticipated.

Acknowledgement

The authors would like to thank the management of OryxMine for their support of the shotcrete support systemprojects mentioned and permission to publish this paper. ◆

Application of shotcrete at Oryx Mine

▲125The Journal of The South African Institute of Mining and Metallurgy MAY/JUNE 1998

The South African Institute of Mining and MetallurgyFounded in 1894

OBITUARIESOBITUARIES

Date of Election Date Deceased

B.D. McNeice Retired Fellow 18 March 1968

J. Pope Retired Fellow 8 August 1947 11 March 1998

W.H. Bailie Retired Member 12 September 1952 25 March 1998

A.J. Rossouw Retired Fellow 11 November 1960 18 April 1998

E.P. Gush Member 11 March 1960 8 May 1998

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126 MAY/JUNE 1998 The Journal of The South African Institute of Mining and Metallurgy

There is only one way to describe BAUMA—WOW !!It is so big and comprehensive, that it is difficult to

properly view, and the best way is to only visit specificstands.

BAUMA is the world’s biggest trade fair for constructionmachinery, equipment and products. It started in 1954, andis held every 3 years inMunich (Germany).The last BAUMAattracted over 1700exhibitors and 350 000visitors (from 133countries), during itsone week duration. Togain some idea of thesize, it is worth notingthat the set-up costsfor the exhibitors wasreported to be aboutDm 200 million (almostR650 000 Million !).

This year BAUMAhas moved to a new venue—the newly-opened MunichTrade Fair Centre—that was the old Airport. This year’sBAUMA was held between the 30th March to the 5th April1998, and consisted of 430 000m2 of exhibition space (ofwhich 140 00m2 was enclosed in 12 massive halls).

The venue was also the location of 4 Internationalsymposia:

➤ 5th International Symposium on Tunnel Construction

➤ 4th International Symposium on Micro-tunnelConstruction

➤ 3rd International Symposium on Roads and RoadConstruction

➤ 15th International Symposium on Automation andRobotics in Construction

To keep the visitors to those actually interested andinvolved in construction (and try to make some moneyprobably ) admission is Dm 32/day (about R100).

MBT had a large outside stand with a double storeyportable office erected on the site. The focus of the exhibitswas the New Logica wet shotcrete boom and nozzle that can

automatically spray atunnel to a desiredthickness(and thencross-check that thethickness has beenachieved). This is asignificant advance inshotcrete applicationtechnology—especiallyfor major constructionexcavations.

During my fairlybrief visits around theFair, I was only able tospot one purely SouthAfrican breed exhibitor,

and that was Bell Equipment, who had a relatively largeindoor stand with some heavy equipment on display. I hopethat they did well, and that at the next BAUMA more SouthAfrican exhibitors will be displaying their wares.

On the Thursday, at the close of the Fair, in the earlyevening, the rail line serving BAUMA failed, leaving tens ofthousands of people potentially stranded. The Germanauthorities, however, used their usual magic and managedto produce dozens of buses, and hardly inconveniencedpeople !!!

BAUMA was, and will be, great and more SouthAfricans should visit and exhibit at the next one (in the newmillennium). ◆

* Issued by: Dr A.J.S. Spearing, Senior Manager,Underground Construction Group, MBT International

Visit to BAUMA*

EDITOR REQUIREDThe SAIMM is looking for a suitable person to fill the position of editor for a volume of some tenbooks on current South African Mining Practices. It is envisaged that such a person will be wellacquainted with the mining methods in use in South Africa, (perhaps a retired mine manager). Awealth of information is already available to start the books. The SAIMM will support the editor infinding other suitable persons to fill in unknown areas.

Applicants should address their applications to:

The Secretary, South African Institute of Mining and Metallurgy, P.O. Box 61127, Marshalltown, 2107

The South African Institute of Mining and MetallurgyFounded in 1894