07 - golder associates

Upload: ferdiyan

Post on 07-Jul-2018

223 views

Category:

Documents


0 download

TRANSCRIPT

  • 8/18/2019 07 - Golder Associates

    1/8

    Prosiding Seminar Sehari Kemantapan Lereng di Pertambangan Indonesia III, Bandung, 16 Juni 2003

    LOW WALL INSTABILITIES IN COAL MINES IN INDONESIAFROM A GEOTECHNICAL PERSPECTIVE

    Simon Ballantyne, Davi Nolan an M!liai Me""yGolder Associates, Jakarta

    ABSTRACT

    Indonesian coal mines are at a critical stage o de!elopment" #here are

    large commercial and political pressures to e$tend e$isting mines to greater depths than originall% planned, so as to e$tend their lie because o the diicult% o starting up ne& operations" 'eepening o mines, particularl% those mines&ith steepl% dipping seams presents signiicant geotechnical and h%drogeological challenges" (lread% there ha!e been lo& &all ailures, &hichha!e had a serious impact on production in a number o mines in Kalimantan"#his paper addresses the issues &hich can lead to lo& &all ailure b% irst setting out rele!ant in!estigation methods to identi% the ailure triggers" (lternati!e design methods to accommodate the potential ailure mechanismsare considered, and practical &a%s o implementing the design re)uirementsare then discussed" *round&ater pressures and both intact roc+ and roc+ 

    interace strength are +e% elements in lo& &all stabilit%" ethods o )uanti%ing and monitoring these parameters are co!ered"

    #$ INTROD%CTION

    This paper discusses the potential reasons for coal mine low wallinstability and outlines how the probability of instability can be reduced, whichmay allow established coal mines to be developed to greater depths.

    Due to the difficulty of starting up new operations, many coal mines inIndonesia are reuired to e!tend their e!isting mines deeper than originally

    planned in order to achieve coal production targets. "ith greater depths,geotechnical properties resisting instability tend to remain unchanged, whileproperties driving instability only increase. #y measuring the increase andknowing the factors resisting instability, strategies for balancing the resultingforces can be developed and transformed into practical ways of keeping the lowway stable.

    $ow wall failure represents a critical ha%ard that mines often overlook. As many as & mines in 'alimantan have e!perienced significant low wallproblems since ())(.

    This paper discusses both the geotechnical and hydrogeologicalconditions that can combine to create the ha%ard of low wall failure.

    The paper then looks at methods of evaluating the ground conditions andemphasises the critical step that is often missed in mine geotechnics*

    +

  • 8/18/2019 07 - Golder Associates

    2/8

  • 8/18/2019 07 - Golder Associates

    3/8

    Prosiding Seminar Sehari Kemantapan Lereng di Pertambangan Indonesia III, Bandung, 16 Juni 2003

    and then shear failure of the remaining mudstone layer due to removal of toesupport 59igure (6.

    The resistance to lift off offered by the mudstone layer would bedependent on the thickness and density of the mudstone layer and the tensilestrength between the mudstone and the underlying sandstones. :nce lift off had occurred, the entire length of the mudstone layer would develop stressesparallel to the inclination of the layer as self load would only be supported alongthe inclined layer and into the pit floor.

    ;

    Fi!"e #Fail!"e Se-!en(e . Po**i/ility #

    +. 1ydraulic lift off 5dotted arrows6(. $ayer parallel stresses increase in

    the mudstone 5dashed arrow6

    ;. 3tresses cause shear failure at themost highly stressed %one 0 the toebreakout 5solid arrow6

    2udstone $ayer 

    Fi!"e &Fail!"e Se-!en(e . Po**i/ility &

    +. 1ydraulic pressure 5dotted arrows6(. #uckling failure of the mudstone layer 

    5line showing e!aggerateddeformation profile6.

    ;. 3hear failure of the mudstone due to

    removal of toe support 5solid arrow6.

    2udstone $ayer 

  • 8/18/2019 07 - Golder Associates

    4/8

    Prosiding Seminar Sehari Kemantapan Lereng di Pertambangan Indonesia III, Bandung, 16 Juni 2003

    &$&$ Fo"(e* Involve

    The stability of the low wall is in a state of balance between the drivingforces and the resisting forces 0 these are outlined below7

    Fo"(e* "ivin lo0 0all +ail!"e Fo"(e* "e*i*tin lo0 0all +ail!"e

    9orce derived from groundwater pressure

    9orce available from adhesionbetween layering.

    9orce from the component of theweight of the low wall acting in the

    downslope direction 5dependant onheight and inclination of low wall6.

    9orce from the component of theweight of the low wall acting across

    the layering 5dependant on heightand inclination6.

    9orce developed from shear resistance between layers and

    across layers 5at breakout areas intoe6

    These forces are discussed below7

    Groundwater pressures

    Depending on the rock material and mass permeability, natural drainagemay occur within the low wall as the pit floor becomes deeper, andconseuently, the water pressures within the low wall will be lowered.If depressurisation by mining alone does not occur, then the ratio betweenoverburden pressure available in the low wall and water pressure within the lowwall increases. 3implistically, when this ratio drops below +, failure will occur.

    Therefore, there is some critical ratio between thickness of the coalmeasure rocks and the height of elevated water from the floor of the pit. The

    overburden pressure that needs to be considered is related to the depth toimpermeable layers within the low wall. The component of weight representedby this depth that acts across 5normal to6 the layering develops the pressurethat needs to balance the water pressure.

    #oth the confining pressure from the overburden and the water pressureare simple parameters to calculate and measure in the coal mine.

    Rock interface strength

    9or groundwater pressures to lift off low wall layers due to buoyancy

    alone, the groundwater pressure must first overcome the tensile strengthbetween layers. #ased on simple 2ohr4

  • 8/18/2019 07 - Golder Associates

    5/8

    Prosiding Seminar Sehari Kemantapan Lereng di Pertambangan Indonesia III, Bandung, 16 Juni 2003

    3andstones in 'alimantan pits are often only weakly cemented andstrengths in tension in the order of only + or ( k8a.

    3heared surfaces are often evident between layers in coal measurerocks in 'alimantan and also represent very low strengths in lift off mechanismsof separation.

    The shear strength at the interface between layers cannot be affordedlarge components due to cohesion and should be modeled using a fairly straightfailure envelope.

    =ock interface strength can be easily measured in the mine.

    Rock strength in breakout areas (across bedding)

    $ow wall failures seem kinematically attracted to the direction parallelwith bedding, although must break out through the bedding at some stage sothat the failure surface can daylight into the pit.

    The resistance to failure travelling across the layering in the rock mass isdependent on the rock material strength in this direction as well as any affectfrom structure within the rock mass.

    In many mines in 'alimantan, two >oint sets e!ist normal to the layeringand form a trelice pattern when the layers are viewed flat. In the low wallconfiguration, these >oints are inclined so that they dip back into the wall. If breakout failure in the low wall were to e!ploit these >oints, then initialdisplacement would need to be upwards.

    9aulting parallel to these >oints sets is apparent in some mines in'alimantan and may represent a lower strength and more persistent surface for failure breakout.

     At least ( mines in kalimantan show an upwards movement of coal at thetoe of low wall areas that have moved. It seems possible therefore, that therock mass strength is influenced by these >oints and that these represent pathsof lower resistance 5despite the unfavorable kinematics6 compared withbreakout through intact rock mass.

    Typical mudstone rocks in 'alimantan are in the order of from + to ; 28ain ?nconfined

  • 8/18/2019 07 - Golder Associates

    6/8

    Prosiding Seminar Sehari Kemantapan Lereng di Pertambangan Indonesia III, Bandung, 16 Juni 2003

    The answer is probably due to the way in which the e!isting investigationdata is analysed and used to come up with recommendations that can be

    incorporated practically into mine design and operation.:ften, a mine@s idea of geotechnical investigation is to drill a couple of 

    holes and take some samples to send to a lab. #asing a mine design on thisinformation is risky business 0 either the mine will be developed conservatively,or it will fall down 0 and there is only a fine line in between.

    In many cases, mine design is then left to the contractor, who willdevelop a feel for the ground and optimise the stripping ratio based on that feel.This approach is better than taking ( samples for ?ective of the drilling is not to make a hole 0 it is so that the corecan be characterised and documented in a language that can be interpreted bythe engineer. :nly then can the information gained from the drilling be used tohelp keep the mine from falling in. The word BdrillingC should really be replacedby the word BloggingC when used in relation to geotechnical investigations.

    3amples should also be taken from diamond core for later laboratorytesting. these samples should be representative of layers that can be mappedout in section 5and along the pit6 and more than one per unit should be taken.

    /

  • 8/18/2019 07 - Golder Associates

    7/8

    Prosiding Seminar Sehari Kemantapan Lereng di Pertambangan Indonesia III, Bandung, 16 Juni 2003

    the samples must be removed from the splits immediately and protected tomaintain original conditions.

    $aboratory testing should comprise determination of bulk density,unconfined compressive strength and perhaps some direct shear testing alonge!isting discontinuities within the core.

    =eal analyses can begin at this stage. ?se of anisotropic modeling,1oek4#rown rock strength parameters and pie%ometric heads in specific layerscan avoid over4conservative results. 8it design parameters should be providedas relationships between probability of failure and slope angle E height.

    Fnough drilling should have been carried out by the #ankable feasibilitystage to provide a high level of confidence to be developed in the modeling. Analyses should be evolved to a stage to a high level of confidence in design.

    During production, inpit mapping should be maintained by e!perienced

    geotechnical personnel. 2onitoring for cracking must be carried out. Goodcoordination should e!ist between the mining contractor, the geotechnicalpersonnel and the pit designer.

    If failures occur, then these should be used to BcalibrateC e!isting modelsand should provide a basis for possible pit design revision 0 or mine seuencingrevision.

     A design for ground investigation and coordination is provided in theattached table.

    1$1$ P"a(ti(al Sol!tion* +o" Lo0 Wall P"o/lem*

    'nowing the factors that cause low wall failures is not enough to stopthem. In order to reduce the risk of low wall problems, we need to find practicalways of increasing resisting forces and decreasing driving forces. If the oppositehappens, then stability is reduced.

     Almost invariably the low wall will be developed parallel with the bedding.If the bedding is undercut, then layer parallel stresses are relieved and shear resistances along the layer interface are tested against the downslope weight of the block. In 'alimantan 5as in any place6, undercutting of layers in the low walldoes happen. If this cut is not taken back up to the crest, then the layer willoften slide.

    8erhaps the simplest and most cost effective method of decreasingfactors that cause sliding is to provide drainage to the low wall. Drilling into thewall is becoming more popular in 'alimantan and high groundwater pressuresare nearly always found in the floor. Depressurisation can offer a much reducedcost option to unloading options 0 however, how many and how deep Theseuestions can be predicted from first principals during feasibility level work, butreally need to be reassessed based on the results of measurements madeduring mining.

    8ie%ometers need to be installed behind the footwall and far enoughback from the crest of the footwall to demonstrate that water pressures behindthe base of the low wall are less than available overburden pressures left in thelow wall 5much, much less6.

  • 8/18/2019 07 - Golder Associates

    8/8