Geotechnical Slope Stability

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GEOTECHNICAL SLOPE STABILITY 1.0 SCOPE “Agreed” refers to a standard, level or criterion which if achieved ensures that no significant adverse environmental impact is likely to occur. Such standards, levels or criteria may be drawn from published sources or proven practice but, in all cases, must be to the satisfaction of the relevant Responsible Authority; “Angle of repose” is the angle of steepest slope at which material will remain stable when loosely piled; “Cut slope” refers to a man-made s
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  January 1995Geotechnical Slope Stability  1 GEOTECHNICAL SLOPE STABILITY 1.0SCOPE This guideline provides advice on the geotechnicalaspects of designing for stable sloping post-mining landforms. Such landforms include: ã low wall spoil (strip mines), ã out-of-pit dumps, waste rock dumps, reject organgue dumps (strip, open pit and undergroundmines), ã haul ramp batters (strip mines), ã retaining embankments, and ã final void batters.This guideline recognises the different resourcesavailable to small scale miners and larger operators.Accordingly, some generally acceptable geotechnicalslope stability criteria are described in Attachment 1.These criteria are intended to apply (subject to site-specific conditions) to operations which are remote frompopulation centres and involve pits: ã having volumes not exceeding 100,000 m 3 , and ã depths of not greater than 20 m. 2.0OBJECTIVE To ensure the effective management of the risk ofgeotechnical instability in waste dumps, spoil piles andabandoned open pit slopes in the final void. 3.0RELATED GUIDELINES ã Tailings Management ã Open Pit Rehabilitation ã Minesite Decommissioning 4.0INTERPRETATION For the purposes of this guideline, unless the contextindicates otherwise: “Agreed” refers to a standard, level or criterion which ifachieved ensures that no significant adverseenvironmental impact is likely to occur. Such standards,levels or criteria may be drawn from published sources orproven practice but, in all cases, must be to thesatisfaction of the relevant Responsible Authority; “Angle of repose” is the angle of steepest slope atwhich material will remain stable when loosely piled; “Cut slope” refers to a man-made slope created byexcavation into insitu material; “Factor of Safety” (FOS) , in relation to a slope orembankment, is the ratio of total force available to resistsliding to the total force tending to induce sliding. Whenthe slope or embankment is on the point of failure, theresisting and disturbing forces are equal and the FOS is1.0. An FOS greater than 1.0 indicates stability; “Fill slope” refers to a man-made slope formed at theedge of material dumped or placed to create stockpiles,dumps, retaining embankments or similar structures; “Rehabilitation” refers to the measures and actionsused to remediate land disturbed by mining operationsand/or exploration activities; “Responsible Authority” means any State GovernmentDepartment, corporation, statutory authority or localgovernment empowered to determine an application forthe granting of approval for a development proposal orany component of that proposal (by way of generalconsent, licence or permit, etc.). 5.0BACKGROUND The stability of the final land form left at the end of miningoperations is critical to the successful rehabilitation of thesite. There are significant advantages in taking this intoaccount when selecting mining and spoil disposalmethods to be used during the mining operation. Re-shaping, draining and capping of slopes can incursignificant costs. Spreading the cost of such workthrough the project life is to be preferred to one high costclean-up event at the end of the project when cash flow isreduced.Hence working to plans of operation that take intoaccount the final land form, including final void and spoiltips, and provide for progressive rehabilitation ofexhausted and completed areas is to be encouraged.The analyses and investigations of the geotechnicalstability of slopes will incur costs which will normally haveto be borne at project start-up.While geotechnical investigations can appear expensivein the short-term, they can save on the longer term costsof poor slope design. Poor design can lead to: ã lost production and resources, ã reduced personal safety, This guideline is ADVISORY ONLY and isnot intended to prescribe mandatorystandards and practices. This guideline isintended to assist the development ofproject-specific environmentalmanagement practices.  2  Geotechnical Slope StabilityJanuary 1995  ã increased risk of equipment damage, ã damage to rehabilitated areas, and ã unnecessary rehandling of materials duringslope reshaping. 6.0MANAGEMENT STRATEGIES 6.1Geotechnical Stability  Long term geotechnical stability should be maintainedwithin agreed standards dependent on thegeomorphology of the surrounding landform and theproposed post-mining land use. No landform is stable ingeological time. The design and safety of the finallandform should be suitable for the agreed end land use.Geotechnical stability is defined as the stability of anexcavated slope or spoil pile against mass failure.Factors of Safety against failure are generally defined asthe ratio between restoring forces and disturbing forceswithin the slope. Restoring forces are dependent on theavailable shear strength in the materials plus anyintroduced supports (such as anchors or rock bolts),while disturbing forces are a function of applied shearstresses, pore pressures, surcharges and earthquakeloadings within the slope. Conventionally used safetyfactors for temporary and permanent slopes are 1.2 and1.5 respectively. However, some Responsible Authoritiesmay specify different values and these should beconfirmed. 6.2Assessment Procedures  The following steps are recommended in approaching theassessment of the geotechnical stability of slopes:(a)Prepare conceptual mine layout and selectconcept design for open pit and spoil slopes.(b)Collect geotechnical data.(c)Define design parameters.(d)Define Factors of Safety.(e)Analyse geotechnical slope stability.(f)Refine slope geometries to conform with agreedFactors of Safety.During mining operations, slope stability performanceshould be reviewed and designs amended as necessary.When developing concept designs and amendingdesigns, the possibility of future extensions or deepeningof the pit should be taken into account. 6.3Concept Slope Design  At concept design stage, slope geometries should bebased on local experience and with similar materials insimilar environments. All slopes should be identified andcategorised with respect to consequence of slope failureand type of slope. Types of slope may include: ã Abandoned slopes in final void ã Haul ramp batters (strip mines) ã Retaining embankments ã Low wall spoil ã Out of pit dumps, waste rock dumps, reject organgue dumps. 6.4Data Collection  Data collection should be relevant to the type of slopesrequired and should be directed to the relevant factorsaffecting geotechnical stability.(1)Dumps and StockpilesData for dumps and stockpiles is required forassessing:(a)the bearing capacity of the underlyingfoundation materials,(b)the stability of slopes formed in thedumped material, and(c)the permeability and drainagecharacteristics of the dumpData collected should include:(a)A description of the soil profile belowthe dump/stockpile site in terms of: ã soil type ã particle size distribution ã plasticity (Atterberg limits) ã moisture content ã density ã shear strength (total and effectivestress angle of friction andcohesion) ã compressibility ã thickness and depth to rock.(b)Hydrogeological conditions below thedump/stockpile site including: ã groundwater levels ã permeability.(c)Geotechnical properties of thedump/stockpile materials including: ã particle size distribution ã density ã anticipated compacted density ã plasticity (Atterberg limits) ã dispersion index ã mineralogy ã shear strength ã permeability  January 1995Geotechnical Slope Stability  3  and any variations of the above if thematerial is expected to weather ordeteriorate.(d)Any other relevant data such asearthquake loadings and surcharges.(2)Open Pit SlopesData for final slopes and batters remainingwithin the open pit is required for:(a)designing long term pit slopes,(b)assessing long term slope deterioration,and(c)determining hydrogeological effects onthe local groundwater.Data collected should include:(a)a description of soil and rock profilethrough the slope,(b)soil parameters as listed above fordumps and stockpiles,(c)rock density and uniaxial compressivestrength,(d)rock structure including orientation,occurrence and spacing of bedding, joints, faults and other discontinuities,(e)shear strength along discontinuities,(f)groundwater levels,(g)permeabilities,(h)depth of weathering,(i)depth of soil cover andpaleotopography (eg. buried channels),(j)surcharges (during and after miningoperations), and(k)earthquake loadings. 6.5Design Parameters  Design parameters should be selected to represent thecharacteristics of the slope forming materials. Measuredvalues of soil parameters may show a scatter both locallyand spatially. For example, insitu bulk densitymeasurements of one particular spoil pile may vary from15 kN/m 3 to 18 kN/m 3 (local scatter). However, due todifferent dumping methods a second spoil pile of thesame material might vary from 17 kN/m 3 to 20 kN/m 3 (spatial scatter between dumps).Any scatter in raw data may be due to any one of thefollowing: ã a real natural variation of the parameter, ã measurement errors or inaccuracies, or ã spatial variation, such as in the bulk densityexample given above.All new data must therefore be carefully examined andfiltered before being grouped for statistical analysis.In addition to material parameters it is very important toselect the correct groundwater and pore pressuredistribution for the slope. 6.6Stability Analysis - Detailed Design  Typical types of failure that can occur include:(a)Earth, rock fill and spoil dumps andembankments ã circular ã non-circular semi-infinite slope ã multiple block plane wedge ã log spiral (bearing capacity of foundations) ã flow slides.(b)Final void slopes in earth and rock ã block slide ã wedge ã toppling ã circular (normally earth slopes only).Stability analysis and slope design is an iterative processof successive trials whereby potential sliding surfaces arechosen and the Factor of Safety determined. This iscontinued for all kinematically possible surfaces until thecritical surface is found. The critical surface is the onewith the lowest Factor of Safety. If this is below theminimum design Factor of Safety for the project, theslope geometry, drainage, or construction materials needto be varied until the minimum Factor of Safety isachieved or exceeded.Computer programs are commercially available toperform most stability analyses but personnelexperienced in their operation, particularly in theparticular project environment, should be employed tofacilitate the analyses.For preliminary and conceptual design purposes use canbe made of stability charts published in readily availabletexts (see references). However, these designs need tobe confirmed and refined by detailed analysis at finaldesign stage. 6.7Performance and Feedback  Progressive rehabilitation of completed or exhaustedareas allows the performance of early areas to be used inmodifying designs for subsequent areas. This canachieve more effective designs that are suitable for theparticular project environment and that can reducerehabilitation costs.Slope performance monitoring generally includes: ã Selecting several typical profiles normal to theslope contours. ã Driving or concreting-in survey levelling pointsalong the profile. ã Photographing and surveying the profiles onceor twice a year - say at the start and finish of thewet season.  4  Geotechnical Slope StabilityJanuary 1995  ã Installing standpipes and measuring water levelson a similar basis. ã Comparing surveys cumulatively and assessingslope degradation. ã Keeping a record book of any slips and slopefailures that occur on any slope (not necessarilyalong profile lines). 7.0IMPLEMENTATION STRATEGIES 7.1General  Geotechnical investigation, data assessment, analysisand design is a specialised discipline. Depending on thesize of the project, geotechnical input may only berequired at specific and infrequent times. Considerationshould be given to employing geotechnical consultantsfor this work.More detailed guidance on geotechnical slope stabilityapplicable to small scale mining operations remote frompopulation centres is given in Attachment 1. 7.2Data Gathering  Methods of data gathering include: ã surface mapping and sampling ã test pitting and costeans ã borehole sampling of soils, either undisturbed ordisturbed ã continuous rock coring, core orientation,geomechanical logging ã downhole geophysical methods ã groundwater sampling ã insitu testing in boreholes includinga) permeability tests,b) pressuremeter tests for elastic modulideterminations,c) insitu stress measurement,d) standard penetration test (SPT) for relativedensity of soils,e) insitu vane shear test for undrained strengthof soft clays, andf) pumping/dewatering tests, ã electric friction core profiling ã laboratory tests on rock, soil materials and wateraccording to Australian Standards (AS) orinternational rock testing standards (ISRMS) ã Field trials of the proposed works, eg.g) trial mine pits,h) stockpiles,i) spoil piles, and j) compaction trials for engineeredembankments, roadways, causeways.Much of the fieldwork for such data gathering can becarried out as a small extension to an explorationprogramme. Mobilisation costs can be minimised if thetwo activities are carried out together. 7.3Geotechnical Analysis and Design  (1)Open Pits  Analytical methods for cut slopes have been welldocumented in published texts ( Reference 1) and thesemethods include: ã Stereographic projection graphical techniquesfor the analysis of discontinuity data ã Plane failure analysis ã Wedge failure techniques ã Toppling failure analysis ã Circular and non-circular analysis by the methodof slices ã Finite element and finite difference computertechniques.A typical design implementation would be: ã Divide the pit into areas of similar materialground properties, geological structure,stratigraphy, grade of weathering etc. ã Select a typical cross section for each area ã Assess discontinuity data and rock massstrength data and decide on likely failure modes(there may be more than one) ã Select groundwater levels in the slope ã Perform stability analyses ã Re-configure slope geometry if Factor of Safetyis unacceptable. (2)Dumps and Stockpiles  Analytical methods for dumps and stockpiles include: ã the method of slices, circular or non-circular ã multiple wedge/sliding block analyses.A typical design implementation would include: ã Group together all slopes that comprise similardump/stockpile material, similar foundations andwater pressures, and similar geometry ã Select a representative typical cross sectionthrough each group. ã Assign material parameters and groundwaterlevels
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