GEOTECHNICAL SLOPE STABILITY

January 1995 Geotechnical Slope Stability 1

GEOTECHNICAL SLOPE STABILITY

1.0 SCOPE

This guideline provides advice on the geotechnicalaspects of designing for stable sloping post-mining landforms. Such landforms include:

• low wall spoil (strip mines),

• out-of-pit dumps, waste rock dumps, reject organgue dumps (strip, open pit and undergroundmines),

• haul ramp batters (strip mines),

• retaining embankments, and

• 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:

• having volumes not exceeding 100,000 m3, and

• depths of not greater than 20 m.

2.0 OBJECTIVE

To ensure the effective management of the risk ofgeotechnical instability in waste dumps, spoil piles andabandoned open pit slopes in the final void.

3.0 RELATED GUIDELINES

• Tailings Management

• Open Pit Rehabilitation

• Minesite Decommissioning

4.0 INTERPRETATION

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.).

5.0 BACKGROUND

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:

• lost production and resources,

• 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.

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2 Geotechnical Slope Stability January 1995

• increased risk of equipment damage,

• damage to rehabilitated areas, and

• unnecessary rehandling of materials duringslope reshaping.

6.0 MANAGEMENT STRATEGIES

6.1 Geotechnical 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.

6.2 Assessment 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.

6.3 Concept 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:

• Abandoned slopes in final void

• Haul ramp batters (strip mines)

• Retaining embankments

• Low wall spoil

• Out of pit dumps, waste rock dumps, reject organgue dumps.

6.4 Data Collection

Data collection should be relevant to the type of slopesrequired and should be directed to the relevant factorsaffecting geotechnical stability.

(1) Dumps and Stockpiles

Data 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 dump

Data collected should include:

(a) A description of the soil profile belowthe dump/stockpile site in terms of:

• soil type

• particle size distribution

• plasticity (Atterberg limits)

• moisture content

• density

• shear strength (total and effectivestress angle of friction andcohesion)

• compressibility

• thickness and depth to rock.

(b) Hydrogeological conditions below thedump/stockpile site including:

• groundwater levels

• permeability.

(c) Geotechnical properties of thedump/stockpile materials including:

• particle size distribution

• density

• anticipated compacted density

• plasticity (Atterberg limits)

• dispersion index

• mineralogy

• shear strength

• permeability


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 Slopes

Data 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.

6.5 Design 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/m3 to 18 kN/m3 (local scatter). However, due todifferent dumping methods a second spoil pile of thesame material might vary from 17 kN/m3 to 20 kN/m3

(spatial scatter between dumps).

Any scatter in raw data may be due to any one of thefollowing:

• a real natural variation of the parameter,

• measurement errors or inaccuracies, or

• 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.

6.6 Stability Analysis - Detailed Design

Typical types of failure that can occur include:

(a) Earth, rock fill and spoil dumps andembankments

• circular

• non-circular semi-infinite slope

• multiple block plane wedge

• log spiral (bearing capacity of foundations)

• flow slides.

(b) Final void slopes in earth and rock

• block slide

• wedge

• toppling

• 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.

6.7 Performance 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:

• Selecting several typical profiles normal to theslope contours.

• Driving or concreting-in survey levelling pointsalong the profile.

• Photographing and surveying the profiles onceor twice a year - say at the start and finish of thewet season.


• Installing standpipes and measuring water levelson a similar basis.

• Comparing surveys cumulatively and assessingslope degradation.

• Keeping a record book of any slips and slopefailures that occur on any slope (not necessarilyalong profile lines).

7.0 IMPLEMENTATION STRATEGIES

7.1 General

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.

7.2 Data Gathering

Methods of data gathering include:

• surface mapping and sampling

• test pitting and costeans

• borehole sampling of soils, either undisturbed ordisturbed

• continuous rock coring, core orientation,geomechanical logging

• downhole geophysical methods

• groundwater sampling

• insitu testing in boreholes including

a) 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, and

f) pumping/dewatering tests,

• electric friction core profiling

• laboratory tests on rock, soil materials and wateraccording to Australian Standards (AS) orinternational rock testing standards (ISRMS)

• 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.

7.3 Geotechnical Analysis and Design

(1) Open PitsAnalytical methods for cut slopes have been welldocumented in published texts ( Reference 1) and thesemethods include:

• Stereographic projection graphical techniquesfor the analysis of discontinuity data

• Plane failure analysis

• Wedge failure techniques

• Toppling failure analysis

• Circular and non-circular analysis by the methodof slices

• Finite element and finite difference computertechniques.

A typical design implementation would be:

• Divide the pit into areas of similar materialground properties, geological structure,stratigraphy, grade of weathering etc.

• Select a typical cross section for each area

• Assess discontinuity data and rock massstrength data and decide on likely failure modes(there may be more than one)

• Select groundwater levels in the slope

• Perform stability analyses

• Re-configure slope geometry if Factor of Safetyis unacceptable.

(2) Dumps and StockpilesAnalytical methods for dumps and stockpiles include:

• the method of slices, circular or non-circular

• multiple wedge/sliding block analyses.

A typical design implementation would include:

• Group together all slopes that comprise similardump/stockpile material, similar foundations andwater pressures, and similar geometry

• Select a representative typical cross sectionthrough each group.

• Assign material parameters and groundwaterlevels


• Perform stability analyses to determine therequired slope angles to ensure Factors ofSafety are acceptable.

As a general guide, circular stability analysis shouldalways be carried out. If the foundations soils below thedump or stockpile contain soft or weak layers sandwichedbetween stronger layers, then multiple wedge/slidingblock type analyses should also be carried out to assesssliding along these weak layers.

Short term slope stability can be assessed using theundrained total stress shear strength parameters of thesoils (Cu, φu). However, since slopes must be stable inthe long term, an effective stress analysis using effectivestress parameters (c’and φ’) and pore water pressuresshould be used for final design.

These stress parameters are referred to as follows:

Cu = cohesion intercept for undrained, totalstress conditions,

c’ = cohesion intercept for effective stresses,

φu = internal angle of friction for undrained,total stress conditions, and

φ’ = internal angle of friction for effectivestresses.

7.4 Construction

The construction programme should minimise the needto return to an area more than once. Within the open pitor quarry distinguish between production faces and finalslopes. A production face does not need to fulfil thestability requirements of a final slope and should bedesigned to maximise mine productivity.

As a production face approaches the limits of the mine,the mining method may have to be modified to form thefinal slope. Modifications may include reducing theheight between benches, or using smooth blastingtechniques as the final round. Returning to an area toclean up final slopes is expensive since it often requiresactivities outside of the normal mine operation. Henceprogressive formation of final slopes is considered mostdesirable.

8.0 REFERENCES

1. Hoek, E., and Bray, J.W. (1981). Rock SlopeEngineering, IMM, London.

2. Department of Mines, Western Australia. (1991). Guidelines on Safety Bund Walls AroundAbandoned Open Pits, Perth.

3. International Committee on Large Dams. (1982). Manual on Tailings Dams and Dumps.ICOLD Bulletin No.45.

4. Australian Standard AS 1289. (1991). “Methods of Testing Soils for EngineeringPurposes”.


ATTACHMENT 1

SOME GEOTECHNICAL SLOPE STABILITY CRITERIA FOR SMALL-SCALE MINING OPERATIONS INAREAS REMOTE FROM POPULATION CENTRES

(1) APPLICATION OF CRITERIAFor small scale mining operations in Queensland, certain generalised criteria can be provided to assist the selection ofacceptable slope angles. The conditions controlling stability will always be site specific, hence due care should be takenwhen applying these criteria. In particular, if there is a history of unstable slopes in the area, particularly during miningoperations, then specific geotechnical studies should be carried out.

These criteria are intended to apply to operations which are remote from population centres and involve pits:

• having volumes not exceeding 100,000 m3, and

• depths of not greater than 20 m.

(2) GEOTECHNICAL SLOPE STABILITYA slope is geotechnically stable if it does not physically collapse. The Factor of Safety is a measure of the confidence thatcollapse will not occur.

(3) SITE CLASSIFICATIONAny mine operation will contain several types of slope, eg. cut pit slopes and waste dumps slopes. The first step in ensuringstable slopes is to categorise areas on the mine site according to slope type.

Groups of similar slope types can then be assessed separately. The procedure is as follows:

(a) Prepare a site plan indicating all areas of excavation (pits), filling, stockpiles, dumps, and dams.

(b) Allocate to each area one of the slope classifications given in Table 1.1.

(c) For each slope classification, refer to the guidelines given in the following sections.

TABLE 1.1SLOPE TYPE CLASSIFICATION

Slope Type Description

C1 Cut slopes above areas which will be open toaccess by the public and by stock.

C2 Cut slopes above areas where access isprohibited.

F1 Slopes to dumps and other stockpiles.

F2 Low wall spoil slopes (strip mines).

F3 Fill slopes to retaining embankments (eg.tailings dams, water dams).

(4) CUT SLOPES

General

The two main cut slope categories depend on whether access is allowed below the slope. Where access is prohibited,allowing gradual collapse to a stable state may be acceptable and feasible, and hence initial steeper slopes may bepermitted. This approach needs to incorporate barriers above and behind the cut slopes to physically prevent access to thepotentially unstable pit edge zone. More severe restrictions are required for cut slopes which remain above areas freelyaccessible to people and stock.

Type C1 Slopes

Generally, overall slope angles in unweathered (unoxidised) rock should not be steeper than 1 vertical to 1 horizontal(1V:1H), with individual bench faces no steeper than 2V:1H. If there is a history of stable slopes at steeper angles supportedby documented evidence and, (preferably), still standing old and abandoned faces, then steeper slopes may be acceptable.

If the rock slopes are cut by unfavourable geological features such as weak fault zones or joints (or bedding planes dippingsteeply out of the face), and along which the overlying rock could slide, then the slopes should be assessed on a case bycase basis. Unstable slopes will require battering back to a safe angle or made inaccessible and treated as if a Type C2Slope.


Slopes cut in weathered (oxidised) rock, overburden or soil should not be steeper than the following:

• 1 (V) : 1.5 (H) for slopes less than 5 m high and without groundwater seepage.

• 1 (V) : 2.5 (H) for all other slope conditions.

For slopes which cut through a mixed profile, (for example, of weathered rock/overburden overlying unweathered rock), theabove maximum angles should be applied to each relevant material. These slope angles assume that no instabilityproblems have been previously experienced in the area or during operations.

Type C2 Slopes

For cut slopes to which access will be prevented, bund walls should be provided which:

• are located at least 10 m outside the area overlying the potentially unstable rock mass, ie. the total of the width ofthe “potentially unstable pit edge zone” plus 10 m away from the existing pit edge (Figure 1.1);

• have a minimum height of 2 m and a minimum base width of 4 m; and

• wherever possible, are constructed from unweathered, freely draining, end dumped rockfill. When only oxidisedrock is available for construction of the safety bund wall, the least weathered or hardest material should be used. Inthese cases, the bund wall may need to be supplemented by a properly constructed fence.

Suitable signs, clearly stating public safety risk and prohibiting public access, should be erected outside the safety bundwall. Additionally, shrub and/or tree planting at the outside edge of the bund wall should be implemented where practicable,to lessen the visual impact of the wall.

(5) FILL SLOPES

Type F1 Slopes

Dumps and stockpiles tend to be constructed by end tipping, hence operating slope angles approximate the angle of reposeof the dumped material. Coarse rock dumps will have side slopes steeper than dumps of finer grained and clayey materialssuch as soil and (oxidised) weathered rock.

In many situations, desired end landform and other environmental requirements will determine the final slope angle. Suchrequirements generally involve flatter slopes than those needed to ensure geotechnical stability.

Where geotechnical stability is the determining requirement, two options are available:

(a) Long term steep side slopes can be developed by constructing a perimeter bund of compacted waste or stockpilematerial within which the bulk of the dump or stockpile can be end tipped or placed in the usual mine operationmethod. Typical compacted bund slopes would be:

(i) unweathered rock 1 (V) : 1.75 (H)

(ii) weathered rock, overburden, up to 15 m high - 1 (V) : 2.5 (H)

(iii) weathered rock, overburden, greater than 15 m high- 1 (V) : 3 (H)

These maximum slope angles apply to relatively level sites (ie. with ground slope less than 10%), with stable foundationsoil and rock materials. Sites that are steeper or have weak foundation conditions should be individually assessed bygeotechnical investigation.

(b) Alternatively, end tipped and hence poorly compacted materials can be dozed and graded out to flatter slopes atthe end of operations. As a general guide, final slopes should be no greater than half the angle of repose of the endtipped material as measured on-site. Since most materials have angles of repose of less than or equal to1(V):1.5(H), final graded slopes should not exceed 1(V):3(H).

Again, dumps and stockpiles on sites which are steep or have weak foundation conditions should be individuallyassessed.

Type F2 Slopes

Low wall spoil slopes commonly adopted in Queensland coal fields are generally in the order of 1(V):1.5(H). However,actual slopes depend on the nature of the spoil and its degree of compaction. Poorly compacted mudstone/shale spoils canbecome saturated and breakdown after repeated wet season exposure. Flatter slopes will result.

Type F3 Slopes

All embankments retaining lagoons of tailings, water or other materials should be designed and constructed using soundengineering practice. As a general guide, slopes of 1(V):2.5(H) are commonly adopted for embankments up to about 15 min height and which also incorporate internal drainage blankets.


FIGURE 1.1EXAMPLES OF PREFERRED SAFETY BUND WALL LOCATIONS (from Reference 2)

GEOTECHNICAL SLOPE STABILITY

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