GEOTECHNICAL SLOPE STABILITY

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GEOTECHNICAL SLOPE STABILITY

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GEOTECHNICAL SLOPE STABILITY

  1. 1. GEOTECHNICAL SLOPE STABILITY “Agreed” refers to a standard, level or criterion which if 1.0 SCOPE achieved ensures that no significant adverse environmental impact is likely to occur. Such standards, This guideline provides advice on the geotechnical levels or criteria may be drawn from published sources or aspects of designing for stable sloping post-mining land proven practice but, in all cases, must be to the forms. Such landforms include: satisfaction of the relevant Responsible Authority; • low wall spoil (strip mines), “Angle of repose” is the angle of steepest slope at which material will remain stable when loosely piled; • out-of-pit dumps, waste rock dumps, reject or gangue dumps (strip, open pit and underground “Cut slope” refers to a man-made slope created by mines), excavation into insitu material; • haul ramp batters (strip mines), “Factor of Safety” (FOS), in relation to a slope or embankment, is the ratio of total force available to resist • retaining embankments, and sliding to the total force tending to induce sliding. When the slope or embankment is on the point of failure, the • final void batters. resisting and disturbing forces are equal and the FOS is This guideline recognises the different resources 1.0. An FOS greater than 1.0 indicates stability; available to small scale miners and larger operators. “Fill slope” refers to a man-made slope formed at the Accordingly, some generally acceptable geotechnical edge of material dumped or placed to create stockpiles, slope stability criteria are described in Attachment 1. dumps, retaining embankments or similar structures; These criteria are intended to apply (subject to site- specific conditions) to operations which are remote from “Rehabilitation” refers to the measures and actions population centres and involve pits: used to remediate land disturbed by mining operations and/or exploration activities; • having volumes not exceeding 100,000 m 3, and “Responsible Authority” means any State Government • depths of not greater than 20 m. Department, corporation, statutory authority or local government empowered to determine an application for the granting of approval for a development proposal or any component of that proposal (by way of general This guideline is ADVISORY ONLY and is consent, licence or permit, etc.). not intended to prescribe mandatory standards and practices. This guideline is intended to assist the development of 5.0 BACKGROUND project-specific environmental management practices. The stability of the final land form left at the end of mining operations is critical to the successful rehabilitation of the site. There are significant advantages in taking this into account when selecting mining and spoil disposal methods to be used during the mining operation. Re- 2.0 OBJECTIVE shaping, draining and capping of slopes can incur significant costs. Spreading the cost of such work To ensure the effective management of the risk of through the project life is to be preferred to one high cost geotechnical instability in waste dumps, spoil piles and clean-up event at the end of the project when cash flow is abandoned open pit slopes in the final void. reduced. Hence working to plans of operation that take into 3.0 RELATED GUIDELINES account the final land form, including final void and spoil tips, and provide for progressive rehabilitation of • Tailings Management exhausted and completed areas is to be encouraged. • Open Pit Rehabilitation The analyses and investigations of the geotechnical stability of slopes will incur costs which will normally have • Minesite Decommissioning to be borne at project start-up. While geotechnical investigations can appear expensive 4.0 INTERPRETATION in the short-term, they can save on the longer term costs of poor slope design. Poor design can lead to: For the purposes of this guideline, unless the context indicates otherwise: • lost production and resources, • reduced personal safety, January 1995 Geotechnical Slope Stability 1
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  3. 3. • increased risk of equipment damage, • Low wall spoil • damage to rehabilitated areas, and • Out of pit dumps, waste rock dumps, reject or gangue dumps. • unnecessary rehandling of materials during slope reshaping. 6.4 Data Collection Data collection should be relevant to the type of slopes 6.0 MANAGEMENT STRATEGIES required and should be directed to the relevant factors affecting geotechnical stability. 6.1 Geotechnical Stability (1) Dumps and Stockpiles Long term geotechnical stability should be maintained Data for dumps and stockpiles is required for within agreed standards dependent on the assessing: geomorphology of the surrounding landform and the proposed post-mining land use. No landform is stable in (a) the bearing capacity of the underlying geological time. The design and safety of the final foundation materials, landform should be suitable for the agreed end land use. (b) the stability of slopes formed in the Geotechnical stability is defined as the stability of an dumped material, and excavated slope or spoil pile against mass failure. (c) the permeability and drainage Factors of Safety against failure are generally defined as characteristics of the dump the ratio between restoring forces and disturbing forces Data collected should include: within the slope. Restoring forces are dependent on the available shear strength in the materials plus any (a) A description of the soil profile below introduced supports (such as anchors or rock bolts), the dump/stockpile site in terms of: while disturbing forces are a function of applied shear • soil type stresses, pore pressures, surcharges and earthquake loadings within the slope. Conventionally used safety • particle size distribution factors for temporary and permanent slopes are 1.2 and 1.5 respectively. However, some Responsible Authorities • plasticity (Atterberg limits) may specify different values and these should be confirmed. • moisture content • density 6.2 Assessment Procedures • shear strength (total and effective The following steps are recommended in approaching the stress angle of friction and assessment of the geotechnical stability of slopes: cohesion) (a) Prepare conceptual mine layout and select • compressibility concept design for open pit and spoil slopes. (b) Collect geotechnical data. • thickness and depth to rock. (c) Define design parameters. (b) Hydrogeological conditions below the (d) Define Factors of Safety. dump/stockpile site including: (e) Analyse geotechnical slope stability. • groundwater levels (f) Refine slope geometries to conform with agreed • permeability. Factors of Safety. (c) Geotechnical properties of the During mining operations, slope stability performance dump/stockpile materials including: should be reviewed and designs amended as necessary. When developing concept designs and amending • particle size distribution designs, the possibility of future extensions or deepening of the pit should be taken into account. • density • anticipated compacted density 6.3 Concept Slope Design • plasticity (Atterberg limits) At concept design stage, slope geometries should be based on local experience and with similar materials in • dispersion index similar environments. All slopes should be identified and categorised with respect to consequence of slope failure • mineralogy and type of slope. Types of slope may include: • shear strength • Abandoned slopes in final void • permeability • Haul ramp batters (strip mines) • Retaining embankments 2 Geotechnical Slope Stability January 1995
  4. 4. and any variations of the above if the 6.6 Stability Analysis - Detailed Design material is expected to weather or deteriorate. Typical types of failure that can occur include: (d) Any other relevant data such as (a) Earth, rock fill and spoil dumps and earthquake loadings and surcharges. embankments (2) Open Pit Slopes • circular Data for final slopes and batters remaining • non-circular semi-infinite slope within the open pit is required for: • multiple block plane wedge (a) designing long term pit slopes, • log spiral (bearing capacity of foundations) (b) assessing long term slope deterioration, and • flow slides. (c) determining hydrogeological effects on the local groundwater. (b) Final void slopes in earth and rock Data collected should include: • block slide (a) a description of soil and rock profile • wedge through the slope, • toppling (b) soil parameters as listed above for dumps and stockpiles, • circular (normally earth slopes only). (c) rock density and uniaxial compressive Stability analysis and slope design is an iterative process strength, of successive trials whereby potential sliding surfaces are (d) rock structure including orientation, chosen and the Factor of Safety determined. This is occurrence and spacing of bedding, continued for all kinematically possible surfaces until the joints, faults and other discontinuities, critical surface is found. The critical surface is the one (e) shear strength along discontinuities, with the lowest Factor of Safety. If this is below the minimum design Factor of Safety for the project, the (f) groundwater levels, slope geometry, drainage, or construction materials need (g) permeabilities, to be varied until the minimum Factor of Safety is achieved or exceeded. (h) depth of weathering, (i) depth of soil cover and Computer programs are commercially available to paleotopography (eg. buried channels), perform most stability analyses but personnel experienced in their operation, particularly in the (j) surcharges (during and after mining particular project environment, should be employed to operations), and facilitate the analyses. (k) earthquake loadings. For preliminary and conceptual design purposes use can be made of stability charts published in readily available 6.5 Design Parameters texts (see references). However, these designs need to Design parameters should be selected to represent the be confirmed and refined by detailed analysis at final characteristics of the slope forming materials. Measured design stage. values of soil parameters may show a scatter both locally and spatially. For example, insitu bulk density measurements of one particular spoil pile may vary from 6.7 Performance and Feedback 3 3 15 kN/m to 18 kN/m (local scatter). However, due to Progressive rehabilitation of completed or exhausted different dumping methods a second spoil pile of the areas allows the performance of early areas to be used in same material might vary from 17 kN/m3 to 20 kN/m3 modifying designs for subsequent areas. This can (spatial scatter between dumps). achieve more effective designs that are suitable for the particular project environment and that can reduce Any scatter in raw data may be due to any one of the rehabilitation costs. following: Slope performance monitoring generally includes: • a real natural variation of the parameter, • Selecting several typical profiles normal to the • measurement errors or inaccuracies, or slope contours. • spatial variation, such as in the bulk density • Driving or concreting-in survey levelling points example given above. along the profile. All new data must therefore be carefully examined and • Photographing and surveying the profiles once filtered before being grouped for statistical analysis. or twice a year - say at the start and finish of the In addition to material parameters it is very important to wet season. select the correct groundwater and pore pressure distribution for the slope. January 1995 Geotechnical Slope Stability 3
  5. 5. • Installing standpipes and measuring water levels i) spoil piles, and on a similar basis. j) compaction trials for engineered • Comparing surveys cumulatively and assessing embankments, roadways, causeways. slope degradation. Much of the fieldwork for such data gathering can be • Keeping a record book of any slips and slope carried out as a small extension to an exploration failures that occur on any slope (not necessarily programme. Mobilisation costs can be minimised if the along profile lines). two activities are carried out together. 7.3 Geotechnical Analysis and Design 7.0 IMPLEMENTATION STRATEGIES (1) Open Pits 7.1 General Analytical methods for cut slopes have been well Geotechnical investigation, data assessment, analysis documented in published texts ( Reference 1) and these and design is a specialised discipline. Depending on the methods include: size of the project, geotechnical input may only be required at specific and infrequent times. Consideration • Stereographic projection graphical techniques should be given to employing geotechnical consultants for the analysis of discontinuity data for this work. • Plane failure analysis More detailed guidance on geotechnical slope stability applicable to small scale mining operations remote from • Wedge failure techniques population centres is given in Attachment 1. • Toppling failure analysis • Circular and non-circular analysis by the method 7.2 Data Gathering of slices Methods of data gathering include: • Finite element and finite difference computer • surface mapping and sampling techniques. • test pitting and costeans A typical design implementation would be: • borehole sampling of soils, either undisturbed or • Divide the pit into areas of similar material disturbed ground properties, geological structure, stratigraphy, grade of weathering etc. • continuous rock coring, core orientation, geomechanical logging • Select a typical cross section for each area • downhole geophysical methods • Assess discontinuity data and rock mass strength data and decide on likely failure modes • groundwater sampling (there may be more than one) • insitu testing in boreholes including • Select groundwater levels in the slope a) permeability tests, • Perform stability analyses b) pressuremeter tests for elastic moduli • Re-configure slope geometry if Factor of Safety determinations, is unacceptable. c) insitu stress measurement, (2) Dumps and Stockpiles d) standard penetration test (SPT) for relative Analytical methods for dumps and stockpiles include: density of soils, • the method of slices, circular or non-circular e) insitu vane shear test for undrained strength of soft clays, and • multiple wedge/sliding block analyses. f) pumping/dewatering tests, A typical design implementation would include: • electric friction core profiling • Group together all slopes that comprise similar dump/stockpile material, similar foundations and • laboratory tests on rock, soil materials and water water pressures, and similar geometry according to Australian Standards (AS) or international rock testing standards (ISRMS) • Select a representative typical cross section through each group. • Field trials of the proposed works, eg. g) trial mine pits, • Assign material parameters and groundwater levels h) stockpiles, 4 Geotechnical Slope Stability January 1995
  6. 6. • Perform stability analyses to determine the 8.0 REFERENCES required slope angles to ensure Factors of Safety are acceptable. 1. Hoek, E., and Bray, J.W. (1981). Rock Slope Engineering, IMM, London. As a general guide, circular stability analysis should 2. Department of Mines, Western Australia. always be carried out. If the foundations soils below the (1991). Guidelines on Safety Bund Walls Around dump or stockpile contain soft or weak layers sandwiched Abandoned Open Pits, Perth. between stronger layers, then multiple wedge/sliding block type analyses should also be carried out to assess 3. International Committee on Large Dams. sliding along these weak layers. (1982). Manual on Tailings Dams and Dumps. ICOLD Bulletin No.45. Short term slope stability can be assessed using the 4. Australian Standard AS 1289. (1991). undrained total stress shear strength parameters of the “Methods of Testing Soils for Engineering soils (Cu, φu). However, since slopes must be stable in Purposes”. the long term, an effective stress analysis using effective stress parameters (c’and φ’) and pore water pressures should be used for final design. These stress parameters are referred to as follows: Cu = cohesion intercept for undrained, total stress conditions, c’ = cohesion intercept for effective stresses, φu = internal angle of friction for undrained, total stress conditions, and φ’ = internal angle of friction for effective stresses. 7.4 Construction The construction programme should minimise the need to return to an area more than once. Within the open pit or quarry distinguish between production faces and final slopes. A production face does not need to fulfil the stability requirements of a final slope and should be designed to maximise mine productivity. As a production face approaches the limits of the mine, the mining method may have to be modified to form the final slope. Modifications may include reducing the height between benches, or using smooth blasting techniques as the final round. Returning to an area to clean up final slopes is expensive since it often requires activities outside of the normal mine operation. Hence progressive formation of final slopes is considered most desirable. January 1995 Geotechnical Slope Stability 5
  7. 7. ATTACHMENT 1 SOME GEOTECHNICAL SLOPE STABILITY CRITERIA FOR SMALL-SCALE MINING OPERATIONS IN AREAS REMOTE FROM POPULATION CENTRES (1) APPLICATION OF CRITERIA For small scale mining operations in Queensland, certain generalised criteria can be provided to assist the selection of acceptable slope angles. The conditions controlling stability will always be site specific, hence due care should be taken when applying these criteria. In particular, if there is a history of unstable slopes in the area, particularly during mining operations, 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 m 3, and • depths of not greater than 20 m. (2) GEOTECHNICAL SLOPE STABILITY A slope is geotechnically stable if it does not physically collapse. The Factor of Safety is a measure of the confidence that collapse will not occur. (3) SITE CLASSIFICATION Any mine operation will contain several types of slope, eg. cut pit slopes and waste dumps slopes. The first step in ensuring stable 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 1.1 SLOPE TYPE CLASSIFICATION Slope Type Description C1 Cut slopes above areas which will be open to access by the public and by stock. C2 Cut slopes above areas where access is prohibited. 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 be permitted. This approach needs to incorporate barriers above and behind the cut slopes to physically prevent access to the potentially unstable pit edge zone. More severe restrictions are required for cut slopes which remain above areas freely accessible 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 supported by 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 dipping steeply out of the face), and along which the overlying rock could slide, then the slopes should be assessed on a case by case basis. Unstable slopes will require battering back to a safe angle or made inaccessible and treated as if a Type C2 Slope. 6 Geotechnical Slope Stability January 1995
  8. 8. 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), the above maximum angles should be applied to each relevant material. These slope angles assume that no instability problems 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 of the “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 oxidised rock is available for construction of the safety bund wall, the least weathered or hardest material should be used. In these 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 bund wall. 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 repose of the dumped material. Coarse rock dumps will have side slopes steeper than dumps of finer grained and clayey materials such as soil and (oxidised) weathered rock. In many situations, desired end landform and other environmental requirements will determine the final slope angle. Such requirements 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 stockpile material within which the bulk of the dump or stockpile can be end tipped or placed in the usual mine operation method. 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 foundation soil and rock materials. Sites that are steeper or have weak foundation conditions should be individually assessed by geotechnical investigation. (b) Alternatively, end tipped and hence poorly compacted materials can be dozed and graded out to flatter slopes at the end of operations. As a general guide, final slopes should be no greater than half the angle of repose of the end tipped material as measured on-site. Since most materials have angles of repose of less than or equal to 1(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 individually assessed. 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 can become 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 sound engineering practice. As a general guide, slopes of 1(V):2.5(H) are commonly adopted for embankments up to about 15 m in height and which also incorporate internal drainage blankets. January 1995 Geotechnical Slope Stability 7
  9. 9. FIGURE 1.1 EXAMPLES OF PREFERRED SAFETY BUND WALL LOCATIONS (from Reference 2) 8 Geotechnical Slope Stability January 1995

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