center4533265FINAL YEAR PROJECT
00FINAL YEAR PROJECT
left2295525INFLUENCE OF SPATIAL DISTRIBUTION OF DISCONTINUITY ORIENTATION ON BENCH STABILITY IN JWANENG MINE
00INFLUENCE OF SPATIAL DISTRIBUTION OF DISCONTINUITY ORIENTATION ON BENCH STABILITY IN JWANENG MINE
40576507858125PREPARED BY: Vincent Tirelo G. Rachata 13000293Thabang Godwin Olefhile 13000817
00PREPARED BY: Vincent Tirelo G. Rachata 13000293Thabang Godwin Olefhile 13000817
1714503524250FACULTY OF ENGINEERING AND TECHNOLOGY
DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING
028000FACULTY OF ENGINEERING AND TECHNOLOGY
DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING
CHAPTER 1 : INTRODUCTION1.1 BACKGROUND 1.1.1 BACKGROUND OF COMPANY AND STUDY AREADebswana Jwaneng mine is a diamond mine located in south-central Botswana about 120 kilometers west of the capital city Gaborone as shown in REF _Ref461525581 h * MERGEFORMAT Figure 1. The mine is owned by Debswana which is a partnership between the government of Botswana and De Beers Company. It was discovered in 1972 by DeBeers geologists and became fully operational in August 1982.
Figure SEQ Figure * ARABIC 1: Google map showing location Jwaneng mine, Botswana
There are currently 3 main kimberlite pipes mined in Jwaneng mine located in the south,center and north part of the pit respectively . Two additional small kimberlite bodies have also been intersected within the pit.The strata dips between 10 and 40 towards the southwest. Currently the pit is 2.5km by 1.5 km and was approximately 600m deep by the time our attachment ended.The type of mine method employed is split shell mine ,i.e mining in cuts .Currently Jwaneng mine is mining Cut 6,7 and 8 , with ongoing feasibility study of mining Cut 9 .
1.1.2 STATEMENT OF THE PROBLEMTALK ABOUT CUT 8 BENCHES AND THEIR ROCK UNIT CHARACTERISTICS(JOINT ORIENTATION) AS THEY PERTAIN TO STABILITY GEO CONSIDERATIONS FOR THE ORIENTATIONS :JOINTS , BEDDING picture ya pitSlope stability point of view depends on frequency of joints , joint surface conditions and the strength of the rock mass. The degree of fracturing of a rock mass is controlled by the number of joints in a given direction thus their orientation determines the shape of individual blocks thereby controlling the likelihood of the type of failure likely to occur.1.2 AIMS OF THE PROJECT
To define sets of discontinuities which control stability on slopes in Cut 7 and Cut 8
To find out the probability for different types of structurally controlled slope failure
To determine slope failure mechanisms and their variations based on discontinuity orientation
1.3 METHODOLOGY 1.3.1 Extensive literature survey / review 1.3.2 Field work comprising of site investigation, discontinuity survey , geotechnical mapping 1.3.3 Data analysis : stereographic analysis (polar and equatorial , pole and contour plot ) and kinematic analysis ( sensitivity analysis ,slope failure mode assessment )
1.4 EXPECTED OUTCOMESThis research is intended to give a detailed study of the role discontinuities play in slope stability through kinematic analysis .Stability will be quantified by probability distribution of the difference between resisting and displacing forces. Results will be provided in terms of stereographic projections for the analysis of structural geology . These diagrams will show the type of block failure and will be used to examine the direction of slide giving an indication of stability conditions.
Furthermore contour heat maps showing each failure mode and overall failure mode mechanisms likely to be experienced in different sections of the pit will be provided. Thus this research serves a means of determining pit stability status and monitoring to enable safe work practices , economic mining , as well as risk assessment in Jwaneng mine .
FACILITIES TO BE USEDMaptek I-site studioDIPS and stereographic analysis softwareLaser scan data
CHAPTER 2 : GEOLOGY2.1 GEOLOGY OF JWANENG MINEThe Jwaneng area is underlain by the Transvaal supergroup which is covered by paleoproterozoic-aged sedimentary rocks. The lithological sequence is as follows, at the top is the Kalahari Group which comprises of sand and scree (alluvial material) on the upper part and calcrete or caliche which is cemented calcium carbonate surficial gravel. This formation is underlain by the Boshoek Formation that is composed of felsic volcanics and graphitic shale with sandstone intercalations, then follows the Time Ball Hill Formation with laminated sandstone and black shale on the upper part and laminated sandstone, mudstone, greywacke and black shale interbedded with thin, mafic tuff bands on the lower part. Underlying the Time Ball Hill Formation is the Rooihoogteor or Duitschland Formation on the upper part is fine grained to medium grained, poorly sorted, argillaceous quartzite as well as grey-wacke to sub-wacke with silty mudstone. Separating the upper and lower Duitschland formation is the chert pebble conglomerate consisting of angular to rounded chert clasts in grity matrix. On the lower part of this formation is chert pebble conglomerate (flat) well known as bevets conglomerate. At the base of this sequence is the Malmani dolomite recorded to be 15cm thick massive, polymitic diamictite unit (Beukes 2006). The stratigraphic representation below shows a cross-section along the E-E’ cross-sectional line, (Basson et al., 2015) illustrating different lithology.
In general the sequence goes as follows, Kalahari sand is at the very top of the stratigraphy followed by calcrete (Cal) which is whitish to reddish in colour which do not form any part of the formations then comes the laminated shale (LS) which is easily identifiable by the numerous bedding present. From there we have carboneous shale (CS) which is characterized by polished joint surfaces and their dark colour. Next is the quartzitic shale (QS) with chert pebble conglomerate or bevet (BVT) occurring between the QS. These bevets act as a marker horizon because of their persistence and because they result from glacial deposit environment. Carboneous shale (CS) then comes again with diamictite occurring within the unit itself. Dolomite then follows as the second last and then finally the basement rock which is igneous, mostly likely granite.
Stratigraphic Name Rock Type (Mine Rock Code) Typical Thickness (m)
Kalahari Sequence Sand and Calcrete (Cal) 55-60
Timeball Hill Formation Laminated Shale (LS) Residual
Lower Timeball Hill Formation Carbonaceous Shale (CS) 30
Rooighoogte Formation Quartzitic Shale (QS) 135
Chert Pebble Conglomerate Bevets (BVT) 0-4
Rooighoogte Formation Quartzitic Shale (QS) 375
Lower Rooighoogte formation Carbonaceous Shale 10
Malmani Subgroup Dolomite (DM) Residual
Table SEQ Table * ARABIC 1 Stratigraphic Column of the Jwaneng Mine (After SRK Country Rock) CITATION Tun10 l 1033 (Tunono, 2007)
Picture ya pit : identify units
CHAPTER 3: LITERATURE REVIEWMass movement processes of slopes are highly dependent on orientations of structural discontinuities within rock mass . Associated hazards are defined by the orientation of structures and associated mechanisms of slope failure i.e. planar , wedge , toppling : block and flexural. Typical rock mass with multiple weak surfaces or discontinuities may form a consistent pattern over a range of spatial scale .That is to say there are different discontinuity sets in a bench face thus the need to account for their variability and their respective orientations during slope design.
HOW DISCONTINUITY ORIENTATIONS AFFECT SLOPE STABILITYStability of rock slopes is significantly influenced by structural discontinuities in which the slope is excavated. CITATION Wyl04 l 1033 (C ; W, 2004) . Discontinuities are surfaces or planes that mark a change in physical or chemical characteristics of a rock mass , in the form of joints , bedding planes or faults.. They define the structural weakness plane where movement can take place and contribute effectively to displacement . As such , they control the type of failure which may occur in a rock slope , thus the need to understand them and the influence they have on the engineering behavior of rock mass.
Furthermore , discontinuities have a major role in mine slope design , as such dominant geological structures i.e joints , beddings are used for optimum pit design.The orientation of discontinuities refers to the dip and dip direction . The dip is the maximum inclination to the horizontal and its direction is the direction of the horizontal trace of the line measured clockwise from north . Geological Structures
There are various discontinuities of geological origin that can affect the stability of rock slopes.
Arising from the deposition of sediments in layers, are distinct physical discontinuities. They may occur at the interface between different rock types at various spacing within a single rock unit. They may be persistent and generally extend over greater areas than any other type of discontinuity. In some rock types, movements along bedding planes may have developed weakened shear zones. The nature and inclination of bedding is always of prime importance when considering slope stability in sedimentary rocks.
Joints are developed to some degree in most rocks. They are planar fractures formed to relieve stresses, across which there has been little or no movement. Jointing plays some part in the majority of slope failures in rock masses since intact rock is generally stronger than the discontinuities.
Faults occur less frequently than joints and may have undergone substantial displacements. Faulting often produces continuous or persistent planes of weakness. Fault zones may develop in which the fault is not a single clean break, but occurs as a series of displacement surfaces in an area of distorted, crushed and often weathered material (termed ‘gouge’). Faulting can occur in any rock type. Faults can provide the shearing or release surfaces for several modes of failure.4.Unconformities
Unconformities are surfaces representing breaks in the sedimentary process. Such breaks are only structurally significant where some erosion or tilting of rocks has occurred before the deposition of overlying material (an angular unconformity). Angular unconformities typically occur over a wide area. The surface is often irregular with sudden changes in inclination. An unconformity typically marks a change in rock properties. Where the old weathered zone has been preserved, this may also constitute a zone of weakness.
5.Slip planes and tension cracks
These may result from ancient or recent ground movements and can significantly affect stability, particularly if loaded or undercut by mining. (Sarma, 1973).
The importance of these features when preparing mine designs depends on:
· Orientation with respect to both the slope and to other discontinuities within the slope;
· Resistance to movement along the surfaces or planes;
· Persistence and spacing of the features; and
· The ease with which water can penetrate, accumulate or flow along them.In most rock masses (except where discontinuities are widely spaced or impersistent), intact material strength usually exceeds the strength of discontinuities and hence geological structure is the dominant influence on slope stability and design. Certain critical information regarding the rock mass must be recorded and available to the geotechnical engineer and mine designer in assessing slope stability and its influence on mine development.
The following should be considered :5.1 Discontinuity features
The ease with which displacement can occur along a discontinuity is affected by its openness, infilling, roughness and continuity. Openness is the separation between the faces of the intact rock blocks. In many cases where the separation is large, the void will have been infilled. Sliding resistance along a discontinuity may be increased due to mineralization forming the infilling, or decreased where the infilling includes clay materials. Surface roughness includes irregularities on the discontinuity which may effectively increase the overall resistance to sliding. This resistance is decreased when the discontinuity is open. If wall to wall contact in the discontinuity is lost, the shear strength of the discontinuity will be that of the infilling material. Where persistence or continuity of a single feature is high (e.g. bedding planes, faults, shear zones and master joints), there may be potential for large movements.
Rocks may be broken down by the combined effects of temperature, air, water and associated chemical activity. Wetting and drying are essentially physical effects and may only affect the outer margins of exposed rock slopes (sometimes leading to rockfall). Chemical weathering is a generally slower process, but has the potential to significantly affect rock mass strength. It often has no effect during the working life of the mine, but may be a factor affecting the long term stability of slopes on abandonment. Weathered rock may commonly be present prior to excavation.
5.3 LithologyThe mechanical properties of intact rock depend on the physical properties of the constituent minerals and their bonding to one another. In some circumstances, petrographic analysis may identify the presence of minerals that may influence stability (including unstable weathering products or soluble cements). Unstable minerals can lead to rapid weathering of exposed rock and reduce its intact strength.5.4 Critical geometry
The location and orientation of discontinuities must be determined. These items often dictate the position of potentially unstable parts of an excavation. Combined with the slope geometry, the dip and dip direction of discontinuities frequently govern the style and extent of potential instabilities.
5.5 Moisture content
In addition to the effects of pore water pressure, many rock materials may be weakened when water is present and the materials are saturated. Strength reduction varies with rock type (e.g. granite and sandstone are subject only to minor strength reductions, but clay rich rocks can be much weakened).3.1 SLOPE FAILURE MECHANISMSGenerally ,there are four mechanisms of slope failure being :
According to the Markland’s test, a plane failure is likely to occur when a discontinuity dips in the same direction (within 200) as the slope face, at an angle gentler than the slope angle but greater than the friction angle along the failure plane (Hoek and Bray, 1981).
A wedge failure may occur when the line of intersection of two discontinuities, forming the wedge-shaped block, plunges in the same direction as the slope face and the plunge angle is less than the slope angle but greater than the friction angle along the planes of failure (Hoek and Bray, 1981) . It is particularly common in the individual bench scale but can also provide the failure mechanism for a large slope where structures are very continuous and extensive.
A toppling failure may result when a steeply dipping discontinuity is parallel to the slope face (within 300) and dips into it (Hoek and Bray, 1981). According to Goodman (1989), a toppling failure involves inter-layer slip movement. The requirement for the occurrence of a toppling failure according to Goodman (1989) is “If layers have an angle of friction ?j, slip will occur only if the direction of the applied compression makes an angle greater than the friction angle with the normal to the layers. Thus, a pre-condition for interlayer slip is that the normals be inclined less steeply than a line inclined ?j above the plane of the slope. If the dip of the layers is ?, then toppling failure with a slope inclined ? degrees with the horizontal can occur if (90 – ?) + ?j < ?”.
A circular failure can occur in soil slopes, the circular method occurs when the joint sets are not very well defined. When the material of the spoil dump slopes are weak such as soil, heavily jointed or broken rock mass, the failure is defined by a single discontinuity surface but will tend to follow a circular path.(Hoek et al, 1994) .
3.2 GENERAL SLOPE CONFIGURATIONIn open-pit design to ensure better stability, consideration should be given to the dip of the overall pit wall. Better stabilization is obtained if the faces are so cut that the strata are inclined away from the overall pit wall dip. (Samui, 2008)As such , the following factors are considered:Optimum design of bench parametersAs per Metalliferous Mines Regulations (MMR) 1961, in open cast workings in alluvial soil, morum, gravel, clay, debris or other similar ground, the sides shall be sloped at an angle of safety not exceeding 45° from the horizontal. The sides shell be kept benched, and height of any bench shall not exceed 1.5 m and the breadth thereof shall not be less than the height.
Where “ore” or other similar deposit is worked by manual means on a sloping face, the face shall be benched and sides shall be sloped at an angle of not more than 60° from the horizontal. The height of any bench shall not exceed 6 m and the breadth thereof shall not be less than the height. In an excavation any hard and compact ground or in prospecting trenches and pits, the sides shall be adequately benched, sloped or secured so as to prevent danger from all of sides.(Gong et al., 2007).
Excess slope angle both in the benches and overall pit wall, poses danger of slide. In unconsolidated strata, if the slope angle exceeds the angle of repose, this leads to a dangerous position and causes slope failure.
Size of benches
The size of bench floor should be always more than the height of the bench. The floor width should be capable to accommodate the inadvertent broken rocks and mineral. During lay-out stage, if it is allowed to cut benches in the strata dipping in the same direction as the original dip- direction of the strata, the situation leads to slope instability, even if, a proper slope angle has been maintained. lf the bench slope is not designed properly complying a suitable gradient (say 5°) for automatic drainage of rain water by gravity, chances become conspicuous for slope-instability as the water accumulate in the bench floor especially in soft strata.
Height of benches
If the height of the faces exceeds a certain value i.e. the critical height, stability is reduced and favourable conditions for failure strata are created.Accomplishing stepping shape
The best method for achieving more stability is to design the benches in a stepped form along the strike direction of the ore body. Selection of favourable profiles and contours is of utmost important in ensuring overall pit-wall stability. In designing very large excavated slopes which are increasingly common in both civil and mining projects, the engineer is met with two conflicting requirements. On one hand, vast sums of money can be saved by steepening the slopes, thereby reducing the amount of material to be excavated. On the other hand, loss of life and serious damage to property can result from failure induced by excessive steepening of a particular slope. The engineer achieves an optimum design by a compromise between slopes which is flat enough to be safe, because the rock mass behind each slope is unique.Depth of working pit
When the depth of working- pit exceeds the critical value, the stability of the wall, forces of cohesion and the internal co-efficient of friction are reduced drastically and cause slope failure.Material Fails
If the failure is in a non-critical area of the pit, the easiest response may be to leave the material in place. Mining can continue at a controlled rate if the velocity of the failure is low and predictable and the mechanism of the failure is well understood. However, if there is any question about the subsequent stability, an effort should be made to remove the material. Large-scale failures can be difficult and costly to clean up. Often, a mining company will choose to leave a step-out in the mine design to contain the failed material and continue mining beneath the step-out. The value of the ore that is lost needs to be evaluated against the costs of clean-up to determine if this is a feasible solution. The size of the blasts may also need to be reduced to minimize impacts on the unstable zone.
To prevent small-scale failures from reaching the bottom of the pit, both the number of catch benches and the width of catch benches can be increased. Catch fences have also been installed at some operations to contain falling material.3.3 ROCK SLOPE STABILITY ANALYSIS3.3.1 LIMIT EQUILIBRIUM ANALYSIS
Limit equilibrium methods have been used for decades to safely design major geotechnical structures (Fredlund et al., 1977). Bishop’s simplified method, utilizing a circular arc slip surface, is probably the most popular limit equilibrium method (Lovine et al., 2011). Although Bishop’s method is not rigorous in a sense that it does not satisfy horizontal force limit equilibrium, it is simple to apply, and in many practical problems, it yields results close to rigorous limit equilibrium methods (Weia et al., 1998).
In order to carry out Limit Equilibrium Analyses, Rocsience Slide v.6.0 and Slope/W of GeoStudio 2012 are utilized.For the sake of consistency, only the strength parameters, namely cohesion and friction angle, are used as random variables. Other variables are kept at their mean values. It results in several slip surfaces and it can also generate the most critical failure surface.3.3.2 FINITE ELEMENT ANALYSISIn order to carry out a Finite Element Analysis, Rocscience Phase2 v.8.0 is utilized. After defining the slope geometry in the software, both deterministic and probabilistic analyses are conducted separately.
For the probabilistic analysis, Rosenblueth’s Point Estimate Method, which is the only probabilistic method available in version 8.0 of the software, is used.
The initial and boundary conditions are defined, and Mohr-Coulomb model is used for the constitutive model of the slope.For the probabilistic analysis, effective cohesion and effective internal friction angle of the slope are used as random variables. Other parameters are considered to be known with more certainty and used as deterministic input variables. The magnitude of Young’s Modulus and Poisson’s ratio do not affect the location of the critical failure surface and the value of the strength reduction factor (SRF) of the slope, since displacements and stresses are out of interest and location of the critical slip surface are determined by incremental shear stress values.
3.3.2 RANDOM SAMPLING3.3.3 KINEMATIC ANALYSISKinematic analysis, which is purely geometric, examines which modes of slope failure are possible in a jointed rock mass (Wright et al.Z, 1984). Angular relationships between discontinuities and slope surfaces are applied to determine the potential modes of failures (Yoon et al., 2002). Stereographic representation (stereonets) of the planes and lines is used. Stereonets are useful for analysing discontinuous rock blocks. Program DIPS (Visualization software) allows for visualization of structural data using stereonets, determination of the kinematic feasibility of rock mass and statistical analysis of the discontinuity properties (Zulfu et al., 2008).3.4 ACCEPTANCE CRITERIA
3.4.1 FACTOR OF SAFETYFactor of safety is describing the structural capacity of a system beyond the expected loads or actual loads. Essentially, how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated to know the structure’s ability to carry load using detailed analysis and it must be determined to a reasonable accuracy. The FOS is chosen as a ratio of the available shear strength to that required to keep the slope stable. For safer conditions, FOS should be greater than 1(Zhao et al.,2002). For highly unlikely loading conditions, factors of safety is as low as 1.2-1.25 (Zhang et al., 2004).3.4.2 PROBABILITY OF FAILUREREFERENCES