Slopestability

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slope and dump stability, diffent slope failures, factora affecting slope stanbility, analysis of slope stability, monitoring of slopes, stabilisation of slopes

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Slopestability

  1. 1. SLOPE STABILITY AND DUMP STABILITY U.Siva Sankar Sr. Under Manager Project Planning Singareni Collieries Company Ltd E-Mail :ulimella@gmail.com or uss_7@yahoo.com Visit at: www.slideshare.net/sankarsulimellaIntroduction Slope stability Analysis MethodsTypes of slope failure Stabilizing methodsFactors Affecting Slope Stability Monitoring and instrumentation 1
  2. 2. Slope Stability Introduction Introduction: Slopes either occur naturally or are engineered by humans An understanding of geology, hydrology, and soil properties is central to applying slope stability principles properly. Analyses must be based upon a model that accurately represents site sub surface conditions, ground behavior, and applied loads. Time of Analysis Safe and economic design of excavations, embankments, earth dams, landfills, and spoil heaps .Slope Stability IntroductionSlope stability problem is greatest problem faced by the open pitmining industry. The scale of slope stability problem is divided in totwo types:Gross stability problem:It refer to large volumes of materials which come down the slopesdue to large rotational type of shear failure and it involves deeplyweathered rock and soil.Local stability problem:This problem which refers to much smaller volume of material andthese type of failure effect one or two benches at a time due to shearplane jointing, slope erosion due to surface drainage. 2
  3. 3. Slope Stability IntroductionAim of slope stability: To understand the development and form of natural and man made slopes and the processes responsible for different features. To assess the stability of slopes under short-term (often during construction) and long-term conditions. To assess the possibility of slope failure involving natural or existing engineered slopes. To analyze slope stability and to understand failure mechanisms and the influence of environmental factors. To enable the redesign of failed slopes and the planning and design of preventive and remedial measures, where necessary. To study the effect of seismic loadings on slopes and embankments. Aim of slope stability: Safe, properly designed, scientifically engineered slope. Profitability of open cast mines. Design engineer/ scientist •Excessive steepening: Slope failure Loss of production, extra stripping costs to remove failed material, DGMS may close the mine 3
  4. 4. TYPES OF ROCK SLOPE FAILURES Failure in Earth and Rock mass Plane Failure Wedge Failure Circular Failure Toppling Failure Rock fall Failure in Earth, rock fill and spoil dumps and Embankments Circular Non-circular semi-infinite slope Multiple block plane wedge Log spiral (bearing capacity of foundations) Flow slides and Mud flow Cracking Gulling Erosion Slide or SlumpFigure. Simplified illustrations of most common slope failure modes. 4
  5. 5. Fig. Failure mechanisms for the sliding failure mode (After Brown,1994): a) single block with single plane; b) single block with stepped planes; c) multiple blocks with multiple planes; d) single wedge with two intersecting planes; e) single wedge with multiple intersecting planes; f) multiple wedges with multiple intersecting planes; and g) single block with circular slip pathPlane FailureSimple plane failure is the easiest form of rock slope failure to analyze. It occurswhen a discontinuity striking approximately parallel to the slope face anddipping at a lower angle intersects the slope face, enabling the material abovethe discontinuity to slide. 5
  6. 6. Plane Failure Geometrical Conditions for sliding on single Plane failure: The plane on which sliding occurs must strike parallel or nearly parallel (±200) to the slope face The failure plane must “daylight” in the slope. This means its dip must be smaller than the dip of the slope face The dip of the failure plane must be greater than angle of internal friction Release surfaces which provide negligible resistance to sliding must be present in the rockmass to define the lateral boundaries of the slide. Alternatively, failure can occur on a failure plane passing through the convex “nose” of a slope.Wedge failureWedge failure can occur in rock masses with two or more sets ofdiscontinuities whose lines of intersection are approximatelyperpendicular to the strike of the slope and dip toward the plane of theslope. 6
  7. 7. Toppling FailureToppling failures occur when columns of rock, formed by steeplydipping discontinuities in the rock structure and it involves overturningor rotation of rock layers Circular Failure Circular failures are generally occur in weak rock or soil slopes. Failures of this type do not necessarily occur along a purely circular arc, some form of curved failure surface is normally apparent. Circular shear failures are influenced by the size and mechanical properties of the particles in the soil or rock mass. Fig: Circular Failure types 7
  8. 8. Types of circular failureCircular failure is classified in three types depending on the area that is affected by the failure surface. They are:- Slope failure: In this type of failure, the arc of the rupture surface meets the slope above the toe of the slope. This happens when the slope angle is very high and the soil close to the toe posses the high strength. Toe failure: In this type of failure, the arc of the rupture surface meets the slope at the toe. Base failure: In this type of failure, the arc of the failure passes below the toe and in to base of the slope. This happens when the slope angle is low and the soil below the base is softer and more plastic than the soil above the base. Rock FallIn rock falls, a mass of any size is detached from a steep slope or cliff,along a surface on which little or no shear displacement takes place, anddescends mostly through the air by free fall, leaping, bouncing, or rolling 8
  9. 9. Cracking It is due to differential settlement of the mine waste and suction level, exceeding the tensile strength, is reached. Due to further drying, or in subsequent dry periods, cracks can grow until finally, the complete thickness of the sealing layer is penetrated Gulling The gulling was observed in many dumps and it is quite dominant erosion mechanism. Gullies involve incision to depths often well in excess of a metre, and remove large quantities of soil 9
  10. 10. Gully formation Formation of gullies due heavy rain water flowSlide or Slump Shallow failures involving slumping of saturated or partially saturated dump materials. Concentrated surface flows discharging over the dump crest. Slides, either in rock or soil, will have rotational or translational movement. The sliding of material along a curved surface called a rotational slide or slump. A common cause of slumping is erosion at the base of a slope 10
  11. 11. Extensive soil erosionLong term impacts of river Ber m a lo H ig ng th e hes unb t f lo rok o d le en a ve l in rea mon so on 11
  12. 12. Weathering 12
  13. 13. A First Incident Begins. A 170 Ton capacity rear dump truck flees the effect of some oncoming miscalculation The Coal face has begun to fallHere it is cargo that is moving transport equipment! 13
  14. 14. There is no escape from this slide of the coal benchesSlope Stability Factor affecting slope stability FACTORS AFFECTING SLOPE STABILITY Geological discontinuities of Rock Mass Geotechnical Properties of slope Groundwater and Rainfall (Force Due To Seepage of Water ) Geometry of slope (Gravitational Force ) State of stress Erosion of the Surface of the Slopes due To Flowing Water Seismic effect (Forces Due To Earthquakes ) Dynamic Forces due to Blasting and HEMM Movement Slope modification, Under cutting Temperature and Spontaneous Heating Presence of UG galleries 14
  15. 15. Slope Stability Factor affecting slope stabilityGeological discontinuities of Rock Mass Joints Bedding Joints Joint spacing Joint direction and dipping FaultsFig: Idealized diagramshowing transition fromintact rock to jointed rockmass with increasing samplesize Factors Affecting Slope StabilityGeological Structure: The main geological structure which affect the stability of the slopes in the open pit mines are: amount and direction of dip intra-formational shear zones joints and discontinuities Reduce shear strength Change permeability Act as sub surface drain Plains of failure faults weathering and alternation along the faults act as ground water conduits provides a probable plane of failure 15
  16. 16. Spacing, Persistence, ApertureSlope Stability Factor affecting slope stability Geotechnical Properties of slope Shear strength of rock mass Cohesion (C) Angle of Internal friction (Ø) Density Permeability Moisture Content Particle size distribution Angle of Repose “Angle of repose” is the angle of steepest slope at which material will remain stable when loosely piled; 16
  17. 17. Factors Affecting Slope Stability• Cohesion : It is the characteristic property of a rock or soil that measures how well it resists being deformed or broken by forces such as gravity. In soils/rocks true cohesion is caused by electrostatic forces in stiff over consolidated clays, cementing by Fe2O3, CaCO3, NaCl, etc and root cohesion. However the apparent cohesion is caused by negative capillary pressure and pore pressure response during undrained loading. Slopes having rocks/soils with less cohesion tend to be less stable• Angle of Internal Friction: Angle of internal friction is the angle (Ø), measured between the normal force (N) and resultant force (R), that is attained when failure just occurs in response to a shearing stress (S). Its tangent (S/N) is the coefficient of sliding friction. It is a measure of the ability of a unit of rock or soil to withstand a shear stress. This is affected by particle roundness and particle size. Lower roundness or larger median particle size results in larger friction angle. It is also affected by quartz content. Factors Affecting Slope Stability Lithology • The rock materials forming a pit slope determines the rock mass strength modified by discontinuities, faulting, folding, old workings and weathering. • Low rock mass strength is characterized by circular raveling and rock fall instability like the formation of slope in massive sandstone restrict stability. • Pit slopes having alluvium or weathered rocks at the surface have low shearing strength and the strength gets further reduced if water seepage takes place through them. These types of slopes must be flatter. Ground Water • It causes the following: • alters the cohesion and frictional parameters and • reduce the normal effective stress • Ground water causes increased up thrust and driving water forces and has adverse effect on the stability of the slopes. Physical and chemical effect of pure water pressure in joints filling material can thus alter the cohesion and friction of the discontinuity surface. • Physical and the chemical effect of the water pressure in the pores of the rock cause a decrease in the compressive strength particularly where confining stress has been reduced. 17
  18. 18. Groundwater and Rainfall Water in Crack Presence of water – Flow of water - Not a big problem. Water flow checked – water storage- hydro. pressureGroundwater and Rainfall : Water in pores 18
  19. 19. Slope Geometry: The basic geometrical slope design parameters are height, overall slope angle and area of failure surface. With increase in height the slope stability decreases. The overall angle increases the possible extent of the development of the any failure to the rear of the crests increases and it should be considered so that the ground deformation at the mine peripheral area can be avoided. Generally overall slope angle of 45° is considered to be safe by Directorate General of Mines Safety (DGMS). Steeper and higher the height of slope less is the stability. Fig: Typical Pit slope Geometry Figure: Typical slope failure and relationships between critical slope heights and slope angles 19
  20. 20. Figure: Typical slope failure and relationships between critical slope heights and slope angles Factors Affecting Slope StabilityMining Method and Equipment Generally there are four methods of advance in open cast mines. They are: strike cut- advancing down the dip strike cut- advancing up the dip dip cut- along the strike open pit working • The use of dip cuts with advance on the strike reduces the length and time that a face is exposed during excavation. Dip cuts with advance oblique to strike may often used to reduce the strata • Dip cut generally offer the most stable method of working but suffer from restricted production potential. • Open pit method are used in steeply dipping seams, due to the increased slope height are more prone to large slab/buckling modes of failure. • Mining equipment which piles on the benches of the open pit mine gives rise to the increase in surcharge which in turn increases the force which tends to pull the slope face downward and thus instability occurs. Cases of circular failure in spoil dumps are more pronounced. 20
  21. 21. Slope Stability Factor affecting slope stability State of stressIn some locations, high in-situ stresses may be present within therock mass. High horizontal stresses acting roughly perpendicular toa cut slope may cause blocks to move outward due to the stressrelief provided by the cut. High horizontal stresses may also causespalling of the surface of a cut slope. Slope Stability Factor affecting slope stability Erosion Two aspects of erosion need to be considered. The first is large scale erosion, such as river erosion at the base of a cliff. The second is relatively localized erosion caused by groundwater or surface runoff. 21
  22. 22. Seismic effect Seismic waves passing through rock adds stress which could cause fracturing. Friction is reduced in unconsolidated masses as they are jarred apart. Liquefaction may be induced. One of the major hazards of earthquakes is the threat of landslides. This is particularly so because the most unstable parts of the earth are at the plate boundaries and it is also here that young fold mountain belts are formed and there are high relief and steep slopes Most open pit operators are familiar back break form blast, but most people only consider the visible breakage behind the row of holes of the blast.Dynamic Forces Blasting has a significant influence upon stability of slopes. Uncontrolled blasting- over breaks, overhangs and extension of tension cracks. Opening & loss of cohesion between weak planes. shattering of slope mass and allowing easier infiltration of surface water unfavourable ground-water pressures. Due to effect of blasting and vibration, shear stresses are momentarily increased and as result dynamic acceleration of material and thus increases the stability problem in the slope face. It causes the ground motion and fracturing of rocks. 22
  23. 23. Slope Modification –Modification of a slope either by humans or by natural causes can result inchanging the slope angle so that it is no longer at the angle of repose. A mass-wasting event can then restore the slope to its angle of repose. Undercutting - streams eroding their banks or surf action along a coast can undercut a slope making it unstable. 23
  24. 24. What do you do with a burning Coal face?Coal Face on fire 24
  25. 25. Dynamite was used toloosen the Coal forcollection by apowerful electricShovels.But heat from theexplosion & anexposed Coal seamcan sometimes be abad combination.Fire erupts from theCoal face! Fig. Plot of slope displacement versus time for prediction of failure. A. Plot of fastest moving point in the slope. B. Plot of slowest moving point in the slope. C. Prediction of slope failure date based on existing data (extrapolation). D. Predicted and actual date of failure. 25
  26. 26. DGMS Guidelines for Benches or slopes designManual or Conventional Opencast MinesIn alluvial soil, morum, gravel, clay, debris or other similar ground – the sides shall be sloped at an angle of safety not exceeding 45 degrees from the horizontal or such other angle as permitted by Regional Inspector of mines the sides shall be kept benched and the height of any bench shall not exceed 1.5 m and the breadth thereof shall not be less than the height: In coal, the sides shall either be kept sloped at an angle of safety not exceeding 45 degree from the horizontal, or the sides shall be kept benched and the height of any bench shall not exceed 3m and the width thereof shall not be less than the height. In an excavation in any hard and compact ground or in prospecting trenches or pits, the sides shall be adequately benched, sloped or secured so as to prevent danger from fall of sides. However the height of the bench shall not exceed 6 m. No person shall undercut any face or side or cause or permit such undercutting as to cause any overhanging. DGMS Guidelines for Benches or slopes design Mechanized opencast working.- Before starting a mechanized opencast working, design of the pit, including method of working and ultimate pit slope shall be planned and designed as determined by a scientific study. The height of the benches in overburden consisting of alluvium or other soft soil shall not exceed 5 m and the width thereof shall not be less than three times the height of the bench The height of the benches in overburden of other rock formation shall not be more than the designed reach of the excavation machine in use for digging, excavation or removal. The width of any bench shall not be less than – (a) the width of the widest machine plying on the bench plus 2m, or (b) if dumpers ply on the bench, three times the width of the dumper, or (c) the height of the bench, whichever is more. 26
  27. 27. DGMS Guidelines for Formation of Spoil Banks and Dumps(1) While removing overburden, the top soil shall be stacked at a separate place, so that, the same is used to cover the reclaimed area.(2) The slope of a spoil bank shall be determined by the natural angle of repose of the material being deposited, but shall in no case exceed 37.5 degrees from the horizontal. The spoil bank shall not be retained by artificial means at an angle in excess of natural angle of repose or 37.5 degrees whichever is less.(3) Loose overburden and other such material from opencast workings or other rejects from washeries or from other source shall be dumped in such a manner that there is no possibility of dumped material sliding.(4) Any spoil bank exceeding 30m in height shall be benched so that no bench exceeds 30m in height and the overall slope shall not exceed 1 vertical to 1.5 horizontal.(5) The toe of a spoil-bank shall not be extended to any point within 45m of a mine opening, railway or other public works, public road or building or other permanent structure not belonging to the owner. 27
  28. 28. Methods for Slope Stability Analysis Limit equilibrium - Analytical (software), Chart methods Kinematic analysis, To determine the types of above mentioned failure. Sensitivity analysis Classification method –SMR Probabilistic method, and Numerical modelling method. Stability Analysis of Mine SlopesLimit equilibrium method, It is the most widely accepted and commonly performed design tool in slope engineering Sliding occurs when a limit equilibrium condition is reached, i.e., when the resisting forces balance the driving forces. These methods are the most widely accepted and commonly used design methods and they permit a quantification of slope performance with the variations in all the parameters involved in the slope design. The basic idea behind the limit equilibrium approach is to find a state of stress along the critical surface so that the free body, within the slip surface and the free ground surface, is in static equilibrium. This state of stress is known as the mobilized stress, which may not be necessarily the actual state along this surface. This state of stress is then compared with the available strength, i.e. the stress necessary to cause failure along the slip surface. 28
  29. 29. To represent the slope performance other than the equilibriumcondition, it is necessary to have an index and the widely used indexused to be factor of safety.Factor of safety is calculated as the ratio of shear strength to theavailable shear stress required for equilibrium, integrated through thewhole slide.It is constant throughout the potentially sliding mass. Due to scatter oftest results and the uncertainty of these input parameters, a factor ofsafety greater than one is necessary to ensure an acceptably lowchance of failure. Guidelines for the Equilibrium of a Slope Plane Sliding – Stability AnalysisFig. Effect of ground water on rock slope (source: Abramson, 1995) 29
  30. 30. Slope Stability Stability Analysis of SlopePlanar failure Analysis With no tension crack and no water pressure Block A R ShearStrength Factor of safety = ShearStress W sinθ W cosθ W c + σ tan φ Factor of safety = τs w sin(θ ) Normal Stress; σ = A w cos(θ ) Shear Stress , τ= A w cos θ c+ tan φ A cA + w cos θ tan φ Factor of safety = w sin θ = w sin θ ASlope Stability Stability Analysis of Slope Tension crack present in upper slope surface Depth of tension crack; Z = H + b tan α c − (b + H cot α ) tan θ Weight of unstable block; W = 2 (H cot αX + bHX + bZ ) ) 1 2 X = (1 − tan θ cot α ) Area of failure surface; A = ( H cot α + b) sec θ 1 Driving water force; V= γ wZ w 2 2 1 Uplift water force; U= γ wZw A 2 30
  31. 31. Slope Stability Stability Analysis of Slope Tension crack present in slope face Depth of tension crack; Z = ( H cot α − b)(tan α − tan θ ) 1  2  Z 2  Weight of unstable block; w = γH 1 −  cot θ (cot θ tan α − 1) 2   H   Area of failure surface; A = ( H cot α c − b) sec θ 1 Driving water force; V= γ wZw 2 2 1 Uplift water force; U= γ wZw A 2 cA + ( w cos θ − U − V sin θ + T cos β ) tan φ Factor of safety = W sin θ + V cos θ − T sin βSlope Stability Numerical Circular Failure Analysis W 31
  32. 32. Slope Stability Numerical Circular Failure Analysis FOS = c+σ tan φ τsSlope Stability Stability Analysis of Slope Circular Failure Analysis FOS = c+σtan φ τ = c + σ tan φ τs w cos θ c+ tan φ Wn ∆L c∆L + w cos θ tan φFOS = = w sin θ w sin θ ∆L n= p ∑ [c∆L n =1 n + Wn cos α n tan φ ] FOS = n= p ∑ [W n =1 n sin α n ] 32
  33. 33. Software based on Limit equilibrium Method SLIDE (rocscience group) GALENA GEO-SLOPE GEO5 GGU SOILVISIONOverview of GALENA 33
  34. 34. Software for water pressure simulation HYDRUAS GEOSLOPE/ SEEP (GEOSTUDIO) SOILVISION /Water GMS FEFLOWSoftware based on Numerical modeling PHASES2 PLAXIS FLAC-SLOPE / UDEC / PPF ANSYS FEFLOW GEOSLOPE/SIGMA SOIL-VISION 34
  35. 35. Kinematic AnalysisThe average orientations of the discontinuity sets determined from thegeotechnical mapping were analysed to assess kinematically possiblefailure modes involving structural discontinuities Slope Unfavourable Slope favourable Kinematic Analysis to know Type of Failure 35
  36. 36. Sensitivity analysis The sensitivity analysis was done with an aim to know the influence of water on the factor of safety. This study is highly beneficial to choose the best method of remedial measure for any critical slope. The influence of groundwater on factor of safety is remarkable. The stability analyses of highwall slope have been conducted in undrained geo-mining condition also It is evident that the highwall slopes are stable in drained condi-tion with cut-off safety factor of 1.3 is unstable, if the slopes are subjected to undrained condition with safety factor less than 1.3. In order to avoid undrained condition, attention must be paid to avoid entry of rain/ surface water in the slope by providing suitable drainage in and around the quarry, failing which the slope can become unstable. It should be taken up well before the onset of monsoon. Slope Mass Rating (SMR) 36
  37. 37. Adjustment rating of F1, F2, F3 and F4 for joints Classification of Rock Slope according to SMT 37
  38. 38. Slope Stability Stabilization TechniquesSTABILIZATION OF SLOPE Drainage System Stabilization through Support Rock Mass Improvement and Stabilization MethodsDrainage System Surface drainage Subsurface Drainage Fig: Slope Drainage and depressurization methods Slope Stability Stabilization TechniquesSurface Drainage Systems: Surface drains and landscape design are used to direct wateraway from the head and toe of cut slopes and potential landslides, and to reduceinfiltration and erosion in and along a potentially unstable massSub-Surface: The main functions of subdrains are to remove subsurface water directlyfrom an unstable slope, to redirect adjacent groundwater sources away from the subjectproperty and to reduce hydrostatic pressures beneath and adjacent to engineeredstructures Objective Decrease water pressure Effective garland drain, directed away from excavated pit. Proper and effective drainage 5 to 10 deg. increase in slope angle 95% slide triggered by poor water management. 38
  39. 39. Slope Stability Stabilization Techniques Stabilization through Support • Ground Inclusions Ground anchor Soil Nails Rock Bolt Ground inclusion: It is a metal bar that is driven or drilled into competent bedrock (rock which is not highly fractured or broken up) to a provide stable foundation for structures such as retaining walls and piles, or to hold together highly fractured or jointed rock.Slope Stability Stabilization Techniques Stabilization through Support Piles• Piles are long, relatively slender columns positioned vertically in the ground or at an angle (battered) used to transfer load to a more stable substratum.• Piles are often used to support or stabilize structures built in geologically unstable areas.• Piles used as foundation for structures constructed on compressible soil or weak soil.• Grouped piles used as a retaining wall: Anchors are generally used to increase the effectiveness of pile walls 39
  40. 40. Slope Stability Stabilization Techniques Stabilization through Support • Retaining Walls Engineered structures constructed to resist lateral forces imposed by soil movement and water pressure Retaining walls are commonly used in combination with fill slopes to reduce the extent of a slope to allow a road to be widened and to create additional space around buildings Slope Stability Stabilization Techniques Rock Mass Improvement and Stabilization Methods Geosynthetics Grouting Chemical Stabilization Biological Stabilization 40
  41. 41. Slope Stability Stabilization Techniques Rock Mass Improvement and Stabilization Methods Geosynthetics are porous, flexible, man-made fabrics which act to reinforce and increase the stability of structures such as earth fills, and thereby allow steeper cut slopes and less grading in hillside terrain. Geosynthetics of various tensile strengths are used for a variety of stability problems, with a common use being reinforcement of unpaved roads constructed on weak soils. Grout is a cement or silicate based slurry, fluid enough to be poured or injected into soil and thereby fill, seal, or compact the surrounding soil. Grouting is the pressure injection of this slurry through drilled holes into fissured, jointed, permeable rocks and compressible soils to reduce their permeability and increase their strength.Slope Stability Stabilization Techniques Rock Mass Improvement and Stabilization Methods Chemical stabilization is a soil improvement method that increases the load bearing capability by mixing the soil with powders, slurry, or chemicals. Stability is developed in a number of ways; for example, the admixtures can fill soil voids, bond together individual grains, change the permeability of the soil Biological Stabilization 41
  42. 42. Dump Slope StabilityControlled placement of spoil Impermeable material increases water pressure. weak top layer – swelling minerals, base of the dump – permeable material. Improving drainage at the base of the dumps, •Blasting/ ripping of the floor, • Garland drain/ bund near toe of dump, • all along the periphery of dump edges, •5 m away from the toe of the dump – toe cutting. Dump Slope Stability Proper spoil levelling To check rainwater ponding at top, Dumping in depressed zone, Liquefaction of dump toe, Planting of self-sustaining grass and plants to check the soil erosion, to avoid the formation of deep gullies, form terraces, 1 m wide at the height of each about 6m. Rejection dump – near crest of slope – dead wt. on slope No unplanned dump – Near the crest. 42
  43. 43. Factor of safety 1.25 Stability analysis of active mine slope without overlying dump Factor of safety 1.1Stability analysis of active mine slope with overlying dump 43
  44. 44. Slope MonitoringObjective & why desiredIf detected in the early stage and later stage.Techniques Instrumentation Photogramammetric GPS Satellite imageries Survey based techniques Most widely used, Precision, Repeatability, Direct displacement. Slope Stability Slope Monitoring SLOPE MONITORING INSTRUMENTS Extensometers Time domain reflectometry (TDR) Inclinometers Piezometers Crack Meters Fig: slope with Extensometer Extensometers Borehole extensometers consists of tensioned rods anchored at different points in a borehole Changes in the distance between the anchor and the rod head provides the displacement information for the rock 44
  45. 45. Slope Stability Slope MonitoringTime domain reflectometry * lower installation costs * no limits on hole depth * immediate determination of movement * remote data acquisition capabilityIn TDR, a cable tester sends a voltage pulse waveform down a cable grouted ina borehole, If the pulse encounters a change in the characteristic impedance ofthe cable, it is reflected. This can be caused by a crimp, a kink, the presence ofwater, or a break in the cable. The cable tester compares the returned pulse withthe emitted pulse, and determines the reflection coefficient of the cable at thatpoint. The change in impedance with time corresponds qualitatively to the rateof ground movement.Slope Stability Slope Monitoring InclinometersMonitoring slopes and landslides to detect zones of movementMonitoring dams, dam abutments, and upstream slopes.Monitoring the effects of tunneling operations 45
  46. 46. Slope Stability Slope MonitoringPiezometers • Vibrating wire • Pneumatic • Standpipe piezometersSlope Stability Slope Monitoring Crack Meters Crack meters can be very useful tools in the early detection of deforming mass movements. These devices measure the displacement between two points on the surface that are exhibiting signs of separation. 46
  47. 47. Prism Monitoring based on survey techniquesPrisms are installed on the highwalls at a regular spacing, 50m horizontally and45m vertically, and on critical areas throughout the open pits. Surveyors collectand store data, while the rock engineers then analyse the data, looking forsignificant movement, and report any potential areas of slope failure to the miningpersonnel.Laser MonitoringMounted laser scanners will scan the entire pit walls by dividing them into zones.A camera is attached to the side of the laser and takes photographs at the start ofscanning. The data transmitted by laser scanner was downloaded to a computerand analysed using software. Radar Monitoring The GroundProbe slope stability Radar (SSR) uses differential interferometry to measure sub-millimetre movements on a rough rock face Digital photogrammetry SiroVision is a digital photogrammetry software program that enables safe and comprehensive mapping of dangerous and inaccessible highwalls, which are being captured in photographs with the use of high resolution digital camera. Seismic Monitoring Seismic monitoring aims to predict slope deformation by measuring micro seismic events caused by brittle movements within a rock slope. Analysis of micro seismic events using multiple tri axial geophones enables the location of source and therefore the discontinuity on which movement is occurring. 47
  48. 48. Slope Stability Slope Monitoring Monitoring by Observational Techniques : Total Station Total station instruments consist of a device to measure horizontal and vertical angles, and some form of Electromagnetic Distance Measurement (EDM) capability to measure distances. These instruments allow the surveyor to measure 3D coordinates of points remotelySlope Stability Slope Monitoring LASER - Remote controlled Monitoring 48
  49. 49. Slope Stability Radar Technology The Ground Probe SSR is a technique for monitoring open pit mine walls based on differential interferometry using radar waves. The system scans a region of the wall and compares the phase measurement in each region with the previous scan to determine the amount of movement of the slope. An advantage of radar over other slope monitoring techniques is that it provides full area coverage of a rock slope without the need for reflectors mounted on the rock face. The system offers sub-millimetre precision of wall movements without being adversely affected by rain, fog, dust, smoke, and haze. The system is housed in a self contained trailer that can be easily and quickly moved around the site. It can be placed in the excavation, or on top of a wall or on a bench to maximize slope coverage whilst not interfering with operations. The scan area is set using a digital camera image and can scan 320 degrees horizontally and 120 degrees vertically. The system provides immediate monitoring of slope movement without calibration and prior history. Scan times are typically every 1-10 minutes. Slope Stability Radar Technology Data is uploaded to the office via a dedicated radio link. Custom software enables the user to set movement thresholds to warn of unstable conditions. Data from the SSR is usually presented in two formats. Firstly, a colour “rainbow” plot of the slope representing total movement quickly enables the user to determine the extent of the failure and the area where the greatest movement is occurring. Secondly, time/displacement graphs can be selected at any locations to evaluate displacement rates. Additional software can also be installed to allow the data to be viewed live at locations remote to the SSR site such as corporate offices and at the offices of geotechnical consultants. 49
  50. 50. Fig: Slope Stability radarTypical problems, critical parameters, methods of analysis and acceptability criteria for slopes. 50
  51. 51. Thank You 51

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