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SLOPE STABILITY AND FAILURE

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SHIBASOUMYA GHOSH
SHIBASOUMYA GHOSH

This document discusses slope stability and failure in open pit mines. It notes that as mining depths increase, slope design becomes more important for economic reasons. Slope stability problems can be either gross or local failures. Factors that affect stability include slope geometry, geology, groundwater, lithology, dynamic forces, and mining methods. Common failure types are planar, wedge, circular, and toppling. Slope stability is assessed using limit equilibrium methods or numerical modeling techniques. Numerical models divide the rock mass into zones to simulate complex slope behavior.

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SLOPE STABILITY AND FAILURE  PRESENTED BY- SHIBASOUMYA GHOSH NIT RAIPUR, MINING ROLL- 14121061
Importance of the subject • Open pit mining is a very cost-effective mining method allowing a high grade of mechanization and large production volumes. It is therefore possible to mine mineral deposits of a very low grade which could not be mined economically using underground methods. • Mining depths in open pits have increased steadily during the last decades, and open pits with mining depths of up to and exceeding 500 meters are not unusual. • Design of open pit slope angles is becoming more and more important as the mining depths of open pits continuously increase. Small changes in the overall pit slope angle have large consequences on the overall economy of the mining operation.
• The actual slope angles used in the mine depend upon (i) the presence of haulage roads, or ramps, necessary for the transportation of the blasted ore from the pit (ii)possible blast damage (iii) ore grades and (iv)economical constraints
SLOPE STABILITY It is one of the greatest problem which any open cast mine encounters. Scale of this problem is divided in: • Gross stability problem: It refers to the overall problem of stability of major parts of slope due to large shear failure and it generally occurs in deeply weathered rock. • Local stability problem: It refers to problem which is much lower in scale and it generally doesn’t affect more than a couple of benches at one time. It mainly occurs due to shear plane joints or slope erosion due to surface drainage.
FACTORS AFFECTING SLOPE STABILITY Following factors affect slope stability: • Slope Geometry • Geology and geological structure • Ground water • Lithology • Dynamic forces • Method of mining and equipment used • Angle of internal friction • Cohesion
Slope Geometry – Height, overall slope angle and area of failure surface are the elementary geometrical slope design parameters. Slope stability decreases sharply if the height of slope increases. The chances of development of failure increases on increasing the overall slope angle. Geological Structure – The main geological structure affecting the stability of the slopes are: • Amount and direction of the dip • Joints and discontinuities • Faults Lithology – The rock materials forming a pit slope determines the rock mass strength
Ground water – The presence of ground water causes following altercations is the strata: • It alters the friction and cohesion parameters. • It reduces the normal effective stress. Dynamic Forces- Due to vibration caused by blasting process shear stress increased, it maximizes the dynamic acceleration Of material which causes instability in the slope plane. Due to blast damage and back break, these factors also affect the failure in rock mass i.e. the bench face angle, vibrations from blasting.
TYPES OF ROCK SLOPE FAILURE – Following are the different types of failure that a slope can undergo: • Planar failure • Wedge failure • Circular failure • Toppling failure
Planar failure – Failure where a block of rock slide on a single plan dipping out of the face.
Conditions for plane failure – (a) The plane on which sliding occurs must strike parallel or nearly parallel (within approximately ±20◦) to the slope face. (b) The sliding plane must “daylight” in the slope face, which means that the dip of the plane must be less than the dip of the slope face, that is, ψp < ψf .
Wedge Failure – This failure happens when discontinuity plane striking obliquely to the slope face where sliding takes place along the line of intersection of two such planes.
Conditions – • The dip of the line of intersection is smaller than dip of slope face. • The two sets of discontinuities must intersect each other. • The sliding will take place along the dip of intersection of two discontinuities
Circular failure – The circular failure 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. Toppling failure- It occurs in slopes having near vertical joint sets, very often the stability depends on the stability of one or two key blocks, when they collapse, failure occurs. This type of failure involves rotation of blocks of rocks about some fixed base.
Toppling failure-
Slope Stability Assessment- Slope stability analysis is performed to assess the safe design of a human-made or natural slopes (e.g. embankments, road cuts, open-pit mining, excavations, landfills etc.) and the equilibrium conditions. Two major analytical approaches for stability analysis and design of open pit slope: 1. Limit Equilibrium Method 2. Numerical Modelling
Numerical modelling • These are computer programs that represent the mechanical response of a rock mass subjected to a set of initial conditions such as in situ stresses and water levels, boundary conditions. • Numerical models divide the rock mass into zones. Each zone is assigned a material model and properties • Numerical modelling techniques have been widely used to solve complex slope problems, which otherwise, could not have been possible using conventional techniques. These models are used to simulate rock slope as well soil slope with complex conditions.
Types of numerical modelling • Continuum modelling • Discontinuum modelling • Hybrid modelling
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SLOPE STABILITY AND FAILURE

  • 1. SLOPE STABILITY AND FAILURE  PRESENTED BY- SHIBASOUMYA GHOSH NIT RAIPUR, MINING ROLL- 14121061
  • 2. Importance of the subject • Open pit mining is a very cost-effective mining method allowing a high grade of mechanization and large production volumes. It is therefore possible to mine mineral deposits of a very low grade which could not be mined economically using underground methods. • Mining depths in open pits have increased steadily during the last decades, and open pits with mining depths of up to and exceeding 500 meters are not unusual. • Design of open pit slope angles is becoming more and more important as the mining depths of open pits continuously increase. Small changes in the overall pit slope angle have large consequences on the overall economy of the mining operation.
  • 3. • The actual slope angles used in the mine depend upon (i) the presence of haulage roads, or ramps, necessary for the transportation of the blasted ore from the pit (ii)possible blast damage (iii) ore grades and (iv)economical constraints
  • 4. SLOPE STABILITY It is one of the greatest problem which any open cast mine encounters. Scale of this problem is divided in: • Gross stability problem: It refers to the overall problem of stability of major parts of slope due to large shear failure and it generally occurs in deeply weathered rock. • Local stability problem: It refers to problem which is much lower in scale and it generally doesn’t affect more than a couple of benches at one time. It mainly occurs due to shear plane joints or slope erosion due to surface drainage.
  • 5. FACTORS AFFECTING SLOPE STABILITY Following factors affect slope stability: • Slope Geometry • Geology and geological structure • Ground water • Lithology • Dynamic forces • Method of mining and equipment used • Angle of internal friction • Cohesion
  • 6. Slope Geometry – Height, overall slope angle and area of failure surface are the elementary geometrical slope design parameters. Slope stability decreases sharply if the height of slope increases. The chances of development of failure increases on increasing the overall slope angle. Geological Structure – The main geological structure affecting the stability of the slopes are: • Amount and direction of the dip • Joints and discontinuities • Faults Lithology – The rock materials forming a pit slope determines the rock mass strength
  • 7. Ground water – The presence of ground water causes following altercations is the strata: • It alters the friction and cohesion parameters. • It reduces the normal effective stress. Dynamic Forces- Due to vibration caused by blasting process shear stress increased, it maximizes the dynamic acceleration Of material which causes instability in the slope plane. Due to blast damage and back break, these factors also affect the failure in rock mass i.e. the bench face angle, vibrations from blasting.
  • 8. TYPES OF ROCK SLOPE FAILURE – Following are the different types of failure that a slope can undergo: • Planar failure • Wedge failure • Circular failure • Toppling failure
  • 9. Planar failure – Failure where a block of rock slide on a single plan dipping out of the face.
  • 10. Conditions for plane failure – (a) The plane on which sliding occurs must strike parallel or nearly parallel (within approximately ±20◦) to the slope face. (b) The sliding plane must “daylight” in the slope face, which means that the dip of the plane must be less than the dip of the slope face, that is, ψp < ψf .
  • 11. Wedge Failure – This failure happens when discontinuity plane striking obliquely to the slope face where sliding takes place along the line of intersection of two such planes.
  • 12. Conditions – • The dip of the line of intersection is smaller than dip of slope face. • The two sets of discontinuities must intersect each other. • The sliding will take place along the dip of intersection of two discontinuities
  • 13. Circular failure – The circular failure 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. Toppling failure- It occurs in slopes having near vertical joint sets, very often the stability depends on the stability of one or two key blocks, when they collapse, failure occurs. This type of failure involves rotation of blocks of rocks about some fixed base.
  • 15. Slope Stability Assessment- Slope stability analysis is performed to assess the safe design of a human-made or natural slopes (e.g. embankments, road cuts, open-pit mining, excavations, landfills etc.) and the equilibrium conditions. Two major analytical approaches for stability analysis and design of open pit slope: 1. Limit Equilibrium Method 2. Numerical Modelling
  • 16. Numerical modelling • These are computer programs that represent the mechanical response of a rock mass subjected to a set of initial conditions such as in situ stresses and water levels, boundary conditions. • Numerical models divide the rock mass into zones. Each zone is assigned a material model and properties • Numerical modelling techniques have been widely used to solve complex slope problems, which otherwise, could not have been possible using conventional techniques. These models are used to simulate rock slope as well soil slope with complex conditions.
  • 17. Types of numerical modelling • Continuum modelling • Discontinuum modelling • Hybrid modelling