Question

1. Using pictures and/or diagrams, describe and explain three (3) methods used to monitor pit slope stability. (20 points)

question contains 20 marks, so answer should be accurate and explained clearly(2pages without diagrams)

please type your answer, please do not write on the paper and post it

Answer 1

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 problem is greatest problem faced by the open pit mining industry. The scale of slope stability problem is divided in to two types: Gross stability problem: It refer to large volumes of materials which come down the slopes due to large rotational type of shear failure and it involves deeply weathered rock and soil. Local stability problem: This problem which refers to much smaller volume of material and these type of failure effect one or two benches at a time due to shear plane jointing, slope erosion due to surface drainage.

Aim 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.

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.

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 Slump.  

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 failure:

Wedge failure can occur in rock masses with two or more sets of discontinuities whose lines of intersection are approximately perpendicular to the strike of the slope and dip toward the plane of the slope.

Types of circular failure

Circular 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 Fall: In 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, and descends mostly through the air by free fall, leaping, bouncing, or rolling.

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.

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.

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.