Question

1. Using pictures and/or diagrams describe and explain four (4) basic modes of rock failures and the factors contributing to pit slope instability. (12 points)

Please type your answer, Please do not write on the paper and post it (minimum 2 page)

Answer 1

1. Four basic modes of rock failure are as follows-

• Plane failure-

A rock undergoes this mode of failure by structural discontinuities like bedding planes, faults between weathered rock and wedges between the rock which are free to move . Plane failure leads to sliding action along the failure surface.

Favourable Conditions for plane failure are -

i. The plane on which sliding occurs must lie parallel (±20°) to slope face.

ii. The dip of the sliding plane must be greater than angle of internal friction.

iii. The dip of the sliding plane must be less than dip of slope face.

• Wedge Failure-

Wedge failure can occur in rock mass to slide along two intersecting discontinuities. Wedge failures can happen very fast in a matter of fraction or it can happen slowly and gradually in several months.

• Topping Failure-

Topping failure occurs when column of rocks steep discontinuities which eventually leads to slippage of the layers. It includes overturing of rock layers.This failure action causes due to the column’s center of gravity lying outside the base dimensions. This failure can be further classified depending upon the modes as-

i. Flexural

ii. Block

iii. Block Flexural.

• Circular Failure-

In circular slips shape of failure surface may be circular arc or non-circular curve, it may occur in weak rock or soil slope. Sliding of material along curve is called circular failure. This failure necessarily does not occur on circular arc, sometimes surface can be apparently normal.

Types of circular failure are-

i. Slope failure

ii. Base failure

iii. Toe failure.

2. Factors contributing to pit slope instability are as follows-

• Geotechnical Discontinuities-

The main geotechnical discontinuities affecting pit slope instability are fault, joint, amount and direction of dip , bedding plane of rock mass. Instability may occur if the strata dip into the excavations even if the clay band comes between two rocks.

• Geotechnical parameters -

The main geological parameters which affect the stability of the slopes in the open pit mines are shear strength of rock mass, cohesion, moisture content.grain size, moisture content, atterberg limit, etc

• Lithology and Ground Water -

Pit slopes having soil alluvium or weathered rocks ground water, drainage pattern, rainfall, permeability, aquifer, etc are the Lithology and Ground Water which also comes in consideration affecting pit slope instability.

• Slope Geometry -

Slope geometry is the important factor which affecting the slope instability. Factors which comes into consideration of slope geometry are height and angle of slope, bench height and angle, area of failure.

• Dynamic forces -

Dynamic forces affecting pit slope instability are Blasting, Seismic activity.

• Method of construction -

The method by which construction is carried out like Shovel, dumper, BWE or combination, can also affect the stability of the slopes in the open pit mines.

Summary- Four basic modes of rock failure are Plane failure, Wedge Failure, Topping Failure, Circular Failure.