SLOPE STABILITY

           Key words  slope stability , causes, shearing strength

Introduction

Land  slides are the downward and outward  movements of  materials on the slope. The reason for this is the lack of required shear strength. The slope materials may be composed of natural rock and soil, artificial fills. Slides can occur in natural slopes as well as man- made structures and embankments.

Great care should be taken in the selection of good construction material and also suitable construction methods to avoid sliding of slopes.

Causes of slope failures

The causes of slope failures may be external or internal.

External: Steepening of the slopes, depositing of material along the edge of the slopes and earthquakes.

Internal: Decrease in shearing resistance due to excess pore water pressure, leaching of salts, softening and breakage of cementation bonds.

The other activities which may provoke land slides are: Rapid draw down, seepage of water, erosion, heavy rainfall, excavation at the toe of the slope, excavation at the head of the slope, blocking of the drainage.

Basic concepts of stability analysis

All sloping surfaces are subjected to shearing stresses .The shear available  should overcome the developed stresses. If the shearing stresses are more, then a surface of rupture will be formed at the verge of failure.

1. Finding the most severely stressed internal surface and the connected shearing stress along the surface   Finding the shearing strength along the surface

Instead of resorting to rigorous mathematical analysis, a reasonable shape of failure is assumed and then the stability of the soil above the assumed surface is analysed.

Rate of land movement

The movements associated with land slides can be grouped under four heads.

-  Creep, pre-failure movements, movements during slides, post failure movements.

Creep may range from 1 to  few millimeters per year. Heavily over consolidated clays experience post failure movements, the speed of movements ranging from 0 to 6 m per year.

Importance of the Factor of Safety on slope stability analysis.

It is a measure of limiting the stress or strain so as to ensure the safety of the structure. As such, the working stress in any earth structure is much less than the shear strength of the soil.

The Factor of Safety may be defined as the ratio of shearing resistance available (τf ) along a slip surface to the total mobilized or developed shearing stress.( τ ).

F= τf / τ

Both cohesion and friction contribute to the shearing strength

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