Stability of Slopes, Chapter 16, VNS Murthy.

Hello readers, let's start this chapter 16, of VNS Murthy Book.

16.1  Introduction : Slopes of earth are of two types:

1. Natural slopes             2. Man made Slope 

• Natural slopes exist in hilly areas. 
• The sides of cuttings, the slopes of embankments constructed for roads, railway lines, canals etc. and the slopes of earth dams constructed for storing water are examples of man made slopes.

The slopes whether natural or artificial may be  : 

1. Infinite slopes                 2. Finite slopes 

• The term finite slope is used to designate a constant slope of infinite extent. 

Causes of Failure of Slopes : The important factors that cause instability in a  slope and lead to failure are

1. Gravitational Force             2. Force due to seepage water             3. Erosion of the surface of slopes due to flowing water                 4. The sudden lowering of water adjacent to a slope

5. Forces due to earthquakes 

16.2 General Considerations and Assumptions in the analysis : 

1. Testing of samples to determine the cohesion and angle of internal friction.

2. The study of items which are known to enter but which cannot be accounted for in computations

3. Computation

16.3 Factor of Safety : 

In stability analysis, two types of factors of safety are normally used. They are 

1. Factor of safety with respect to shearing strength.

2. factor of safety with respect to cohesion. 

16.4 Stability Analysis of Infinite Slopes in Sand 

The factor fo safety of infinite slopes in sand may be written as

16.5 Stability Analysis of Infinite Slopes in Clay 

16.6 Methods of Stability Analysis of Slopes of Finite Height 

1. Swedish Circle Method ( Fellenius, 1927 )

2. Bishop Method ( 1955 )

3. Bishop and Morgenstern Method ( 1960 )

4. Spencer Method ( 1967 )

16.7 Plane Surface of Failure 

•  Culmann ( 1875 ) assumed a plane surface of failure for the analysis of slopes which is mainly of interest because it serves a test of the validity of the assumption of plane failure.

16.8 Circular Surfaces of Failure 

• The investigations carried out in Sweden at the beginning of this century have clearly confirmed that the surfaces of failure of earth slopes resemble the shape of a circular arc. 
• The types of failure that normally occur may be classified as : 

       1. Slope failure         2. Toe failure             3. Base failure 

16.9  Failure under undrained conditions 

    In this, two types of failures are considered. They are:         

        

        1. Slope failure            2.Base failure

16.10 Friction Circle Method 

    The forces considered in the analysis are : 

        1. The total weight W of the the mass above the trial circle acting through the centre of mass.

        2. The resultant boundary neutral force U. 

        3. The resultant intergranular force, P, acting on the boundary.

        4. The resultant cohesive force C.

16.11  Tayor's Stability Number 

16.12  Tension CracksIf a dam is built of cohesive soil, tension cracks are usually present at the crest.

16.13 Stability analysis by method of Slices for steady Seepage
16.14 Bishop's Simplified method of Slices 
16.15 Bishop & Morgenstein method for Slope Analysis
16.16 Morgenstein Method of Analysis for Rapid DrawDown Condition
16.17 Spencer Method of Analysis 



Thanks learner for reading it. I will try to bring a series on it as our exams are near. 


References :   1. Textbook of Soil Mechanics and Foundation Engineering,  V.N.S. Murth
2. Slideshare