Shear strength of soil.

Shear force:-

Shear force is the force applied along or parallel to a surface or cross section , instead of being applied perpendicular to the cross section .

Due to compression , the particle's become more and more closely packed , densification increases strength increase and thus , compression failure of soil is also rare.

Field situation of shear failure:-

  The field situation where shear failure can occurs are given below 

1. Upstremt slope of earthern dam , specially during sudden draw down.
2. Earth behind retaining wall , especially surcharge.
3. Under foundation along planes of maximum shear .

4 .Subgrade of road.

Concept of shear strength of soil:-

  It can be defined as the maximum value of shear stress that can be mobilized within a soil mass .

  If this value is equalized by the shear stress on any plane or surface at a point ,failure will occure in the soil because of movement of portion of the soil mass along that plane or surface.

  The soil is then said to have failed in shear.

Factor affecting shear strength of soil :-

1.   Type of soil 
2. Water content 
3. Cohesion 
4. SInternal friction 
5. 5. Compaction

Component of shearing resistance of soil:-

1. Cohesion
2. Internal friction 
3. Angle of repose.

Cohesion :-
 The force of attractions between the soil particles is called the cohesion .
 Cohesion means attraction between similar particle.

  The particle of soil at contact points have bonds of attraction with each other . The sum of total of these small forces of attractions will cause cohesion of soil.

 In some soil this force of cohesion is more prominent .These are called cohesive soil . e.g. clay and organic soil.

Internal friction :-

When the particle moves against one another , a force of friction comes into play in the opposite direction of motion .
  Soil having microscopic particles often have zero or negligible internal friction  e.g.- clay , some soil have maximum or complete shear resistance due to internal friction e.g. sand , murum  etc.
Shear resistance of most of the soil is made up partly from internal friction .

Angle of repose :-

It is the angle made by soil mass with inclined surface when it just start sliding in down word direction due to self weight.


Mohr’s Circle of Stress for Soils

Otto Mohr, a German scientist devised a graphical method for the determination of stresses on a plane inclined to the major principal planes. The graphical construction is known as Mohr’s circle. In this method, the origin O is selected and the normal stresses are plotted along the horizontal axis and the shear stresses on the vertical axis.



To construct a Mohr circle, first mark major and minor principal stress on X axis, Mark the centre point of that as C. A circle is drawn with c as centre and CF as radius. Each point on the circle gives the stresses ? and ? on a particular plane. The point E is known as the pole of the circle.

Mohr’s circle can be drawn for stress system with principal planes inclined to co-ordinate axes
Stress system with vertical and horizontal planes are not the principal planes

Mohr-Coulomb Theory 

The soil is a particulate material. The shear failure in soils is by slippage of particles due to shear stresses. According to Mohr, the failure is caused by a critical combination of normal and shear stresses. The soil fails when the shear stress on the failure plane at failure is a unique function of the normal stress acting on that plane.

Since the shear stress of the failure plane is defined as the shear strength (s) the equation for that can be written as

S= f (theta)

The Mohr theory is concerned with the shear stress at failure plane at failure. A plot can be made between the shear stresses and the normal stress at failure. The curve defined by this is known as the failure envelope.

The shear strength of a soil at a point on a particular plane was expressed by Coulomb as a linear function of the normal stress on that plane as,

In this C is equal to the intercept on Y axis and phi is the angle which the envelope make with X axis.


Shear strength determination :-
 Various test are available for determination of shear strength , either in the field or in the laboratory .

• Direct shear test 
• Unconfined compression test
• Vane shear test.

Direct shear test - Laboratory method .

 This is the oldest shear test method in use and still most common because of its simplicity . It also known as shear box test.
 The soil specimen is confined in a metal box that is split horizontally
  If the specimen is fully or partially saturated , perforated metal plates and porus stones are placed above and below the specimen for drainage .
  If the specimen is dry , solid metal plates are used .
  A pressure pad is placed on top and the entire box is placed in a trolley .
  The upper half of the box is fixed to a support through a proving ring and the lower half of the box is pushed at a constant rate of strain .
 A vertical load is applied on the pressure pad . At time of failure , shear stress is measured by the proving ring.
 Then the test is repeated for another sample of the same soil, for a different vertical load on the the pressure pad.
 4-5 repetition are made for 4-5  different normal load .
 By deciding normal load and corresponding shear load at failure by the internal horizontal area of the shear box , the normal and shear stress values can be obtained .
  These values are plotted and a best fit . straight line through these points gives the strength envelope .form this C and (Fi) can be determined .

Advantages :- 

• Test is simple and fast .
• Drainage is quick due to less thickness .

Disadvantages :-

• Actual field conditions ,  is not  simulated in the set up.
• Measurement of pore pressure is not possible.
• Failure of soil specimen is always along a horizontal plane, which may not be very reliastic.
• If any large soil particle or stone etc , are present at failure plane , it will give wrong result.