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Chapter: Civil : Soil Mechanics -Shear strength

Important Question And Answer: Sher strength

Civil - Soil Mechanics -Shear strength

Important Question And Answer:  Sher strength


1. What is shear strength?


It is the principle engineering property which controls the stability of a soil mass under loads. The shear stringth of soil is the resistance to deformations by continuous shear displacement of soil particles.




2. What are the factors that influence shear strength?


      resistance due to interlocking of particle


      frictional resistance between the individual soil grains which may be sliding friction, rolling friction.


      Adhesion between soil particle or cohesion.



3. What is principle plane and principle stress?


A principle plane is defined as a plane on which the stress is wholly normal on one. Which does not carry shearing stress.


From mechanics it is known that there exist three principle planes at any point in a stressed material. The normal stress acting on the principal plane are known as principal stresses.

4. What are shear stringth parameters?


c and ?


where c is known as apparent cohesion


?is called angle of internal friction.


5. What are the limitations of rohr coulomb theory?


      It neglects the effect of the intermediate principle stress.


      It approximates the curved failure envelope by a straight line, which may not give correct results.


6. What are the difference to measure the shear stringth of soil?


1.   Direct shear test

2.     Friaxial shear test


3.     Unconfined compression test

4.     Vane shear test



7. What is rohr coulomb theory?


The rohr coulomb theory of shecuring stringth of a soil, first propounded by coulomb (1976) and later generalized by rohr, is the most commonly used concept. The functional relationship between the normal stress on any plane and the shearing strength available on that plane was assumed to be linear by colomb. Thus the following is usually known as coulomb's law.


S= C+?tand


C = apparent cohesion


?= internal friction


9. What is strength envelope?


It the normal and shear stress corresponding to failure are plotted, then a curve is obtained. The plot or the curve is called strength envelope.




10. What do you know about undrained and drained test?


In the underained test, no drainage of water is permitted. Hence there is no dissipation of pore pressure during the entire test. In the drained test, drainage is permitted throughout the test during the application of both normal and shear stress.

11.            What are the field test to determine shear strength test?


      Field test


      Penetration test









12.            What are the different types of soil based on shear stringth?


Cohesionless soil : These are the soils which do not have cohesion ie c=oo. These soils derive shear stringth from the intergranular friction. These soils are also called frictional soils.


eg : sand, gravel


Purely  wherive  soil  :  These  are  the  soils  wh




eg: silts, saturated clay


13. What are the factors that affect shear stringth of cohesionless soils?


Shape of particles, gradation, confining, pressure, deviator stress, vibration and repeated loading, type of minerals.



14. what are the factors that affect shear strength of cohesive soils?


Structure of clay, clay content, drainage condition, rate of strain, repeated loading, confining pressure, plasticity index, disturbance.


15. What are the merits and demerits of direct shear test?




a.     This is the only test where both the shearing stress and the normal stress on the plane of failure are measured directly.

b.     Volume changes during the test can be measured easily.








a.     The  shear  stress  distribution  over  the  pl


b.     The drainage cannot be controlled, and so the pore pressure behaviour cannot be obtained from the test.



16. What are the different types of failure of a triaxcal compression test specimen?


Brittle failure


Semi plastic failure


Plastic failure


17.  What  do  you  mean  by  stress?path?


A stress?path is a curve or a straight li depicting the changes in stress in a test specimen or in a soil eleme or unloading.






18. What is peak shear strength? West out the factors it depends on?


Peak shear strength of a soil is the max shear stress that can be rested by the soil. It depends on percent day contact, drainage condition, stress level, anisotropy.




19.  What  is  rohr's  circle?  What  are  the  char


The graphical method for the determination of stresses on a plane inclined to the principal stress is called rohr's wide.


The characteristics are


1.     The maximum angle of obliquity ?max is obtained by drawing a tangent to the circle from the origin o.


2.     Shear stresses plane at right angle to each other are numerically equal but are of opposite signs.



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