Question

Three direct shear tests were conducted on a dry soil.

Assume the soil obeys the Mohr-Coulomb failure criterion: ?? = ? ′ + ???′ tan?′

Since water ingress in the soil would reduce the soil strength. By repeating the four direct shear tests using saturated soil, would you expect the values of ??, c’ and ?′be different from those obtained from the dry test? Explain why.

Answer 1

Yes the values of shear parameter () will differ because we know that shear strength of the soil through direct shear test is dependent on cohesion, phi- value, effective stress on soil .

Now considering these three variables on dry and wet soil we will see the effects

1.

a- Cohesion( )of soil in dry condition is more as compared to wet hence it will decrese the shear strength ()

b. phi-values of soil is also more for dry soil so it will also decrease shear strength () of soil.

c. As we can see that shear strength () of soil is also dependent on effective stresses on plane of failure

so we know that effective stresses are more in case of dry soil as pore water is also introduced in wet condition.

and we n=know effective stresses = total stresses - pore pressure

so it will also result in decrement of shear strength ().

so finally we can conclude that , Yes the shear parameter ( ) will differ in wet condition from that of dry condition.

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