Geotechnical Engineering

DHARM

264 GEOTECHNICAL ENGINEERING

after the occurrence of complete consolidation and consequent dissipation of pore water

pressure. Thus, complete consolidation, dependent upon the dissipation of pore water pressure

and hence upon the increase in the effective stress, leads to increase in the shearing strength

of a soil. In other words, it is the effective stress in the case of a saturated soil and not the total

stress which is relevant to the mobilisation of shearing stress.

Further, the density of a soil increase when subjected to shearing action, drainage being

allowed simultaneously. Therefore, even if two soils are equally dense on having been consoli-

dated to the same effective stress, they will exhibit different shearing strengths if drainage is

permitted during shear for one, while it is not for the other.

These ideas lead to a statement that ‘‘the strength of a soil is a unique function of the

effective stress acting on the failure plane’’.

Equation 8.26 may now be modified to read:

s = c′ + σf tan φ′ ...(Eq. 8.27)

where c′ and φ′ are called the effective cohesion and effective angle of internal friction, respec-

tively, since they are based on the effective normal stress on the failure plane. Collectively,

they are called ‘effective stress parameters’, while c and φ of Eq. 8.26 are called ‘‘total stress

parameters’’.

More about this differentiation and other related concepts will be seen in later sections.

*8.6 HVORSLEV’S TRUE SHEAR PARAMETERS

Hvorslev (1960), based on his experimental work on remoulded cohesive soils, proposed that
the shearing strength, s, can be represented by the following general equation, irrespective of
the stress history of the soil:

s = f(ef , σf) ...(Eq. 8.28)

where f(ef, σf) = a function of the void ratio, ef , at failure, and the effective normal stress on

the failure plane, at failure.

This may be written more explicity as follows:

s = ce + σf tan φe ...(Eq. 8.29)

where ce = ‘true cohesion’ or effective cohesion, and φe = ‘true angle of internal friction’ or
effective friction angle.

The true angle of internal friction is found to be practically constant. However, the true
cohesion is found to be dependent upon the water content or void ratio of the soil at failure. In
fact, it is found to be directly proportional to the ‘equivalent’ consolidation pressure or the
pressure from the virgin impression curve corresponding to the void ratio at failure.
That is to say:
ce = K. σe ...(Eq. 8.30)

where ce = true cohesion,

σe = equivalent consolidation pressure,

and K = constant of proportionality, called ‘‘cohesion factor’’ or ‘‘coefficient of cohesion’’.