Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 293

It is observed that the shear strength is proportional to the normal pressure. The strength
envelope passes through the origin, giving an angle of shearing resistance φcu.

However, it is fallacious to assume that the shear strength is related to the normal
pressure during the application of shear. This may be demonstrated by consolidating all the
samples under one particular pressure and testing them in shear under a different pressure.
In such a case the results will appear somewhat as shown in Fig. 8.38.

t

Shearing strength

Normal pressure

s 1 s 2 s 3 s

Consolidation pressures 3

Consolidation pressures 2

Consolidation pressures 1

0

Fig. 8.38 Consolidated undrained tests in direct shear on remoulded, saturated,
and normally consolidated clay (consolidated under normal pressures
σ 1 , σ 2 , and σ 3 , and sheared under different normal pressures)
It is observed that the shearing strength is independent of the normal pressure during
shear but is dependent only on the normal pressure during consolidation or consolidation
pressure. The process of preconsolidation may thus be viewed simply as a method of changing
the consistency of the clay, the strength at a given consistency being practically indpendent of
normal pressure during shear.

Similarly, consolidated undrained tests may be conducted in triaxial compression by
either of the following procedures:
(i) The specimens of saturated, remoulded, and normally consolidated clay are
consolidated under different cell pressures and sheared, without permitting drainage,
under a cell pressure equal to the consolidation pressure. This approach is more
commonly used.
(ii) The specimens are consolidated under the same cell pressure σc, and then sheared
under undrained conditions with different cell pressures by increasing the axial
stress; different series of these tests may be performed with different values of cell
pressure for consolidation, which will be constant for any one series, as stated above.
The results from the first method appear somewhat as shown in Fig. 8.39; total stress
envelopes as well as effective stress envelopes are shown.
The failure envelopes pass through the origin, giving ccu = ccu′ = 0, and values of φcu and
φcu′ such that φcu′ > φcu. If the tests are conducted starting with a very low consolidation pres-
sure, the initial portion of the envelope is usually curved and shows a cohesion intercept. The
straight portion when extended passes through the orign.

An overconsolidated clay shows an apparent cohesion; the equation for shear strength
is:

s = ccu + (σ – σc) tan φcu ...(Eq. 8.54)