Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 297

is often observed. This is an important phenomenon which is quantitatively characterised by

‘Sensitivity’, defined as follows:

Sensitivity, St = Unconfined compression strength, undisturbed

Unconfined compression strength, remoulded

...(Eq. 8.56)

A comparison of stress-strain curves for a sensitive clay in the undisturbed and re-
moulded states is shown in Fig. 8.44.

t

Shearing stress

Undisturbed

Remoulded

Failure point (arbitrary)

20% Shearing strain % s

Fig. 8.44 Stress-strain curves for a sensitive clay in the

undisturbed and remoulded states

Sensitivity classification is given in the table below:

Table 4.1 Sensitivity classification of clays (Smith, 1974)

Sensitivity St Classification

1 Insensitive

1–2 Low

2–4 Medium

4–8 Sensitive

8–16 Extra-sensitive

Greater than 16 Quick (St can be even up to 150)

Overconsolidated clays are rarely sensitive, although some quick clays have been found

to be overconsolidated.

8.13 ILLUSTRATIVE EXAMPLES

Example 8.1: The stresses at failure on the failure plane in a cohesionless soil mass were:

Shear stress = 4 kN/m^2 ; normal stress = 10 kN/m^2. Determine the resultant stress on the

failure plane, the angle of internal friction of the soil and the angle of inclination of the failure

plane to the major principal plane.

Resultant stress = στ^2 +^2

= 1022 + 4 = 10.77 kN/m^2