Geotechnical Engineering

DHARM

72 GEOTECHNICAL ENGINEERING

A qualitative classification based on activity is given in Table 3.6:

Table 3.6 Activity classification

Activity Classification

Less than 0.75 Inactive

0.75 to 1.25 Normal

Greater than 1.25 Active

3.11 UNCONFINED COMPRESSION STRENGTH AND SENSITIVITY

OF CLAYS

The unconfined compression strength of a clay soil is obtained by subjecting an unsupported

cylindrical clay sample to axial compressive load, and conducting the test until the sample

fails in shear. The compressive stress at failure, giving due allowance to the reduction in area

of cross-section, is termed the ‘unconfined compression strength’ (qu). In the field, a vane-

shear device or a pocket penetrometer may be used for quick and easy determination of strength

values, which may be related to qualitative terms indicating consistency. (These and other

aspects of strength will be studied in greater detail in Chapter 8).

It has been established that the strength of a clay soil is reacted to its structure. If the

original structure is altered by reworking or remoulding or chemical changes, resulting in

changes in the orientation and arrangement of the particles, the strength or the clay gets

decreased, even without alteration in the water content. (It is known that the strength of a

remoulded clay soil is affected by water content).

‘Sensitivity (St)’ of a clay is defined as the ratio of the its unconfined compression strength

in the natural or undisturbed state to that in the remoulded state, without any change in the

water content:

St =

q

q

u

u

()

()

undisturbed

remoulded

...(Eq. 3.63)

The classification of clays based on sensitivity, in a qualitative manner, is given in
Table 3.7:

Table 3.7 Sensitivity classification

Sensitivity Classification Remarks

2 to 4 Normal or less sensitive Honeycomb structure

4 to 8 Sensitive Honey or flocculent structure

8 to 16 Extra-sensitive Flocculent structure

> 16 Quick Unstable