Geotechnical Engineering

DHARM

214 GEOTECHNICAL ENGINEERING

was extra external loading acting on the area. Thus, the maximum stress to which the soil

sample was ever subjected is the current over burden pressure σv 0 ( = γ′.z).

Depth z Unform soil

sv 0 with unit wt.g¢

Sample

Ground surface

Recompression

Compression

sg¢v 0 =z

Pressures(log scale)

Void ratio e

(arithmetic scale)

(a) Location of sample taken for compression test (b) Compression test results for the sample
Fig. 7.13 Conditions applying to compression test sample
The results of a compression test performed on this sample are shown in Fig. 7.13 (b).
For laboratory loading less than σv 0 , the slope of the compression curve is less than it is for

loads greater than σv 0 , since, in so far as this soil is concerned, it represents a reloading or

recompression. Thus, the portion of the curve prior to pressure σv 0 represents a recompression

curve, while that at greater pressures than σv 0 represents the virgin compression curve.

It is obvious that a change in the slope of the compression curve occurs when the previ-
ous maximum pressure ever imposed on to the soil is exceeded. If the ground surface had at
some time is past history been above the existing surface and had been eroded away, or if any
other external load acted earlier and got released, σv 0 , the existing over-burden pressure,
would not be the maximum pressure ever imposed on the sample. If this greatest past pres-
sure is σvmax, greater than σv 0 , compression test results would be as shown in Fig. 7.14.

sv 0

Void ratio e

Pressures(log scale)

Break in slope

of curve

sv max

Fig. 7.14 Compression test results where past pressure

exceeds present overburden pressure

7.2.6 Normally Consolidated Soil and Overconsolidated Soil

In view of the marked difference in the compressibility behaviour of clay soils which are being
loaded for the first time since their origin in relation to the behaviour of clay soils which are
being reloaded after initial loading and unloading, as depicted in Figs. 7.11 and 7.12, it becomes