COMPUTATION OF COLLAPSE SETTLEMENT: Foundations on Collapsible Soils.

The double oedometer method was suggested by Jennings and Knight (1975) for determining a quantitative measure of collapse settlement. The method consists of conducting two consolidation tests. Two identical undisturbed soil samples are used in the tests. The procedure is as follows:

1. Insert two identical undisturbed samples into the rings of two oedometers.
2. Keep both the specimens under a pressure of 1 kN/m2 (= 0.15 lb/in2) for a period of 24 hours.
3. After 24 hours, saturate one specimen by flooding and keep the other at its natural moisture content
4. After the completion of 24 hour flooding, continue the consolidation tests for both the samples by doubling the loads. Follow the standard procedure for the consolidation test.
5. Obtain the necessary data from the two tests, and plot e-log p curves for both the samples
as shown in Fig. 18.5 for normally consolidated soil.
6. Follow the same procedure for overconsolidated soil and plot the e-log p curves as shown
in Fig. 18.6.

From e-log p plots, obtain the initial void ratios of the two samples after the first 24 hour of loading. It is a fact that the two curves do not have the same initial void ratio. The total overburden pressure p0 at the depth of the sample is obtained and plotted on the e-log p curves in Figs 18.5 and 18.6. The preconsolidation pressures pc are found from the soaked curves of Figs 18.5 and 18.6 and plotted.

Normally Consolidated Case
For the case in which pc/p0 is about unity, the soil is considered normally consolidated. In such a
case, compression takes place along the virgin curve. The straight line which is tangential to the
soaked e-log p curve passes through the point (e0, p0) as shown in Fig. 18.5. Through the point
(e0, p0) a curve is drawn parallel to the e-log p curve obtained from the sample tested at natural
moisture content. The settlement for any increment in pressure Δp due to the foundation load may
be expressed in two parts as

Overconsolidated Case
In the case of an overconsolidated soil the ratio pc/p0 is greater than unity. Draw a curve from the
point (e0, p0) on the soaked soil curve parallel to the curve which represents no change in moisture
content during the consolidation stage. For any load (p0 + Δp) > pc, the settlement of the foundation
may be determined by making use of the same Eq. (18.4). The changes in void ratios Δen and Δec
are defined in Fig. 18.6.

Figure 18.5 Double consolidation test and adjustments for normally consolidated
soil (Clemence and Finbarr, 1981)

Figure 18.6 Double consolidation test and adjustments for overconsolidated soil
(Clemence and Finbarr, 1981)