### COMPUTATION OF COLLAPSE SETTLEMENT: Foundations on Collapsible Soils.

**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)

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