Civil and Building Engineering.

One problem remains: whether or not the laboratory tests give properties that can be used directly in the field. More explicitly, does taking a soil sample from the ground and putting it into the consolidation cell in the laboratory cause a change in properties? Field sampling of soils will be discussed with respect to the effects of disturbance on the results of tests in the laboratory.

The most extreme disturbance would be obtained by physically remolding a soil. An e-log omega curve for a hand-carved sample (minimum sampling disturbance) is shown in Figure 3.23 for a sample of Leda clay from Toronto, Canada, and also is a curve is shown for a sample that was completely remolded prior to testing. Clearly, using a seriously disturbed sample of this soil for a field design would lead to large errors in predicting total settlement.

As a simple example, a clay is considered that is 40 ft thick in the field. To simplify the calculations (with some loss of accuracy), the layer will not be subdivided. An average initial effective stress of 1000 psf is assumed along
with an increase in effective stress of 3000 psf. Eq. 3.60 will be used to calculate settlements. The calculated settlement for the hand-carved specimen is
If the curve for the remolded sample is used (which started at the same E0 but settled under the first applied load), the settlement is
Figure 3.25 One-dimensional consolidation curve for a specimen of clay that was sedimented from suspension.

Figure 3.26 Casagrande’s construction to find the maximum previous consolidation pressure.

Schmertmann (1955), was used to estimate the location of the field curves in Figures 3.24 and 3.27.
The time rate of settlement is controlled by the estimated value of the coefficient of consolidation in the same way that the total settlement is controlled by the consolidation curve. The effect of complete remolding on the
cv of a sensitive clay is shown in Figure 3.28 (the same test as for Figure 3.23). At low confining pressure, remolding caused a severe reduction in cv. If time rate or settlement calculations are important on a project, then highquality undisturbed samples should be used for testing.