CALCULATION OF PRIMARY CONSOLIDATION SETTLEMENT
Effects of Unloading/Reloading of a Soil Sample Taken from the Field:
Let us consider a soil sample that we wish to take from the fi eld at a depth z (Figure ). We will assume that the groundwater level is at the surface. The current vertical effective stress or overburden effective stress is
(a) Soil sample at a depth z below ground surface.
(b) Expected one-dimensional settlement response.
and the current void ratio can be found from gsat using Equation. On a plot of s9z (log scale) versus e, the current vertical effective stress can be represented as A, as depicted in Figure .
To obtain a sample, we would have to make a borehole and remove the soil above it. The act of removing the soil and extracting the sample reduces the total stress to zero; that is, we have fully unloaded the soil. From the principle of effective stress [Equation], s9z 5 2Du. Since s9 cannot be negative— that is, soil cannot sustain tension—the porewater pressure must be negative. As the porewater pressure dissipates with time, volume changes (swelling) occur. Using the basic concepts of consolidation described in Section , the sample will follow an unloading path AB (Figure. The point B does not correspond to zero effective stress because we cannot represent zero on a logarithmic scale.
However, the effective stress level at the start of the logarithmic scale is assumed to be small (<0). If we were to reload our soil sample, the reloading path followed would depend on the OCR. If OCR 5 1 (normally consolidated soil), the path followed during reloading would be BCD (Figure 9.4b). The average slope of ABC is Cr. Once s9zo is exceeded, the soil will follow the normal consolidation line, CD, of slope Cc. If the soil were overconsolidated, OCR . 1, the reloading path followed would be BEF because we have to reload the soil beyond s9zc before it behaves like a normally consolidated soil. The average slope of ABE is Cr and the slope of EF is Cc. The point E marks the past maximum vertical effective stress. Later in this chapter, we will determine the position of E from laboratory tests (Section ).