Unit 7 Secondary Consolidation

One-dimensional consolidation of soils is conveniently divided into three portions, viz., (1) initial compression, (2) primary consolidation, and (3) secondary consolidation. Such a classification involves the clear implication that the three effects proceed in that order. Initial one-dimensional compression of saturated clays is negligible because of the low compressibility of the pore water. Initial compression in the laboratory is likely to result from apparatus deflections, partial saturation of the soil, and improper seating of the loading head. Primary consolidation (Fig. 7.1) is the more or less S-shaped curve on a plot of settlement vs. log time and is assumed to result from the gradual dissipation of excess pore water pressures. Secondary consolidation is then taken to mean all of the compression that follows completion of primary consolidation.