The effect of secondary compression during the primary consolidation phase

Abstract

It is well known that there is discrepancy in the measured field rate and magnitude of consolidation when compared with those estimated from Terzaghi’s theory based on laboratory test results using small soil specimens.

The objective of the study is to characterize the compressibility of a marine clay near Chek Lap Kok with emphasis placed on the effect and contribution of the secondary compression of the soil.

Various hypotheses and theories regarding the relations between primary consolidation and secondary compression were examined in this study, notably the creep hypotheses A and B and the concept of isotaches.

A series of laboratory tests, consisting of oedometer cell, Rowe cell and triaxial cell, was devised to investigate the validity of some proposed hypotheses/theories using the marine clay near Chep Lap Kok. The study focused on the possible occurrence of the secondary compression during the primary consolidation stage and the characteristics of secondary compression under different specimen thicknesses and load increment ratios. To avoid the presumption that secondary compression comes after primary consolidation, Rowe cell and triaxial cell were employed to register the degree of primary consolidation based on the measured excess pore water pressure of the specimen.

A main conclusion of the study was that secondary compression occurred in the primary consolidation stage and the extent of which depends on a number of factors such as the thickness of the specimen. The conclusion partly explained the discrepancy between the laboratory and field consolidation behaviour. Recommendations were also given to alert the designers involving in the estimation of consolidation settlement.