Report No. : GEO Report No. 72

Report Title : Long-term Consolidation Tests on Clays from the Chek Lap Kok Formation (1998), 89 p. (Reprinted, 2002)

Author : D.O.K. Lo & J. Premchitt

Abstract

The settlement of a saturated clay layer consists of two main stages: primary consolidation settlement during which dissipation of pore water pressure occurs in the clay resulting in an increase in effective stress, and secondary compression under practically constant pore water pressure and effective stress over the long-term.

In Hong Kong, there are very few data on the compressibility characteristics of the firm to stiff clays from the Chek Lap Kok Formation, in particular the secondary compression characteristics. For this reason, the Airport Authority (AA) has requested the Geotechnical Engineering Office (GEO) to conduct two studies on the clay samples obtained from boreholes at the new airport site at Chek Lap Kok. The samples were taken at approximate elevations of -15 mPD to -29 mPD. The work has been carried out as a project under the GEO R&D Theme on Marine Geotechnology. The first study involved determination of the primary consolidation properties of the clays under constant rate of strain testing using a hydraulic consolidation cell. The results of that study are given in GEO Report No. 55.

The second study, as presented in this report, investigates the secondary compression behaviour of the clays experimentally using the oedometer. In this study, clay specimens were subjected to sustained effective vertical stresses at levels similar to those encountered in the field over various durations up to eighteen months. The tests carried out constitute some of the longest duration oedometer tests in Hong Kong and elsewhere. For comparison, the secondary compression parameters are commonly estimated from conventional oedometer tests over a 24-hour duration only. The tests in this study were designed to provide information on the variation of rate of secondary compression with time. This report presents a literature review on the previous studies on secondary compression, including factors affecting the secondary compression index and its determination, as well as the results of the tests carried out in this project.

The value of Ca/Cc for the clay samples tested is found to be about 0.044. This Ca/Cc value together with the e-log sv' curves from conventional oedometer tests may be used to estimate the secondary compression of firm to stiff clays from the Chek Lap Kok Formation at different levels of sustained effective vertical stress. The results also show that under the test conditions used in this study, the secondary compression index, Ca, decreases significantly with time in the initial period but the rate of decrease becomes smaller as the test continues. Due to this general decline with time, the Ca values estimated from data taken within the conventional 24-hour oedometer test duration are found to be typically considerably greater than the values derived from data taken over a period of testing of several months.

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