Report No. : GEO Report No. 55

Report Title : Conventional and CRS Rowe Cell Consolidation Test on Some Hong Kong Clays (1996), 93 p.

Author : J. Premchitt, K.S. Ho & N.C. Evans

Abstract

It is often necessary to carry out special laboratory consolidation tests using techniques other than the conventional oedometer testing for the design and construction of reclamation and related projects. In response to this need, two series of Rowe cell consolidation tests were conducted, on two common Hong Kong clays.

The first series was conducted in Phase I work using soft marine clay obtained form seabed near Soko Islands. A step-loading method was used throughout but drainage conditions were varied to obtain both the coefficient of consolidation in the vertical direction and that in the horizontal direction. The results showed that the coefficient of consolidation in horizontal direction for the marine clay samples tested is significantly greater than the coefficient in vertical direction.

The second series was conducted in Phase II work using stiff alluvial clay obtained from the site ate the new airport at Chek Lap Kok. The tests were performed employing a Rowe cell which was controlled and monitored by a computerised hydraulic loading system. Constant rate of strain (CRS) test was carried out in this test series principally to determine preconsolidation pressure of the clay. For comparison, small-step loading tests using conventional oedometer were also conducted. For the seven samples tested, it was determined that six of them have low preconsolidation pressures and they are basically normally consolidated. The other sample was estimated to be significantly over-consolidated based on the CRS test results.

The tests indicated that the procedures for the Rowe cell are relatively straightforward and good results can be readily obtained from the tests. The CRS test offer several advantages over conventional incremental loading test. The time required for a CRS test is considerably less than the conventional test; the test results can be presented in a continuous plot; and the smooth loading procedure reduces sample disturbances. The fully automated Rowe cell system provides reliable and high quality test results, and the overall work is efficient and cost effective.

For stiff clay such as alluvial clays, CRS test may be the only means to find definitive values of preconsolidation pressure. The conventional tests on such a clay often results in quite a flat e-log(p) curve and estimated of preconsolidation pressure is uncertain. CRS test is a standard option in many countries, such as USA, Norway and Sweden. Wider usage of CRS test in Hong Kong will be beneficial.

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