Newcastle Chapter

Characteristics of Vacuum Consolidation

Prof. Jin-Chun Chai, Department of Civil Engineering, University of Japan

Laboratory consolidation tests under odometer conditions were conducted under both vacuum pressures and surcharge loads. The characteristics of vacuum consolidation have been investigated by comparing vacuum pressure and surcharge load induced consolidations. Firstly, regarding the magnitude of consolidation settlement, vacuum pressure induced less or the same settlement as that by a surcharge load with the same magnitude. If a vacuum pressure alone is larger than the lateral pressure required to maintain a k0 condition (no lateral displacement), there will be inward lateral displacement and the vacuum pressure will induce less settlement than that of a corresponding surcharge load. Secondly, regarding the effects of drainage boundary conditions, under a vacuum pressure, two-way drainage resulted in less settlement than that of one-way drainage. Theoretically, under a vacuum pressure one-way and two-way drainage cause consolidation at the same rate because for both cases the water in the soil is only allowed to drain out from the top surface of the samples. Thirdly, regarding the effect of the order of soil layers in a two-layer system, under one-way drainage conditions, for both surcharge load and vacuum pressure cases, the order of soil layers only influenced the rate of consolidation but not the final settlement. When a layer with a relative lower value of hydraulic conductivity (k) was located immediately adjacent to the drainage boundary, the consolidation rate was lower. However, for vacuum pressure applied under two-way drainage conditions, the order of the soil layers not only influenced the rate of consolidation but also the magnitude of settlement. Based on the laboratory test results, a method of predicting vacuum pressure induced deformation has been proposed. The method has been applied. This method has been applied to three case histories reported in the literature, two cases in China and one case in Japan. It is shown that the field-measured data are simulated reasonably well, suggesting that the method is useful for the design of vacuum consolidation projects.

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