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THE PORE AIR-WATER CONFIGURATIONS AND THEIR EFFECTS ON THE MECHANICAL PROPERTIES OF PARTIALLY SATURATED SOILS

YU PEI-JI CHEN YU-JIONG (Institute of Hydrotechnical Research)  
The configuration of pore air and water as well as the structure of soil skeleton has great influences on the mechanical properties of partially saturated soils. In this paper three basic systems with different air-water configurations are postulated, namely, closedair system, bi-opened system and closed-water system. A closed-air system is defined as a partially saturated soil of such high degree of saturation that the air in the pore space forms isolated bubbles which are entirely enclosed by surrounding pore water. On the contrary, a closed-water system is a partially saturated soil with low degree of saturation, in which the pore water is not continuous. The intermediate case is the bi-opened system, in which both the pore air and water are continuous. The existence of those three basic systems is indicative from test results and critical values of degree of saturation between two neighbouring systems are given. The experimental investigation verifies the authors' original idea, that the mechanical properties of the closed-water system and bi-opened system are essentially similar to each other, but they are quite different from that of closed-air system. For the closed-air system the value of pore pressure coefficient is high, the dissipation of pore pressure is slow and the shear strength parameters in terms of effective stress is far greater than that in terms of total stress. For the closed-water system and bi-opened system the values of pore pressure coefficient are low, the pore pressure dissipates rapidly and the difference between the shear strength parameters in terms of total stress and that in terms of effective stress is small. The results of investigation of the air-water configurations lead to the suggestions that, for the closed-air system the effective stress method with the consideration of the dissipation of pore pressure should be used in the analysis of stability of earth slopes, but for the other two systems the total stress method can be used.
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