The Practical Computation Method For Strength And Deformation Of Unsaturated Soils

The strength formula and the constitutive relation of unsaturated soil are hardly used in the project because of the difficulties of measuring and calculating suction pressure. Water content which can be gained easily is selected to analyze the effects on strength and on deformation by moisture of unsaturated soil. The strength and deformation tests under different values of constant water content have been done. Based on the tests results, one total stress strength formula and one non-linear model of unsaturated soil are establishedIt is necessary to keep the water content constant during the test. The pore air pressure dissipates quickly and the pore water pressure does not have such behavior under construction. It is also necessary to guarantee that the pore air pressure has dissipated completely during the test to simulate that characteristic. The test procedure and control measures to realize the above two test aims are put forward. The tests results indicate that the strength and the power to resist deformation decrease with the increase of water content.It can be easily found that the cohesion and the angle of internal friction of unsaturated soil decrease linearly with the increase of water content by the test results. Then the total stress strength formula of unsaturated soil is established. The model parameters of unsaturated soil with different water content can be gained by the Duncan-Chang model method. The non-linear model of unsaturated soil is developed on the basis of analyzing the relationship between these parameters and water content. If the water content distribution of soil strata is known, the strength can be easily obtained by the presented formula, and the deformation under loading can be calculated by the model. The formula and the model are convenient and practical.On the other hand, the rigorously developed methods for consolidation of unsaturated soils are hardly used and the simplified methods have some disadvantages. The practical methods are advanced on account of the trait of the water flow and the air flow under different degree of saturation.For unsaturated soil with medium degree of saturation, it is assumed that the compaction at initial stage and the pore air dissipation are completed instantaneously. Then, the process of water pressure dissipation can be studied separately. The simplified one-dimensional consolidation method is advanced. On the basis of the one-dimensional method, the practical two or three-dimensional method is developed.For unsaturated soil with high degree of saturation, because the air and the water flow away from the soil together in consolidation stage, they are regarded as mixed compressed fluid. It is also assumed that the water flow only depends on the water pressure. The simplified one-dimensional consolidation method is suggested. The suction pressure, the water pressure, the degree of saturation, and the compression can be calculated by this method. The calculation results of one case show the presented two simplified approaches are reasonable.