Investigation of shear strength of compacted fine grained soils in drainage conditioons

The shear strength of soils can be defined as the strength of the soil particles against shear forces resulting from internal friction. Determination of shear strength of soils is necessary for calculation of bearing capacity, stability analysis of geotechnical structures and determination of stress-strain characteristics of soils. The shear strength of the soils depends on the level of deformation. Residual shear strength is a a constant strength value that does not change with increasing deformation level of the soil and it occurs due to the parallel orientation of the particles to the shear surface and the polishing effect created by the water entering the shear surface. Residual shear strength is based on overconsolidated clays, collapsed soils for geotechnical designs. One of the significant factors affecting the shear strength of soils is saturation. As it is known, the floors composed of three-phase structure consisting of water, air and grain. The situation in which all of the voids in the soil are filled with water is considered to be saturated. In classical soil mechanics, it is considered that the ground is saturated when maximum or residual shear strength parameters are obtained. If the soil is not saturated, it is the main research area of the unsaturated soil mechanics. In this study, it is aimed to investigate how the maximum and permanent shear strength parameters of the compacted fine grained soils change under drainage conditions in a saturated and a unsaturated situations. By using two different test methods (consolidated-drained triaxial test and multistage direct shear test), the change of shear strength parameters in the case of saturation and unsaturation is investigated and it is aimed to observe the differences between the results. For these aims, classification experiments are carried out on different samples, the index properties of the samples are determined according to the classification test results, and samples with favourable properties are selected. The selected samples are subjected to the compaction tests at three different compression energies; the standard proctor stiffness, the modified proctor stiffness, and the specified stiffness. For each energy level, the optimum water content of the samples and the maximum dry unit volume weights of samples corresponding to these optimum water content values are determined. The used samples are prepared at the first compaction energy (the specified stiffness), the second compaction energy (the standard proctor stiffness) and the third compaction energy (the modified proctor stiffness) in the optimum water content and subjected to the odometer test to calculate pre-consolidation pressures at each energy level. Samples with pre-consolidation pressures which calculated at each energy level by two different methods (Casagrande (1936) and Da Silva (1970) methods) are subjected to consolidated-drained triaxial tests and multistage direct shear tests in case of saturated (S=%100) and unsaturated (according to the saturation of their energy levels) conditions, under the loading of pre-consolidation pressures at three compaction energies. In the direction of the obtained results, maximum and residual shear strength parameters are determined in saturated and unsaturated condition. The main purpose of this method is to ensure that the samples prepared for the three compressing energies are protected at the beginning of the experiment and that the sample can be tested in the unsaturated state without changing the saturation level. In order to obtain the shear strength parameters in the non-saturating case, the samples are prepared in optimum water contents at three energy levels and consolidation is made under the pre-consolidation pressure for each energy level. This is why it is not desirable to allow any deformation of the samples in the positive or negative direction by loading under pre-consolidation pressure when the samples are ready for consolidated-drained triaxial testing and multistage direct shear testing. The void ratio and the saturation ratio change because the samples under the over pre-consolidation pressure are subject to compaction. It is aimed to keep the saturation ratio at the initial level because used consolidated-drained triaxial pressure test system and the multistage direct shear test systems do not measure the change in pore air pressure. In the samples loaded under the pre-consolidation pressure, the void ratio and the saturation ratio are different from the conditions under which they are prepared, since they are swollen. It is aimed to calculate the shear strength parameters of unsaturated situation as result of providing a boundary condition by maintaining the void ratio and water saturation ratio in the conditions that the samples are prepared at three energy levels. Since the samples are unlikely to be prepared at a certain saturation level and at a certain pre-consolidation pressure, the compaction energy is kept constant (the energy applied to compress the samples at three energy levels) and the saturation ratio and pre-consolidation pressure at the optimum water content at this level are determined. The samples are subjected to the consolidated-drained triaxial test and the multistage direct shear test in the first stage and in the saturated condition. In the second stage, the same specimens were subjected to the consolidation-drainage three-axis pressure test and the repetitive shear box experiments at the initial saturation rates without saturating the water. According to the obtained results, the variation of maximum and permanent shear strength parameters in water saturated and unsaturated cases is investigated by two different test methods in accordance with the purpose of study. The consolidated-drained triaxial tests of the unsaturated samples were carried out with a computer-assisted triaxial testing instrument in the laboratory. In this way, computer-assisted triaxial testing device in Istanbul Technical University Soil Mechanics Laboratory has been made capable of making experiments efficiently. In this study, the variation of the parameters of maximum and residual shear strength in the water saturated and unsaturated condition of the compacted finely grained soils under drainage conditions was investigated. For this purpose, consolidated-drained triaxial experiments and multistage direct shear experiments were carried out on three different compaction energies on specimens buried in the laboratory, in a saturated and unsaturated state, with consolidation and drainage. As a result of the experiments performed, the maximum and residual shear strength parameters obtained by consolidated-drained triaxial tests and repeated shear box multistage direct shear tests in saturated condition were compared. It was observed that the values of the residual shear strength obtained from consolidated-drained triaxial tests were greater than those of the shear strength obtained from the multistage direct shear tests. The variation of maximum and residual shear strength parameters in saturated and unsaturated condition was observed. According to the results obtained, saturation effected the maximum shear strength parameters, whereas the residual shear strength parameters had no significant effect. Several correlations have been proposed to be used in the direction of practical purposes.