Study On Stability Of Soft Clay Slope

In the east coastal areas of our country, most of the ports, docks and dikes are built on the soft clay foundation. Therefore, it is an important problem for the slope stability consisting of soft clay, which has lower shear strength. The proper shear strength parameters are needed to estimate the slope stability of the soft clay. However, because of the complex physical and mechanical properties, it is difficult to know exactly the shear strength of the soft clay. Among the common shear strength indexes, the shear strength parameters by CU test and those by UU test cannot be used to calculate the stability of the soft clay correctly. The former is usually over eatimated the safety of the slope, and the latter is consistently under estimated the safety of the same slope. The in-situ vane shear test is frequently performed to determine shear strength for slope stability analysis. But without considering the increase of the shear strength, the safety factors of stable slopes are always calculated less than 1.0. In this thesis, for the sake of revising the Code for Soil Foundations of Port Engineering, the shear strength for soft clay slope stability analysis is studied. The main research work consists of the following parts.1. Calculating results show that between the shear strength parameters and calculation methods, the safety factor of soft soil slopes are more sensitity to the shear strength parameters than to the calculation methods.2. According to the analysis of the work mechanism of the vane shear test and the characters of the sliding surfaces, it is shown the in-situ vane shear strength may not represent the shear strength mobilized along the analyzed sliding surfaces, which is the shear strength of the vertical plane. It is known to all that the shear strength on vertical plane is the smallest shear strength in all plane of the soil element. Therefore, the in-situ vane shear strength is less than the real shear strength on the slipping surface of the slope. Using the vane shear strength, the safety factor would be under estimated.3. Based on the analysis of the work mechanism of the vane shear test, and using the linear relation between the vane shear strength and the depth, this thesis presented a method of converting in-situ vane shear strength into undrained shear strength parameters. The Mohr-Coulomb theory is used on the vane shear strength calculation equation. The shear strength parameters determined for in-situ vane shear strength are subjected to statistical regression analysis, which is taken into consideration the anisotropy, the lateral compression coefficient and the average consolidation degree in the soft clay.4. Using the regressed shear strength parameters, the slope stability analysis method is performed for some existing slopes. Based on the results, it can be concluded that the presented method is effective to convert in-situ vane shear strength into undrained shear strength parameters for slope stability analysis. Simultaneously, the regressed analysis method is used to calculate the stability of cylindrical foundation. The results are closed to the practical engineering. Therefore, the method has a wide application.5. The upper bound and lower bound theorem is used to analyze the process of the progressive failure for the slope of the soft clay, and the calculation method is feasible to calculate the effective broken range of the slope of the soft clay.