Theories and methods for undrained strength and deformation of saturated soils

The calculation of undrained strength and deformation of saturated soils is a complex and difficult issue in geotechnical engineering, and there are still contradictory opinions regarding its many aspects. In this study, a systematic and in-depth exploration of this topic is carried out according to the author's years of relevant studies. Firstly, the total stress method and effective stress method for undrained analysis as well as their respective limitations are discussed. The treatment of pore water pressure is explained from the basic equation of finite elements, and it is pointed out that regardless of the method used, attention should be paid to distinguishing between the two types of the pore water pressures and those of the total stresses. Then, the accuracy of Skempton-Henkel's formula for calculating the excess pore pressure under different stress paths is examined, and then it is combined with the MC strength criterion to form an undrained strength model for saturated soils. The undrained strength characteristics of saturated soils under isotropic and anisotropic consolidations are analyzed, which can provide reference for the selection of strength parameters for engineering calculations. It is suggested that the generalized Tresca strength criterion should be used for practical undrained calculations. To prevent computation errors in undrained analysis using the MC criterion together with the effective stress method, a method of using the equivalent strength parameters is proposed, which is a better alternative to the direct inputting of the undrained strength. Based on the above discussions, the shortcomings of the consolidated undrained strength index (CU index) are analyzed, and it is pointed out that only when the total stress path in the soils is the same as that in the tests for determining the CU index, this index can be directly used for calculation to give accurate results. Especially for the calculation of bearing capacity of foundations, direct use of the CU index will give serious erroneous results. The drawbacks of using the CU index for calculating the active and passive soil water pressures in excavation engineering are analyzed, and the reasonable method and corresponding formula are proposed. The issue of the inclination angle of the sliding surface when the undrained ultimate soil pressures occur, puzzled to many, is also discussed in depth. Finally, regarding the calculation of bearing capacity and short-term settlements of saturated foundation, the deficiencies of the conventional methods are analyzed, and new methods are proposed based on the deep understanding of the influencing mechanisms of the relevant factors. The proposed formula for bearing capality can more accurately calculate the bearing capacity under both drained and undrained conditions, which can thus better ensure the safety and economy of the design. The new method for settlement can effectively calculate the short-term settlement of saturated soils, and also shows good prospective for drained conditions.