Developing Mechanism Of Residual Deformation And Pore Pressure During Liquefaction Process Of Saturated Loess

Liquefaction of saturated soil is one of the most important problems in the field of soil dynamics. For soil liquefaction, there are three relative key issues, i.e. residual deformation, void water pressure, and their quantitative interrelation. The interrelation is essential not only to understand physical process of liquefaction, but also to describe the increase of void water pressure and development of large liquefied deformation. Because of its particularity of provenance factors and formation environment, natural loess has special engineering geologic features comparing with other soils, which still reflect in liquefaction of saturated loess. Generally it is the key problem understanding soil mechanical response behavior under external loadings of developing mechanism of soil residual deformation and void pressure, and of their quantitative interrelation and controlling effect. For the process of soil liquefaction, unfortunately, its developing mechanism and generating forces of void water pressure are still unclear.This thesis analyzed the developing process of the micro-cracks by analyzing the ratio of volumetric strain and axial strain, as well as the influence of micro-crack development. These results disclosed detail features of the development of soil residual deformation under external loadings. Based on analyzing quantitative relationship between residual deformation and void pressure, the author revealed the effect of volume residual deformation controlling void pressure by through two aspects, i.e. the uniform of response process and causation of void air/water pressures, and the difference of two kinds of void pressures in absolute value. Combining dynamic triaxial tests of saturated sand, the thesis analyzed the developing features of the maximum void pressure ratio in process of saturated soil liquefaction. This analysis disclosed the developing law of void pressure during liquefaction process and then confirmed the issue of the maximum void pressure ratio exceeding 1.0 is whether a real physical phenomenon or not. The innovative results of this thesis are summarized as follows.(1) Under the effect of the external load, the variation trend of volumetric strain and axial strain of unsaturated and saturated loess shows stepwise which may have relationship with the micro-crack state change from open to close. On the stage of micro-crack development and expansion, the ratio of bulk strain and axial strain reduce; on the contrary, when the micro-crack is close the ratio increase. The development degree of saturated loess micro-crack state change reduce quickly under load deformation, and its incremental ratio has similar representation.(2) The response process and cause of void water pressure has consistency with void air pressure. The absolute value of void water pressure and void air pressure have obvious difference in the dependence to confining pressure, the absolute value of water contribution to void water pressure is the absolute value of air contribution to void air pressure 10 times. The response process and cause of void pressure depends on the interaction mechanism among solid, water and air three phase mediums, and the absolute value of void pressure relies on the inherent characteristics of water and air phase mediums such as permeability and compressibility.(3) Relative testing data by means of traditional analysis method of pore pressure ratio showed that the maximum void pressure of loess is about 0.8, which of the sand is about 1.2 in the process of saturated soil liquefaction. Based on the logic relationship analysis of calculating effective confining pressure, and considering the axial stress in the test, the maximum void pressure of sand can be modified to the value of 1.0. Analysis results illustrate that the phenomenon of maximum void pressure ratio over 1.0 when soil is liquefied may have relationship with calculation analysis but not the actual physical phenomenon.