Theoretical And Experimental Studies On One-Dimensional Nonlinear Consolidation Of Soft Soil

The conventional consolidation theories often neglected the non-linearity of soil for practical purposes. Studies of nonlinear consolidation behavior of soft soil started from about 50 years ago. Many efforts have been made to derive solutions for different kinds of one-dimensional consolidation theories considering more realistic assumptions about soil behavior. However, there seem quite few researches on constitutive relationship and experimental determination of related parameters, as well as nonlinear consolidation theory considering stress history. One-dimensional nonlinear consolidation theories of normally consolidated (NC) and over-consolidated (OC) soil are discussed in detail through laboratory test and theoretical analysis in the dissertation.A specially instrumented, automated hydraulic consolidation device (GDS ADVCTS) has been developed, which is fully controlled by a computer-data acquisition system that monitors load, pore pressure, total stress, and deformation. The load is applied by a stepping motor. One-dimensional consolidation and permeability tests and CRS consolidation tests are performed on soft Xiaoshan soil specimens. Results from the tests conducted show the effectiveness of the apparatus. Effective stress-void ratio and the permeability-void relationships of the material are summarized to investigate the nonlinear consolidation behavior of soft soil. Non-Darcy seepage models are obtained to analyze the permeability velocity-hydraulic gradient relationships. It is shown that the compressibility and permeability of OC soil during consolidation exhibits nonlinear behavior with the development of effect stress and there is essentially a piecewise linear relationship (e versus lg k_v or lge versus lg k_v) near the permeability coefficient corresponding to the pre-consolidation pressure.The results of constant rate of strain (CRS) consolidation test based on linear and nonlinear CRS theory are compared to those of conventional incrementally-loaded consolidation and permeability (ILP) tests on the same soil, and soft Xiaoshan soil specimens are loaded using one of three vertical strain rates(0.05, 0.01 and 0.005%/min) to analyze the strain rate effect. An empirical approach has been developed to estimate the strain rate for use in CRS test. Results show that nonlinear CRS theory is better with the ILP test results than linear CRS theory, and thereby validity of nonlinear CRS theory is supported. The CRS behavior of soft Xiaoshan soil is virtually independent of strain rate across the range tested. CRS tests have a number of distinct advantages over conventional ILP test. Very much more data are obtained allowing better identification of consolidation parameters. In addition they can usually be carried out in a shorter time than would be required for an equivalent stage loading test. Analytical solutions are derived for one-dimensional nonlinear consolidation of NC soil subjected to time-independent or time-dependent loading on the basis of nonlinear compression relationships including Hyperbola Relationship, lge-lgσ' Relationship and lg(1 + e)-lgσ' Relationship respectively. Those solutions are compared with Davis's nonlinear consolidation theory and Terzaghi's linear consolidation theory, and one-dimensional nonlinear consolidation behavior of NC soil is then illustrated in detail.Semi-analytical methods are established for solving the problem of one-dimensional nonlinear consolidation of OC soil, including e-lgσ' piecewise-linear function and e-lgk_v piecewise-linear function, as well as lge-lgσ' piecewise-linear function and lge—lgk_v piecewise-linear function respectively. Two corresponding computer programs named ODNCALOS-IV and ODNCALOS-III are developed, in which the difference in permeability and compressibility behavior between OC and NC soil is taken into consideration for the first time. The programs are verified through comparisons with available analytical solutions including the one incorporating lge-lgσ' Relationship in the dissertation. The nonlinear consolidation behavior of OC soil is illustrated by using ODNCALOS-IV. A variety of consolidation curves are plotted, and the influence of various factors on the development of settlement and the dissipation of pore pressure are discussed, such as pre-consolidation pressure, applied load, nonlinear variations of compressibility and permeability, etc. Especially, parameter C_ke / C_k represents the difference in permeability behavior between OC and NC soil and is found to play a significant role in deciding the rate of the consolidation process, though it does not influence the final settlement. Results further indicate that neglecting the difference will overestimate the consolidation rate.The constitutive relationships obtained in the study are used in the semi-analytical method and analytical solution. Consolidation and permeability tests are performed on OC and NC soil specimens to access the validity of these theories. Using material properties obtained from those tests, the semi-analytical method simulates the vertical displacement and the base pore pressure of NC soil, better than the analytical solution. It is concluded from the research that semi-analytical method can predict correctly the behavior of the NC soil, and the analytical theory for nonlinear consolidation of NC soil herein provides a direct and efficient tool for the verification of numerical methods. Furthermore, semi-analytical solution program ODNCALOS-IV and ODNCALOS-III are used to predict the settlement magnitude, and time rate of settlement as well as pore pressure of OC soil. Simulations using ODNCALOS-IV indicate better agreement with consolidation tests than those using ODNCALOS-III, and thereby the nonlinear consolidation theory for OC soil incorporating e-lgσ' piecewise-linear function and e - lgk_v piecewise-linear function is more suitable for soft Xiaoshan soil.