The study of undrained and drained behavior of unsaturated soils
Rahardjo, H. (Harianto)
Compacted soils, expansive soils and collapsible soils are typical unsaturated soils that are commonly considered as problematic soils. The behavior of unsaturated soils during undrained loading and consolidation processes have not been extensively studied from an experimental standpoint in comparison to saturated soils. The primary objective of this research program was to study the pore pressure development and the consolidation behavior of unsaturated soils resulting from the application of external loads and changes in pore-water pressure. The study dealt with one-dimensional loading (Ko-conditions) and consisted of theoretical and experimental programs. The theoretical program was started with a brief literature review on unsaturated soil behavior. The relevant theories were developed by first introducing the physics involved during undrained and drained loadings. Boyle's gas law and the constitutive equations for unsaturated soils were used in the derivation of pore-pressure parameters for undrained loading. Darcy's law and the constitutive equations for unsaturated soils were used in deriving the governing flow equation for the water phase. The stress state variables for an unsaturated soil were used in the formulation of the constitutive equations. An experimental program was established and the program involved the development of appropriate equipment for testing, the selection of a suitable soil and testing procedure; and eventually the carrying-out of the undrained and drained tests. A Ko-cylinder that allowed undrained and consolidation tests to be performed on an unsaturated soil specimen was designed and built. Simultaneous measurements of pore-air and pore-water pressures were made throughout the soil specimen during tests. Details on the design and construction of the equipment are outlined in the thesis. Criteria used for selecting the soil and testing procedure are described. There were four types of tests performed on the soil throughout the experimental program. The undrained or constant water content loadings were conducted for obtaining the pore pressure parameters for an unsaturated soil. The drained tests consisted of consolidation and increasing matric suction tests. The drained tests were conducted in order to study the pore pressure and volume change behavior of an unsaturated soil during a transient process. The above tests were performed alternately in an experimental series. There were five experimental series carried-out in the program. The experimental data were analyzed and interpreted in the light of the formulated theory. Theoretical simulations for matching the experimental data were also performed. Comparisons between the experimental data and theoretical simulations are presented and discussed in the thesis. The experimental pore pressure parameters obtained from the undrained and constant water content loadings agree reasonably well with the formulated theory. The pore-air pressure dissipation was found to be essentially instantaneous when the air phase is continuous. The pore-water pressure dissipation during the consolidation test was found to be faster than the pore-water pressure decrease during the increasing matric suction test. The differing rates of dissipation were attributed to the differing coefficients of water volume change for both tests. The water volume changes during the consolidation test were considerably smaller than the water volume changes during the increasing matric suction tests for the same increment of pressure change.