No abstract

No abstract

Practical applications of unsaturated soil mechanics still lag behind the state-of-the-art knowledge. The main stumbling block is the time-consuming processes involved in the measurement of the unsaturated soil parameters required for the.constitutive models. Recent research bas shown that the soil-water characteristic curves of a soil can be used in the establishment of a number of die unsaturated soil parameters. In many applications it has become obvious that a satisfactory equation for describing the soil-water characteristic curve will simplify the detennination of the soil parameters. Over the years a number of equations have been suggested. Most of these equations have limited success depending on soil types..1bis paper evaluates the more popular equations that have been suggested and shows that all the equations can be derived from a single generic form. One equation has been identified that performs very well for all soil types. If this equation is in common usage, useful databases on unsaturated soil parameters can be more easily established for practical applications of unsaturated soil mechanics.. . Typical soil-water characteristic curves for a sandy soil, a silty soil, and a clayey soil are shown in Fig. 1. From the soil

No abstract

This paper examines the three categories of permeability functions for unsaturated soils, including empirical macroscopic, and statistical models. The theoretical backgrounds and performance of each category are examined against various experimental data. The paper also shows that it is possible to degenerate statistical models to macroscopic models and then to empirical models. A new empirical equation for the permeability function is suggested. The statistical model demonstrates good performance and can be readily applied. In some cases the performance of the statistical model can be further improved with the i n d u c t i o n of a correction factor. INTRODUCTION Many GeotechnicaI and geoenvironmental problems involve water flow through unsaturated soils. Water coefficient of permeability in an unsaturated soil is a function of pore-water pressure or water content. Direct measurements of permeability in the laboratory can be time-consuming. especially for low water content conditions. Indirect measurements of permeability are commonly performed by establishing permeability functions through the use of the relationship between water content and pore-water pressure (i.e., soil-water characteristic curve). There are numerous permeability functions available for unsaturated soils. In general, the various permeability functions can be categorized into three groups: empirical macroscopic, and statistical models. It is important that the assumptions and theoretical backgrounds associated with each category of function be known prior to its use in application. A review of these backgrounds is presented in the Paper. The performance Of each Category of function is also examined against various experimental data. Advantages and limitations associated with each model are illustrated in the paper. PERMEABILITY OF UNSATURATED SOILS Permeability of saturated soils is a function of void ratio e only. For unsaturated soils, the coefficient of permeability with respect to water i s a function of both void ratio e and water content w. Since void ratio e, water content w, and degree of saturation S are interrelated, can be expressed as a function of any two of the three parameters. i.e. measure such a wide range of permeability values efficiently. permeability measurements can be performed either in the field or in the laboratory. However, field measurements are usually more. variable due partly to macroscopic features and partly from the assumptions made. In this paper only laboratory measurements are discussed In direct measurement there are steady-state and unsteadystate methods (Fredlund and Rahardjo 1993). In the steadystate method, a matric suction is first imposed on a soil specimen using the axis-translation technique (Hilf 1956). at equilibrium, denoted by a constant water content, a hydraulic gradient is then imposed across the soil specimen The flow rate is measured and the permeability is obtained via Darcy's law. using the unsteady-state method or instantaneous profile method, a cylindrical soil specimen is subjected...

Multistage direct shear tests have been performed on saturated and unsaturated specimens of a compacted glacial till. A conventional direct shear apparatus was modified in order to use the axis-translation technique for direct shear tests on unsaturated soils. The soil can be subjected to a wide range of matric suctions. The testing procedure and some typical results are presented. Nonlinearity in the failure envelope with respect to matric suction was observed. Suggestions are made as to how best to handle the nonlinearity from a practical engineering standpoint. Key words: shear strength, unsaturated soils, negative pore-water pressures, soil suction, direct shear.

Shallow landslides in natural residual soils slopes are common all over the world. The slip surfaces associated with these landslides are often situated above the groundwater table. Therefore, it is important to quantify the contribution of negative pore-water pressure to the shear strength of soil. The shear-strength characteristics of residual soil from the Jurong Formation in Singapore were assessed using multistage, consolidated drained triaxial tests. These tests involved shearing under either a constant net confining pressure and varying matric suctions or under a constant matric suction and varying net confining pressures. An extended form of the Mohr–Coulomb equation was used to interpret the test results. The test results show that for matric suctions up to 400 kPa, the angle of internal friction associated with the matric suction, [Formula: see text], is similar to the effective angle of internal friction, [Formula: see text], which averages 26° for the residual soil of the Jurong Formation. The residual soil can maintain a high degree of saturation for matric suctions as high as 400 kPa. Examples involving stability analyses of a residual soil slope with varying pore-water pressure profiles indicate that soil suction contributes significantly to the factor of safety, particularly for shallow slip surfaces. Key words : residual soil, unsaturated soil, matric suction, shear strength, multistage triaxial test, slope stability.

No abstract