|M.Sc Student||Shay Nachum|
|Subject||The Relationship Between Three Dimensional Stresses|
and Strains During One Dimensional Loading,
Unloading and Swelling of Unsaturated
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Talesnick Mark Lloyd|
|Professor Emeritus Frydman Sam|
|Full Thesis text - in Hebrew|
One of the central issues in the field of geomechanics is the ability to predict changes that will take place in soil volumes and stresses based on the knowledge of external changes to the system.
In the case of unsaturated clay, one of the most important predictive capabilities is the ability to predict and describe deformations within the soil and changes in soil-induced stresses, in response to wetting.
An understanding of unsaturated soil response to wetting, and the development of a constitutive law are based on the ability to experimentally measure changes in soil during controlled wetting. This study offers a method for performance of reliable measurement, and highlights issues that have not previously received sufficient attention. A major problem addressed in the study is the effect of the friction between the soil and the test cell. This subject is rarely addressed in the literature, and the solutions offered in the standards are inadequate. The study offers a practical method to measure and minimize side wall friction, and demonstrates its effect on measured results.
An improved cell was built in which the side walls are not connected to the base. The base was placed upon a load cell. The difference between the force applied to the sample top and the force measured at its base is the frictional force that is transferred from the soil to the cell walls.
Standard tests do not measure horizontal stresses. In this study, the horizontal stresses were measured using null gauges. Hence, the strains and stresses were measured triaxially, forming a basis for development of a tensorial model that describes the state of strains and stresses in a three-dimensional state.
The first stage of the study focused on the development of a reliable testing and measurement system. A fine-grained dune sand was used for this purpose. In order to examine the effect of wall friction on the results, friction reduction measures were applied; polyethylene strips covered with grease were placed on the test cell walls. It was found that the use of friction reduction measures decreased the frictional force from about 30% of the applied force to about 7%. The effect of friction reduction on the value of K0 during loading and on the hysteresis of horizontal stress during unloading is significant.
The conclusions reached in the initial phases of the research were applied in testing the swelling of compacted clay samples when wetted. A series of specimens was prepared to the same conditions of density and moisture content, and were wetted under the same vertical stress. The outcomes of tests performed in the cell developed in this study were compared to the outcomes of tests performed in a standard ring oedometer. The swell obtained in the research cell was about 20% larger than that obtained in the standard ring.
The relationships between the vertical swelling deformation, and vertical, horizontal and mean stresses, are well described by semi-logarithmic lines with R2 = 0.99. These curves may serve as a basis for the development of a constitute law.