| Abstract | This thesis presents the results of an investigation on the
shear strength characteristics of isotropically consolidated
Bangkok clay in the weathered zone. (The weathered zone is the
zone in which the soil consistency has increased by weathering . Actually, for unweathered marine clay the consistency increase with depth)
Undisturbed samples were taken from depths of 3.5 ft, 4.75 ft, 8.2 ft, 13.0 ft, and 17.5 ft. At each depth, at least 6 samples were tested under normally consolidated conditions. The stress-strain and strength properties were determined by consolidated undrained triaxial compression tests. The physical and chemical properties of the soil at various depths were
also determined. It was found that at consolidation pressures less than a certain pressure, the soils behaved in some aspects like slightly over consolidated clay. For the samples consolidated at pressures
greater than this pressure the soil behaved like normally consolidated clay. This pressure is called the "critical pressure"
or "apparent pre-consolidation pressure". It is the minimum
consolidation pressure required to break down the cemented soil
skeleton. The existence of this pressure is the result of an
increase in the structural strength of the skeleton by weathering processes. Its magnitude depends on the degree of weathering processes. Its which was found
magnitude depends on the degree of weathering to dimish with depth. The magnitude of the critical pressures determined by the results of triaxial compression tests were approximately the same as those obtained from consolidation tests. The critical pressure or the apparent pre-consolidation pressure was found to decrease with depth and at a depth of approximately 17 ft. the soil behaved as a normally consolidated clay.
The significant effects of change in the physical and chemical
properties of Bangkok clay in the weathered zone are, an increase
in soil consistency which diminishes with depth, a decrease in soil density (and increase in natural water content) as the depth increases, and changes in chemical properties. The effect on the
chemical properties are 1) a lowering of the pH-value in the upper part of the zone, 2) an increase in organic matter content and 3) a constant salt concentration throughout the depth. The results of triaxial tests showed that the strength para meters and the pore pressure parameter at failure were dependent upon the rigidity of the soil skeleton. This, in turn, was affected by weathering processes which caused the development of
chemical bonds between the clay particles and an increase in soil density. A rigid soil skeleton resulted in higher strength para¬ meters and a lower pore pressure parameter at failure. |
