Comparison of sample disturbance between standard sampling and current practices in Bangladesh

Abstract

Shelby tubes are usually used in Bangladesh for collecting undisturbed samples. Shelby tubes are cheaper as compared to any other tubes and easy to handle. However, the tubes used in Bangladesh do not conform to the standard specifications. The Conventional Shelby Tubes have highly frictional undulated surfaces, high area ratio and higher cutting edge angle. Thus Conventional Tube samples are expected to be disturbed during intrusion and extrusion. To overcome these shortcomings, a Modified Shelby Tube sampler was fabricated which have thin smooth wall surfaces, low area ratio and sharp cutting edge. To compare the extent of sample disturbance of both Conventional and Modified Shelby Tube samples, soil samples were collected by using both the samplers from the same location and depth, and taken to the laboratory to determine strength and compressibility characteristics of both samples. From the test results it was observed, in general, Modified Shelby Tube samples had larger undrained shear strength as compared to those of Traditional Shelby Tube samples. Also, the Modified Shelby Tube samples showed less compressibility than those of Traditional Shelby Tube samples. Due to more disturbance, Traditional Shelby Tubes samples had larger compression index, coefficient of consolidation and coefficient of volume compressibility. Considering vertical permeability characteristics, Modified Shelby Tube sample showed greater permeability than those of Traditional Shelby Tube samples indicating that sample disturbance may reduce the permeability of clay. Finally it might be concluded that Traditional Shelby Tube samples yield more disturbed samples than Modified Shelby Tube.