Comparative study on engineering properties of block and tube samples of Dhaka clay

Abstract

This thesis contains a comparative study of engineering properties and behaviour of "block" and "tube" samples of reconstituted normally consolidated soft Dhaka clay (Liquid limit = 52, Plasticity index = 34). Reconstituted samples of soft Dhaka clay were prepared in the laboratory by Ko-consolidation of shIny in a large cylindrical consolidation cell (230 n1l11diameter by 230 mm high). A pressure of 100 kPa was used during consolidation. "Block" samplcs wcrc prcparcd by hand trimming of small blocks. "Tubc" samplcs wcrc oblaincd by stcadily pushing s,unpling lubcs of varying arca ralic and extemal diameter to thickness (Djt) ratio but of constant outside cutting edge angle (OCA) into the large diameter sample in the consolidation cell. The area ratio, Delt ratio and OCA of the tube samples were 9.7 'x. to 73.1 %, 8.3 to 44.5 and 5°, respectively. Unconsolidated undrained (UU) triaxial compression tests, unconfined compression tests, unconsolidated quick direct shear tests and one-dimensional consolidation tests were conducted on "block" and "tube" samples to evaluate their strength, deformation, stiffness, compressibility/expansibilty and peIllleability properties. In both UU triaxial test and unconfined compression test, compared with the "block" sample, the values of undrained shear strength (su),and initial tangent modulus (Ej) of the "tube" samples decreased considerably. The values of axial strain at peak deviator stress (Ep) of the "tube" samples, however, increased. In UU triaxial compression and unconfined compression test, the values of Sudecreased by 17% to 62% and the values of Ej were reduced by 7 % to 76 % due to increase in area ratio of sampler from 16.4 % to 73.1 % (or decrease in Dolt ratio from 27.3 to 8.3). Values of Ep have been found to increase by 33 % to 200 %. It has been found that compared with the "block" samples undrained shear strength of "lube" samples obtained from unconsolidated quick direct shear tests decreases by 10 'Yu to 28 % due to increase in area ratio from 9.7 % to 41.4 % (or decrease in Delt ratio from 44.5 to 12.6). Compared with the "block" sample, the values of initial void ratio (eo), compression index (Ce) and expansion index (C,) of the "tube" increascd by 16 % to 55 %, 13 % to 30 % and 20 % to 40 %, respectively due to increase in area ratio of sampler from 9.7 % to 41.4 % (or decrease in Delt ratio from 44.5 to 12.6). The values of coefficient of volume compressibility (mv) and coefficient of consolidation (ev) of the "tube" samples either increased or decreased compared with the "block" sample. However, beyond the preconsolidation pressure of 100 kPa, there are insignificant changes in the values of mv and Cv between the "block" and "tube" samples. Little change in the values of coefficient of penneability between "tube" and "block" samples up to preconsolidation pressure of 100 kPa has been observed. Beyond preconsolidation pressure, however, there is practically no change in permeability in relation to changes in vertical effective consolidation stress. At a particular void ratio there is a trend of reduction in the value of the coefficient of vertical permeability of the "tube" samples compared with the "block" sample. Effect of t"st method on the undrained shear properties has also been assess"d. The values of su, Ep and Ej of "block" and "tube" samples obtained from UU triaxial compression tests are, in general, the maximum. The values of Su of "block" samples obtained from unconfined compression test and unconsolidated quick direct shear tests are 76"1<, and 71 % oftha! ohtained in lJlJ trinxial compression test. The values ofcp and Ej of "block" samples obtained from unconfined compression tests are 75 % and 54 % of that obtained in UU triaxial compression test. It has been found that the values of Suof "block" samples obtaincd from unconfined compression tests arc higher (about 6 %) thar. that obtained from unconsolidated quick direct shear tests. For the "tube" samples, however, the values of s" obtained from unconsolidated quick direct shear tests are higher (2 % to 44 %) than those obtained from unconfined compression tests.