Experimental investigation of sampling disturbance in Dhaka clay

Abstract

This thesis contains an experimental investigation on the effects of sample disturbance using reconstituted Dhaka clay. Reconstituted samples of Dhaka clay (LL = 45, PI = 23) have been prepared in the laboratory by K,,-consolidationof slurry in a large cylindrical consolidation cell (260 mrn dia by 305 mm high). A pres~'ureof 100kPa was applied during consolidation. "In situ" samples were prepared by hand trimming of small blocks. "Tube" samples have been obtained by pushing sampling tubes of varying area ratio (AR) and outside cutting edge angle (OCA) in to the large diameter sample in the consolidation cell. The AR and OCA of the tube samples varied from 10.8% to 55.2% and 4° and 15° respectively. "Perfect" samples were prepared from "in situ" samples by undrained release of total stress in a triaxial cell. UU, CIU and CKoUtriaxial compression tests were carried out on "in situ", "tube" and "perfect" samples having nominal dimensions of 38 mm dia. by 76 mm high in a triaxial cell with pore pressure measurements. Anisotropic reconsolidation under Ko-conditionbeyond in-situ stresses of the "tube" and "perfect" samples were also carried out to investigate the suitability of use of SHANSEP procedure of consolidation to reduce effecl~ of sampling disturbance . Onedimensional incremental loading consolidation tests were also carried out on "in situ" and "tube" samples. Compared with "in situ" sample, the initial effective stress of the "tube" samples reduced significantly (5.4% to 32.1 %) because of disturbance caused by penetration of tubes. The undrained strength, s, and the stiffness of the "tube" samples decreased with increasing AR and OCA. Initial tangent modulus,E, and secant stiffness at half of the peak deviator stress, E", also reduced considerably because of increasing AR and OCA. Depending on AR and OCA, s" E, and E", decreased from 2% to 23%, 3% to 54% and 3% to 42% respectively. Axial strain at peak deviator stresS,E. of "tube" samples increased considerably (14% to 47%). Compared with "in situ" samples, the pore pressure changes of the "tube" samples during undrained shearing to failure are less, resulting in lower values of Ap. Values of A, decreased upto a maximum of 40%. The degree of disturbance (D) of "tube" tube samples increased with increasing area ratio and out~ide cutting edge angle. For an increase in area ratio between 10.8% and 55.2%, the degree of disturbance increased up to 121%. The degree of disturbance increased up to about 68% for an increase of outside cutting edge angle from 40 to 150. Because of disturbance, the changes in the compressibility and permeability properties have been found to be insignificant. In comparison with "in situ" sample, "perfect" sampling produced appreciably ditferent effective stress paths. Because of "perfect" sampling undrained strength ratio decreased and the stiffness parameters increased compared with "in-situ" sample. The normalized stiffnesses E,I(J~' and E",I(J~' have been increased by about 67% and 42% respectively. The value of Ap reduced considerably (about 53%) because of "perfect" sampling. Compared with "in situ" sample, the stiffness and strength of "tube" and "perfect" samples reconsolidated using SHANSEP-2.5 procedure were significantly less. The values of Ep and A. were increased considerably in both "tube" and "perfect" samples. The result~ of Koreconsolidation of both "tube" and "perfect" samples using the SHANSEP procedure, therefore, appear to indicate that SHANSEP procedure of reconsolidation to restore "in-situ" behaviour may not be applicable to Dhaka clay. From the UU, cm and CKoU triaxial copression tests carried out in this research to investigate the effect~ of sampling disturbance, it has been found that compared with CKoU triaxial compression test, CIU triaxial compression test grossly overestimates the strength, stiffness while UU triaxial compression test underestimates strength and stiffness.