Effects of sampling disturbance on mechanical properties of reconstituted normally consolidated and over consolidated Dhaka clay

Abstract

The present study has been carried out to investigate the effects of sampling disturbance on mechanical behaviour of reconstituted normally consolidated and overconsolidated Dhaka clay (OCR values of 2, 5, and 10). Disturbed Dhaka clay samples were conected from Rupnagar Housing Project, Mirpur-ll, Dhaka. The clays were low to medium plasticity (ll = 47, PI= 26). Reconstituted normally consolidated samples of Dhaka clay were prepared in the laboratory by Kc-consolidation of slurry in a large cylindrical consolidation cell using a consolidation pressure of ISO kN/m2 while overconsolidated samples having OCR values of2, 5 and 10 were prepared by reducing the maximum pressure of 150 kN/m2 to 75 kN/m2 , 30 kN/m2 and IS kN/m2 respectively. "In situ" samples were prepared (by consolidating 38 mm diameter by 76 mm high specimens under Ko-conditions in the triaxial cell) to its "in situ" stress state. "Tube" samples of various OCR values were prepared from the large diameter consolidated sample by inserting samplers of different area ratio, external diameter to thickness ratio (Djt) and out~ide cutting edge angle (OCA). The area ratio, Delt ratio and OCA of the samplers varied from 16.4% to 73.10/0, 27.3 to 8.3 and 5° to 200 respectively. Normally consolidated and overconsolidated "perfect" samples were prepared from respective "in situ" samples by WK!rainedrelease of the total stress in the triaxia1 cell. Undrained triaxial compression tests were carried out on "in situ", "tube" and "perfect" samples. "Tube" and "perfect" samples having OCR values of 2 and 10 were also reconsolidated isotropically and anisotropically under Ko-condition using Bjerrum (CKoU-1.00'vc) and SHANSEP (-1.50'vc and -2.50' vc)procedures. Disturbance due to perfect and tube sampling have significant influence on the mechanical properties of norrnally consolidated and ovcrconsolidated Dhaka clay. Disturbance due to perfect sampling led to reduction in the values of s" and Ap while Ei. Eso and f>pincreased caused due to total stress relief. Because of perfect sampling undrained strength (s,,) reduced up to 8.2% while, axial strain at peak deviator stress (f>p)increased up to 20.9"""for samples of OCR value of 10. Due to perfect sampling pore pressure parameter at peak deviator stress (Ap) reduced up to 90% while, initial tangent modulus (~) and secant stiffness at peak half deviator stress (Eso) increased up to 14.0"/0and 19% respectively for normally consolidated sample. Due to total stress relief for perfect sampling disturbance, the reduction in Su increases with increase in OCR while, the increase in Ei Eso reduce with increase in OCR. The initial effective stress (o'j) of "tube" samples reduced considerably because of disturbance caused by penetration of tubes. Compared with" in situ" samples, values of s•• Ei, Esoand Ap of the "tube" samples reduced while 6p increased. Changes in measured soil parameters between the "in situ" and "tube" samples have been found to depend significantly on the sampler characteristics (area ratio, D,It ratio and OCA) used for retrieving the "tube" samples. The values of 0';, Su, and Ei were reduced up to 26.20/0, 43% and 62% respectively for normally consolidated sample due to increase in area ratio from 16.4% to 73.1% (or reduction in D,It ratio from 27.3 to 8.3). The respective reductions for normally consolidated sample due to increase in OCA from 4° to 15° are 21.90/0,38% and 60%. Values of &pfor OCR values of 1 and 5, increased up to 57.7% and 52.6% due to increase in area ratio and OCA respectively. A quantitative increase in the degree of disturbance (Dd) has been obtained due to increase in area ratio and OCA. The results indicates that compared with normally consolidated reconstituted Dhaka Clay, tube sampling causes relatively little degree of disturbance in overconsolidated reconstituted Dhaka Clay. The reduction in initial effective stress due to tube sampling reduces with increase in OCR. The increase in value of &pdue tube sampling reduces with increasing OCR. For tube sampling disturbance, trend of small decrease in the reduction of Su I rr' Ye, Ej I rr' vc and Eso I rr' vc obtain with increasing OCR. However, significant increase in reduction of Ap with increasing OCR has been observed It appeared that for good quality sampling, a sampler ought to have a well combination of area ratio and OCA. In order to reduce disturbance due to sampling in soft Dhaka clay, area ratio and OCA of sampler should be kept practically as low as possible. A correction curve has been provided from the plot of strength ratio versus overconsolidation ratio for samples of Dhaka clay. This correction curve can be used to find the perfectly undisturbed strength of the tube samples retrieved of Dhaka clay for use in analyses and designs. Isotropic reconsolidation (CnJ-l.Oo've) has the effect of gross overestimation of "in situ" strength, 6p and E; for the "tube" and "perfect" samples. It has been found that compared with SHANSEP procedures reconsolidation using Bjerrum procedure (CKoU-l.Oo'vo) for both "tube" and "perfect" samples of overconsolidated Dhaka clay, produced the best overall estimate of the "in situ" properties in terms of undrained strength, stiffness, strain and pore pressure response.