Effects of sampling disturbance on engineering behaviour of coastal soils from Chittagong

Abstract

The present study has been carried out to investigate~e ~ffet~~r~~~~g disturbance on engineering behaviour of reconstituted normally conso i a coastal soils from Chittagong. Disturbed soils from three selected coastal sites, namely Patenga, Fakirhat and Kumira situated nearby Chittagong city were collected. The soils were clays of low to high plasticity (plasticity index between 18 and 33). Reconstituted samples of coastal soils have been prepared in the laboratory by Ko-consolidation of slurry in a large cylindrical consolidation cell using a consolidation pressure of 100 kN/m'. "In situ" samples were prepared by consolidating 38 mm diameter by 76 mm high specimens under Ko-conditionsin the triaxial cell to its "in situ" stress state. "Tube" samples were prepared from the large diameter consolidated sample by inserting samplers of different area ratio, external diameter to thickness ratio (DJt) and outside cutting edge angle (OCA). The area ratio, DJt ratio and OCA of the samplers varied from 10.8% to 55.2%, 10 to 40 and 4" to 15° respectively. "Perfect" samples were prepared from "in I situ" samples by undrained release of the total stress in the triaxial cell. Undrained triaxial compression tests were carried out on "in situ", "tube" and "perfect" samples. "Tube" and "perfect" samples from two locations (Patenga and Kumira) were also reconsolidated isotropically and anisotropically under Ko-condition using SHANSEP procedure. One-dimensional consolidation tests were also carried out on "in situ" and "tube" samples. Disturbance due to tube and "perfect" sampling have significant influence on the mechanical properties of coastal soils. The nature of the effective stress paths and pore pressure responses of both "tube" and "perfect" samples are markedly different from the "in situ" samples. The "tube" and "perfect" samples adopt stress paths and showed pore pressure responses which are more typical of overconsolidated clays. Because of "perfect" sampling undrained strength (s,,)and Skempton's pore pressure parameter A at peak deviator stress (A,,) reduced up to 13% and 83% respectively. Axial strain at peak deviator stress (ep) and initial tangent modulus (E;), however, increased up to 32% and 47% respectively. The initial effective stress (ui') of "tube" samples reduced considerably because of disturbance caused by penetration of tubes. Compared with " in situ" samples, values of so, E; and Ap of the "tube" samples reduced while ep increased. The changes in measured soil parameters between the "in situ" and "tube" ii samples have been found to depend significantly on the sampler characteristics (area ratio, Dolt ratio and DCA) used for retrieving the "tube" samples. The values of (f;', 5,., and Ei were reduced up to 34%, 56%, and 76% respectively due to increase in area ratio from 10.8 % to 55.2 % (or reduction in Dolt ratio from 40 to 10). The respective reductions due to increase in DCA from 4" to 15" are 24 %, 51 % and 69 %. Values of e, increased up to 79% and 70% due to increase in area ratio and DCA respectively. A quantitative increase in the degree of disturbance has been obtained due to increase in area ratio and DCA. The compressibility and expansibility parameters, e.g., C, C" m" m" and c, , have not shown any noticeable trend with tube sampling disturbance. The extent of disturbance due to tube sampling has been found to depend on the plasticity of the samples. The less plastic samples suffered larger reductions in s, and (fi' than the more plastic samples. The degree of disturbance have also been found to be higher in less plastic samples than in more plastic samples. It appeared that for good quality sampling, a sampler ought to have a well combination of area ratio and DCA. In order to reduce disturbance due to sampling in soft coastal clays area ratio and DCA of sampler should be kept practically as low as possible. A correction curve has been developed from the strength data of the "tube" and "perfect" samples which can be used to find the perfectly undisturbed strength of the tube samples retrieved from coastal region studied for use in analyses and designs. Isotropic reconsolidation has the effect of grossly overestimation of "in situ" strength, e, and Ei for the "tube" and "perfect" samples. It has been found that Ko-reconsolidation of the "tube" samples and "perfect" samples to 2.5 times (f~' and 1.5 times (f~' respectively produced the best overall estimate of the "in situ" strength, stiffness and pore pressure parameter A,