Stress-deformation characteristics of selected coastal soils of Bangladesh and their sampling effects

Abstract

A considerable development activity within the Coastal Region of Bangladesh has necessitated an understanding of the geotechnical behaviour of soils from this region. With this objective in view a study into strength-deformation, compressibility and intrinsic properties of reconstituted samples of Chittagong coastal soils were undertaken. This thesis presents stress-deformation characteristics of three selected coastal soils and their sampling effects. The soils were collected from Banskhali, Anwara and Chandanaish in Chittagong coastal belt of Bangladesh. The soils are low to medium plasticity (Liquid limit = 34 to 45 and Plasticity index = 10 to 20). Reconstituted samples of the three soils were prepared in the' laboratory by Koconsolidation of slu~ in a large cylindrical consolidation cell using a consolidation pressure of 150 kN/m . Overconsolidated samples were prepared in the triaxial cell by releasing the maximum isotropic consolidation pressure of 150 kN/m2 to appropriate values to achieve overconsolidation ratios (OCR) of 1.5,2, 5, 10,20 and 30. The stress-deformation-strength, stiffness and pore pressure characteristics of reconstituted isotropically normally consolidated and overconsolidated "block" samples of the three coastal soils were investigated in the laboratory by performing undrained triaxial compression tests. Models for the prediction of undrained shear strength of normally consolidated and overconsolidated samples have been developed. To develop intrinsic models of compressibility, intrinsic compression lines (ICL) for the three soils under Ko and isotropic stress conditions have been established which can be used to determine compressibility indices of soils at any depth of known overburden pressure. State boundary surfaces (Roscoe and Hvorslev state boundary surfaces) and critical state lines of the three coastal soils have been established. The critical state parameters of the soils have also been evaluated. Constitutive models relating critical state soil parameters and plasticity index of the soils have been proposed. Applications of these models to undisturbed natural clays will require further investigation. The experimentally observed stress-strain behaviour of reconstituted normally consolidated samples of the three coastal soils have been compared with those predicted using two critical state models, namely, "Cam clay model" and "Modified Cam clay model". It has been found that the results predicted by using "Modified Cam clay model" compared more favourably with the observed experimental results than "Cam clay model" for the three coastal soils. The present st\ldy has also been carried out to investigate the effects of "perfect" sampling disturbance and tube sampling disturbances on engineering properties of reconstituted normally consolidated samples of the three coastal soils. Undrained triaxial compression tests were carried out on "in situ", "perfect" and "tube" samples. "In situ" samples were prepared by consolidating reconstituted specimens of 38 mm diameter by 76 mm high under Ko-condition in the triaxial cell to its in situ stress state. "Perfect" samples were prepared from "in situ" samples by undrained release of the total stresses in the triaxial cell. "Tube" samples were prepared from the large diameter consolidated samples by inserting samplers of different area ratios, external 'diameter to thickness ratio (De/t) but of constant outside cutting edge angle (OCA) and internal diameter (D,). Area ratio, Delt ratio, OCA and Dj of the samplers were 16.4% to 73.1%, 27.3 to 8.3, 50 and 38 mm, respectively. Undrained triaxial compression tests were carried out on reconsolidated "perfect" and "tube" samples of the three coastal soils to assess the suitability of various reconsolidation techniques to minimize sampling disturbance effects. Experimental results indicate that disturbances due to perfect and tube sampling have significant influence on the mechanical properties of coastal soils. The nature of the effective stress paths and pore c pressure responses of both "perfect" and "tube" samples are markedly different from those of the "in situ" samples. The "perfect" and "tube" samples adopted stress paths and showed pore pressure responses which are more typical of overconsolidated clays. Disturbances due to perfect sampling led to reduction in the values of undrained shear strength (su), Skempton's pore pressure parameter A at peak deviator stress (Ap), initial tangent modulus (Ej) and secant stiffuess at half the peak deviator stress (Eso) while axial strain at peak deviator stress (cp) increased due to total stress relief. Due to total stress relief, the reduction Su,Ej and Eso increased with the decrease of plasticity while the increase in cp increased with the decrease of plasticity of the soils. It is also evident that the decrease in mean effective stress (p') due to perfect sampling increases with decreasing plasticity of the soils. The initial effective stress (a'j) of "tube" samples reduced considerably because of disturbance caused by penetration of tubes. Compared with the "in situ" samples, values of su, Ej, Eso and Ap of the "tube" samples decreased while .cp increased. The changes in measured soil parameters between the "in situ" and "tube" samples have been found to depend significantly on the sampler characteristics, i.e., area ratio, Delt ratio, used for retrieving the "tube" samples. The values of a'j, su, Ej and Eso were decreased due to increase in area ratio (or reduction in Delt ratio). The values of cp, however, increased due to increasing area ratio. A quantitative increase in the degree of disturbance (Dd) has been obtained due to increase in area ratio, while the values of Dd increased with the decrease of Delt ratio of sampler. Disturbance due to tube sampling has been found to depend on the plasticity of the samples of the three coastal soils. The highest reductions in a'j, Su,Ej and Eso occurred in the least plastic samples, whereas the minimum reduction in a'j,su, Ej and Eso occurred in the most plastic samples. Among the samples of the coastal soils, the least plastic sample produced higher degree of disturbance than the most plastic sample. It appeared from the present investigation that for good quality sampling, a sampler ought to have an area ratio as low as possible, preferably less than 10 %. A correction curve has also been developed from the strength data of "perfect" and "tube" samples for estimating the perfectly undisturbed undrained shear strength of the tube samples retrieved from the coastal region studied for use in geotechnical analyses and designs. Isotropic reconsolidation to a pressure equal to vertical in situ pressure a'vc (CIUl. Oa'vc) has the effect of producing large overestimation of in situ strength su, cp, Ej and Eso of the "perfect" and "tube" samples. Isotropic reconsolidation to a'vc also overestimated the values of Ap. However, isotropic reconsolidation to a pressure equal to isotropic effective stress a' ps(CIU-I.Oa'ps) of the "perfect" sample underestimated the values of sU, E" Eso and Ap, while overestimated the value of cp for "perfect" samples. It has been found that compared with SHANSEP procedures, Koreconsolidation up to in situ state of stress, i.e., Bjerrum procedure (CKoU-I.Oa'vc) produced the best overall estimate of the in situ properties in terms of the undrained strength, strain, stiffuess and pore pressure response.