Evaluation of geotechnical properties of regional soils with particular emphasis on coastal region

Abstract

This study is carried out to develop an understanding of the geotechnical behaviour of four Coastal soils from three locations. Undisturbed block samples from shallow depths were collected from Gohira [Chittagong], Kalapara [PatuakhaliJ and Mognama [Cox"s Bazar] of the Coastal region. Physical and index properties of all the four samples were determined. Laboratory tests were performed using SHANSEP method (Ladd and Foott, 1974) to check whether the strength and stress-strain characteristics of these soils can be normalized with respect to the consolidation stress. For this purpose consolidated undrained triaxial compression (CIU) test under isotropic condition on undisturbed and reconstituted samples in normally loaded as well as pre loaded states were performed and the results for undisturbed and reconstituted samples were compared. One dimensional compression characteristics, normalized strength parameters, modulus of deformation and pore pressure parameters were established for the four Coastal soils in normally loaded and overconsolidated states both for undisturbed and reconstituted samples. The soils studied consisted of clays of high plasticity (CH), clays of low plasticity (CL) and silts of low plasticity (ML) with clay contents varying from 30 to 41 percent with little or no sand. For undisturbed sample Cc varies from 0.214 to 0.426 and for reconsti tuted sample Cc varies from 0.29 to 0.404. The average undrained strength ratio (Su/ac") in isotropic stress conditions for normally loaded undisturbed Coastal soils varies between 0.30 and 0.35. For these soils, it was found that empirical relations between undrained strength ratio and plasticity index proposed by Skempton (1957) is not applicable as it underestimates the undrained shear strength. Results indicate that strength ratio for these soils is dependent on consolidation pressure. Higher consolidation pressure yields lower stress ratio and the difference increases with increasing OCR values. A comparison of the Su/oc' versus OCR curves for undisturbed and reconstituted samples shows that undisturbed curve lies above the reconstituted curve for all soils except for the sample of Gohira (3.5 m) for which the curves overlap. Results demonstrate that undrained strength ratio versus OCR curves for reconstituted soil can be used with appropriate corrections to determine undrained shear strength of undisturbed soils if field consolidation stress and OCR are known. From one dimensional consolidation test results on reconstituted samples, Intrinsic compression lines (ICL) are drawn for thp. four Coastal soils. These coincide with the ICL for most clays furnished by Burland (1990). Sedimentation compression lines (SCL) drawn for the four undisturbed Coastal soils show that the sample of Gohira (3.5 m) is located above the SCL line while the other soils lie below the SCL line furnished by Burland (1990). This suggest that strength and compressibility properties of the sample of Gohira (3.5 m) is controlled by its in-situ structure.