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.
