Abstract:
The bearing capacity of subsoil is crucial for designing the foundations of any structure. Shear strength plays a vital role in the development of soil bearing capacity. For non-plastic soils (sand-silt mixture), the shear strength is expressed in terms of angle of internal
friction (ф). A process for determining the angle of internal friction is to collect undisturbed soil samples from required depths and perform appropriate laboratory tests, which is
time-consuming and expensive. Another way, widely practiced for non-plastic,
i.e., cohesionless soils, is to assess the angle of internal friction from Standard Penetration Test (SPT) N-value. Particularly, this approach is relevant to design of deep foundations in and around Dhaka City, where the sub-soil profile typically consists of an upper layer
(5 to 10 m thick) of brown to reddish brown medium stiff to stiff clay underlain by a deep deposit of medium dense to dense silty sand. Several researchers have suggested empirical correlations to assess the angle of internal friction from SPT N-values and other factors, namely, fines content (% passing #200 sieve), relative density, maximum and minimum density. Empirical correlations are also presented in BNBC (2020), representing angle of internal friction in terms of SPT N-value only. Some of these correlations, including those in BNBC, are widely used in Bangladesh mainly due to the unavailability of equipment and expertise besides reducing cost and time. However, the appropriateness of these equations has not been verified for any non-plastic deposits in Bangladesh.
In the present study, available data from several projects at various locations in Dhaka city on SPT N-value of non-plastic deposits along with other parameters such as fines content, uniformity coefficient, coefficient of curvature, fineness modulus, and angle of internal friction (from laboratory tests on disturbed samples) were collected. These data were analyzed to identify the significant factors that govern the angle of internal friction of the soil deposits. Obviously, SPT N-value is found as the most important factor affecting
ф values. For SPT (N1)60-value of 15, the ф values ranged within 16.20 to 28.20 and for
SPT (N1)60-value of 40, the ф values ranged within 21.50 to 33.50. ф values increased almost linearly with SPT (N1)60. The fines content appeared to be the next important factor after SPT N-value governing the ф values. For a given fines content, the ф values are found to vary as much as 100. Correlations available in the literature, established for non-plastic soils of several other countries, appeared unsuitable for the local soils. Therefore, an empirical correlation for ф in terms of SPT (N1)60 and fines content is first established using the available data.
To verify the established empirical equation, 3 (three) exploratory boreholes (30 m deep) were drilled at Mirpur-2, Banani, and Golapbag area of Dhaka city. Standard Penetration Tests were conducted and disturbed soil samples were collected from the non-plastic layers. Direct Shear tests were conducted on the collected samples to obtain the angle of internal friction along with determination of physical properties. It is found that the correlation developed in this study may be applied to the non-plastic subsoil deposit in Dhaka in which there is a large variation (as much as 90%) of fines content.