Study on Selection of Suitable Foundation for Khulna City Corporation Area

Abstract

The soil condition of Khulna City Corporation (KCC) area is different from other places due to its very soft nature up to a great depth and the existence of peat layer. As a result, the selection of suitable foundation type is a challenging job for the geotechnical engineer. In this region, even for a small project, a sub-soil condition is required to reveal completely for the safe design of structure. Since sub-soil exploration is expensive and time consuming, an evaluation of sub-soil condition in a comprehensive way may help designer in this context for the selection of suitable foundation for KCC area.

In this study, samples were collected by auger boring from I 09 points within KCC. From test results of these samples and using correlation equations, the parameters are calculated. The samples from auger are disturbed sample from which moisture content, liquid limit, plastic limit and specific gravity are obtained. Unconfined compressive strength (qu) and compression index (Cc) are the important parameters for design purpose for which undisturbed samples are required. For these tests sample collection, preparation and testing need considerable amount of refinement, precaution and time, with the result that the procedure is costly. This situation calls for an alternative method by which this issue can be addressed. Correlation equation is available in literature verified in the context of Khulna soil. For this purpose available existing soil reports (202 nos.) were studied for where 617 nos. value· for qu and 2 IO nos. value for Cc were collected. Using existing correlation equations q0 and Cc were calculated from corresponding values of moisture content, liquid limit, plastic limit and specific gravity. Comparing the actual and calculated values, it was observed that the actual value of qu is approximately 1.98 times more than the correlation equations qu value and Cc is approximately 1.09 times more than the correlation equation Cc value, which means the q, and Cc from equation were much conservative and safe for structure. From the result of collected existing 112 nos. bore log and 109 nos. auger boring, the position and depth of peat layer are described. By combination auger boring and existing 202 nos. soil reports, the profile of q, and Cc are prepared. From this profile, q, and Cc may be measured from different places. The position of peat layer and instant observation water table are shown.

The results accomplished in the study can be used by the practicing engineers as a background information for the selection of suitable foundation in KCC areas for the construction of relevant structures.