Geotechnical characterization of the subsoil in Khulna city corporation (KCC) area

Abstract

Erratic soft subsoil deposits and presence of organic layer in the Khulna City Corporation (KCC) area pose potential challenge to the design and construction of foundation for building structures within the area. Therefore, it is essential to develop a general understanding of the characteristics of the subsoil formation of the KCC area. In this study, an attempt has been made to perform a systematic geotechnical investigation consisting of ,different field and laboratory tests extending from South to North of the KCC area. Standard Penetration Test (SPT) has been carried out at six selected locations in order to collect disturbed. and undisturbed samples and also to record SPT blow counts. Laboratory investigations have been performed in order to identify the index properties, shear strength properties and '..- compressibility properties of different soil layers. Field investigations and laboratory index test results reveal that the upper layer of north part of the KCC area i.e. Goalkhali and Khalishpur consists of subsequent layers of elastic silt, organic clay layer, elastic silt to silty clay and fat clay. On the other hand, upper layer of south non-riverside area i.e. Chotta Boyra, Sonadanga and Farazipara area consists of fat clay to clayey silt which is underlain by organic clay layer. This organic clay layer is followed by elastic silt with fine sand layer which is underlain by silt with fine sand and. mica. The top layer of south riverside area is predominantly clayey silt. Unlike other areas, no organic layer exists below this top layer. The following layers of this part of the ;KCC area consist of .'t- elastic silt underlain by silt with mica. The natural moisture content as well as degree of saturation for both organic and inorganic soil samples is high. Both SPT - N value and unconfined compressive strength of the upper layer are low and it is difficult to draw any correlation between them. Unlike upper layer, the type of soil below the organic layer can be classified as c-<t>type in which angle of internal friction dominates. Measured angle of internal friction value corresponding to relative density obtained by SPT -N value is low. The compressibility of inorganic clay and silt is moderate to high while the compressibility for orgamc clay layer is very high. The coefficient of secondary compressibility is also high for both organic and inorganic layers. Based on the geotechnical characteristics identified in this study, it is suggested that Rammed aggregate pier (RAP) may be useful ground improvement technique for low to medium rise buildings for both north and south part. Preloading with vertical drains may be considered to be the suitable ground improvement technique for the north part i.e. Goalkhali and Khalishpur area. Piled raft foundation system may be suitable for tall buildings where basement is required. Commonly practiced piled foundation may be employed for buildings where basement is not required or in situations where excavation is difficult.