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.
