Characterization of Khulan Subsoil and Settlement Response of This Soil Improved by Granular Fills

Abstract

The study area, Khulna City Corporation (KCC) is one of the important urban areas in the South-West part of Bangladesh. In the present study the whole KCC area was subdivided into three zones: North, Middle and South. In the North Zone (Afilgate, Shiromoni, Fulbarigate, Mohesshorpasha, etc) it is found that the soil layers between 0 12 m from EGL are mostly clayey silt. In this stratum there exists an organic clay layer with the variation of 3 m to 6 m depth in most of the locations. The soil layers between the depth of 12 in to 18 m at the northeast part of the North zone, which is near Bhairab river, contain predominantly sand, while in the areas namely Fulbarigate, KUET campus the soil at this depth contains predominantly silt. In the middle zone (Khalishpur, Daulatpur, Boyra, Mujgunni, etc.) it is found that the sub-soil in the western side contains an organic layer of 3m to 4.5 m thickness starting from about 1.5m to 3m depth to downward. Below this depth the soil consists of predominantly silt up to about 18 in depth. In the eastern side of this zone the soil contains predominantly silt up to about 6m to 7.5m depth. Below this depth the soil contains mainly fine sand. In the South zone (Sonadanga, Nirala, Gollamari, Sheikhpara, West side of Rupsha, Khulna University, etc) the sub-soil contains organic clay up to about 3m to 4.5m depth from EGL. Below this level the soil mainly consists of silt with small portion of clay and organic matters up to about 18 m depth from EGL. From the model test it was found that the load carrying capacity of soft soil can be increased significantly by adopting ground improvement technique with filling sand under foundations. The rate of settlement of the test footing on untreated ground increased substantially after the settlement of the ground exceeds 20mm. It is also evident that the load bearing capacity of treated grounds do not depend on the depth of filling only but also on the width of granular fills. From the experiment, it was seen that the load bearing capacity was increased by replacement of soft soil with granular materials (sand) at the bottom of the foundation up to 1.95 times than that of untreated soil. The load bearing capacity of treated soil was significantly increased with the increase of depth of sand filling under foundation. The bearing capacity was increased by 30%, 72% and 85% with the increase of filling depth by 1.0, 1.5 and 2 times the width of foundation respectively when area of foundation is equal to area of sand filling. For the granular fills of depth 150 mm(D1Bf , the degrees of improvement were found as 30%, 56% and 59% higher than that of untreated condition when width of filling were 1 Bf, 21 Bf, and 313f respectively. So, it was exhibited that for constant depth ((Ds= 1 Bf) of compacted sand, bearing capacity was increased significantly up to the width of sand filling equal to twice the width of foundation. In case of sand fills of depth 225 mm (Ds=1 .5Bf), the degrees of improvement were found as 72%, 79% and 8 1 % higher than that of untreated condition when width of filling were 1Bf, 2.51Bf, and 4Bf respectively. From this it is observed that for constant depth (Ds=1.5Bf) of compacted sand, bearing capacity was increased significantly up to the width of sand filling equal to 2.5 times the width of foundation. When the depth of granular fills was increased up to 300 mm (Ds=2Bf , the degrees of improvement were observed as 85%, 92% and 95% higher than that of untreated condition when width of filling were 1 Bf, 3 Bf and 5Bf respectively, it is observed from this result that for constant depth ((Ds=2Bf) of compacted sand, bearing capacity was increased significantly up to the width of sand filling equal to 3 times the width of foundation.