Padma Multipurpose Bridge Project

With increasing depth density increases as well. In the direction to the char there is loose sandhaving a thickness of 30 m below the clay layer. In a depth of -27 m PWD to -37 m PWD stiff claywas detected. However, this clay was not found in the other boreholes. Borehole ABBH 02 incomparison with ABBH 01 demonstrates the change between the more stable deposits on themainland and the continuously redeposited material in front of the riverbank and on the char.14. The near-surface clay layer was deposited in very recent times. The sand deposits arecoarser than those at the north bank and the silt fraction is smaller. Mica content at south bank isprobably higher than at north bank. At the South Bank the stabilizing clay layer does not exist. It onlyexists locally. The soil profile is formed by silty sand/sandy silt exclusively. The grain sizes arecoarser than at the North Bank. The upper side of the strata profile has predominantly loose (tomedium) density. Also here density increases to medium density below riverbed. At South Side,however, the riverbed is situated at lower depth. According to bathymetric survey it is situatedapproximately at -10 m PWD. The test results show a clearly higher content of mica minerals atSouth Bank than at North Bank.3.1.2.4 Char in front of Janjira15. The investigations of the main bridge showed silty sand on the surface down toapproximately 15 m (Figure 3-5. Silty sand, medium dense, follows. Density increases with the depthand finally the relative density gets more and more medium dense to dense. All deposits containmica.Figure 3-5: Cross-Section along Bridge Axis South Bank16. Textural analysis of soils has been carried out during geotechnical investigations from thesoils collected from test pits on both the banks and Charland. Results of this analysis are shown inTable 3-2. Soils in the left bank have higher silt content than the soils of Charland and right bank.Generally soils on the left bank are very stable and less prone to River erosion. Soils in the Charlandcan be classified as ‘silty clay loam’ with sand and silt content of 70% and 27%, respectively. ThisCharland is a relatively stable Charland formed about 20 years ago. Soils on the right bank nearservice area are ‘loamy sand’ with a very high sand content of 82%. Hence, soils in the right bank aremore prone to River erosion. Most of the soils in the Project area are highly fertile from agricultureviewpoint and suitable for agricultural crop production and tree plantation.Table 3-2: Texture of soil samples obtained from test pitsClose to the Charland Close to the Right Bank nearLeft Bank (Charland Right Bank Service Area 3Janajat)Test Pit Number TP-1 TP-2 TP-3 TP-4Depth (m) 2.4 ~ 3.0 2~2.5 2.4~2.8 2.4~3.0Water content (%) 30 28 24 32Specific Gravity 2.74 2.74 2.74 2.66Gradation:Sand (%) 2 27 13 82.53-6