Padma Multipurpose Bridge Project

3.1.2.2 Strata along North Bank9. Eight boreholes were sunk down to 35 and 70 m depth. Disturbed and undisturbedsamples were taken in regular distances; Standard Penetration Tests were performed as well. Thevisual soil classifications were corrected by the classification tests in laboratory. On the basis of thesecorrections, together with the number of SPT-values, the strata profile along the north bank wasprepared (Figure 3-4). River banks, as those of Padma River, are characterized by continuous erosionand deposition. Stratification is not coherent and homogenous; the deposits vary in composition,density and consistency even in short distances. Soils in adjacent boreholes, at similar level and withsimilar composition, do not belong necessarily to the same “stratum”. They were deposited atdifferent times and on different conditions. For the assessment a certain generalization is required.For this reason the generalized Longitudinal Strata Profile cannot show all local variations.10. The boreholes conducted on the mainland did not show clay at this depth, but siltysand/sandy silt. For further considerations a working hypothesis is derived: It is clay from the thicksurface layer. This clay layer rests on top of the fine grained silty sand/sandy silt which is verysusceptible to erosion. It can be easily washed out and the clay breaks off in big and high clods.These clods of clay lay now above the silty sand/sandy silt, medium dense to dense (layer 3) andreplaced layers 2 in front of the river bank step by step.Figure 3-4: Longitudinal Strata Profile11. At North Bank a thick layer of clay stabilizes the riverbanks. The clays belong to the groupof floodplain clays. Floodplain clays are usually weakened by joints and fissures due to depositionfollowed by cyclic expansion and contraction from wetting and drying. Many soft seams, embedded inthe layer, confirm this observation. The presence of fissures produces some permeability even in theclay layer. This explains the observation that water level in fishery ponds reacts in short time to thevariations of the river level.3.1.2.3 Strata along South Bank12. In the course of the bridge design 4 boreholes (ABBH 01, ABBH 02, ABBH 03, ABBH 04)were performed in the bridge axis from ~140 m behind the present riverbank to the Charland in frontof the riverbank. Laboratory testing was carried out on samples collected below -80 m PWD. On theflood plain there is 2 – 3 m thick layer of young clay. Consistency ranges from very soft to soft.13. On the mainland below this clay layer (ABBH 01) sand is following having medium density.3-5