Soil characteristics and liquefaction potential of selected reclaimed areas of Dhaka city

Abstract

Reclaimed areas are being developed in and around Dhaka city in Bangladesh by filling low lands. There are four methods for developing such areas in practice. In general, soil is collected from riverbeds by cutter-suction dredging into a barge, which is carried to the nearest river site. Soil is then pumped through pipes in a slurry form mixing with water, and transferred to the point of deposition. However, in this method of filling, segregation of particles occurs. Dredged materials are collected from riverbed and riverbanks of the rivers near the city. It was found that mean grain size, fines content, uniformity coefficient and fineness modulus of the source materials for developing such areas varied from 0.002 to 0.34 mm, 2 to 15%, 2.60 to 4.30 and 0.15 to 1.45, respectively. Field density tests were conducted near the surface at two reclaimed sites of Dhaka city. It was observed that the field density varied from 12.84 to 17.54 kN/m3. Relative density (Dr) of the samples varied from 29 to 107%. However, the relative density was more than 50% in general. About one hundred borehole data had been collected from different government and non-government agencies and sixteen borings were conducted. It was found that ground water table exists 1.5 to 2.5 m below existing growld level (EGL). Filling depth found to vary from 3 to 8 m. The SPT N-value of the filling depth varied from 1 to 13. In most cases, the soil was silty sand up to filling depth of reclaimed sites. The soil beneath the fill varied significantly. The variability in sub-soil characteristics indicates that the detailed sub-soil investigation is necessary for proper foundation design in such areas. Liquefaction potential analysis was conducted at 38 locations based on Japanese Code of Bridge Design and Chinese Criterion. The liquefaction potential analysis was not conducted at otller locations as the soil of those locations was clayey type. The value of peak horizontal acceleration, amax was taken as O.l5g according to seismic micro zonation map of Bangladesh. According to Japanese Code of Bridge Design, there are only two locations where liquefiable depth exists. Liquefiable depth of those locations is 2.5 to 5.0 m and 5.0 m from EGL. On the other hand, according to Chinese Criterion, there are 28 locations where liquefiable depth exists. The liquefiable depth varied from 2.0 to 13.0 m from EGL. However, the liquefaction potential determined by Chinese Criterion does not consider the effects of fines content. It is clear that the liquefaction potential of such areas in Dhaka city is low because the filling soil has fines. But, the presence of fines in the hydraulic fill means greater compressibility and difficulty in compaction. It also reduce permeability and hence the rate of drainage. Relative density tests can be conducted at larger depths to determine correlation between Dr and SPT N-value. Laboratory tests can be conducted to determine the cyclic strengths of the soils. More detailed study can be conducted to determine the liquefaction potential precisely using other methods and models. Liquefaction potential map of the areas can be determined using the results obtained in this study. Keywords: Reclaimed area, soil characteristics, liquefaction potential.