Foundation alternatives in fill soils overlaying organic layer

Abstract

The main objectives of this research are to determine the sub-soil characteristics (especially the properties of the soft organic layer beneath the filling layer) of selected reclaimed areas and to propose suitable alternative foundation systems for such sub-soil condition. In order to identify the geotechnical parameters, investigations have been carried out at some selected reclaimed areas. Total fourteen soil borings have been conducted at four reclaimed areas of Dhaka city. Besides, data have been collected for six reclaimed areas within Dhaka city. A silty clay or a dense sand layer exists under the filling layer in such areas. The depth of filling layer of sand varies from 1.5~7.5 m from existing ground level (EGL). The thickness of the soft organic clay varies from 3.0~13.5 m from EGL. Beneath this soft organic layer, a soft silty clay and dense sand have been found up to 18 m depth from EGL. Groundwater table exists at 0.6~7.0 m below the EGL. The uncorrected SPT N-value of the filling sand layer varies from 1~11. The mean grain size (D50) and fine content (Fc) was found to vary from 0.015~0.210 mm and 12~31%, respectively. The uncorrected SPT N-value of the organic clay varies from 1~4. Dry unit weight, moisture content and organic content have been found to vary from 4.6~12.1 kN/m3, 29~140% and 7.9~29.4%, respectively. It indicates that, this clay is very soft in consistency. Liquid limit (LL) and plasticity index (PI) of organic clay varies from 38~190% and 11~62%, respectively. From the above results, it is observed that this layer contains organic silt (OL) and organic clay (OH) which are medium to highly compressible. Unconfined compressive strength of the clay varies from 6~66 kPa. It has been found that initial void ratio and compression index of soft organic layer vary from 0.87~3.88 and 0.28~1.25, respectively. The coefficient of consolidation (cv), coefficient of volume compressibility (mv) and permeability (k) vary from 0.25~17.25 m2/yr, 0.06×10-3~7.30×10-3 kN/m2 and 9.06×10-12~4.97×10-9 m/sec, respectively. Attempt has been made to correlate unconfined compressive strength (qu) with SPT Nvalue, plasticity index (PI) and organic content (OC). Correlation does not exist between qu and SPT N-value. No definite correlation has been found between qu and PI as well as OC. However, it has been found that qu increases with increase of PI and decreases with the increase of OC. Attempt has also been made to correlate compression index (Cc) with OC and initial void ratio (eo). In this case also no definite correlation was obtained. The increasing tendency of Cc has been observed with the increase of OC. However, a correlation between Cc and eo has been found. Total consolidation settlement and time dependent settlement of organic layer due to the surcharge of the filling layer have been estimated. It has been found that total consolidation settlement of the study area varies from 447~1734 mm. It has been observed that settlement time (considering single drainage) varies from 6~106 years. Performance of foundation systems in similar sub-soil condition has been studied. Some foundation systems are suggested for the study areas. Spread footing with Rammed Aggregate Pier (RAP), buoyancy raft foundation and pile foundation have been suggested. The sub-soil condition indicates that negative skin friction may develop in case of pile foundation in such sub-soil condition. Possible mitigation methods for negative skin friction have been discussed. Keyword: Reclaimed area, fill soil, soft organic clay, strength and compressibility characteristics.