Geotechnical characterization of soft soil along Mollahat- Noapara road section at Bagerhat

Abstract

A considerable part of Bangladesh, especially the southwestern part of the country near the Sundarban is covered by problematic soil. The problem is intensified by the presence of substantial proportion of organic matter. A major road is being constructed in Mollahat - Noapara, under Southwest Road Network Development Project (SRNDP) at Bagerhat, through such problematic soil. The scarcity of reliable data is a vital hindrance for the development of a proper methodology to handle such soils in Bangladesh. To characterize the soil deposit, several boreholes were drilled in the project site along the road section and soil samples were collected from various depths of each borehole. Moisture content, organic content, atterberg limits, specific gravity, density and grain sIze distributions of the collected samples are determined in the laboratory. It is observed that geotechnical properties of the soil in the study area vary with depth and location. In general, moisture contract, organic content, liquid limit and plasticity index decrease with the increase of depth. Moisture content varies from 30% to 165%, organic content varies from 5% to 30%. Liquid limit is in the range of 35% to 68% and plasticity index is in range of 17% to 35%. From the test results, it is observed that more than 90% of the soils are finer than 0.075mm. A 0.3 m to I m thick peat or peaty soil layer exists at upper 0.5 to 3 m depth. Organic substance is extended up to 12 m depth. Up to 12 m depth from existing ground level, soil fall in the OL, OH and OL-OH group according to the Unified Soil Classification System (USCS). It indicates' that up to 12 m, the soil is mainly organic silt and organic silty clay of low plasticity and organic clay of medium . to high plasticity or mixture oflow to high plastic silt and clay. In the next 12 to 20 m depth, the soil is mainly in the ML group according to USCS, indicating inorganic silt and silty or clayey fine sand, or clayey silt with slight plasticity and elastic silts. However a few soil sample fall in OL group. From 20 m to 35 m depth, the soil is coarse grained and is classified as SM, SW, SW-SM according to USCS, which is mainly silty sand and sand-silt mixture or well-graded sand with little or no fines. Unconfined compression tests are conducted for fifty eight samples collected from different locations and depths. It is observed that unconfined compressive strength varies in the range from 10 to 150 kPa for SPTP-N value in the range from I to 5. It is also observed that q, increases with the increase of SPT-N value. However, q, and SPT-N value increase with the increase of depth. Again it is observed that there is no definite relationship between organic content and q,. But in general, q, decreases with the increase of organic content. From consolidation characteristic it is observed that compression index varies in the wide range from 0.156 to 0.628. Initial void ratio varies in the range from 0.95 to 2.13. It indicates that the soil is highly porous. The coefficient of consolidation varies in the range from 0.48m'/year to 26.98 m'/year. Surcharge load was applied in two trial sections to accelerate the consolidation rate of the sub soil. Settlement due to surcharge at the end of 180 days is 225 mm. But the anticipated settlement was 336 mm. It indicates that the estimation for the settlement is in the good agreement. Key words: Compression Index, Consolidation Settlement, Initial Void Ratio, Organic Content, Peat, Soft Soil, SPT -N value, Trial Section, Unconfined Compressive Strength.