Geotechnical Interpretation Of Soil For The Proposed Underground Utility Tunnel From Kuril To Malibagh In Dhaka City
For the proposition of establishment of upcoming 7.7 km common utility tunnel under Progoti Sarani in Dhaka between Kuril flyover and Malibagh Chowdhurypara, Dhaka North City Corporation (DNCC) has stepped forward to complete a feasibility study report accumulating all soil test data around Dhaka city found from different secondary organizations. Besides that, four boreholes to a depth of about 30m each were drilled at different locations and consequent laboratory tests are also done as per the requirement. In that perspective, a new aspect of geological and geotechnical site of Dhaka city soil profile is prepared based on those primary and secondary data focusing a single part of city from Kuril to Malibagh. The prepared soil profile is compared with the geomorphological map of Dhaka city published by Geological Survey of Bangladesh (GSB) and found that most of the soil in Dhaka city is of Madhupur terrace (clayey silt and silty clay) of Pleistocene era. The range of liquid limit of maximum data in the project is between 40 and 60, plasticity index is greater than 4 and limit values between 'A' line [0.73(LL-20)] and 'U' line [0.9(LL-8)] of plasticity chart, so the soil criteria will be CL (Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays) and CH (Inorganic clays of high plasticity, fat clays). The soil is found Over Consolidated Clay (OCC) because in most of the cases natural water content (W) is close to Plastic limit (PL). Most of the clay minerals are found Illite and Kaolinite which is classified between Inactive and Normal clays (Activity ratio, A = 0.38 to 0.90) which means that there may be no significant swelling and shrinkage characteristics in the project area along with changing of water contents as well as the possibility of structural damage is also negligible. But a matter of concern is that in some points soils are collapsible according to plasticity index. Results of chemical analysis indicate that the sub-soils are not aggressive to concrete and not corrosive to reinforcement steel.
Index Terms� Dhaka City, Geotechnical Interpretation, Subsurface Soil Profile, Soil Properties.