Summary : The City of Dhaka occupies a unique geological location in terms of the geological set up and ground condition. Geologically the city landmass is a part of an old terrace that is raised considerably up than the surrounding... more
Summary :
The City of Dhaka occupies a unique geological location in terms of the geological set up and ground condition. Geologically the city landmass is a part of an old terrace that is raised considerably up than the surrounding floodplains. The general elevation in the axial part of the city reaches up to about 6 to 7m above mean sea level (AMSL) whereas the surrounding floodplains are at about 4m AMSL. The ground of the terrace is composed of Madhupur Clay (Pleistocene), consisting of very stiff to hard, reddish brown, highly ferruginous, plastic Clay-Silt with some minor contents of fine quartz-sand and iron concretions. The shear strength properties are considerably high and the average value of un-drained shear strength is ≈ 2 kg/cm2. The settlement characteristics indicate that the red clay to be an over-consolidated stratum. The geological history indicates that due long run erosion about 20m of materials have been removed from the top of the city ground. This indicates that an amount of ≈ 1 kg/cm2 of vertical stress has removed from the top of the existing ground surface. At present the average thickness of Madhupur Clay in the main urban area is about 7 meters. The upper clay-silt stratum overlies a firm sand bed, geologically known as Dupitila Formation (Upper Tertiary). The geological and geotechnical properties of Dupitila Sand (Upper Dupitila Sandstone) tends to behave as soft rock. This soft sedimentary rock acts as dense bedrock to hold most of the pile tips of all deep foundation which is uniformly distributed throughout the axial part of the city.
In terms of hydro-geological environment the Madhupur Clay Formation acts a wonderful cap over the Dhaka terrain. The top material is low in permeability and no ground water recharge occurs in the city area at ambient condition. The surface materials and natural topography with unique natural drainage system the flash-flood water has excellent run-off characteristics. The permanent and confined ground water level is located below 70m from the surface. The existing geo-hydrological environment provides better options for underground excavation.
The existence of Madhupur Clay and Dupitila Sand below the urban feet turns the ground condition of Dhaka city to the best comparing the ground condition of many other cities of the world. Though Bangladesh is located at edge of most seismically active global zone, in terms of the ground condition the apparent seismic hazard* is relatively low in Dhaka city than most of other parts of the country (expected maximum Peak Ground Acceleration is 0.5g). A correlation on Shear Wave Velocity (Vs) of the geological materials underneath Dhaka city indicates that the value of Vs would be > 350 m/sec. Considering the geological and geotechnical aspects, the city of Dhaka has advantageous ground condition for construction of underground engineering structures including underground rapid transportation system.
