Geology of Dhaka
Dhaka is situated at the southern tip of a Pleistocene terrace, the madhupur tract. Two characteristic geological units cover the city and surroundings, vizMadhupur Clay of the Pleistocene age and alluvial deposits of Recent age. The Madhupur Clay is the oldest sediment exposed in and around the city area having characteristic topography and drainage. The major geomorphic units of the city are: the high land or the Dhaka terrace, the low lands or floodplains, depressions and abandoned channels. Low lying swamps and marshes located in and around the city are other major topographic features.
The subsurface sedimentary sequence, up to the explored depth of 300m, shows three distinct entities: one is the Madhupur Clay of the Pleistocene age, characterised by reddish plastic clay with silt and very fine sand particles. This Madhupur Clay unconformably overlies the dupitila formation of the Plio-Pleistocene age, composed of medium to coarse yellowish brown sand and occasional gravel. The incised channels and depressions within the city are floored by Recent alluvial floodplain deposits and are further subdivided into Lowland Alluvium and Highland Alluvium.
Geotechnical Characteristics
Geotechnical Characteristics of the Madhupur Clay in Dhaka city and its surroundings vary significantly both aerially and vertically. The evaluated parameters, particularly its low strength and high compressibility values indicate that the clay, to some extent, is problematic for engineering construction. The moisture content and plastic limit results show that Madhupur Clay is normally consolidated to over-consolidated. The clay is normal to active and has intermediate to high plasticity. The compressibility values suggest that the clay is very low to highly compressible at different locations.
Hydrogeology
The DupiTila sands aquifer is the main source of water in Dhaka city. Madhupur Clay overlies the aquifer with a thickness of 8 to 45m (averages 10m). The aquifer varies in thickness from 100 to 200m (averages 140m). The groundwater level has dropped by 6m in the last seven years. The water table was 52 meters below mean sea level in 2011. Under the present conditions the peripheral rivers act as sources of recharge as the DupiTila sands are exposed along the riverbeds. Other sources of recharge are vertical percolation of rain and flood water, leakage from water mains and the sewer system and seepage from standing water bodies within the city.