SUSTAINABLE UNDERGROUND CONSTRUCTIONS INCLUDING TUNNELS FOR DHAKA CITY, BANGLADESH: A GEOLOGICAL AND GEOTECHNICAL INSIGHT

Cities without the use of underground space are unthinkable in a modern urban environment. Mega-city Dhaka in Bangladesh has yet to step into the underground construction world to take advantage of its underlying unique and firm soil, the stiff-to-hard Madhupur Clay Residuum, which overlies dense-to-very dense sandstone of the Dupi Tila Sandstone Formation. Dhaka is very densely populated and continues to be among the fastest growing city in Asia. Uncontrolled growth including skyward expansion have caused tremendous overcrowding and exasperating traffic congestions. However, use of underground space is still ignored and largely unexplored. The possibility of building an underground rapid transit system will relieve many of the present urban stresses.

The city was developed on an uplifted tectonic block (anticlinorium) of Plio-Pleistocene age, which is isolated from the surrounding floodplains of active Ganges-Brahmaputra delta system. The top 20 ft of ground is composed of stiff-to-hard over-consolidated clayey soil (Layer 1), that overlies thick moderately lithified sand (Layer 2, drilled to 200 ft). A 3D geological model is prepared to illustrate the spatial distribution of these stratigraphic layers. Extensive geophysical and geotechnical exploration data including hydrogeological conditions are compiled to give insights and engineering options for shallow and deep underground construction. The geotechnical and geophysical properties of the geological strata include vertical consistency, density, undrained shear strength, consolidation and settlement characteristics, and shear wave velocity. A plot of geotechnical properties with depth indicates improved ground conditions occur with the increasing depth. The 3D geological model shows that strata are homogenous and uniformly distributed. The increase of density, shear strength and shear wave velocity (>1400 ft/s below 20 ft) with depth is a strong advantage for planning of safe tunnels and underground structures.