Study on application of cut and cover method for the construction of metro rail tunnel in Dhaka city

Abstract

A feasibility study for a metro rail tunnel for Dhaka was performed in this thesis. A tunnel alignment along the existing surface railroad system from Tongi to Kamalapur was chosen for the feasibility study. Infrastructures close (4 to 40 m from edgc) to the existing railway that may be effected due to tunnel construction were identified. Density of building close to the road line is highest between the the Basabo, Moghbazar ill1d Mohakhali areas. Howevcr, some unauthorized structures exist close to the rail lines. These arc unlikely to create problem during constructions as these may be demolished. Geo technical characterization of subsoil condition along the proposed tunnel alignment revealed variability of the subsoil. The subsoil was characterized on the basis of available literature. Considering the soil profile and the soil parameters, four general cases of ground condition has been chosen for ill1alysis of earth retention system for tunnel construction to cover a wide range of ground conditions. Tunnel structure and the station building were chosen as those used for typical metro lines. Design of the earth retention system revealed that cantilever sheet pile wall will not be suitable for the ground conditions along the rail alignment and for the depth of excavation expected for the tunnel in Dhaka. Earth retention systems consisting of sheet pile, contiguous drilled piles ill1d diaphragm walls with lateral bracing have been designed for the proposed railway tunnel. The effect of nearby structures on the design of the retention system revealed that if the structure is located at 8 ft or beyond, the effect of the building structure on the earth retention system would be negligible. Among the earth retaining systems, cost of diaphragm wall was found to bc the minimum of the three systems i.e, sheet piling, contiguous piling and diaphragm walling. The cost of diaphragm wall for the proposed tunnel ranged from Tk. 29.6 crore to Tk. 46 crore per km. The costs of diaphragm wall is 17 to 21 percent less than those for braced-cuts, and 15 to 23 percent less than those for drilled contiguous pile system. The cost/km of tunneling using diaphragm wall system appear to be 26% less than the cost of Flyover Structure at Mohakhali. The study revealed that metro tunnel system is feasible for Dhaka city and would appear as an effective solution for the current traffic condition. Diaphragm wall system appeared to be the most effective for construction of the tunnel.