Study on application of NATM method for construction of metro system in Dhaka city

Abstract

A study on the application of various methods of construction of underground metro for Dhaka city is undertaken in this thesis. A tunnel route alignment along the existing road surface starting from Pallabi to Sayedabad is chosen for this study. Infrastructure nearby the route that may affect the total tunnel construction or that may get affected by underground tunneling is identified in order to propose the best possible metro construction system for the route. A high density of the buildings nearby the selected route is found to be located between Bijoy Sarani and Bangla motor intersection. Also some rare and large structures are located in the Pallabi area. Based on the nature of the methods discussed in this study none of the existing structures in the route create a big problem. Identification of challenges to be encountered on the route of the Metro construction is undertaken in this study. Availability of unmanaged and unarranged network of utility lines lying beneath the road surface in the selected route makes the management of the city utility networks in case of tunnel construction a bit difficult as cut and cover method of construction is concerned. Also present situation of the network of utility line which means dislocation and relocation for construction of metro is a great cost in this aspect. Another challenge to be faced in this route is application of future development programs in the Dhaka city. In two points of the route there are very large conflicts among the three projects meeting which they need synchronization today in order to construct all the projects in future for specified point. Application of any proposed project without synchronizing with other two makes the application of other project very difficult or even impossible. The third challenge identified is the presence of large buildings nearby which they will affect the earth retention systems design. In this study an analysis regarding the metro route, the stations and the proper construction method is presented. Proposed 15.21 km of the route, 12 stations and tunnel section as well as the section suitable for the New Austrian Tunneling Method (NATM) proposed. The construction method details about the steps required in New Austrian Tunneling Method (NATM) is included in this study as well. A cost analysis is undertaken in order to assess the total costs needed for any considered method of tunnel construction. This analysis is restricted to the cost of tunneling only. The detailed costs like construction of station utilities etc are not considered in this study. Also in this thesis NATM and Cut and Cover costs are compared. Based on findings and calculations, NATM method shows to be 25.12 % cheaper than Cut and Cover method by the total cost of Tk7, 507.2 million for NATM and Tk 9,392.99 million of Cut and Cover. Due to large capital cost for Tunnel Boring Method (TBM) as compare to NATM and Cut and Cover, this TBM method was not considered in this analysis. Selected route is subjected to large utility networks of WASA Titas Gas, Electricity and BTCL located all along the route with their specific behavior to the tunneling process. Some of these utility lines need to be relocated, displaced, and replaced during the construction. Also proposed projects by DCC, STP 2004 and Bangladesh Bridge Authority (BBA) are to be constructed along the route, which synchronizations of the present at meeting points due to conflicts should be considered. As the conflicts, challenges and flexibility of the construction method is concerned, NATM shows more practicality as compare to Cut and Cover. The NATM has no conflict with Utility networks, and there is no limit in deep excavations to eliminate the effect of surrounding elements which the Cut and Cover shows a lot of difficulties in this aspect. This study revealed that despite all of the present challenges in the route, application of Metro tunnel construction is feasible for Dhaka city. Based on this study the best method of contraction is NATM in aspects of cost, technology, ease of application and independency of science. Also due to less amount of conflict between NATM and the surrounding elements and due to ease of synchronization by environment, application of NATM method for Dhaka city underground metro construction is suggested.