ABSTRACT
The application of numerical limit analysis to the stability of shallow tunnels has been pioneered by Sloan and Assadi who investigated the undrained stability of a plane strain square tunnel in a cohesive soil (Assadi & Sloan, 1991) and the stability of square and circular tunnels in cohesive soil with shear strength varying linearly with depth (Sloan & Assadi, 1991, 1992) using linear programming techniques. The stability of the tunnel was described conveniently by two load parameters
1 INTRODUCTION
Rapid increase in the volume of constructions in densely populated urban areas brings the task of accurate estimation of the stability of soils with multiple underground openings to the priority list of geotechnical engineers. On the other hand it appears that there is no generally accepted design or analysis method available to assess the stability of the structures resting on a stratum with shallow underground openings of square or rectangular cross-section. The efficient utilization of underground space dictates that the non-circular openings and tunnels should be preferred in the design as quadrilateral objects like trains and buildings are usually used there. Furthermore, large size non-circular tunnels are quickly becoming a widespread building technology by virtue of the development of advanced tunneling machines. The merits of the construction of non-circular underground openings compared to circular ones would be the reduction of the area of excavated section, the reduction of the surplus soils, thus diminished environmental issues, and the reduction of rented space for the construction of tunnels. Although the top, bottom, right and left spaces of circular tunnels are usually useless, circular tunnels have been predominantly used so far since their construction is easier from the standpoint of excavation and maintenance of the heading.
