ABSTRACT
One of the key design and construction parameters in urban tunnelling projects with the use of a closed face Tunnel Boring Machines (TBM), is the appropriate range of face pressure to be applied by the TBM, so as to meet all the relevant strict and many times conflicting requirements. On the one hand, the face pressure needs to be high enough to ensure not only the tunnel face stability, but also the control of induced ground deformations within predefined, acceptable limits. On the other hand, maintaining a conservative value of the face pressure that is too high, results in a slow boring process, when all urban tunnel projects face challenges with very demanding construction schedules and may also lead to blowout risks. Although numerous analytical methodologies provide a satisfactory degree of investigation of the required face pressure for face stability, the estimation of the face pressure that will additionally result in limited deformation around the tunnel and hence on the ground surface, still practically relies only on execution of parametric 3D numerical analyses. This article briefly presents an analytical method that can be applied on soft ground urban tunnels to assess the level of active face support to be considered in numerical analysis in order to ensure elastic relaxation in the advance core and thus effective ground deformation control. The method is then compared with three (3) urban tunnelling case studies from India and Ecuador, where TBMs have successfully underpassed urban areas and structures with EPB machines, through soft soils under small overburden. Available design and construction data from the applied face pressure and the recorded deformational field around the tunnels are presented, followed by a comparative discussion on the results and the applicability range of the method.
