Keywords: tunnel junction, sprayed concrete, face stability, 3D finite element analysis.

The paper presents a real case of a complex underground tunnel junction at shallow cover, analysed with three dimensional finite element program ZSoil 3D [1]. The main objective is to describe the practical application of three dimensional finite element analysis as a design tool for a complex underground constructions. The purpose of the simulation is to identify lining stresses induced by multi-stage excavation/construction cycles, to compute required reinforcement within the sprayed concrete shell and to assess face stability.

The principal tunnel is a twin bore 9.1m diameter road tunnel. The road tunnel is of a precast concrete lining construction. There is requirement for a sprayed concrete sump cross-passage to be constructed between the two bores. The distance between the road tunnels is 16m. The passage and associated openings will be constructed within stiff, highly overconsolidated clay.

In this study focus is on the sprayed concrete sump crosspassage, which is a circular tunnel with changing diameter. The diameter enlarges from the 5 m diameter opening at the road tunnel up to the maximum of 6.6 m diameter opening in the middle of the crosspassage. In the middle of the cross passage a 10 m deep shaft (sump) is to be constructed. The shaft diameter changes from 3.5 m at the junction to 6.5m at the bottom. Tunnel axis of road tunnel is about 20 m below ground level. The construction of the cross-passage including sump will proceed at the completion of the first segmentally lined tunnel drive but before the second. Therefore the effects of the tunnel machine passing the end of the cross-passage need to be ascertained. The cross passage construction will cease 3m from the extrados of the proposed second drive and a temporary headwall will be constructed. The final 3m connection into the second drive is carried out at the completion of TBM drive from within the second segmentally lined tunnel.

The FE-simulation starts with construction of first segmentally lined tunnel. Then beam element propping around future cross passage opening is installed to take loads from the segments during cross passage excavation. The next steps are cyclic removal of soil continuum elements and construction of sprayed concrete shell elements. This procedure is adopted for cross passage and shaft. After completion of all sprayed concrete works installation of reinforced ring beam at openings and removal of steel propping is carried out.

Due to low permeability it is assumed that drainage conditions do not change during construction process. A total stress approach has been chosen with undrained shear strength and undrained Young's modulus both being a function of depth below ground. For assessment of face stability at selected construction stages the "c,? reduction" algorithm is used which is implemented in the analysis software ZSoil . The paper illustrates that nonlinear three-dimensional numerical modelling of complex underground can be a valuable design tool for engineers in the construction industry. Tunnelling as well as all the other disciplines linked with ground engineering is subject to a variety of uncertainties, so that simplified material models for soil and sprayed concrete can be justified for design purpose. To scope with the uncertainties which are inherent to the ground characteristics, lining geometry and lining material, construction program and workmanship several sets of computations with varying key parameters are required to learn about the sensitivity of the design with respect to the chosen parameters.

1

Z_Soil.PC User Manual, Elmepress International, Zace Services Ltd, Lausanne (2001)

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