The UDEC modeling of tunnel excavation and support is invesgated in several main considerations including setting of computational model, Hybrid DEM/BEM scheme, tunnel excavation and support and judgement of tunnel stability to obtain reliable computational results.
The discrete element code, UDEC, is specially designed to model jointed rock mass and has been widely used to simulate tunneling, Deep excavations and rock slopes (Cundall 1980, Lemos 1987, Barton et al. 1994). A UDEC model is an assemblage of blocks and contacts, respectively, representing rock material and rock joints. The blocks are further subdivided into triangular finite difference elements to present the deformability of rock material. The rock joints may deform and fail and thus the UDEC provides A way to model possible separation of rock material blocks due to joint failure. Different from other continuity- based numerical methods, more attention should be paid in UDEC modeling to obtain reliable results. E.g., in situ stress is usually not uniform due to the existence of rock joints. Applying a uniform in situ stress to the model does not reach force equilibrium and a consolidation stage is required to avoid extra displacement being transferred to the tunnel excavation stage. To include major joints as more as possible in the model, the model size cannot be so large as other numerical methods do and the hybrid DEM/BEM scheme would be a very practical alternative to limit the model size rather than using traditional fixing condition at far field boundary. Since the UDEC is a 2-D oriented program, 3-D space cannot be physically involved but working face effect must be taken into account when applying shortcrete and rockbolts. This paper is to investigate the UDEC modeling of tunnel excavations and supports. Main considerations in the UDEC modeling are examined including setting of computational model, hybrid DEM/BEM scheme, tunnel excavation and supports, and judgement of tunnel stability.
The computational model would reflect the geological condition (joint geometry), in situ stress and boundary conditions to make The modeling reasonable. 2.1Joint geometry and selection of models Rock joints dominate the tunnel stability and should be reflected Properly in the computational model. As stated by Hart (1993), major joints possibly affecting tunnel stability should be introduced in the computational model. As the rock joints at the crown have more risk to fail, extra care must be taken in introducingthe joints at the crown. Natural rock masses are usually multiply jointed with several joint sets and some random joints. The introduction of the rock joints in the model would include three steps. First, the joint distribution can be traced from borehole logging data and joint mapping at current and adjacent constructions. Site seismic investigation may provide additional information for identifying weak zones and faults. Second, a statistical analysis on the joint strikes and dip [Ingles with deviations would be conducted to classify the joint sets. Finally, a filtering should be carried out to identify those major Joints.