Stability of near-surface excavations in weak rock and soil

To comprehend the deformation behavior of near-surface excavations in weak rock and soil subjected to magnetic forces and in situ stresses for the Superconductive Magnetic Energy Storage (SMES), several axisymmetric and three-dimensional models using the finite element method (FEM) were applied. For axisymmetric jointed rock model, the finite element formulation coupling solid deformation with fluid diffusion was based on Biot's theory and the equivalent continuum principle. A new computer code was developed for the elastoplastic and elasto-viscoplastic models for soft rock and soil by applying the technique of reduced nodal freedom array to store the stiffness matrix. The three-dimensional jointed rock model comprises intact rocks and joints with gouge material. The gouge material was modeled as a weak material by reducing its stiffness.; Computation was based on the data from eastern Wisconsin with given magnet load, measured in situ stress and assumed parameters. The results based on the equivalent continuum jointed rock model indicate that the instantaneous radial deformation of the outer trench wall in water-saturated jointed rock will be around 20% less than that in dry rock.; The results from the elastoplastic model show that the range of radial deformation under the given magnet load does not appear to endanger the trench in the short term. As a result of creep behavior, the radial deformation of the trench wall will reach steady state in a matter of months. This creep behavior is not significant in weak dolomite under the given magnet load.; The results of the analysis for soil indicate that the trench wall may not support the given magnet load if there is no artificial support in the trench wall. A magnet load of 0.54 Mpa is the limit load for an unsupported trench in soil.; A hypothetical three-dimensional jointed rock model was also considered in this study. The results from this model show that the fault striking at S25{dollar}spcirc{dollar}W has the most serious effect on the trench deformation among given oriented-fault cases due to a larger in situ shear stress. A very small diplacement (in the order of 10{dollar}sp{lcub}-8{rcub}{dollar} to 10{dollar}sp{lcub}-10{rcub}{dollar} m) of the faulted gouge material relative to the surrounding rock in each oriented-fault case was found. Thus, the whole trench structure would be stable.