Computation of strains and pressures for tunnels in swelling rocks

Abstract

For the Stuttgart 21 project in Germany, tunnels have to be driven in unleached Gypsum Keuper. This rock mass consists of a horizontal alternating sequence of sulphate-containing clay- and siltstone and pure sulphate rock. The sulphate is found in two different modifications, as gypsum and as anhydrite. If water gains access to the rock, the anhydrite transforms into gypsum. This chemical process results in an increase of the solid volume by some 61%. Correspondingly major swelling deformations can be the consequence. In the past, this has led to major floor heave in many railway and road tunnels. In cases in which it was attempted to prevent or impede swelling strains by means of a stiff tunnel lining, swelling pressures often developed that were high enough to lead to the destruction of the tunnel lining. To avoid such failures a constitutive law to describe the swelling phenomenas in a realistic way was developed. It describes the development of time-dependent strains and stresses due to swelling considering elastic and viscoplastic behaviour. Furthermore the anisotropic swelling behaviour is considered. This constitutive model was implemented within a three-dimensional numerical computation program according to the finite element method and calibrated with the aid of back-analyses of monitoring results in the exploration gallery of the Freudenstein tunnel which is completely located in the unleached Gypsum Keuper and which is especially interesting because different support principles were tested. A good agreement between numerical solutions and monitoring results could be achieved for all support principles with only one set of parameters. We therefore assume that the constitutive law describes the conditions in the exploration gallery of the Freudenstein tunnel well, and that it should be applied in combination with the 3D-finite-element-software as a basis for the design of tunnels in swelling Gypsum Keuper. The principles of resisting support and yielding support respectively are presently used for the design of tunnels in anhydritic, swelling rock. In the first case, the internal concrete lining is designed to resist the occurring swelling pressure. In the second case a yielding zone is installed underneath the invert of the concrete lining of the tunnel. This zone leads to a reduction of the swelling pressure and thus can lead to a reduction of reinforcement and thickness of the concrete lining in comparison to the principle of resisting support. It is combined with drainage of the yielding zone to prevent groundwater from reaching the rock above the tunnel roof and thus to prevent swelling of the rock in this area. In both design principles it is assumed, that water to initiate the maximum swelling load and full swelling, respectively, is available in sufficient quantities. Consequently the designed measures have to be carried out over the full length of the tunnel in swelling rock. Based on observations the hypothesis was made, that in the area of the transition from water bearing to anhydritic rock, self-sealing due to swelling occurs around the tunnel if the resisting principle is applied. As a consequence of this self-sealing effect seepage through the rock parallel to the tunnel and thus also swelling is interrupted at a certain distance from the water bearing formation. In a research project carried out by WBI a rock mechanical and hydraulical model and a corresponding 3D-FEM-code have been developed which describe the corresponding phenomena. It is expected that the results of this work will lead to remarkable cost savings along with the design of tunnels in swelling rock. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.