Gotthard base tunnel rock burst phenomena in a fault zone ...

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Cross cavern East Tube EON West Tube EWN Figure 1 Schematic original layout of the multifunction section at Faido. 0 km 57 km 420 MFS Faido Figure 2. Geology along the Gotthard Base Tunnel consisting mainly of gneiss and granite. MFS Faido is located in the Leventina Gneiss of very good quality 1.3 Actual Geology in the area of the MFS Faido During construction of the cross cavern (Figure 1) a break down of fine grained quartz occurred in the cavern’s roof forming a cavity of 8 meters in height. The results of an extensive drill holes campaign together with seismic reflection measurements effectuated during construction revealed an unknown large fault system in the area of the MFS Faido. The main kernel of this fault strikes at an average angle of about 15° to the tunnel axis and dips at about 80°. The fault system encountered during construction is shown in Figure 3. In the fault’s kernel, layers of partially completely decomposed rock (kakitrite) are embedded. Adjacent to the east of the fault hard and brittle Leventina gneiss is located. To the west of the fault the rock mass consists of hard but less brittle Lucomagno gneiss. 2. DIFFICULTIES DURING CONSTRUCTION During the first rebuilding of the EWN tube to the north of the cross cavern (Figure 1) a support consisting of steel arches HEM 200 backfilled with 40 cm concrete was installed immediately after each excavation step of 1 m. This rigid support was intended to cater for the heavy pressure and especially to protect workforce from break outs at the face. In the rebuilt section, on a length of 250 m (Figure 3) the loading of the support however gave rise to radial displacements of up to 1 m. Strain
North EON tube EWN tube Area of increased rock bursts 800 m Cross section for 2D modeling 421 Fault’s kernel Cross Cavern Access tunnel East West Rescue Station South Figure 3. Actual geology recognized by January 2003. Fault’s kernel with intensively jointed rock to both sides of the kernel. As a consequence the crossover has been shifted to the south (Figure 1). measurements revealed yielding of the steel arches already Figure 4 days after backfilling[1]. Severe damage of the support developed (Figure 4) and the critical section had again to be rebuilt with an enlarged excavation radius of 1.5 m to allow for additional displacements. A flexible support has been installed using TH profiles with sliding connections. Shotcrete slots were left open for unhindered sliding of the arches and for avoiding damage of the shorcrete lining due to further movements (Figure 5). Figure 4. Sheared off steel arch in the invert 3. NUMERICAL SIMULATION OF THE STATIC LOAD CASE In order to better understand the failure mechanisms in the area around the tunnels of the MFS numerical modeling was carried out. The model section is Figure 5. Rebuilding of the critical section with enlarged profile and slots in the concrete lining indicated in Figure 3. From the known geology it was obvious that a discontinuum model with deformable blocks had to be used in order to recognize the influence of the discontinuities and the kakirite zones on the stability of the tunnel section. For this purpose the program UDEC, developed by ITASCA was used. The investigation was based on a parametric study comprising the variation of rock and joint properties. The model is shown in Figure 6. The different fault regions in the model have been selected according to the geologist’s findings based on interpretation of numerous borehole results. From overburden, rock mass density and gravity a primary principal stress level of σyy=42 MPa and σxx = 21 MPa was defined at the top of the model. In Figure 6 the vertical stresses yy are represented. The results
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Gotthard base tunnel rock burst phenomena in a fault zone ...

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