Underground Singapore 2011 • After the concrete of the ventilation shaft structure has gained the sufficient design strength as required, the remaining reinforced concrete work of the ventilation shaft structure will be constructed upon removing the remaining temporary steel support frames. • Subsequently the abovementioned steps will be repeated for the last cell, Cell 2.
38m
12m
7m
Figure 10(a) Typical longitudinal section of the mined tunnel
Cell 2 Cell 1
Cell 3
Figure 10(b) Typical cross sections of mined tunnel
3.3 Geotechnical Analyses for the Excavation Sequence for Mined Tunnel Design The excavation sequence of the mined tunnel was modeled with Plaxis. Figure 11 shows the Plaxis model used for the design. The slope cut will be made prior to advance the excavation of the ground ahead. The slope stability analyses using un-drained and drained parameters have been performed using PLAXIS 2D FE analysis – -c reduction method for 3 critical sections along the longitudinal direction to determine the factor of safety against face stability of the tunnel excavation i.e. At the beginning, middle and at the end of the tunnel excavation mostly within the Estuarine Clay (E) layer. The high tensile steel dowel (Deformed Bar Type II) bars will be installed to stabilize the ground ahead of the excavation. It has been modeled by increasing the cohesion value of the original ground ahead of the excavation due to the presence of those dowel bars which is assumed to be of minimum 3 round lengths. The dowel bars will be overlapped by at least 2 round lengths. The minimum factors of safety of 1.2 for drained and 1.5 for un-drained condition have been met in accordance to the design criteria.
203