| Popis: |
The paper describes the technology of blasting works during the excavation of the “ Brzet” Tunnel, with a special review of a secure seismic blasting regime, taking into account the delicate geological situation of the surface over the tunnel. Overlaying rock consists of limestone boulder. Single unstable rock boulder can be found on the surface above the entire tunnel route. There is a fault at the tunnel portal, which presents additional difficulty in the execution of works. Taking into account the vicinity of the highway and residential area beneath the tunnel, it was necessary to establish a secure blasting regime. Maximum instantaneous charge in the tunnel-blasting field must not result in such ground vibrations, which would cause falling of the boulders. Therefore, measuring of ground vibration velocities in the rock mass were conducted, as well as measurement of relative displacement of boulders by using LVDT displacement sensors. In accordance with the safety requirements, the permitted ground vibration velocity level is set at 15 mm/s. Based on the results of measurement of ground vibration velocities and the maximum instantaneous charge, individual ground vibration velocity zones were determined with regards to the thickness of rock between the side of the tunnel and the slope of the hill. At each blasting, the permitted ground vibration velocities will be monitored. In case the measured velocities reach the limit permitted value, blasting patterns will be corrected and advance per single blasting will be decreased. Additional safety measures include: controlled removal of fully detached boulders and execution of protective embankments under the tunnel route. |
