Abstract
Tunnel blast-induced vibration probably causes damage to the rock mass surrounding the tunnel surface and also to the rock mass of the slope at the tunnel entrance. It is important to simultaneously monitor the vibration on the tunnel surface and on the tunnel entrance slope face, especially when the blasting work face is close to tunnel entrance. During blasting excavation of the traffic tunnel at Baihetan hydropower station, vibration monitors were installed both on the tunnel surface and on the tunnel entrance slope face. Based on the monitoring data, a comparative study is conducted on the peak particle velocity (PPV) and frequency characteristics of the vibrations at these two locations. A three-dimensional FEM simulation of the tunnel blast is then performed to verify the field test results. The field monitoring and the numerical simulation show that there is significant difference between the vibration on the tunnel surface and that on the tunnel entrance slope face. The vibration on the tunnel surface has greater PPV and faster attenuation, while the tunnel entrance slope face has higher frequency and faster reduction rate in the center frequency. These differences are attributed to the different wave types and wave propagation paths. The tunnel surface is mainly surface waves transmitted through the damaged rock mass around the tunnel profile, while the tunnel entrance slope face originates mainly from the body waves transmitted via the undamaged rock mass inside the mountain. The comparisons of the monitored vibrations with the velocity limits specified in the Chinese standard show that the most dangerous vibration that may exceed the limit occurs on the tunnel surface. Therefore, the maximum charge weight used in the tunnel blast is determined by the vibration on the tunnel surface. Under different control standards, the allowable maximum charge weight per delay is further discussed.
Highlights
Construction of hydropower stations in the areas of high mountains and deep canyons involves large-scale excavation of underground caverns, such as powerhouses, diversion tunnels, tailrace tunnels, and traffic tunnels. e drilling and blasting method is the most commonly adopted for underground cavern excavation [1]
From the blasting work face to the tunnel entrance, 30 observation points are arranged on the tunnel floor along the tunnel axis to observe the blasting vibration on the tunnel surface. e longitudinal distance between the adjacent observation points is 2−5 m. e distance from the observation points to the side wall of the tunnel is set to 1.5 m to keep it consistent with the field monitoring. ere are 30 observation points selected on the ground to record the vibration on the tunnel entrance slope face. ese external observation points are all located exactly above the observation points inside the tunnel
At the distance d 10 m, the peak particle velocity (PPV) on the tunnel surface exceeds the velocity limit specified in the standard when the charge weight is more than 35 kg. is indicates that the rock mass surrounding the tunnel surface is more adversely affected by the tunnel blast vibration. en, during the blasting excavation of the No 2 traffic tunnel, the criterion of blasting vibration control is that the PPV on the tunnel surface must not exceed 20 cm/s
Summary
Construction of hydropower stations in the areas of high mountains and deep canyons involves large-scale excavation of underground caverns, such as powerhouses, diversion tunnels, tailrace tunnels, and traffic tunnels. e drilling and blasting method is the most commonly adopted for underground cavern excavation [1]. When explosives in blastholes are detonated, a part of the explosion energy is used for rock fragmentation, and the rest of the energy is dissipated in the form of seismic waves (blasting vibration), air blasts, flying rocks, and noise. Among these negative effects, the blasting vibration is a major concern for designers and constructors, because it will cause the maximum damage to the surrounding structures [2]. Ramulu et al [3] assessed the rock damage due to repeated blasting in a railway tunnel through borehole imaging observation and vibration velocity monitoring. During the blasting vibration monitoring for tunnel blasts, the monitors are typically arranged on the tunnel surface to measure the vibration responses of the surrounding rock masses or linings. e vibration monitoring for the slopes at the tunnel entrances is
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