A case study on the cavity effect of a water tunnel on the ground vibrations induced by excavating blasts

- Cavity effect is the vibration amplifications with the actions of an underground void space.
- The ground vibrations behind the tunnel advancing face are greater than those ahead of it.
- Amplifying effect of ground vibrations first rises, then falls with the distance to tunnel working face.
- Cavity effect has a limited action range in shallow tunnel excavation.
- Tunnel radius and its buried depth are two influencing factors of cavity effect.

The cavity effect means the amplifications of blast vibrations when propagating through a cavity inside the rock mass. It has been investigated by a series of blast tests in an underground water tunnel. Monitoring points were set up in pairs on the ground surface and symmetrically about the tunnel face. The amplifying coefficient of blast vibrations was defined as the PPV (peak particle velocity) ratio of the two symmetry points. It is found that the amplifying coefficient first rises to a maximum of 3.5, and then falls down with the distance in the tunnel axial direction. Meanwhile, tunnel radius and its buried depth are two other influencing factors. The coefficient linearly increases with tunnel radius, while decreases with the depth in a power function. These results suggest that the cavity effect only works within a limited scope on the ground, and mainly occurs in shallow tunnel excavations. By introducing a ratio of the tunnel depth to its radius, an empirical formula was proposed to calculate the amplifying coefficient. It can be used to predict the PPV amplifications and therefore define a vibration control area for the safety of ground structures and facilities. It could also be applied to more site conditions by choosing appropriate parameter values in the formula.