In the preliminary investigations of mountain tunneling, a seismic refraction survey from the ground surface is conventionally conducted, and a tunnel support pattern is designed based on the seismic wave velocity of the ground. The resolution of the velocity profile by this technique, however, is poor as the tunnel overburden becomes large. In response to this problem, the authors developed a new survey method in which drilling vibration data were used, called tunnel seismic probe drilling (T-SPD), to estimate the seismic wave velocity distribution ahead of the tunnel face so that it could be used in the design of tunnel support patterns. We conducted field demonstration tests of the T-SPD at three tunneling projects. In this study, we will report the results of the additional verification for the remaining section of a tunnel project previously reported.
In conventional planning of mountain tunnels, seismic refraction surveys from the ground surface are performed as preliminary surveys in accordance with the Japanese Standard (JSCE 2006). Tunnel support patterns are then designed based on the seismic wave velocity of the ground, as obtained by the seismic refraction survey. The resolution of the velocity profile estimated by this technique, however, is poor as the tunnel overburden becomes large. In such conditions, a survey ahead of the face during tunnel construction is considered to be more effective than the seismic survey from the ground surface. Two methods are generally used in the surveys ahead of the tunnel face: seismic reflection method known as tunnel seismic prediction (TSP) or horizontal seismic profiling (HSP), and borehole logging known as drilling survey system (DRISS). Though we can obtain information on the position and direction of the discontinuous surface using TSP or HSP, it is not possible to obtain the width of the fracture zone, which is important for designing tunnel support patterns. The drill machine data provides the distribution of specific energy, which is related to the strength of the rocks, and is defined as the energy consumed to drill the rock of a unit volume. Although probe drilling provides logging data and drainage for groundwater at the same time, its disadvantages are that the drilled cross-sectional area is small relative to that of the tunnel face, and that it is difficult to correlate the drilling specific energy with the quality (integrity) of the rock.