Accurate prediction of the geological conditions ahead of the tunnel face is essential for safe and economical tunnel construction. As an application of the drill logging, a new method for predicting the distribution of seismic velocity ahead of the tunnel face has been developed. The method makes it possible to determine the travel-time from transmission to reception by determining the transmission time at which the rock is hit by the drill bit and processing the wave form of the vibration received by the receivers on the tunnel face. Field test results have shown that the newly developed method is useful in predicting the seismic wave velocity distribution ahead of the tunnel face.

1 Introduction

Non-core drilling exploration (Kuwahara, 2012) is one of the exploration methods used for the ground ahead of the tunnel face. In this method, advancing drilling is carried out for 30 to 50 m ahead of the tunnel face, and we predict the ground condition based on the energy required for excavation per unit volume (specific energy) as an index. The specific energy used as an index varies depending on the change in the working pressure of the hydraulic rock drill for drilling, characteristics or discharge conditions of drilled rock fragments, etc.; thus, the accuracy of the specific energy may also vary. In addition, we can only determine the distribution of the specific energy in the direction of the advancing drilling, and it is hard to determine the ground conditions in a two-dimensional plane.

Considering the above circumstances, we have developed an exploration method for a hydraulic rock drill mounted on a drill jumbo used for mountain tunnel construction, as shown in Figure 1. In this method, we accurately obtain the propagation time (travel-time) for the vibration generated when the drill bit strikes the ground during drilling to reach a geophone placed at the tunnel face. We developed the non-core drilling exploration method to determine the distribution of elastic wave velocities in the ground ahead of the tunnel face.

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