Detecting boulders using the horizontal cross-hole seismic method

Detecting boulders before tunnel excavation has become essential routine work to ensure safe, economic and efficient tunnelling. Here, a horizontal cross-hole seismic method to determine the distribution of boulders in tunnelling areas is developed by integrating the horizontal directional drilling technique and cross-hole seismic method. Compared to the cross-hole seismic method, the proposed method has obvious advantages of higher resolution and less limitation by ground conditions. The basic techniques and process of this method are introduced in detail. To verify the feasibility and efficacy of the proposed method, a 220 kV power cable tunnel crossing the Guangming River is taken as an example for detecting boulders beneath the river. A four-borehole detection plan is implemented, and four profiles are obtained. The results reveal several high-velocity anomalies, which are identified as dense sand according to additional logging surveying. This example indicates that the proposed method can provide sufficient information about boulders in underground engineering.