The use of seismic techniques to identify hazardous ground conditions associated with cavities

The identification of civil engineering hazards such as cavities, mine shafts, etc., is an integral part of site investigations carried out prior to the construction of roads, tunnels and other civil engineering structures. The use of geophysical methods to identify these hazards is becoming increasingly important. An investigation into the effectiveness of three seismic methods to delineate the possible anomalous ground conditions associated with the presence of cavities has been evaluated. The three methods are: surface, interborehole and a single hole method. The surface seismic method (refraction) was used successfully over a disused railway tunnel to identify the presence of a cavity, and a technique was developed to generate images of a theoretical model, which can be compared to a field survey. The interborehole seismic method was used across a disused railway tunnel to study the effect of a cavity on changes in seismic parameters, such as compressional wave velocity and attenuation. Both of these parameters were sensitive to the presence of a large cavity, and the successful application of the technique is demonstrated in the Maidstone survey, where the presence of loosely packed sand (due to sinkholes in the vicinity) was located under a house. The use of the single hole method for detecting cavities is a new technique (ACDER) in seismics, and it is analogous to methods of radar detection. A sparker source, directional receiver and associated instrumentation were developed in the laboratory before field trials at East Fleet. Of the three methods, the interborehole technique was the most successful, followed by the surface method. The single hole method looks promising but requires further work in the design of directional receiving transducers, followed by more field trials