Measuring shear wave velocity, Vs, of a hidden layer: an application to soil improvement under roads

In a number of practical cases, a typical one being the investigation of the subsoil properties below roads or foundations, one faces the problem of measuring the elastic properties of a geological layer (here called “hidden layer”) underlying a more compact and rigid surface layer. In such cases, the effectiveness of common surface seismic methods is poor for different reasons, but mostly linked to the reflection–transmission properties of the waves at a stiff-to-soft interface. Borehole methods are more efficient, but expensive and only provide vertical information at certain points. Attempts carried out in the past to characterize the hidden layer properties through surface seismic techniques consisted in placing the seismic source on the surface alongside, but off the stiff artificial layer (road or foundation). An alternative approach is presented based on placing the seismic source just below the stiff artificial layer. In cases where soil improvement–compaction are carried out through injection of expanding resins, then the hidden layer can be easily reached via the injecting tools and in some cases (e.g., urban settings characterized by laterally continuous artificial layers or roads constructed on embankments) this can be the only viable option. The results obtained from this approach using a number of practical cases where roads affected by differential sinking have later been compacted will be presented. The average soil improvement that can be achieved with the specific kind of expanding resin used in this study is then quantified.