Direct Interpretation of Phase Lags (DIPL) of MASW data: An example for evaluation of (Jet grouting) for soil stiffening enhancement against soil liquefaction

The evaluation of the soil stiffness is routinely performed by direct investigation, using geotechnical tests such as CPT-CPTu. A low cost complement to the direct investigation would be the use of seismic surface wave methods such as, for example, the “Spectral Analysis of Surface Waves” (SASW) or the more updated “Multi-channel Analysis of Surface Waves” (MASW) which use the dispersive nature of surface waves to retrieve the shear velocity (Vs) profile of the subsurface. Unfortunately, available inversion algorithms assume the subsurface model as a stack of homogeneous parallel layers hence of limited practical use when subsoil is known to be laterally heterogeneous. In the following, we shall introduce a novel strategy based on the Direct Interpretation of Phase Lags (DIPL) among pairs of seismic signals, based on a combination of SASW and MASW work-flows, which aims to retrieve the 2-D Vs subsurface profile and in particular to tackle the lateral heterogeneity issue. As an example of application we shall retrieve the 2-D Vs profiles before and after a Jet-Grouting (JG) intervention performed at a test site studied in the context of the reconstruction and prevention phase following the seismic sequence that struck the Emilia Region (Northern Italy) in 2012. Soil stiffening due to the cement mixture injection is then retrieved in term of Vs increase.