Time-lapse surface waves to detect the stiffness of a grouted pile wall
We adopt the surface waves method to assess the stiffness of a grouted pile wall in the ground. The acquisition is performed in time-lapse mode, col¬lecting data soon after the grouting and a few months later and comparing the results. We use a 48-geophone single array, shooting in different positions along the array, in order to extract several dispersion curves to compare, provid¬ing a set of curves referring to different subsurface portions. We applied a spatial windowing based on a set of Gaussian windows with different shapes, optimising lateral and wave¬number resolutions. The comparison between the time-lapse dispersion curves shows an increase of the phase-velocity on the portion of the ground interested by the grouted pile wall. Surface waves data was collected also on a transect close to the grouted wall but not influenced by it, to obtain a reference line. The results demonstrate the reliability of time-lapse sur¬face wave data analysis to assess the changing of the mechani¬cal properties of the ground after consolidating works. The reliability and performance of surface waves have been tested to assess the stiffness of a grouted pile wall. The grouting injection in a urban area interested a depth ranging from 2 m up to 15 m, in a geological environment of a calcareous bedrock 15 m deep, with alluvium overburden. The project is based on the collaboration between Rambøll Denmark and Gamut srl, which is part of the Politecnico di Torino. Seismic data have been collected at two different periods to evaluate the changes of the stiffness of the consolidating ground; the approach is cost-effective because it is non-invasive and is capable of operating in urban areas. As the ambient noise limits the use of methods based on the travel¬time analysis (detection and picking of first time arrivals), surface waves, more energetic than P or S-waves, ensure a high data quality. The challenge is to obtain different surface waves of data along a wall, using a single seismic array.