Case History: Monitoring Resin Injections With The Aid Of 4D Geophysics

Ground improvement is an important practice in areas where the combined presence of expansive soils and fluctuating moisture content often result in differential heaving and settling soils and variable confining forces exerted on foundations. In fall of 2009, three-dimensional (3D) seismic pressure wave (p-wave) refraction and 3D electrical resistivity tomography (ERT) surveys were performed to determine the effectiveness of seismic refraction tomography in imaging the permeation of resin injected during ground improvement efforts within problematic soils. A test site near Piacenza, Italy was selected for this undertaking: Data were collected around the perimeter of an existing house suffering damage due to differential ground settlement. Three independent yet identical surveys were conducted using both methods, producing 3D images of the spatial distribution and variations in subsurface material properties before, during and after resin injection. The site is underlain primarily by a two to three meter thick layer of silt and clay with some sand lenses over an alluvial silty sand base layer. Prior to resin injection, substantial variations in both electrical resistivity and p-wave velocity are evident beneath the particular corner of the structure that exhibits the highest extent of damage (e.g., cracking and settling). During and after resin injections and curing, resistivities and velocities can be seen to change in the surrounding vicinity of injection points, and the magnitude of change in both data sets appears to be related to the amount of resin injected at each point. Geophysical measurements confirm the heterogeneous nature of the near-surface soil at this site, results from both methods correlate well with each other and additional field data (i.e., CPT data), and resultant 3D models are clearly useful in siting injection points as well as time-lapse monitoring the extent of resin permeation.