Geophysical Investigations to Assess the Outcome of Soil Modification Work: Measuring Percentile Variations of Soil Resistivity to Assess the Successful Modification of Foundation Soil by Jet Grouting

Jet grouting as a "soil-modification" procedure is one of the most controllable injection systems, capable of modifying pre-designed volumes of soil with pre-designed volumes of grout. However, the degree of soil modification achieved is a function of the ratio of jetted grout to original soil and is controlled by many variables like: adjusting jetting pressure and duration, jetted volumes and power of penetration, soil volumes injected per minute, number of nozzles used, nozzle diameters, nozzle positioning and orientation, grout composition, grout viscosity and more. One of the most important if not the most vital check to be performed upon completing any jet grouting operation is the verification that the soil has been actually modified as specified and required by the engineering design. Electrotomography investigations (one of many types of geophysical investigations) coupled with advanced computer software procedures were used to assess the effectiveness, success, extent and quality of jet grouting work performed to provide foundations for new structures. The assessment of the extents of jetting dispersion and of grout permanence capable of effectively increase the load bearing capacity of large pockets of compressible peat and of soft silts were of particular concern. Electrotomography was performed to ascertain and confirm in advance the overall geotechnical characteristics of the soil to be modified by jet grouting; check the physical formation, the final location, the shapes, the interactions and the overall integrity and continuity of a set of jet-grouted columns laid on an equilateral triangular grid pattern as foundations of a new harbor terminal building; assess the final dimensions and the yield at various depths of a number of jet-grouted test columns; and provide tri-dimensional models (constructed by computer data elaboration) visually showing the percentile variations of the resistivity finally achieved by the soil modified by jet grouting.