Soil Mapping and Soil Type Identification Related to Large Residential Development

SEI was contacted by the clients’ geotechnical soils engineers to provide a cost-effective geophysical method of mapping the extent of soil materials within an approximate 400-acre residential development site.  The primary goal of the study was to specifically identify areas within the site that contained highly plastic clays that may adversely impact the performance of shallow residential foundations due to their potential for shrink-swell development. 

SEI recommended the Capacitively Coupled Resistivity (CCR) technique as the most cost-effective method for site preparation and proper design of the residential foundations.  The CCR geophysical method was selected based on the advantage of rapid acquisition of high-resolution soil data at shallow depths.

CCR is a geophysical method of imaging three-dimensional subsurface soil layers through the direct measurement of soil resistivity values.  Rapid data collection is accomplished by towing a dipole-dipole array of variable transmitter-receiver offsets behind an all-terrain vehicle (ATV).  Horizontal distance control was accomplished with real-time GPS mapping using a Trimble® Pro-XR differential GPS unit. 

Test studies were initially conducted over a small area to allow for direct correlation of physical soil data with that obtained using the CCR method.  Good correlation was obtained between physical soil properties and soil resistivity data; highly conductive (low resistivity) soils were found to correlate well with highly plastic clays, and soils that were highly resistive were found to correlate well with cohesionless, sandy soils.  The effective depth of investigation by the CCR method was approximately 3.3 meters (10 feet). Results were presented graphically as three-dimensional soil layer slices at increasing depths.        

Based upon the favorable results of the initial test studies, the project was expanded to encompass the entire proposed development tract of 400 acres.  The results of the CCR survey over the entire site provided similar results to those demonstrated in the test areas, allowing the geotechnical project engineers to perform proper site preparation and foundation design, thus limiting potential impacts from expansive, highly plastic clay soils.  In addition, the CCR method was able to delineate the horizontal and vertical extent of sandy soil zones that were subsequently used to provide ideal fill material for site preparation, which resulted in considerable cost savings for the client.