Development of an Electrical Probe for Rapid Assessment of Ground Improvement: Fid-Bau Portal

Development of an Electrical Probe for Rapid Assessment of Ground Improvement

M. A. Mooney / R. Bearce

This reports details the findings of the National Cooperative Highway Research Program project 186 (NCHRP-186) titled “Development of an Electrical Probe for Rapid Assessment of Ground Improvement.” Soil improvement via jet grouting and deep soil mixing has numerous applications in transportation construction including embankment stabilization, underpinning bridge foundations and stiffening bridge approach embankments, excavation and shaft support for transit and traffic tunnels, slope stability, and creating earth retaining or hydraulic barrier walls and bottom seals. The realized geometry of the mixed ground, often called soilcrete, depends greatly on the jet grouting parameters and on the soil stratigraphy at a site. Resulting geometries are highly variable and therefore require detailed verification. The lack of rapid technologies to assess jet grout column geometries is a significant barrier. This need was the impetus for this study. A jet grout push probe has been developed for rapid assessment of jet grout column diameter. The probe is inserted into a freshly jet grouted column immediately after removing the jet grout monitor. The test requires 20-30 minutes to collect sufficient data to estimate column diameter. The probe has been implemented on multiple jet grout construction project sites, primarily granular soil sites (sands, silty sands). In all cases, the estimated diameter was found to be within 5% of the actual constructed diameter. Extensive computational modeling was performed to provide the basis for the design and for the resistivity methodology used to determine column diameter. Continued testing of different diameter jet grout columns constructed in a variety of soil types is recommended. In developing a plan for technology implementation, current specifications for jet grouting on transportation-related projects were examined and important consideration given to the customer for the technology, intellectual property, and the business model. Given current specifications for jet grouting, the immediate customer for the probe is the jet grout contractor community. Contractors are tasked with performing QCQA and meeting acceptance criteria. The main benefit that the probe provides over current techniques is in time savings. Instead of waiting 7 days after jet grouting to perform coring and another 7-21 days for UCS test results, a contractor can assess the diameter within 30 minutes of jet grouting. This provides immediate actionable feedback as the contractor can modify jet grouting parameters as needed within the same work shift. Further, the ability to verify diameter immediately and move on to production can save significant time and money. This ultimately benefits the owner (and tax payers) because jet grout bid prices will decrease. The suggested business model for push probe implementation involves the university licensing the technology to one or a select group of testing companies. These companies, with well trained personnel, should carry out the testing for contractors. There is ample precedent for this in transportation construction.