Engineering Geophysics (John Nicholl Memorial Session)

Abstract

Jet grouting is a geotechnical method of ground improvement to increase shear strength and stiffness of soils. The method is typically used to construct in-situ geometries of grouted soil such as panels or columns. The diameter of grouted columns and its material strength depend on various process parameters and the subsurface soil properties. It is only vaguely possible to predict the final column diameter. Therefore, it is a general practice to excavate a test column and perform a visual examination. However, an excavation to control the in situ diameter is often impossible, especially under complex site conditions, such as a high ground water table. Therefore, as part of a research project, borehole seismic measurements (crosshole, downhole and tomography) were tested as a quality control to verify the extent of the column and to monitor the influence of the jet grout injection on the soil over time. The field surveys were conducted before and after the jet grouting process at different time intervals. The acquired seismic data show clear traveltime differences which allow the determination of the specific column depth and diameter. The tomogram measured in the natural soil and the tomograms of the measurements after the injection process were used to visualize the time dependent effects of the jet grout injection on the soil. AbstractLevee and dam failures due to flood from hurricanes or heavy rainfalls occur without early warning and cause catastrophic damage. Therefore, the development of rapid assessment system of levees is greatly required to delineate weak locations and prioritize compromised locations. The Francis Levee Site is located 0.5 miles west of Francis, Mississippi. During the 2011 Mississippi river flood event, three main sand boils were observed at the toe of the clay apron on the landside and mitigated by the construction of sand bag berms. After the initial mitigation, the US Army Corps of Engineers extended the berm of the levee and placed 16 relief wells. Multiple geophysical surveys including seismic refraction tomography (SRT), multichannel analysis of surface waves (MASW), electrical resistivity tomography (ERT), and electromagnetic survey (EM34) were conducted at the site. These geophysical surveys were conducted to identify locations of preferential flow paths through the subsurface of the levee that might have led to the formation of the sand boils. These geophysical surveys are combined by identifying the traits of different compromised zones in the individual techniques and finding common traits using cross-plot analysis to integrate the strengths of the individual methods. Using this method a map with a more simplified visual representation of the subsurface structure is produced. [This research was funded by the Colorado School of Mines and National Science Foundation Award #OISE -1243539/400512.]