The Use of Shear Wave Velocity in Shallow Foundation Analysis and Design
AdvisorNorris, Gary M.
Civil and Environmental Engineering
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ABSTRACT Standard penetration test (SPT) blow count values N (N<sub>60</sub> and N<sub>1,60</sub>) have been relied upon in the development of foundation design for over 100 years. The purpose of this research was to evaluate whether shear wave velocity, measured via the refraction microtremor method (via SeisOpt® Refraction Microtremor, © 2013 Optim SDS), could be used to assess the SPT N<sub>60</sub> as an alternative or supplementary approach for design of shallow spread foundations and assessments of immediate settlement for near surface soils (i.e. in the upper 10 feet). In addition to obtaining shear wave velocity (V<sub>s</sub>) and SPT blow counts (N) during our investigation, soil index properties (gradation, plasticity, moisture content, and in-situ density) were also determined to see if they could be used to refine trends in V<sub>s</sub> and N to lead to increased reliability. Given the heterogeneity of the soils at the chosen research site, and the inability of both V<sub>s</sub> and N to reflect the fine and variable layering of the sediments, further refinement incorporating index properties proved unsuccessful. This led to consideration of the direct use of shear wave velocity as the means for shallow foundation analysis relative to heterogeneous unsaturated c-φ soil profiles. An analysis procedure is presented that provides assessment of immediate settlement and the safety factor against bearing capacity failure as a function of the assumed stress level and the associated bearing pressure applied to the foundation for the assessed shear wave velocity within the depth of twice the foundation width below the base of the foundation.The refraction microtremor method was also examined during performance of this study in order to gauge a confidence level for the practicing geotechnical engineer in the use of refraction microtremor analysis for foundation analysis and design. The following were considered:* Different but compatible seismographs; * Different geophone lines paired with the different seismographs; all geophones were 12 Hz;* Different array lengths (22 feet and 55 feet) with different geophone spacing (2 feet and 5 feet, respectively);* Different operators; and * Different noise sources, i.e. walking along the array versus a hammer strike to a steel plate.All of the various field practices produced equivalent refraction microtremor Vs results, within an acceptably narrow range.