Geotechnical site characterization in the greater Memphis area using cone penetration tests

Abstract

The determination of seismic ground hazards in Memphis and Shelby County, Tennessee is facilitated by the use of electronic cone penetration tests that can provide up to four independent readings with depth from a single sounding. One series of soundings is being performed for site-specific mapping to determine the presence and extent of potentially-liquefiable sediments, in-situ soil resistance to liquefaction, and initial soil stiffness for ground motion amplification studies. Another series of soundings is being conducted in conjunction with field paleoliquefaction mapping in the New Madrid seismic zone to better define the intensity, magnitude, and geographic extent of ground failures caused by large past earthquake events, as well as information about the source sands. In this paper, an overview is given on the types of multi-channeled penetrometer data that are being collected, including vertical profiles of cone tip stress (qt), sleeve friction (fs), penetration porewater pressure (u1 or u2), downhole shear wave velocity (Vs), and/or electrical conductivity (ke). Representative soundings are presented from select sites to illustrate repeatability, data post-processing methods, and that derived downhole Vs profiles are generally in good agreement with non-invasive surface techniques at two Memphis test sites. Cyclic stress based procedures for liquefaction are discussed with relation to data from a paleoliquefaction site in Germantown, Tennessee, and estimates of the minimum magnitude of the historic event are discussed. While estimates of the earthquake magnitude are preliminary pending additional study on attenuation relationships and site response in the deep soils of the Mississippi Embayment, it is inferred from evaluation of in-situ test data using cyclic stress based techniques that the December 1811 New Madrid earthquake was likely larger than a Mw=7.5 event.