Bender elements and bending disks for measurement of shear and compressional wave velocities in large sand specimens

Abstract

This thesis reports on research that is part of a larger project funded by the National Science Foundation Grant Number CMS-0509894 aimed at evaluating induced-partial saturation in sands as an effective liquefaction mitigation measure. The focus of this thesis is using bender elements and bending disks for shear and compressional wave velocity measurements in large sand specimens to be used in shaking table tests. Shear wave velocity, Vs, and compressional wave velocity, Vp, are two very important parameters used in geotechnical earthquake engineering analysis. The use of bender elements and bending disks in large soil specimens, typically tested in shaking table tests, poses significant challenges. The wave form generated by a source transducer is three dimensional and attenuates quickly as the distance between the source and receiver transducer gets larger. Moreover, due to complex wave forms, wave travel paths, boundary effects, and soil properties it gets harder to obtain reliable accurate test results. Many challenges were experienced during this research starting from manufacturing piezoelectric transducers to get the maximum efficiency to improving the data acquisition and interpretation of the results. Bender elements were found to be suited well for measurement of shear wave velocities in large specimens. For measurement of compressional waves, it was determined that bending disks were more suitable. A special setup was devised including a signal generator, power amplifier, and a multi-channel digital oscilloscope that permitted accurate measurements of simultaneous readings of bender elements and bending disks responses. This thesis presents, the details of the devised experimental setup, preparation of bender elements and bending disks, and example measurements of Vs and Vp in fully and partially saturated sands. A summary and a set of conclusions are presented regarding the challenges of using bender elements and bending disks.