Radial Strain Measurements in Resonant Column and Torsional Shear Tests

Abstract

During resonant column and torsional shear tests using a Stokoe device, conventionally only the change in height of the specimen is measured. To obtain the change in diameter of the specimen, a relationship between radial strain and longitudinal strain is assumed. In this study, the Stokoe device was modified to enable the measurement of change in diameter of the specimen along with the change in its height during resonant column and torsional shear tests. The diameter change measurement system comprises three proximity probes, coaxial cables, proximitor conditioners, and DC power supply. The probes (7.9 mm diameter) are capable of making noncontact displacement measurements with a resolution of 0.025 mm using a metallic target. These probes are fixed along the circumference of the outer chamber at equal 120 degree spacing. The vertical location of these probes is such that they correspond to the midheight of the specimen. Each probe is mounted on a micrometer to enable accurate adjustment of the probe. This system gives a linear output for a distance range of 0.38 to 2 mm between the probe tip and the metallic target. During the tests, the targets (circular copper foils 26 mm in diameter) are fixed on the soil specimen, and output voltage from each of the three proximity probes is recorded directly by a computer. The average readings of the three probes are used to calculate the change in diameter of the specimen. This paper will present the results of tests on seven residual soil specimens.