A Simple Gauge for Local Small Strain Measurements in the Laboratory

Abstract

A simple device named “local deformation transducer (LDT)” was developed to investigate stiffness of soils that can be measured in the laboratory for strain levels ranging from 10-6 to 10-2. The theoretical consideration into the non-linear, but uniquely determined, relationship between the gauge strain (i.e., output voltage) and the axial strain is first described. The accuracy of the strain measurement using LDT is then discussed based on the theory and the results of calibration tests. The stability of the output voltage was examined for a few days using a dummy. In isotropic consolidation tests performed on a fine clean sand in triaxial cells, the internal axial strains measured using LDT agreed well with those associated with the condition of no bedding error, which were extrapolated from the results of tests using the specimens with various heights. Besides, in plane strain compression, the internal axial strain measured using the LDT was close to that determined by means of a laser speckle method. The capability of the LDT has been demonstrated in triaxial tests in which the variations of Young’s modulus of a gravel, a cement-treated sandy soil and a soft rock were successfully measured for the prescribed range of strains as the specimens were subjected to both monotonic and cyclic loadings.