A REVIEW OF INSTRUMENTATION FOR MEASURING SMALL STRAINS DURING TRIAXIAL TESTING OF SOIL SPECIMENS

Conventional procedures for determining deformations during triaxial t ests are based on measurements made externally to the cell. This pract ice is inadequate, particularly if the small strain stiffness of the s oil is being investigated, because errors are introduced which limit t he accuracy and resolution of the measurements. The errors can only be eliminated if axial strains are determined internally, within the cel l, and locally over the central one third of the specimen. Likewise, t he radial dimensions should be monitored at specimen midheight. The la st ten years has seen the introduction of a diverse range of small-str ain measuring devices. This has been largely in response to recognitio n of the importance of achieving strain measurement accuracy of at lea st 10(-3)% for small-strain stiffness evaluation. This paper summarize s the types of instrumentation currently available and their modes of operation, benefits, capabilities, and limitations. Typical results co mparing external and internal strain measurements are presented. A sys tem for classifying internal-strain measuring devices is presented. It is anticipated that test results based on small strain determinations will be a more frequent requirement in the future. Accordingly, the p aper is intended to assist prospective users in becoming familiar with the various techniques that have been used and presents information t hat should enable selection of appropriate equipment for particular ap plications. Given that improvements in existing systems are warranted, it is hoped that the paper will also provide stimulus for further res earch and development.