A review of instrumentation for measuring small strains during triaxial testing of soil specimens

Conventional procedures for determining deformations during triaxial tests are based on measurements made externally to the cell. This practice is inadequate, particularly if the small strain stiffness of the soil is being investigated, because errors are introduced which limit the accuracy and resolution of the measurements. The errors can only be eliminated if axial strains are determined internally, within the cell, and locally over the central one third of the specimen. Likewise, the radial dimensions should be monitored at specimen midheight. The last ten years has seen the introduction of a diverse range of small-strain measuring devices. This has been largely in response to recognition of the importance of achieving strain measurement accuracy of at least 10−3% for small-strain stiffness evaluation. This paper summarizes the types of instrumentation currently available and their modes of operation, benefits, capabilities, and limitations. Typical results comparing external and internal strain measurements are presented. A system 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 paper is intended to assist prospective users in becoming familiar with the various techniques that have been used and presents information that should enable selection of appropriate equipment for particular applications. Given that improvements in existing systems are warranted, it is hoped that the paper will also provide stimulus for further research and development.