Using a direct-drive motor to measure strain in triaxial tests

A newly designed triaxial testing system equipped with a high-precision servo motor and local strain sensors is developed to perform the triaxial test. The servo motor (a direct-drive motor) was used to provide the necessary axial displacement, and to measure the induced strain. A set of accessories for preparation of a specimen was used to eliminate bedding errors and tilting errors. Differences of strains measured through the direct-drive motor and the local strain sensor were studied by conducting triaxial tests on both reconstituted samples and tube samples of Taipei silty clay. The results of the tests show that effective confining pressures or shearing rates cause little difference in the strains measured in the two ways: the difference in strain was less than 6% at small strain. The variations in stiffness measured by the two methods at small strain were in the range −3% to +5%. The ratios of strain measured by the two methods near the failure state of the sample were close to 1. The results have shown that measuring axial strain by a direct-drive motor is reliable, and may be used to improve strain measurement for triaxial tests on soft clay under similar testing conditions.