Influence of Piston Friction on Resilient Modulus of Subgrade Soil

The repeated load triaxial test has been commonly used to evaluate the resilient modulus of base, subbase, and subgrade soils. When conducting the resilient modulus test, if a load cell is placed outside the chamber to measure the axial load, the piston friction is included in the axial load, resulting in inaccuracy on the resilient modulus. Hence, piston friction needs correcting prior to the resilient modulus test when using the external load cell to measure the axial load. In this study, quasi-static and dynamic corrections of the piston friction were conducted under various displacement rates and confining pressures. Test results indicate that the piston friction increases with increasing confining pressure. A correlation model between the piston friction and the confining pressure was established. Besides, the influences of piston friction on the resilient modulus of A-7-5 soil compacted at the optimum moisture content were evaluated. Test findings demonstrate that the resilient modulus is overestimated when ignoring the piston friction, especially at low deviator stress. Procedures for the correction of the piston friction were established and presented.