Small-strain Cyclic Testing With Standard NGI Simple Shear Device

In the standard Norwegian Geotechnical Institute type of direct simple shear (NGI-DSS) test, the horizontal load cell record includes several false loads which do not represent the soil shear resistance. In the case of cyclic loading, the relative magnitude of these false loads increases as the cycle shear strain amplitude gamma(c) decreases. Consequently, in order to obtain accurate cyclic soil behavior at small gamma(c), the NGI-DSS test records must be corrected to address the errors due to these false loads. This paper describes how, with the help of modern computer technologies, the false loads can be identified, quantified, and then subtracted from the load cell record. In an example, the proposed correction procedure is successfully applied to values of gamma(c) as small as 0.026 %. With this procedure, the accurate equivalent viscous damping ratio, the variation of the secant shear modulus with the number of cycles, and other cyclic properties can be obtained at gamma(c) as small as 0.01 %.