In addition to trialing the prototype vacuum mould (see GDS News March 2015), a series of tests provided GDS technical staff with an opportunity to compare the cyclic strength of Toyoura sand obtained from an NGI-style direct simple shear (DSS) apparatus (the GDS EMDCSS system) with that from hollow cylinder (HC) apparatus testing. To make this comparison, four studies in which saturated Toyoura specimens tested in undrained cyclic torsional shear with a constant height condition maintained (i.e., plane strain) were sourced from the literature.
Initial comparisons between the five data sets are given below (bottom-right plot), which shows number of load cycles required to reach a failure criterion (between 7.5 % and 15 % double amplitude shear strain) plotted against the applied cyclic stress ratio (CSR). Here the CSR is defined as the shear stress amplitude, τ, divided by the initial mean effective stress, pʹ0 = (σʹv + 2σʹh)/3. Note as the normal horizontal stress, σʹh, is not directly measured in the EMDCSS, the initial mean effective stress was estimated by assuming a K0 value equal to 0.44. This value was obtained via Jaky’s solution for K0 and an internal friction angle of 34 °, selected based on the relative density range of 45 – 50 %.
Overall it is interesting to see that preliminary comparison of the data suggests the NGI-type DSS test (using the GDS EMDCSS) produces cyclic strengths that agree well with HC response. It is however stressed the data interpretation presented in this news article is only preliminary, and therefore will likely be re-assessed to account for a variety of differences between the specific tests (e.g., specimen preparation method, stress anisotropy etc.)
Disclaimer: The information presented in this news article is written by and is the opinion of GDS Instruments. The article is for informational purposes only and is not reviewed by third parties.