A new index for evaluating liquefaction resistance of soil under combined cyclic shear stresses

• We proposed an new index that narrows scatters of data on liquefaction strength curve caused by different stress paths.
• We conducted cyclic tests of four different stress paths on hollow cylinder samples of Fujian sand and Qiantang silt.
• The data in this study and an existing literature verified the advantages of ESR especially for loose cohesionless soils.

In geotechnical laboratory testing, a cyclic sine wave loading is applied to samples to simulate seismic loading. The shear stress amplitude in cyclic tests, the cyclic stress ratio (CSR) is always used to express the resistance of soil to cyclic loading. However, the cyclic strength of samples subjected to different cyclic stress patterns can exhibit quite differently. Apart from the possibility of the continuous rotation of the principal stress direction, the difference in shear stress due to cyclic changes induced by different cyclic stress paths is also an influential factor. A new parameter is presented in this paper, the equivalent cyclic stress ratio (ESR), which is defined as the ratio of the mean value of the maximum shear stress in a cycle to the initial effective confining pressure. With the hollow cylinder apparatus, several series of cyclic loading tests were carried out on Fujian fine sand and Qiantang silt. The ESR-based data all fall within a very narrow band in the half logarithmic coordinate, even when cyclic stress paths are different. Test results from Ishihara on Toyoura sand were also re-analyzed using the new index, and a similar correlation was evidenced for loose sand under various cyclic stress paths. For dense and very dense sands, the treatment can also narrow the discreteness of results. With this new index, the cyclic strength of soil obtained from relatively simple tests, e.g. cyclic triaxial tests or cyclic torsional shear tests can be easily used to predict the cyclic strength of soil subjected to combined cyclic shear stresses.