Granite residual soil is widely used as a subgrade filler in highway construction. Dynamic loads induced by vehicles and earthquakes are complex and involve multidirectional loads, and the dynamic behavior of soil under multidirectional cyclic loading differs significantly from that under unidirectional cyclic loading. A series of horizontal cyclic direct shear tests under cyclic normal loading were conducted using a large-scale cyclic direct shear apparatus at different shear displacement amplitudes (1, 3, 6, and 9 mm) and normal stress amplitudes (0, 100, and 200 kPa). The test results indicate that under cyclic normal stress, the dynamic shear strength of granite residual soil increased during the forward shear process but decreased during the reverse shear process. The damping ratio increases with increasing shear displacement amplitude and normal stress amplitude. This behavior is associated with higher excess pore water pressure induced by greater normal stress amplitude and larger shear displacement, which drive the soil into the yielding phase. The Granite residual soil exhibited significant asymmetric hysteretic characteristics under bidirectional dynamic loading. However, no model has yet been found to describe the asymmetric hysteretic behavior of soil under bidirectional dynamic loading. To obtain the asymmetric hysteretic curve of granite residual soil under bidirectional cyclic loading conditions in the laboratory without the instruments for bidirectional cyclic direct shear tests, the Hardin-Drnevich model and the second Masing rule were extended to propose two asymmetric hysteretic curve models under bidirectional cyclic loading based on the tests. Both models fit with the test results well.
