In highway construction across the southeastern coastal regions of China, granite residual soil is widely used as subgrade fill material in pavement engineering. Its mechanical behaviour under dynamic loads warrants in-depth investigation. Dynamic events such as vehicular traffic and earthquakes are complex, involving multidirectional loads. The dynamic behaviour of soil under bidirectional cyclic loading differs significantly from that under cyclic loading in one direction. A large-scale bidirectional cyclic direct shear apparatus was utilised to carry on a series of horizontal cyclic direct shear tests on granite residual soil with water contents of 14% and 24% at different normal stress amplitudes (sigma a) (0, 100, 200 kPa). Based on these tests, discrete element method (DEM) models were developed to simulate the laboratory tests. The test results revealed that cyclic normal stress increases dynamic shear strength during forward shear but reduces it during reverse shear. The energy dissipation capacity increases with rising sigma a. The dynamic behaviour of granite residual soil is more significantly affected by cyclic normal stress when the water content is higher. The DEM simulation results indicated that as cyclic shearing progresses, the location of the maximum principal stress (sigma 1) shifts from the top of the specimen toward the shear interface. The distribution of the angle between sigma 1 and the x-axis, as well as sigma 1 and the z-axis, transitions from 'M' distribution to 'Arch' distribution. With increasing sigma a, during forward shear, the magnitude of the maximum principal stress increases, and the orientation of sigma 1 rotates toward the normal direction. Conversely, during reverse shear, the magnitude of the maximum principal stress decreases, and its orientation shifts toward the horizontal shear direction. The material fabric anisotropy coefficient decreases with increasing sigma a, while the anisotropy orientation increases with increasing normal stress.

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.