Study on the Applicability of the Seed Simplified Method in Near-Field and Far-Field Regions

The Seed simplified method is often adopted in dynamic triaxial tests to deal with irregular seismic loads. However, near-fault ground motions usually carry long-period and high-amplitude pulse components, with multiple and complex spectral components, which exceed the scope assumed by the Seed simplified method based on conventional ground motions. Therefore, it is of great importance to explore the applicability of the Seed simplified method to near-fault and far-field earthquakes. Five ground motions from the 1999 Chi-Chi earthquake in Taiwan (China) were selected. The measured ground motions and equivalent cyclic loads were respectively applied to soil samples through the GDS dynamic triaxial apparatus. The test results show that there is a strong correlation between the axial strain generated by the input ground motions and the fault distance of the earthquake records. Under the action of near-fault earthquakes, the maximum residual strain of soil samples is approximately 14 times that under the action of far-field earthquakes. The research findings indicate that the deformation of soil caused by pulse components in near-fault ground motions accounts for about 92% of the total deformation, which is approximately 11 times the deformation caused by residual components, demonstrating that pulse components are the main reason for soil deformation. By comparing the residual strain generated by the measured ground motions with the axial strain generated by the corresponding equivalent cyclic loads, it is found that under the predetermined number of vibration cycles, the axial strain generated by the cyclic loads is greater than the residual strain generated by the far-field ground motions but less than the residual strain of the near-fault ground motions. Therefore, when simulating far-field ground motions, the equivalent number of vibration cycles recommended by Seed is on the safe side. However, under near-fault conditions, the applicability of the Seed simplified method is rather poor. Furthermore, under high confining pressure and large dynamic stress amplitude, the applicability of the Seed simplified method in the near-fault region is significantly reduced.