# Analysis of Bridge Tests on Sandy Overburden Site with Fault Dislocating
## 4 Mechanism of Foundation Failure in Cross Fault Bridge Sites  
Combining global instances of earthquake-induced bridge damage across faults and international seismic codes for bridges, the primary cause of bridge structure impairment and potential collapse on overburdened soil lies in significant deformation damage to both the soil and foundation. Addressing the challenge of rupturing the overburdened soil body during a severe earthquake requires expensive engineering interventions, such as digging deep trenches. However, this method may not guarantee precision in locating the rupture or determining the overall affected area [34; 35; 36]. Hence, understanding the rupture traces and widths within the overburdened soil body's interior profile is crucial. This information directly influences factors such as the required avoidance distance and rupture-resistant measures, forming an integral part of the analysis of site foundation damage mechanisms. Consequently, this paper conducts a comparative analysis of the damage characteristics observed in the foundation soil and bridge structure at both the free site and the bridge site under low-dip reverse fault and high-dip normal fault conditions. This analysis is based on four tests conducted on sandy soil cover under bedrock dislocation. The objective is to investigate and analyze the damage mechanism of the bridge site foundation across the fault:  
1. When bedrock dislocates in a low-dip reverse fault, an extrusion-type shear rupture zone forms within the soil body. This zone extends from the dislocation point toward the dip angle direction, causing raised surface rupture zones. The bridge site's rupture zones pass through the bridge pile foundations along the fault's dip direction, offsetting relative to the free site. Due to the substantial stiffness of the bridge structure, soil and bridge interact, causing structural vibration energy to radiate into the soil layer, inducing a damping effect. This interaction results in larger soil pressure around the structure during rupture, influencing a broader range at the bridge site. It affects the middle of the soil layer from depth, highlighting a more noticeable soil amplification effect. 2. In the context of high-dip normal fault bedrock dislocation, a tensile shear rupture zone forms within the soil body, centered on the bedrock's dislocation point. Cracks appear on the ground surface even with minimal bedrock dislocation, and increased dislocation leads to the derivation of multiple surface rupture zones. Simultaneously, the bridge site's rupture zone passes through the bridge pile foundations along the fault's dip direction, offsetting relative to the free site. Due to soil interaction with the pile and bearing platform of the bridge structure, soil pressure widens in scope and depth. Soil pressure changes in the middle and lower parts of the soil body become obvious, affecting deeper surface rupture cracks in the overburdened soil  
Figure 18: Comparison chart of peak acceleration on both sides of the east and west with height variation in experiments 2 and 4.  
layer. Acceleration of the soil layer in the upper disk is about four times greater than in the lower disk, with a more pronounced effect on the upper disk. 3. In sandy soil cover sites, under low-dip reverse fault dislocation, the foundation damage area is lighter than in high-dip positive fault scenarios, both on the surface and within the soil body. The pile foundation in the main rupture zone's concentrated area on the surface experiences greater stress from surface rupture traces, resulting in severe damage and deformation of the pile foundation, the bottom of the bridge deck slab, and the abutment connection. In Table 3, for the model test with a bedrock dislocation of 70 mm, signifying complete overburden soil body rupture, the actual engineering site's surface rupture width and length parameters are calculated based on observed test phenomena and the similarity ratio (Table 1).