Analysis of Bridge Tests on Sand-Covered Sites Subjected to Bedrock Faulting
Authors: Zhang Jianyi , Ma Jianke , Zhang Haonan , Song YiJie 
Published: August 2023

The seismic design methods based on performance for bridges have made significant progress. However, there is a lack of research on the mechanisms and effects of bridge damage under extreme seismic events such as near-fault and far-field earthquakes. This poses substantial challenges for the construction and operation of bridges in regions characterized by frequent strong earthquakes and intersecting fault zones. In order to investigate the seismic damage mechanisms of bridges subjected to ground surface rupture caused by underlying bedrock fault displacement, as well as to provide fundamental data for the design and construction of bridges near seismogenic fault zones, an analysis of the impact of strong ground surface rupture on bridges was conducted. To achieve this, a large-scale experimental setup consisting of bridge models was developed. The setup incorporated adjacent inclined strong ground surface rupture zones and a synchronous bedrock fault displacement loading system. Four sets of experiments were conducted, yielding the following findings: (1) In the case of low-dipping reverse faults and sand cover soil, the presence of bridge structures altered the direction of rupture propagation, causing the rupture zone to shift eastward while reducing its width. Both the free field and bridge site experienced two areas of uneven settlement in close proximity. The extent of uneven settlement in the free field was approximately 1.5 times wider than that in the bridge site. However, the bridge site exhibited greater differences in settlement and tilting, approximately three times that of the free field. The bottom soil pressure changes were most prominent in the free field, and the presence of the bridge expanded the influence of soil pressure over a wider range, resulting in significant variations in the soil pressure at the mid-section. Acceleration changes in the bridge site soil were more pronounced, whereas the free field soil showed less acceleration near the "V" shaped rupture trace. The presence of the bridge amplified the response of the soil, particularly the top soil layer on the east side, which experienced a four-fold amplification compared to the middle and bottom layers. (2) In the case of high-dipping normal faults and sand cover soil, when the rupture occurred, the width of the rupture zone in the bridge site was narrower than that in the free field, and no subsidence was observed. This indicated that the presence of the bridge partially restrained the occurrence of surface rupture. The influence range of uneven settlement was similar in both the free field and bridge site, within the range of -600mm to 300mm on either side of the main rupture zone. The free field experienced more severe uneven settlement, approximately 1.61 times that of the bridge site. Significant changes in soil pressure were observed in the middle section of the cover soil layer at the bridge site, leading to more severe damage. The variation in soil acceleration near the "V" shaped rupture trace was not significant. However, the acceleration on the top surface of the soil layer on the east side (hanging wall) was amplified nearly four times compared to the west side (footwall), indicating a significant hanging wall effect.