关键词: 优势断裂, 稳定性, 分形, 显微特征

Abstract: In this paper we present the results of the study on the compositions, structures, and seismic reflection of the preferred faults at Jiajiang and Shiyezhou sites of Zhenjiang-Yangzhou highway bridge foundation, as well as the results of the quantitative observation of the microstructural features of fault gouges from these faults by means of scanning electron microscopy (SEM). The segmentation of these faults has been further studied in the light of the preferred plane theory. The compositions and types of cementation of the fault gouges from the eastern segment of the Jiajiang fault are different from those of the fault gouges from the western segment. The main clay mineral of the fault gouges is illite, and chlorite comes to second. The eastern segment of the fault displays a distinct throw in seismic reflection, and here the surface textures of quartz grains from fault gouges can be assigned to group Ⅱ and subgroup Ⅰ_c, which are characterized by subangular shape of grains, subconchoidal texture and medium degree of chemical effect. The western segment of the fault displays a discontinuos and weakened reflection. The fault gouges from this segment contain more quartz grains than those in the eastern segment, while the surface textures of quartz grains belong to subgroups Ⅰ_b and Ⅰ_c, which are characterized by step-like texture and weak chemical effect. The main clay minerals of fault gouge from Shiyezhou fault are illite and montmorillorite, and the granulometric compositions of the gouges can be assigned to group Ⅱ or group Ⅲ. The value of fractal dimension and maturity of the gouges from the northern segment is lower than those of the gouges from the southern segment. The present fault activity in the intersection segment is higher than that in the southern segment. The surface textures of quartz grains in fault gouges from the northern segment belong to subgroup Ⅰ_b , and those from the intersection segment and the southern segment belong to subgroup Ⅰ_c . However, the percentage of perfectly round grain shape and step-like texture is higher in the southern segment than that in the intersection segment. The percentage of indentation and impact pits in the southern segment is higher than those in the intersection segment. It is clear that the Jiajiang and Shiyezhou faults display a significant segmentation in their geometry, activity and structure, while the intersection of the two faults is a combination preferred segment. The results of statistic analysis of surface textures of quartz grains show that both the Jiajiang and Shiyezhou faults have been active for several times, and were characterized by both stick slip and creep slip. The age of latest activity of the Shiyezhou fault is younger than that of the Jiajiang fault. In consideration of the geological condition, therefore, the middle bridge site is not an ideal bridge site. This study has great importance and practical significance to the assessment of fault activity and stability. It provides a geological basis for site selection and design, as well as the safety assessment of oversize bridge project that should be built on the fault zone.

Key words: Preferred fault, Stability, Fractal, Microscopic characteristics