镇扬大桥桥基区优势断裂显微构造特征及分段研究

MICROSTRUCTURAL FEATURES AND SEGMENTATION OF PREFERRED FAULTS AT ZHENJIANG-YANGZHOU YANGTZE RIVER HIGHWAY BRIDGE FOUNDATION SITE

基于SEM和矿物组分分析研究了镇扬大桥桥基优势断裂断层泥的显微结构、显微构造和物质组成等特征 ,并进一步应用优势面理论进行分段研究 ,取得了较好的效果 ,其对大桥桥基断裂活动性和稳定性的评价具有重要意义 ,为建在断裂带上的特大型大桥工程的选址。

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.