Multi-Scale Evolution Mechanism of Sandstone Structure in Baihetan Reservoir Head Region
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摘要: 干湿循环作用下岩石结构多尺度演变机制是工程地质领域热点问题之一，而由于其复杂的结构体系具有随机性和无序性，导致阐述岩石样品同一区域微观结构与宏观力学结构性质之间的关系极具挑战.以白鹤滩水电站近坝址区砂岩为研究对象，采用低场核磁共振（nuclear magnetic resonance，NMR）、亚微米级CT扫描（μCT）、扫描电子显微镜（scanning electron microscope，SEM）等多种精细测试技术探究干湿循环作用下砂岩同一结构位置不同阶段力学性质、孔隙裂隙和矿物结构的演变特征.结果表明：粘土团聚结构中强亲水性矿物吸附水分子后，表面水化膜增厚，溶胀压力导致团聚体破坏；随着干湿循环次数增加，小孔隙数量、孔径增加，而大孔径孔隙并未呈现单调增加趋势；孔-裂隙连通性明显改善，导致砂岩单轴抗压强度呈现指数下降趋势，破裂面裂隙趋于复杂化.最后基于详细的微结构演变证据并结合矿物-水分子模拟揭示了砂岩结构多尺度演变机制.研究成果对由岩石结构劣化诱发的地质灾害研究具有重要意义.Abstract: It is of great significance to elucidate the multi-scale evolution mechanism of rock structures under the action of wetting and drying cycles in the field of engineering geology. However, due to the randomness and disorder of complex structural systems, it is extremely challenging to elucidate the relationship between microstructure and macroscopic mechanical structural properties. Taking the sandstone near the dam site of the Baihetan Hydropower Station as the research object, various fine testing techniques (including NMR, μCT and SEM) were adopted to explore the evolution characteristics of the mechanical properties, pore fracture characteristics and mineral structure of sandstone in different stages of wetting and drying cycles. When water molecules were absorbed by hydrophilic minerals, the surface hydration film thickened, and the swelling pressure led to the destruction of aggregates. With the increase in the number of wetting and drying cycles, the number of small pores and pore size increased, while the number of large pore size did not show a monotonically increasing trend. The pore-fracture connectivity was significantly improved during wetting and drying cycles, resulting in an exponential decline in the uniaxial compressive strength of sandstone, and the fracture surfaces tended to be complicated. Finally, the multiscale evolution mechanism of the sandstone structure was revealed based on detailed microstructure evolution observations and mineral-water molecular simulation results. The research results are of great significance to the study of geological hazards induced by rock structure deterioration.
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图 1 研究区域位置及白鹤滩水电站近坝址区岩体软岩分布区
a.研究区位置；b.白鹤滩水电站近坝址区岩体软岩分布区
Fig. 1. The location of study area and the distribution area of soft rocks in the near dam site area of Baihetan Hydropower Station
图 3 干湿循环作用下砂岩的单轴抗压强度变化
Fig. 3. Change of uniaxial compressive strength of sandstones under cycles of wetting and drying
图 4 不同干湿循环次数砂岩的破坏图
Fig. 4. The failure images of sandstones with different cycles of wetting and drying
图 5 干湿循环作用下砂岩破裂面三维重构
Fig. 5. Three-dimensional reconstruction of the fracture surface of sandstones under cycles of wetting and drying
图 7 不同干湿循环次数下岩石三维裂隙演变特征
Fig. 7. Three-dimensional fracture evolution characteristics of rock under different cycles of wetting and drying
图 8 不同干湿循环次数下岩石孔-裂隙连通性演变特征
Fig. 8. Evolution characteristics of pore-fracture connectivity in rock under different cycles of wetting and drying
图 9 干湿循环作用下粘土团聚体微结构演变特征
Fig. 9. Microstructural evolution characteristics of clay agglomerates under the action of cycles of wetting and drying
图 10 干湿循环作用下岩石裂隙演变特征
Fig. 10. Evolution characteristics of rock fracture under cycles of wetting and drying
图 11 水作用下砂岩结构多尺度演变机制
Fig. 11. Multi-scale evolution mechanism of sandstone structure under water action
表 1 砂岩试样矿物成分及含量
Table 1. Mineral composition and content of sandstone samples
矿物成分 石英 长石 方解石 粘土矿物 伊/蒙混层 伊利石 高岭石 绿泥石 含量（%） 40.8 10.9 9.1 18.9 17.1 1.3 0.9 
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