Investigating weathering features of sandstones in the Yungang Grottoes based on SEM images and micro-scale flow model
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摘要:
风化作用、岩石微观结构、岩石微观渗流特性定量关系的研究是石窟文物有效保护的重要基础，砂岩风化作用严重影响了岩体上石窟文物的有效保护。本次研究使用扫描电镜（SEM）获得云冈石窟不同窟体砂岩的微观图像，根据图像增强和图像分割技术得到了岩石中颗粒和孔隙的数字特征参数，从微观角度建立了反映孔喉连接特性的渗流模型，得到了不同窟体岩石的局部水力传导系数，分析了数字特征参数、水力传导系数、风化作用之间的关系。结果表明，风化作用对石窟砂岩微观结构具有很大影响；砂岩孔隙平均长轴长度大小对应的风化程度分别是全风化或强风化、中等风化、微风化或未风化，长度分别为大于40 μm、25～35 μm、15～25 μm；风化程度越高、水力传导系数越大，随着风化程度的增高、水力传导系数的范围为1 × 10−9～1 × 10−4 cm/s；水力传导系数与孔喉尺寸、孔喉连通性密切相关；孔喉半径比增大时、水力传导系数也增大。
Abstract:The quantitative relationship among the weathering, microscale structure, and microscale flow properties is a fundamental issue for the efficient preservation of the cultural relics in the grottoes. Sandstone weathering may affect the efficient preservation of the cultural relics in the grotto rocks. In this study, the microscopic images of the sandstones of various grottoes in the Yungang Grottoes are obtained by Scanning Electronic Microscopy (SEM), and the digital characteristic parameters of the particles and pores in the rock are obtained by using the image enhancement and image segmentations. A flow model reflecting the characteristics of the pore-throat connection is also established in the microscopic scale. The local hydraulic conductivity coefficients of the rocks at various grottoes are then obtained. The relationship among the hydraulic conductivity coefficients, digital characteristic parameters, and weathering levels is further explored. The results show that the weathering much affects the microstructure of the grotto sandstones; the corresponding great-small order of the weathering for the average long axeses of pores are the complete or high weathering, the moderate weathering, and the slight weathering or fresh sandstones, respectively, with the axeses of greater than 40 μm, 25 to 35 μm, and 15 to 25 μm. The higher levels of weathering will result in the greater hydraulic conductivities ranging from 1×10−9 to 1×10−4 cm/s; the hydraulic conductivity is closely related to the size and connectivity of the pore-throats; and the increases in the pore-throat radius ratios will result in the increase in the hydraulic conductivities.
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表 1 不同窟砂岩样品SEM图像的数字特征参数
Table 1. Digital parameters of SEM images for sandstone samples in various grottoes
石窟编号 颗粒数量/个 颗粒平均粒径/μm 颗粒平均圆度 面孔隙率/% 1 305 10.65 0.42 36.85 3 419 14.74 0.48 30.13 4 214 8.47 0.43 14.32 7 189 4.98 0.40 17.32 8 286 9.48 0.45 31.38 10 79 4.43 0.26 18.38 15 331 16.94 0.52 33.22 17 128 2.47 0.29 19.89 19 72 4.91 0.34 12.20 
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表 2 不同窟体砂岩SEM图像的孔隙特征参数
Table 2. Pore digital parameters of SEM images for sandstones in various grottoes
石窟
编号ri/（10−6 m） ro/（10−6 m） d/（10−6 m） η/（10−6 m2·s−1） 1 61.90 6.33 101.42 1.42 3 49.02 5.27 84.81 1.38 4 33.11 4.21 51.98 1.38 7 27.72 3.62 39.32 1.30 8 41.17 3.93 69.58 1.38 10 18.01 3.86 31.80 1.30 15 58.73 4.33 125.27 1.42 17 21.03 3.64 40.83 1.34 19 21.69 3.98 51.94 1.34
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表 3 不同窟体砂岩的风化特征参数
Table 3. Weathering characteristic parameters for sandstones in various grottoes
石窟编号 孔喉半径比 水力传导系数/（cm·s−1） 1 9.78 1.14 × 10−5 3 9.30 7.71 × 10−5 4 8.80 8.86 × 10−7 7 7.66 9.23 × 10−8 8 10.48 4.23 × 10−6 10 4.01 2.09 × 10−9 15 13.56 8.64 × 10−5 17 5.02 2.14 × 10−8 19 5.44 3.31 × 10−8
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