基于岩石CT扫描的冻融作用对花岗岩细观结构及力学强度影响研究

Mesostructure and strength characteristics of granite under freeze-thaw cycles based on CT scanning

近年来随着西部地区的基础工程建设数量及规模不断增加,西部高原地区的季节性冻融循环效应的影响也随之增强,开展冻融循环作用下岩石细观特性及强度劣化性质研究对指导西部寒区基础工程建设至关重要。首先在偏光显微镜下对岩石薄片进行观察,获取岩石的矿物成分和微结构;接着利用CT扫描技术,对冻融后的花岗岩进行扫描,对扫描图层利用阈值分割进行二值化处理,堆叠得到样品内外结构的高分辨3D数据及影像;结合分形理论计算图像计盒维数并由此对图像复杂度做出量化判断,由此对冻融循环对花岗岩内部结构演化分布特点进行分析;进而揭示其强度演化规律,探究结构演化与强度之间的关系。偏光显微镜下,岩石呈块状构造,具有似斑状粗粒不等粒花岗结构,局部见交代蠕虫结构。似斑晶矿物主要为碱性长石;其他矿物粒径0.25~4.0 mm为主,矿物成分主要为石英、斜长石、碱性长石,次要矿物为黑云母、绿帘石,副矿物有磷灰石、锆石、黄铁矿等,镜下鉴定为似斑状粗粒不等粒黑云二长花岗岩。CT扫描显示,冻融循环效应在影响花岗岩细观结构时,会导致花岗岩内部孔隙率的整体上升,但岩石渗透性变化不大,岩石渗透率仅上升0.003×10^(-3)μm^(2);内部孔隙发育不均匀,试样整体结构改变以萌生较多新的微孔隙为主。冻融循环后岩石内部结构复杂度有所下降,但岩石整体完整性仍然较好,分形维数仍保持在较高水平。分形研究显示,20次冻融循环并未导致花岗岩的结构复杂度发生较大变化,同时试样整体力学特性出现下降,黏性增加以及长期强度出现较大幅度的衰减,进入蠕变试验阶段的应变阈值提高。在评价此类原生结构较致密的岩石的安全性时,仅从结构上进行考量与实际情况往往会出现偏差,应结合必要的强度指标综合评估。岩石在经历冻融循环后,在强度更低的同时会发生更大的变形。该研究可为分形理论在岩石细观结构演化方面的应用及岩石细观结构与强度演化相关研究提供借鉴,并对高寒地区工程施工有指导意义。

[Objective]With the rapid increase in construction projects in the western regions in recent years,the impact of seasonal freeze-thaw cycles in the high-altitude areas of western China has become more pronounced.Conducting research on the microscopic characteristics and strength degradation properties of rocks under freeze-thaw cycles is crucial for guiding engineering construction in these cold,high-altitude regions.[Methods]To study the influence of freeze-thaw cycles on rock structure and mechanical properties,we collected diorite samples from a tunnel in the Kangding area and examined the effects of freeze-thaw cycles on their microstructure and mechanical characteristics.Firstly,thin rock sections were observed under a polarizing microscope to obtain mineral compositions and microstructures.Then,CT scanning technology was used to scan the granite samples after freeze-thaw cycles,and the scanned layers were binarized using threshold segmentation.The scanning images of different layers were binarized using threshold segmentation,and highresolution 3D data and images of the internal and external structures of the samples were obtained by stacking the binary image layers.Fractal theory was applied to calculate the box-counting dimension of the images and quantitatively assess their complexity.This analysis allowed us to examine the evolution and distribution characteristics of the internal structure of granite under freeze-thaw cycles.[Results]Under a polarizing microscope,the rock exhibits a block-like structure with a patchy,coarse-grained,and unequal-grained granite texture,with locally visible metasomatic worm structures.The main phenocryst minerals are alkaline feldspar.Other minerals range in size from 0.25 to 4.0 mm and primarily include quartz,plagioclase,and alkaline feldspar.Secondary minerals include biotite and epidote,while accessory minerals comprise apatite,zircon,and pyrite.Microscopically,the rock is identified as porphyritic,coarse-grained,and unequal-grained biotite diorite granite.Freeze-thaw cycles were applied to the granite samples in the laboratory to study the strength evolution and explore the relationship between structural evolution and strength.The results indicate that the freeze-thaw cycle effect leads to an overall increase in the internal porosity of the granite's microstructure,though the rock's permeability changes minimally,with an increase of only 0.003×10^(−3)μm^(2).The internal pore development is uneven,primarily due to the emergence of new micropores,causing changes in the overall structure of the sample.After freeze-thaw cycles,the complexity of the internal structure of the rock decreases,but the overall integrity remains good,with the fractal dimension staying at a high level.Fractal analysis shows that 20 freeze-thaw cycles do not cause significant changes in the structural complexity of granite.However,the overall mechanical properties of the sample decline,viscosity increases,and long-term strength shows significant attenuation,raising the strain threshold for entering the creep test stage.[Conclusion]When evaluating the safety of rocks with dense primary structures,considering only their structure may lead to deviations from the actual situation.It is essential to combine necessary strength indicators for a comprehensive evaluation.After undergoing freeze-thaw cycles,rocks tend to exhibit more significant deformation while maintaining lower strength.Therefore,appropriate treatments are required for construction in high-altitude areas.[Significance]This study provides a reference for applying fractal theory to the evolution of rock microstructure and the relationship between rock microstructure and strength evolution.It also offers valuable guidance for engineering construction in high-altitude and cold regions.