考虑应力水平和化学腐蚀效应的冻融砂岩细观损伤试验

Experimental Study on Microscopic Damage of Freeze-thawed Sandstone Considering Effects of Stress Levels and Chemical Corrosion

寒区工程岩体在经受冻融作用的同时受到外部荷载和化学溶液的影响,直接体现在孔隙结构的变化上,决定着冻融损伤劣化机制。为研究应力、溶液和冻融共同作用下砂岩孔隙结构的演化特征,开展了不同应力水平和化学溶液的砂岩冻融循环试验,通过核磁共振获得的T2谱和扫描电镜技术获取的微观图像,分析了砂岩孔隙孔径分布、孔隙度、不均匀系数、渗透率和弯曲率以及微观结构随冻融循环的变化情况,探讨了应力水平和化学溶液对冻融过程中砂岩孔隙结构演化过程的影响规律和控制机制。研究结果表明:随冻融循环次数增加,不同尺寸孔隙增加,中孔扩展速率加快,且易扩展为更大尺寸的大孔,引起孔隙度、不均匀系数、渗透率增加,而弯曲率降低;稀硫酸溶液促进小孔发育为中孔和大孔。当轴向应力较低(0.3σ_(f))时,在冻融初期垂直于应力方向的孔隙发生闭合,抑制了化学腐蚀和冻胀力的发挥,不同尺寸孔隙数量较无应力下减少,孔隙度、不均匀系数、渗透率减小,弯曲率增加;之后随化学腐蚀和冻胀力交替累加,再加上轴向应力的附加作用,引起不同尺寸孔隙数量增加。当轴向应力较高(0.7σ_(f))时,受应力主控,中孔和大孔增加明显,沿晶裂纹和穿晶裂纹快速萌生和扩展,引起孔隙度、不均匀系数、渗透率快速增大,弯曲率降低,最终导致砂岩破裂。

Engineering rock masses in cold regions are affected by external loads and chemical solutions during freeze-thaw action,which is directly reflected in changes in the pore structure that determine the damage mechanism.To study the evolution characteristics of sandstone pore structures under the combined action of stress,chemical solution,and freeze-thaw cycles,sandstone freeze-thaw cycle tests were conducted at different stress levels and solutions.The T2 spectra and microscopic images of a sandstone under different conditions were obtained using nuclear magnetic resonance and scanning electron microscopy techniques,respectively.The pore size distribution,porosity,pore size uniformity coefficient,permeability,tortuosity,and microstructural changes in the sandstone were analyzed.The influence and control mechanism of stress levels and chemical solutions on the pore structure evolution process of the sandstone during freeze-thaw cycles were further explored.The results show that as the number of freeze-thaw cycles increases,the number of pores of different sizes increases,and the expansion rate of mesopores accelerates,making the expansion into macropores with larger sizes easier.The porosity,uniformity coefficient,and permeability of sandstone increases,whereas tortuosity decreases with freeze-thaw cycling.The acidic solution promotes the development of micropores into mesopores and macropores.When the sandstone is at low stress,such as at 0.3σf,the pores perpendicular to the stress direction close during the initial stage of the freeze-thaw cycles,suppressing the exertion of chemical corrosion and frost heave force.This leads to a decrease in the number of pores of different sizes compared with a sandstone with no stress,resulting in a decrease in porosity,uniformity coefficient,and permeability,and an increase in tortuosity.Subsequently,with the alternating action of chemical corrosion and frost heave forces coupled with the additional effect of axial stress,the number of pores of different sizes increases.When the sandstone is at high stress,such as at 0.7σf,and mainly controlled by stress,the mesopores and macropores increase significantly,and intergranular and transgranular cracks initiate and propagate rapidly.This results in a rapid increase in porosity,uniformity coefficient,and permeability,and a decrease in tortuosity,and finally resulting in rock failure.