不同冷却条件对高温砂岩物理力学性质的影响

岩石工程可能会经受高温环境。岩石高温后冷却方式的不同往往会导致岩石物理力学性质产生重大变化，这对岩石工程的稳定性、渗透性等都会产生重要影响。采用核磁共振（MRI）、电镜扫描（SEM）和单轴压缩试验对100、300、500、600、800 ℃ 5种不同温度砂岩经两种不同冷却方式（自然冷却和水中冷却）后的孔隙率、孔径分布、峰值强度、峰值应变、应力-应变关系以及微观结构变化等进行研究。试验结果表明：自然冷却时，高温砂岩强度并非随温度升高而持续降低，而水冷却会导致砂岩强度持续降低，且降低幅度远超自然冷却；500 ℃可以看作不同冷却方式对砂岩孔隙率影响的临界值，超过500 ℃，水冷却方式会导致孔隙率急剧增长，大孔径（Ф 10 μm）孔隙所占比例也高于自然冷却，因此，高温砂岩工程采用水冷却方式（如隧道着火后用水灭火）要充分考虑由此可能带来渗透危害；SEM测试表明，当温度 500 ℃时，水冷却对裂纹的增宽和扩展产生促进作用；当温度达到800 ℃时，水冷却砂岩孔洞变大，裂隙更加发育，并贯通连成网络，这会导致透水性大幅提高，同时，这也是该温度水冷却导致强度急剧降低的原因之一。

Effect of different cooling conditions on physical and mechanical properties of high-temperature sandstone

Rock engineering may be subjected to high temperature environment. Different cooling methods of high-temperature rock often lead to significant changes in the physical and mechanical properties of the rock, which will have an important impact on the stability and permeability of rock engineering. Magnetic resonance imaging (MRI), scanning electron microscope (SEM) and uniaxial compression test were used to study the porosity, pore size distribution, peak strength, peak strain, stress-strain relationship and microstructure changes of five temperatures for sandstone samples at 100, 300, 500, 600 and 800℃ under two cooling methods (natural cooling and water cooling). The test results show that: (1) When the rock samples used the natural cooling method, the strength of high-temperature sandstone does not decrease continuously with the increasing of temperature. However, rock samples using water cooling method show continuous decrease of sandstone strength, and the decreasing extent is far greater than that of natural cooling; (2) 500℃ can be considered as the critical value of the influence of different cooling methods on the porosity of sandstone. When the temperature is above 500℃, the water cooling method will cause the rock porosity increase rapidly, and the proportion of pores with large pore diameter (Ф>10 μm) is also higher than that of the natural cooling method. In this consideration, in the field of high-temperature sandstone engineering, the possible seepage hazards should be fully considered when water cooling is used (i.e., fire extinguishing with water after a tunnel is on fire); (3) The SEM test results shows that when the temperature is above 500℃, water cooling promotes the widening and expansion of cracks. When the temperature reaches to 800℃, the pore size of water-cooled sandstone becomes larger, and the fracture is largely developed and connects into a network. This will lead to a substantial increase in water permeability. At the same time, it is one of the reasons for the sharp decrease in rock strength that caused by water cooling at this temperature.