Contrastive Experimental Study On Mechanics And Seepage Characteristics Of Sandstone And Granite After High Temperature

In this thesis, uniaxial compression test and triaxial compression test(which s2=s3=5MPa, 10 MPa, 20MPa) were taken into conducted to investigation the influence of high temperature on the physical and mechanical properties of the rock. Samples were heated to arange of peak temperatures up to 600 ℃ at ambient pressure. The gas permeability was measured during the triaxial compression test. Our study confirms that:(1) The peak strength, peak strain, elastic modulus, cohesive force and internal friction angle of the sandstone after heated on 200 ℃ all increased in varying degrees, while the porosity and initial permeability decreased. Granite shows on the contrary tendency to the sandstone in this period. After heated on 400 ℃~600 ℃, these mechanical properties decreased rapid, and the porosity and initial permeability increased significantly. Thermal damage factor of heated rock was defined with P-wave velocity. The factor increased similar to a quadratic term within the test temperature.(2) The stress threshold, i.e. crack initiation stress sci and crack failure stress scd were taken into account to study the progressive failure of rock, defining stress level ratio, i.e. sci/scd, sci/sf and scd/sf. The result shows that:(a) the sci and scd of sandstone and granite increased with the increase of confining pressure;(b) sci/scd and sci/sf declined with the increase of temperature and increased with the increase of confining pressure while scd/sf didn’t show any obvious regularity. The existence of confining pressure delayed crack generated, and once the crack generated, it can be expanded rapidly, makes the sample turn into the crack failure stage.(3) The permeability evolution during the loading process can be characterized into four different stages: decline stage, stable stage, slowly rise stage and sharply rise stage. There are two differences between sandstone and granite in the permeability evolution:(a) The permeability of the sandstone decreased significantly than that of the granite in the decline stage;(b) The increase order of granite was greater than sandstone after rock failure.(4) Under uniaxial compression condition, the damage form of sandstone changes from tensile failure to shear failure gradually with the increase of temperature, while granite presents tensile failure, and generally have several approximate parallel tensile cracks along the axial and local crushing. Under triaxial compression condition, the confining pressure didn’t show significant effect on the inclination of failure face. After undergone higher heated temperature(particularly 600℃), both sandstone and granite were lots of fragments and powder. The scanning electron micrograph(SEM) was used to observation the damage fracture of samples, found that the cleavage fracture was smooth and flat when the heat treatment temperature below 200 ℃, indicated that brittle failure occurred on these fractures; the cleavage fracture was changed to rough when the heat treatment temperature higher than 400 ℃, indicated that plastic failure occurred on theses fractures. Accordingly, sandstone and granite fracture has a change trend from brittle to ductile with temperature.