Study on Creep Strength Characteristics of Rock-coal Composite Samples

In order to study the creep strength characteristics of rock-coal composite samples, the graded loading creep tests with loading time of 2 h and 6 h were conducted on the sandstone-coal composite samples with the height ratio of 1:1, respectively. And the relationship of creep strength between composite samples and single coal samples was discussed. The results show that the creep strength and creep coefficient of rock-coal composite sample decreased gradually with the increase of the loading time. The average creep strength of group A (loading time of 2 h) was 13.01 MPa and the average creep coefficient was 92.02%. While the average creep strength of group B (loading time of 6 h) was 11.39 MPa and the average creep coefficient was 80.55%. Compared with group A, both the creep strength and creep coefficient of group B decreased by 12.45%. The creep strength of the composite sample was mainly determined by the creep strength of the internal coal sample, but its value was larger than that of the single coal sample. This is mainly because the composite sample was a systematic body. The creep deformation and damage of sandstone reduced the creep deformation and damage of coal sample (the main bearing body in the composite sample). Coal samples in composite samples needed a higher graded stress for accelerating creep failure and to lead to overall failure and instability of the composite sample. At the same time, the development of primary defects in coal samples also affected the creep strength of composite samples and single coal samples.