Experimental study on uneven failure of loose coal and rock composite specimen

In order to study the problem of uneven failure of roadway in loose coal and rock mass under thin seam mining,a series of uniaxial tests of loose coal and rock composite specimens are carried out based on the engineering background of loose coal and rock mining roadway. The test results show that the mean uniaxial compressive strength (MUCS) of loose coal and rock composite specimens is between that of full rock specimens and full coal specimens (2.19 ～ 3.37 MPa). With the increase of constant loading rate,the test results of loose coal-rock composite specimens and full coal specimens are quite different from those of ordinary hard rock specimens:the MUCS and MRS (mean residual strength) of loose coal-rock composite specimens do not increase,but decrease with the loading rate (about 1 MPa). There exists a critical value effect of loading rate for loose coal-rock composite specimen,and the mechanical properties show a law of decreasing first and then increasing. From the contour map of the circumferential strain of the coal part and rock part,the circumferential strain of rock part is relatively stable during the whole loading process,while the strain of coal part fluctuates greatly. The maximum and minimum range of coal part circumferential strain is larger than that of rock part. It can be seen that the overall deformation of the loose coal and rock composite specimen is nonlinear,and the uneven failure between the two parts are obvious.