Development and application of sample preparation and shear test system for soft crushed coal

The study of mechanical properties of soft crushed coal is of great significance for roadway support and prediction of coal and gas outburst, but there is a lack of effective equipment for sampling, sample preparation and mechanical properties analysis of soft crushed coal. Therefore, a set of sample preparation and shear test system for soft crushed coal was developed, which was mainly composed of sampling device, sample preparation device and mechanical property parameter test system. The main functions and characteristics of each part was as following. ① The single-action three-tube coring device in the sampling device was comprised of 3 parts of head assembly, outer tube assembly and inner tube assembly. A stability ring was arranged between the inner and outer tube to ensure that the inner tube and the outer tube were concentric. A liner was arranged inside the inner tube assembly cavity to store coal cores, and the inner tube does not rotate with the outer tube during coring and drilling. When the drilled coal core was full of liner, then the coring device was withdrawn. The liner was disassembled and taken out, and the coal cores and liner were taken out and packaged together. The sampling device was flexible, portable and easy to disassemble and carry. ② The sample preparation device was comprised of 4 prats of stripping instrument, cutting machine, refrigerator and hot fan. The stripping instrument was used to strip the outer lining pipe of coal core. Cutting machine, refrigerator and hot fan was used to cut, freeze and heat shrink coal core. The production of the standard coal core sample is divided into 3 steps of punching and wetting, freezing and cutting, heat shrinking and stripping. Adopting the technology of partial freezing and cutting at the end, and synchronizing the demolding and heat shrinkage uniformly to achieve the sample preparation with various standard size, with perfect sample making method and high success rate. ③ The mechanical property parameter test system was comprised of 3 parts of main structure servo oil source control device and software system, which can realize restrictive and non-restrictive straight shear test function. It was mainly used to carry out straight shear test under different normal stress and triaxial compression test under different confining pressure of soft crushed coal standard specimen. The overall design structure was simple and convenient to operate. A single action three-tube corer was used to obtain soft crushed coal, and a sample preparation device such as a demolding instrument was used to prepare them into standard specimen in this study. Then the straight shear test and triaxial compression test of soft crushed coal standard specimen by using the mechanical property parameter test system were conducted under normal stress of 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 MPa and confining pressure of 0.2, 0.3, 0.4 MPa, respectively. The shear strength parameters such as cohesion and internal friction angle of soft crushed coal was obtained under the two test conditions. The cohesion and internal friction angle obtained from straight shear test were 0.125 kPa and 37.2°, and those obtained from triaxial compression test were 0.121 kPa and 36.4°, respectively. The results show that the system has great practicability and reliability, and provides a better test platform and method for studying the mechanical characteristics of soft crushed coal.