Abstract: To realistically simulate stress states of coal in tunneling laneway frontal and other stress environments, a true triaxial gas-solid coupling testing system is developed to examine the damage deformation and gas seepage of coal under three-dimensional (3D) stress condition. The device is composed of a true triaxial pressure chamber, servo-hydraulic system, gas seepage system, and monitoring and control system. The suitable size of a specimen for the equipment is 200 mm×100 mm×100 mm. The maximum pressures of the axial compression (?1), the principal lateral direction (?2), and the secondary lateral direction (?3), and gas are 70, 35, 10 and 6 MPa, respectively. The characteristics of the device are shown as follows: the axial pressure (?1) and the principal lateral pressure (?2) are loaded by rigid head loading methods, and the secondary lateral pressure (?3) is applied by flexible loading. The 3D stresses are applied independently. To ensure the heads at σ1 and the ?2 directions without disturbing by each other at the same loading time, the rigid-flexible head and pressure transmission slider are designed. The loading control functions of the servo-hydraulic system are more likely to be stable and reliable, and the stress and displacement loading can be controlled accurately. Honeycomb holes are used for ventilation of specimen surface in the gas seepage system to ensure the uniform distribution of gas pressure at the inlet of the specimen. A variety of high-precision sensors are used to monitor and record the stress, the deformation and the gas seepage discharge of coal in real-time. Seepage experiments in two different loading paths are performed to verify the accuracy and reliability of the experiment system which performs well. The device can be used to reveal the coupling mechanism of coal and gas under 3D stress conditions, and to provide a reliable experimental basis for prevention and control of gas disaster and investigations on gas extraction.

Key words: mining engineering, gas-solid coupling, true triaxial, three-dimensional stress, gas seepage