Brittleness is one of the most important mechanical properties in deep rock mass. In order to study the brittle damage evolution mechanism of limestone, the acoustic emission test of limestone under uniaxial compression was carried out, and the relationship between brittleness index and damage variable and b-value was analyzed. The results show that: (1) The ratios of crack closure stress, crack initiation stress and damage stress to peak stress in limestone are 31.3 %, 43.0 % and 85.5 %, respectively. (2) The average brittleness index of limestone is 0.32, and the mean square error is 0.009. With the increase of brittleness index, the damage variable corresponding to the characteristic stress gradually decreases, and the damage variable gradually increases from ‘proportional function’ to ‘concave power function’. The damage acceleration phenomenon exists in the critical failure stage. (3) Under the condition of uniaxial compression, with the increase of brittleness index, the shear failure of limestone decreases and the tensile failure increases, and the brittleness index can better characterize its failure mode. (4) There are four obvious stages in the cumulative ringing count, cumulative energy, cumulative amplitude and time curve of acoustic emission in limestone: slight increase period, quiet period, steep increase period and sharp decrease period. The amplitude of quiet period is less than 80 dB, and the curve shows a step-like upward trend. The larger the brittleness index, the longer the quiet period, the shorter the sharp increase period, and the greater the ratio of the stress corresponding to the b value to the peak stress. (5) The b-value curve shows a trend of rising first and then falling, and the b-value abrupt points are all before the end of the quiet period. These two points can accurately warn the instability and failure of limestone. With the increase of brittleness index, the b-value abrupt point appears later and closer to the peak. The research results provide important reference for surrounding rock stability control and dynamic disaster monitoring and early warning.
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Coal pillars and roadways are subjected to creep by loads and the prolonged action of underground flowing water. This phenomenon directly affects the design and construction of underground reservoirs in coal mines, as well as pumped storage power plants. Previous studies have mainly focused on the mechanical properties of coal rock assemblages or coal and rock monoliths under static, submerged water conditions. These studies did not consider the properties of coal rock fissures and the effects of circulating dynamic water. Therefore, studying the creep properties of coal and rock under dynamic water conditions is of great practical significance. This research can provide theoretical references for grouting, seepage control, and the reinforcement of fissured coal and rock bodies. In response to this demand, a uniaxial creep test monitoring device for coal rock under dynamic water conditions was developed.
We developed a uniaxial creep test monitoring device for coal-rock under dynamic water conditions. The device consisted of three main parts: a servo loading system, a dynamic water control system, and a data acquisition system. The dynamic water system included components such as a water tank, water pipes, and a flow pump. The bottom plate of the water tank was constructed from a 50 mm thick 304 stainless steel plate to ensure it can withstand the pressure during operation. The side plates were made of 4 mm thick acrylic, chosen for both strength against water pressure and the ability to facilitate visual data recording. The upper part of the tank was fitted with an opening through which the pressurized head can pass for loading purposes. To prevent water from leaking out and potentially damaging the pressure tester, the water tank was equipped with a flow channel. A water pipe connected the water tank to the flow pump, and the flow rate was controlled by the pump head and the cross-sectional area of the hose. The flow rate can be adjusted within the range of 0.45~7.00 m/s. Pressure loading was conducted using a microcomputer-controlled electro-hydraulic servo three-axis tester. The loading mode involved axial displacement control, and the loading rate was set at 0.3 mm/min to maintain constant pressure. The water tank was positioned on the loading base of the pressure tester, with the specimen placed inside the tank. The flow pump was activated to establish the desired flow rate within the water tank, facilitating specimen immersion. Concurrently, data such as stress-strain profiles and acoustic emissions were collected. Stress-strain and acoustic emission data were simultaneously collected. Furthermore, we conducted a 165minute experiment to test the pressure loading system and dynamic water circulation system in a field experimental setup.
After the coal-rock creep test monitoring device was assembled under dynamic water conditions, the device was inspected to ensure its proper functioning. The test results indicated the following: During the initial 165 min of continuous testing under the loaded and dynamic water circulation conditions, a consistent load of 18MPa was maintained. The flow rate of circulating dynamic water remained stable at 1.5m/s without any leakage from the built-in water tanks. The creep curve of sandy mudstone under these conditions exhibited three typical stages: initial creep lasting 25 min, followed by steady-state creep for 125 min, and concluding with accelerated creep for 15min. Preliminary tests indicated that the test device can effectively meet the requirements for conducting creep tests under dynamic water conditions.
The coal-rock creep test monitoring device developed for dynamic water conditions offers several advantages, including powerful functionality, stable performance, convenient operation, and low cost. The introduction of new design elements and technologies significantly enhances the performance and reliability of the test device. These improvements enhance the feasibility of conducting uniaxial creep tests on coal-rock under dynamic water conditions.
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