A nonlinear creep damage model for salt rock

Creep behavior is one of the most important mechanical behaviors of coal. In the present study, we systematically carried out a series of creep experiments of briquette samples under triaxial compression and gas pressure. The experimental results showed that the creep deformation characteristics of coal were related to deviatoric stress and gas pressure. The accelerating creep stage initiated when the deviatoric stress reached or exceeded the yield stress until failure took place. In contrast, when the deviatoric stress was less than the yield strength, only transient and steady creep stage occurred. Besides, gas pressure played a positive role in the creep process of coal. The axial strain of coal slightly increased with gas pressure (range from 0.13 to 0.40 MPa) when the effective triaxial stress remained constant. For example, in the steady creep stage (σ 1 = 9 MPa, p = 0.13 MPa), the axial strain of sample S 1 is 2.710% and that of sample S 2 is 3.361%, that is, coal samples with high gas pressure have higher axial strain. Moreover, gas pressure has two main effects on the mechanical behavior of coal. One is that the coal adsorbs gas molecules resulting in swelling strain and swelling stress, and the other is that the gas pressure may compress the coal particles. Considering the swelling stress and gas pressure, we intensively studied the effective stress formula of coal containing gas. Then, the effective stress was taken into consideration in the nonlinear Nishihara creep model. Furthermore, the tertiary creep factor n was introduced in the nonlinear model to control the creep deformation evolution with time so that the nonlinear model could accurately describe the full creep stage of coal. In contrast, due to the lack of such a controlling parameter, the Nishihara model failed to reproduce the accelerating creep stage. K E Y W O R D Scoal, creep model, effective stress, gas pressure, swelling stress, triaxial compression | INTRODUCTIONCreep behavior is one of the fundamental mechanical properties of coal or rock, associated with the stability of many buildings and underground structures in mining and rock engineering, such as roadways, tunnels, and dams. It is important to have a precise understanding of the creep or timedependent behavior of coal or rock, which may prevent coal 502 | ZHANG et Al. Highlights• The coupling effect of triaxial stress and gas pressure on coal was investigated. • The gas pressure played a positive role in the creep process of coal. • The effective stress of gas-filled coal was intensively studied. • The effective stress was considered in the nonlinear Nishihara creep model of coal, which could accurately reproduce the three creep stages, especially the accelerating creep stage.How to cite this article: Zhang L, Li X-C, Gao J-X, An Z-X, Yang X-H, Nie B-S. Creep characteristics and constitutive model of coal under triaxial stress and gas pressure. Energy Sci Eng. 2020;8:501-514. https ://doi.

Section: Three-dimensional Creep Equations For Coal Containing Gasmentioning

confidence: 99%

Section: Three-dimensional Creep Equations For Coal Containing Gasmentioning

confidence: 99%

Creep characteristics and constitutive model of coal under triaxial stress and gas pressure

Zhang

Gao

et al. 2019

“…According to Formulae (1) and (2), it can be seen that the attenuation coefficients of gas emission from slotted, and conventional, boreholes were 0.1273 and 0.111, respectively, with an insignificant difference therein. This was determined by the low permeability coefficient of the coal seam.…”

Section: Changes In Gas Flows and Flow Velocitiesmentioning

confidence: 99%

“…China is a large coal producer and consumer in her medium‐long‐term energy strategy, coal lies at the heart of China's energy structure and will do so for many years . However, China is suffering seriously from coal and gas outburst accidents, which restricts the safe, efficient mining of coal .…”

Section: Introductionmentioning

confidence: 99%

“…China is a large coal producer and consumer in her mediumlong-term energy strategy, coal lies at the heart of China's energy structure and will do so for many years. 1,2 However, China is suffering seriously from coal and gas outburst accidents, which restricts the safe, efficient mining of coal. 3,4 As coal in shallow coal seams is gradually depleted, most coal mines in China have mined deeper coal seams, where there are significantly high geo-stress, high pore pressures, high temperatures, and yet low permeability 5,6 .…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Elimination of coal and gas outburst risk of low‐permeability coal seam using high‐pressure water jet slotting technology: A case study in Shihuatian Coal Mine in Guizhou Province, China

Xie

Zou

et al. 2019

Coal seams in China are mainly characterizsed by low permeability and high risk of coal and gas outburst. Therefore, the elimination of coal and gas outburst risk of low‐permeability coal seam remains a hard challenge. Taking Shihuatian Coal Mine in Guizhou Province, China as an example, boreholes were slotted using high‐pressure water jet to relieve the gas pressure inside the coal seam and increase the permeability of coal mass, which thus improves the gas drainage efficiency and eliminate the outburst risk. The results show that the gas drainage efficiency of the slotted boreholes significantly increases. Compared with the conventional boreholes, the average gas concentration and flow velocity improved by 1.6 and 7.5 times, respectively. Moreover, the prediction index of outburst risk is less than the critical value after high‐pressure water jet slotting, which indicate that the coal and gas outburst risk has been effectively eliminated. The measured values of gas desorption index of drilling cuttings are all lower than the critical value after the application of the high‐pressure water jet slotting technology. The research achievements could provide a practical reference for the effective gas disaster prevention and control in low‐permeability coal seams.

Permeability enhancement mechanism of sand‐carrying hydraulic fracturing in deep mining: A case study of uncovering coal in cross‐cut

Yin

Cheng

et al. 2019

Coal and gas outburst disasters are prone to occur within rock cross‐cut coal uncovering in deep underground coal mines. To reduce the risk of coal and gas burst under rock cross‐cut coal uncovering, a technique of permeability enhancement using hydraulic fracturing sand‐carrying is proposed. Field test was conducted using HF sand‐carrying approach, and the effect of permeability enhancement was investigated. The results show that the evolution of cracks has been subjected three stages: energy slowly increasing and crack initiation, damage localization of coal seam and gradually failure, and crack instability expansion and formation of a complex fracture network system. In addition, it is found that “water‐sand” type injection of high‐pressure fluid carried sand into HF cracks, which creates a support force on crack surface and prevents crack closure. A fully developed fracture network system with high flow conductivity capacity is created and thus increases gas permeability largely. Furthermore, a clearly increasing trend associated with the HF sand‐carrying is found for the treated coal seam area. The permeability coefficient is about 21.5‐30.5 times higher than that from the raw coal seam. A good prevention effect of coal and gas outburst is achieved using HF sand‐carrying method.