Experimental Study on the Creep Characteristics of Coal Measures Sandstone under Seepage Action
Abstract
:1. Introduction
2. Materials and Methods
2.1. Material Characterization
2.2. Testing System
- The axial pressure loading system uses the Electro-hydraulic servo rock mechanics test system of MTS816 to provide a stable and continuous axial load for the specimen during the test, as shown in Figure 2a.
- The confining pressure loading system applies a certain pressure to the specimen by using the pump to press the hydraulic oil into the cylinder.
- The seepage pressure loading system provides a continuous and stable seepage pressure for the specimen based on the injector concept, and this is an important part to achieve the seepage-creep coupling effect, as shown in Figure 2b.
- The rock seepage system is the core part of the rock seepage-creep coupling test system. It is the central system of the axial loading system, confining pressure loading system and seepage pressure loading system. Figure 2c shows the structural compositions of the rock seepage system. As can be seen from the picture, the entire testing system is roughly composed of 14 parts.
- The data collecting and processing system includes a paperless recorder, a pressure transmitter, a flow sensor, and a computer terminal.
2.3. Test Scheme
- Triaxial compression tests of sandstone. Triaxial tests of sandstone will be carried out under the natural state and the saturation state and the confining pressures are 1 MPa, 2 MPa, 3 MPa, and 4 MPa. Then the compressive strengths of sandstone under four confining pressures in two states are determined. According to the triaxial compressive strength, the load classification scheme of sandstone creep experiment can be obtained.
- Triaxial creep tests of sandstone. According to the different loading levels, the creep tests with confining pressures of 1 MPa, 2 MPa, 3 MPa, and 4 MPa are carried out under natural and saturation conditions respectively, and the creep characteristics of sandstone under two conditions are obtained finally.
- Seepage-creep coupling tests of sandstone. Water saturated rock specimens are used as the targets to perform the rock creep tests under different seepage pressures. The confining pressure is 4 MPa, and the seepage pressure p0 is 0.5 MPa, 1.5 MPa, 2.5 MPa, and 3.5 MPa. The effect of seepage pressure on the creep deformation and permeability of sandstone can be obtained.
2.4. Load Grading
2.5. The Method of Microscopic Damage Test
3. Results and Discussion
3.1. Creep Properties of Sandstone under Natural State and Saturation State
3.1.1. Creep Deformation Law of Sandstone under Two States
3.1.2. Creep Deformation Rate of Sandstone under Two States
3.2. Influence of Seepage Pressure on Seepage-Creep Properties of Sandstone
3.2.1. Variation of Creep Deformation of Sandstone with Seepage Pressure
3.2.2. Variation of Seepage Properties of Sandstone with Seepage Pressure
3.3. Macroscopic and Microscopic Mechanism Study on Coal Measures Sandstone Rupture under Seepage-Creep Coupling
3.3.1. Macroscopic Characteristics of Sandstone Rupture
3.3.2. Microscopic Characteristics of Sandstone Rupture
3.3.3. Connection between Macroscopic and Microscopic Mechanism on Sandstone Rupture
4. Conclusions
- Based on the triaxial compression tests of coal measures sandstone, the creep tests of sandstone under natural and saturation state were carried out through the grading loading tests. The results showed that: The creep characteristic curves of sandstone in two conditions were the same, both including decelerating creep and steady creep at lower loading levels. When the loading level was higher, the creep curve included three complete processes: decelerating creep, isokinetic creep and accelerated creep.
- The seepage pressure had a significant effect on the creep deformation and permeability characteristics of coal measures sandstone. When the confining pressure was invariant, the constant creep deformation rate and accelerated creep deformation rate of sandstone increased rapidly with the increase of seepage pressure. With the change of time, the change of sandstone permeability parameters underwent three cycles, and the characteristics of change in each cycle were also divided into two stages: slow change stage and rapid change stage. The change rate gradually increased with the increase of seepage pressure. As the permeation pressure increased, the permeation parameters measured for the first time gradually decreased accordingly.
- When the confining pressure was invariant, with the increase of seepage pressure, the macroscopic damage of sandstone under natural and saturation conditions had a tendency of transition from tensile failure to shear failure. As the seepage pressure raised, the microscopic fracture mode of sandstone had changed from brittle fracture to ductile fracture and finally to brittle fracture. According to the macroscopic characteristics of sandstone rupture, microscopic morphology and fracture modes, the connection between macroscopic and microscopic mechanism on sandstone rupture was established.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Anorthite—Ca(Al2Si2O8) | Microcline Maximum—K(AlSi3) O8 | Kaolinite—Al2Si2O5(OH)4 | Quartz—SiO2 |
---|---|---|---|
45.2% | 31.4% | 13.8% | 9.6% |
ρ/(kg·m−3) | v/(m·s−1) | E/GPa | μ | σu/MPa | σT/MPa | σt/MPa | φ/° |
---|---|---|---|---|---|---|---|
2540 | 2650 | 37.65 | 0.26 | 122.45 | 149.29 | 2.85 | 38 |
Hierarchical Load | Natural Condition | Saturation Condition | ||||||
---|---|---|---|---|---|---|---|---|
p = 1 MPa | p = 2 MPa | p = 3 MPa | p = 4 MPa | p = 1 MPa | p = 2 MPa | p = 3 MPa | p = 4 MPa | |
σ1 (MPa) | 20 | 20 | 23 | 25 | 10 | 14 | 15 | 18 |
σ2 (MPa) | 40 | 40 | 46 | 50 | 20 | 28 | 30 | 36 |
σ3 (MPa) | 60 | 60 | 69 | 75 | 30 | 42 | 45 | 54 |
σ4 (MPa) | 80 | 80 | 92 | 100 | 40 | 56 | 60 | 72 |
σ5 (MPa) | 100 | 100 | 115 | 125 | 50 | 70 | 75 | 90 |
σ6 (MPa) | 120 | 120 | 138 | 150 | 60 | 84 | 90 | 108 |
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Sha, Z.; Pu, H.; Li, M.; Cao, L.; Liu, D.; Ni, H.; Lu, J. Experimental Study on the Creep Characteristics of Coal Measures Sandstone under Seepage Action. Processes 2018, 6, 110. https://doi.org/10.3390/pr6080110
Sha Z, Pu H, Li M, Cao L, Liu D, Ni H, Lu J. Experimental Study on the Creep Characteristics of Coal Measures Sandstone under Seepage Action. Processes. 2018; 6(8):110. https://doi.org/10.3390/pr6080110
Chicago/Turabian StyleSha, Ziheng, Hai Pu, Ming Li, Lili Cao, Ding Liu, Hongyang Ni, and Jingfeng Lu. 2018. "Experimental Study on the Creep Characteristics of Coal Measures Sandstone under Seepage Action" Processes 6, no. 8: 110. https://doi.org/10.3390/pr6080110
APA StyleSha, Z., Pu, H., Li, M., Cao, L., Liu, D., Ni, H., & Lu, J. (2018). Experimental Study on the Creep Characteristics of Coal Measures Sandstone under Seepage Action. Processes, 6(8), 110. https://doi.org/10.3390/pr6080110