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Comparison of mechanical properties of undamaged and thermal-damaged coarse marbles under triaxial compression
International Journal of Rock Mechanics & Mining Sciences 83 (2016) 135–139
Contents lists available at ScienceDirect
International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Short Communication
Comparison of mechanical properties of undamaged and thermal-damaged coarse marbles under triaxial compression Jun Peng a,b, Guan Rong a,b,n, Ming Cai c, Mengdi Yao a,b, Chuangbing Zhou a,d a
State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China Key Laboratory of Rock Mechanics in Hydraulic Structural Engineering (Ministry of Education), Wuhan University, Wuhan 430072, China c Bharti School of Engineering, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 d School of Civil Engineering and Architecture, Nanchang University, Nanchang 330031, China b
art ic l e i nf o Article history: Received 16 April 2015 Received in revised form 8 September 2015 Accepted 30 December 2015 Available online 14 January 2016 Keywords: Rock mechanics Thermal damage Triaxial compression Mechanical property Coarse marble
1. Introduction With increasing demands of underground resource exploitations, many large geotechnical engineering structures such as deep tunnels, boreholes for oil or gas production, underground caverns for storage of radioactive waste, and wells for injection of carbon dioxides, etc., are constructed in a ground with very complex geological conditions. Among these geological conditions, the temperature is one of the key factors that influence the strength and deformation behaviors of rocks. Experimental and numerical characterizations of the effect of temperature on the mechanical properties of rocks are, therefore, of vital importance for proper engineering design and long-term stability maintainence of these structures. In the last two decades, many researchers have studied the mechanical characteristics of rocks under high temperature environments from laboratory testing.1–18 Most of the tests are conducted in uniaxial compression and the results from these tests show that the mechanical properties are significantly affected by thermal heating of the rock specimen. However, due to differences in mineral composition, grain size, and microstructures, etc., the n Corresponding author at: State Key Laboratory of Water Resources and Hydropower Engineering Science Wuhan University, Wuhan 430072, China. E-mail address: [email protected] (G. Rong).
http://dx.doi.org/10.1016/j.ijrmms.2015.12.016 1365-1609/& 2015 Elsevier Ltd. All rights reserved.
observed strength and deformation behaviors are quite complex. Different types of rocks under high temperature treatment exhibit different mechanical characteristics, even for rocks with the same type. For example, the Young's modulus and the compressive strength of granite specimens5,10,14,17 and marble specimens3,13 generally decrease with increase of the heating temperature, while a threshold temperature exists for these properties of some sandstone specimens.6,11 Below this threshold temperature, the Young's modulus and the compressive strength show an increasing trend to some extent. When the treatment temperature exceeds this threshold temperature, the Young’s modulus and the compressive strength gradually decrease. Although numerous experiments have been conducted to study the mechanical behavior of thermal-damaged rocks, the mechanism of thermal treatment influencing the mechanical properties of rocks is still not completely understood. Furthermore, most of the existing researches focused on the mechanical properties of thermal-damaged rocks in uniaxial compression. The study of strength and deformation behaviors of thermal-damaged rocks in triaxial compression is still limited. In the present study, a coarse marble is heated to 600 °C and then slowly cooled down to the room temperature to generate sufficient thermal damage in the specimen. Mechanical properties of undamaged and thermal-damaged specimens under triaxial compression are investigated and compared.
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J. Peng et al. / International Journal of Rock Mechanics & Mining Sciences 83 (2016) 135–139
2. Experimental design 2.1. Description of rock samples The tested rock specimens were collected from a copper mine in Zhenping City, which is located in Henan Province, China. In order to ensure homogeneity of the tested rock specimen, a massive intact marble block with a rough dimension of 60 50 15 cm3 was collected. The rock block was coarse-grained and was relatively isotropic in texture and composition. The grain size is about 3–4 mm. Cylindrical specimens were then drilled out of the large block, with the diameter of 50 mm and the length of 100 mm. The ends of all specimens were finely grinded and polished to meet the specifications recommended by ISRM.19 From a thin section of the rock specimen, it was found that the rock was mainly composed of dolomite and calcite, with slight biotite. Physical properties of the rock specimen were measured prior to testing. The uniaxial compressive strength (UCS) of the rock was about 70.1 MPa and the average density was 2700 kg/m3. The longitudinal wave velocity of the specimen was measured through an ultrasonic pulse transmission technique and the average longitudinal wave velocity was 4478 m/s. 2.2. Testing equipment and procedure The heating device used in this study is a SX3-10-12 box-type resistance furnace, which is composed of the control box and the furnace. The maximum operating temperature is 1200 °C and the rated power is 10 kW. Half of the prepared rock specimens were put into the furnace and heated to a predetermined temperature with a heating rate of 10 °C/min. The predetermined temperature, once reached, was kept constant for 4 h. After that the specimens were left in the furnace to cool down to the room temperature. By referring to a previous work conducted by Rosengren and Jaeger,20 the predetermined temperature used herein was 600 °C in order to generate sufficient thermal damage to the marble specimens. The triaxial compression tests for the undamaged and thermaldamaged specimens were performed using a hydraulic servocontrolled compression system with a maximum load capacity of 2000 kN. The measurement ranges for the axial and lateral extensometers were 8 mm and 4 mm, respectively. The non-linearity of the two extensometers was less than 0.01% of the full scale measuring range. The extensometers were carefully calibrated before testing. The rock specimens were sealed in a rubber sleeve to ensure that the oil in the triaxial cell would not permeate into the specimens. A prescribed confining pressure was firstly applied on the specimen and the axial-displacement controlled loading was used with a loading rate of 0.075 mm/min till the recorded stress–strain curve entered into a stable residual stage. The axial strain, lateral strain, and axial stress were recorded during the loading process. In this study, the maximum confining pressure applied on the specimen was 40 MPa.
3. Experimental results 3.1. Physical properties and microstructures of rock specimens Due to the thermal treatment, the physical properties of the thermal-damaged specimens were quite different with that of undamaged specimens. The color of the undamaged specimen was milk white, with pearly luster. After thermal treatment under the temperature of 600 °C, the color changed to dark gray. By measuring the longitudinal wave velocity of thermal-treated specimens using an ultrasonic pulse transmission technique, it was found that the average value was 746 m/s, which was quite lower
Fig. 1. Optical microscope observations of coarse marbles: (a) Undamaged specimen and (b) thermal-treated specimen under the temperature of 600 °C. The microstructures are enlarged by 25.
than that of undamaged specimen. The result showed that a large amount of microcracks were developed inside the specimen after thermal treatment. However, there was no obvious difference in the volume and the weight between the thermal-damaged and undamaged specimens. Thin sections of both damaged and undamaged specimens were observed using an optical microscope. The microstructures inside the specimen were enlarged by 25 and the results are shown in Fig. 1. It is seen that there is basically no microcracks inside the specimen and the grains are well cemented with each other. However, after thermal treatment, the heat expansion caused separation of the grains at the grain boundaries and a large amount of microcracks, especially the grain boundary microcracks, could be detected. The thermal induced microcrack damage will
J. Peng et al. / International Journal of Rock Mechanics & Mining Sciences 83 (2016) 135–139
Table 1 Summary of test results for the undamaged and thermal-damaged specimens.
Deviatoric stress (MPa)
200 180
40 MPa
160
35 MPa 30 MPa
140 120
25 MPa 20 MPa 15 MPa
100 80 10 MPa
60 5 MPa
40 20 0 0.0
0 MPa
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Axial strain (%) 200 180
Deviatoric stress (MPa)
137
40 MPa 35 MPa 30 MPa
160 140
25 MPa 20 MPa
120
No.
s3 (MPa)
s1–s3 (MPa)
E (GPa)
v
Vp (km/s)
Failure mode
m0-1 m0-2 m0-3 m0-4 m0-5 m0-6 m0-7 m0-8 m0-9 m6-1 m6-2 m6-3 m6-4 m6-5 m6-6 m6-7 m6-8 m6-9
0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40
70.1 88.7 103.3 117.8 130.6 144.0 158.6 166.8 178.8 27.6 60.6 84.0 106.7 121.7 138.5 154.5 163.7 176.8
31.1 30.7 33.7 34.2 28.3 31.8 31.5 33.0 31.1 10.0 13.8 19.2 19.3 19.5 21.1 22.3 23.2 24.7
0.335 0.192 0.261 0.316 0.227 0.290 0.260 0.170 0.346 0.291 0.246 0.252 0.181 0.170 0.282 0.317 0.263 0.234
4.525 4.419 4.567 4.579 4.411 4.411 4.425 4.464 4.504 0.697 0.707 0.697 0.784 0.791 0.796 0.740 0.767 0.734
Splitting Single shear Single shear Single shear Single shear Single shear Conjugate shear Conjugate shear Ductile Splitting Single shear Single shear Conjugate shear Ductile Ductile Ductile Ductile Ductile
Note: In the “No.” column, “m0” denotes the rock specimens without thermal damage and “m6” represents the rock specimens which are heated to the temperature of 600 °C.
100 80
15 MPa 10 MPa
60 40
5 MPa
20 0 0.0
0 MPa
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Axial strain (%) Fig. 2. Stress–strain curves for the tested rock specimens under different confining pressures: (a) Undamaged specimens and (b) thermal-treated specimens under the temperature of 600 °C.
significantly influence the strength and deformation behaviors of the coarse marble which will be discussed in the following two subsections. 3.2. Stress–strain relations and failure modes of rock specimens The tested stress–strain curves under different confining pressures for undamaged and thermal-damaged specimens are presented in Fig. 2a and b, respectively. It is seen that the post-peak behaviors for both kinds of specimens are strongly affected by the confining pressure, showing a similar brittle to ductile transition trend. However, a certain degree of differences exist between the post-peak behaviors of the two kinds of specimens. For undamaged specimen (Fig. 2a), when it is under uniaxial compression, the strength drops rapidly from peak to residual, showing a typical brittle behavior. With increase of the confining pressure (i.e., 5–25 MPa), the curve does not drop down immediately after reaching its peak strength. Instead, the curve remains at the peak value for a while before dropping down, showing a strain-softening manner. The stress–strain curves show a distinct ductile material behavior under high confining pressures (i.e., 30– 40 MPa). For thermal-damaged specimens (Fig. 2b), the ductility of the tested stress–strain curves is much higher than that of undamaged specimens. Under uniaxial compression, the stress– strain curve shows a typical strain-softening behavior. In addition, the ductile plastic deformation behavior can already be detected in the stress–strain curves when the confining pressure exceeds a moderate value (i.e., 20 MPa). The pre-peak deformation behavior is also different between the two kinds of specimens. In the low confining pressures (i.e., 0–
5 MPa), there is strong non-linearity in the initial deformation stage of the stress–strain curve for thermal-damaged specimens. The nonlinear stress–strain behavior in the initial deformation stage has recently been studied,21,22 and can be used to characterize the microcrack density in the rock specimen. It is indicated that a large amount of microcracks reside in the rock specimen by thermal heating, which is consistent with the results from optical microscope observations. However, the non-linearity in the initial deformation stage gradually diminishes with increasing confining pressure. This is due to the fact that the microcracks in the rock specimen are partly closed by the applied confining pressure prior to the application of the axial stress. The test results for two kinds of specimens are summarized in Table 1. It is seen that there is no obvious difference in the Poisson's ratios of the two kinds of specimens. However, the Young's moduli of thermal-damaged specimens are quite lower when compared to that of undamaged specimens. In addition, the failure modes for both kinds of specimens are quite different. For undamaged specimens, the rock specimen fails in a splitting way under uniaxial compression. In moderate confining pressures (i.e., 5–25 MPa), the rock specimens show a single shearing failure. A conjugate shearing or ductile failure can be observed when the specimen is tested under high confining pressures (i.e., 30– 40 MPa). On the other hand, the thermal-damaged specimens fail in a conjugate shear or ductile way in a moderate confining pressure (i.e., 15 MPa or 20 MPa), which is quite different with the failure behavior for undamaged specimens. The results show that the thermal heating can enhance the ductility of rock specimens to a large degree. 3.3. Strength behavior of rock specimens The relationships between the peak strength and the confining pressure for undamaged and thermal-damaged specimens are shown in Fig. 3. It is seen that with increase of the confining pressure, the peak strengths for both kinds of specimens increase. However, the peak strength difference decreases with increasing confining pressure. Due to thermal heating, a lot of microcracks will be generated inside the specimen. The thermal induced microcrack damage can lead to a large strength difference in the low confining pressure. With increasing confining pressure, the microcracks can be partly closed by application of the confining pressure. The effect
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J. Peng et al. / International Journal of Rock Mechanics & Mining Sciences 83 (2016) 135–139
microstructure (i.e., microcracks) that mainly controls the rock strength; however, the confinement will be the key factor that influences the rock strength under high confining pressures. The strength behavior of thermal-damaged rocks had previously been studied by several other researchers.20,23 Variations of peak strength with confining pressure of Wombeyan marble – 196920,24 and Wombeyan marble – 197623 are presented in Fig. 4a and b, respectively. It is seen that the strength behaviors of the two rocks are quite similar with that of the coarse marble studied in this paper, showing a rapid increase in strength of thermal-damaged rock at high confinement.
4. Conclusions
Fig. 3. Variation of the peak strength with confinement for undamaged and damaged rock specimens.
180
Wombeyan marble - 1969 150
σ1 (MPa)
120 90 60
Undamaged Thermal undamaged
30 0
0
5
10
15
20
25
30
σ 3 (MPa)
Acknowledgments The authors gratefully appreciate the editors and two anonymous reviewers for their valuable comments and constructive suggestions in improving this study. The financial supports from the National Natural Science Foundation of China (Grant no. 51579189), the National Basic Research Program of China (No. 2011CB013501), the China Postdoctoral Science Foundation (Grant no. 2015M582273), and the Fundamental Research Funds for the Central Universities are gratefully acknowledged.
200
Wombeyan marble - 1976 160
σ1 (MPa)
The temperature is an important factor that influences the mechanical behavior of rocks and hence affects the stability of engineering structures constructed in or on rock. Based on triaxial compression tests on undamaged and thermal-damaged specimens of a coarse marble, the mechanical properties of the two kinds of specimens are investigated and compared. After thermal heating to the temperature of 600 °C, the color of the specimen changes from milk white to dark gray and the longitudinal wave velocity significantly decreases. From optical microscope observations on thin sections of the specimens, it is seen that a large amount of microcracks, especially the grain boundary microcracks, reside in the thermal damaged specimen. The thermal induced microcrack damage has a great influence on the stress–strain relationships and failure modes of the specimens. The ductility of the post-peak stress–strain curves of thermal-damaged specimens is much enhanced when compared to that of undamaged specimens, and strong nonlinearity is detected for thermal-damaged specimens under low confining pressures. In addition, the strength difference is large in low confining pressures for the two kinds of specimens. However, the strength of thermal-damaged specimen increases rapidly with increasing confining pressure, approaching the same strength with undamaged specimen.
120
80 References
40
0
Undamaged Thermal undamaged 0
5
10
15
20
25
30
35
σ 3 (MPa) Fig. 4. Variation of peak strengths with confining pressures for (a) Wombeyan marble – 196920,24 and (b) Wombeyan marble – 1976.23
of microcracks on the strength will gradually diminish when the confining pressure increases, resulting in a decreasing strength difference. In other words, under low confining pressures, it is the
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limestone and sandstone under high temperature. J Geomech. 2009;15:253–261. 8. Keshavarz M. Pellet FL, Loret B. Damage and changes in mechanical properties of a gabbro thermally loaded up to 1,000 C. Pure Appl Geophys. 2010;167:1511– 1523. 9. Yavuz H, Demirdag S, Caran S. Thermal effect on the physical properties of carbonate rocks. Int J Rock Mech Min Sci. 2010;47:94–103. 10. Chen YL, Ni J, Shao W, Azzam R. Experimental study on the influence of temperature on the mechanical properties of granite under uni-axial compression and fatigue loading. Int J Rock Mech Min Sci. 2012;56:62–66. 11. Ranjith PG, Viete DR, Chen BJ, Perera MSA. Transformation plasticity and the effect of temperature on the mechanical behaviour of Hawkesbury sandstone at atmospheric pressure. Eng Geol. 2012;151:120–127. 12. Inserra C, Biwa S, Chen Y. Influence of thermal damage on linear and nonlinear acoustic properties of granite. Int J Rock Mech Min Sci. 2013;62:96–104. 13. Chen GF, Yang SQ. Study on failure mechanical behavior of marble after high temperature. Eng Mech. 2014;31:189–196. 14. Liu S, Xu J. Mechanical properties of Qinling biotite granite after high temperature treatment. Int J Rock Mech Min Sci. 2014;71:188–193. 15. Tian H, Ziegler M, Kempka T. Physical and mechanical behavior of claystone exposed to temperatures up to 1000 °C. Int J Rock Mech Min Sci. 2014;70:144– 153. 16. Gónzalez-Gómez WS, Quintana P, May-Pat A, Avilés F, May-Crespo J, Alvarado-
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Contents lists available at ScienceDirect
International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Short Communication
Comparison of mechanical properties of undamaged and thermal-damaged coarse marbles under triaxial compression Jun Peng a,b, Guan Rong a,b,n, Ming Cai c, Mengdi Yao a,b, Chuangbing Zhou a,d a
State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China Key Laboratory of Rock Mechanics in Hydraulic Structural Engineering (Ministry of Education), Wuhan University, Wuhan 430072, China c Bharti School of Engineering, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 d School of Civil Engineering and Architecture, Nanchang University, Nanchang 330031, China b
art ic l e i nf o Article history: Received 16 April 2015 Received in revised form 8 September 2015 Accepted 30 December 2015 Available online 14 January 2016 Keywords: Rock mechanics Thermal damage Triaxial compression Mechanical property Coarse marble
1. Introduction With increasing demands of underground resource exploitations, many large geotechnical engineering structures such as deep tunnels, boreholes for oil or gas production, underground caverns for storage of radioactive waste, and wells for injection of carbon dioxides, etc., are constructed in a ground with very complex geological conditions. Among these geological conditions, the temperature is one of the key factors that influence the strength and deformation behaviors of rocks. Experimental and numerical characterizations of the effect of temperature on the mechanical properties of rocks are, therefore, of vital importance for proper engineering design and long-term stability maintainence of these structures. In the last two decades, many researchers have studied the mechanical characteristics of rocks under high temperature environments from laboratory testing.1–18 Most of the tests are conducted in uniaxial compression and the results from these tests show that the mechanical properties are significantly affected by thermal heating of the rock specimen. However, due to differences in mineral composition, grain size, and microstructures, etc., the n Corresponding author at: State Key Laboratory of Water Resources and Hydropower Engineering Science Wuhan University, Wuhan 430072, China. E-mail address: [email protected] (G. Rong).
http://dx.doi.org/10.1016/j.ijrmms.2015.12.016 1365-1609/& 2015 Elsevier Ltd. All rights reserved.
observed strength and deformation behaviors are quite complex. Different types of rocks under high temperature treatment exhibit different mechanical characteristics, even for rocks with the same type. For example, the Young's modulus and the compressive strength of granite specimens5,10,14,17 and marble specimens3,13 generally decrease with increase of the heating temperature, while a threshold temperature exists for these properties of some sandstone specimens.6,11 Below this threshold temperature, the Young's modulus and the compressive strength show an increasing trend to some extent. When the treatment temperature exceeds this threshold temperature, the Young’s modulus and the compressive strength gradually decrease. Although numerous experiments have been conducted to study the mechanical behavior of thermal-damaged rocks, the mechanism of thermal treatment influencing the mechanical properties of rocks is still not completely understood. Furthermore, most of the existing researches focused on the mechanical properties of thermal-damaged rocks in uniaxial compression. The study of strength and deformation behaviors of thermal-damaged rocks in triaxial compression is still limited. In the present study, a coarse marble is heated to 600 °C and then slowly cooled down to the room temperature to generate sufficient thermal damage in the specimen. Mechanical properties of undamaged and thermal-damaged specimens under triaxial compression are investigated and compared.
136
J. Peng et al. / International Journal of Rock Mechanics & Mining Sciences 83 (2016) 135–139
2. Experimental design 2.1. Description of rock samples The tested rock specimens were collected from a copper mine in Zhenping City, which is located in Henan Province, China. In order to ensure homogeneity of the tested rock specimen, a massive intact marble block with a rough dimension of 60 50 15 cm3 was collected. The rock block was coarse-grained and was relatively isotropic in texture and composition. The grain size is about 3–4 mm. Cylindrical specimens were then drilled out of the large block, with the diameter of 50 mm and the length of 100 mm. The ends of all specimens were finely grinded and polished to meet the specifications recommended by ISRM.19 From a thin section of the rock specimen, it was found that the rock was mainly composed of dolomite and calcite, with slight biotite. Physical properties of the rock specimen were measured prior to testing. The uniaxial compressive strength (UCS) of the rock was about 70.1 MPa and the average density was 2700 kg/m3. The longitudinal wave velocity of the specimen was measured through an ultrasonic pulse transmission technique and the average longitudinal wave velocity was 4478 m/s. 2.2. Testing equipment and procedure The heating device used in this study is a SX3-10-12 box-type resistance furnace, which is composed of the control box and the furnace. The maximum operating temperature is 1200 °C and the rated power is 10 kW. Half of the prepared rock specimens were put into the furnace and heated to a predetermined temperature with a heating rate of 10 °C/min. The predetermined temperature, once reached, was kept constant for 4 h. After that the specimens were left in the furnace to cool down to the room temperature. By referring to a previous work conducted by Rosengren and Jaeger,20 the predetermined temperature used herein was 600 °C in order to generate sufficient thermal damage to the marble specimens. The triaxial compression tests for the undamaged and thermaldamaged specimens were performed using a hydraulic servocontrolled compression system with a maximum load capacity of 2000 kN. The measurement ranges for the axial and lateral extensometers were 8 mm and 4 mm, respectively. The non-linearity of the two extensometers was less than 0.01% of the full scale measuring range. The extensometers were carefully calibrated before testing. The rock specimens were sealed in a rubber sleeve to ensure that the oil in the triaxial cell would not permeate into the specimens. A prescribed confining pressure was firstly applied on the specimen and the axial-displacement controlled loading was used with a loading rate of 0.075 mm/min till the recorded stress–strain curve entered into a stable residual stage. The axial strain, lateral strain, and axial stress were recorded during the loading process. In this study, the maximum confining pressure applied on the specimen was 40 MPa.
3. Experimental results 3.1. Physical properties and microstructures of rock specimens Due to the thermal treatment, the physical properties of the thermal-damaged specimens were quite different with that of undamaged specimens. The color of the undamaged specimen was milk white, with pearly luster. After thermal treatment under the temperature of 600 °C, the color changed to dark gray. By measuring the longitudinal wave velocity of thermal-treated specimens using an ultrasonic pulse transmission technique, it was found that the average value was 746 m/s, which was quite lower
Fig. 1. Optical microscope observations of coarse marbles: (a) Undamaged specimen and (b) thermal-treated specimen under the temperature of 600 °C. The microstructures are enlarged by 25.
than that of undamaged specimen. The result showed that a large amount of microcracks were developed inside the specimen after thermal treatment. However, there was no obvious difference in the volume and the weight between the thermal-damaged and undamaged specimens. Thin sections of both damaged and undamaged specimens were observed using an optical microscope. The microstructures inside the specimen were enlarged by 25 and the results are shown in Fig. 1. It is seen that there is basically no microcracks inside the specimen and the grains are well cemented with each other. However, after thermal treatment, the heat expansion caused separation of the grains at the grain boundaries and a large amount of microcracks, especially the grain boundary microcracks, could be detected. The thermal induced microcrack damage will
J. Peng et al. / International Journal of Rock Mechanics & Mining Sciences 83 (2016) 135–139
Table 1 Summary of test results for the undamaged and thermal-damaged specimens.
Deviatoric stress (MPa)
200 180
40 MPa
160
35 MPa 30 MPa
140 120
25 MPa 20 MPa 15 MPa
100 80 10 MPa
60 5 MPa
40 20 0 0.0
0 MPa
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Axial strain (%) 200 180
Deviatoric stress (MPa)
137
40 MPa 35 MPa 30 MPa
160 140
25 MPa 20 MPa
120
No.
s3 (MPa)
s1–s3 (MPa)
E (GPa)
v
Vp (km/s)
Failure mode
m0-1 m0-2 m0-3 m0-4 m0-5 m0-6 m0-7 m0-8 m0-9 m6-1 m6-2 m6-3 m6-4 m6-5 m6-6 m6-7 m6-8 m6-9
0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40
70.1 88.7 103.3 117.8 130.6 144.0 158.6 166.8 178.8 27.6 60.6 84.0 106.7 121.7 138.5 154.5 163.7 176.8
31.1 30.7 33.7 34.2 28.3 31.8 31.5 33.0 31.1 10.0 13.8 19.2 19.3 19.5 21.1 22.3 23.2 24.7
0.335 0.192 0.261 0.316 0.227 0.290 0.260 0.170 0.346 0.291 0.246 0.252 0.181 0.170 0.282 0.317 0.263 0.234
4.525 4.419 4.567 4.579 4.411 4.411 4.425 4.464 4.504 0.697 0.707 0.697 0.784 0.791 0.796 0.740 0.767 0.734
Splitting Single shear Single shear Single shear Single shear Single shear Conjugate shear Conjugate shear Ductile Splitting Single shear Single shear Conjugate shear Ductile Ductile Ductile Ductile Ductile
Note: In the “No.” column, “m0” denotes the rock specimens without thermal damage and “m6” represents the rock specimens which are heated to the temperature of 600 °C.
100 80
15 MPa 10 MPa
60 40
5 MPa
20 0 0.0
0 MPa
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Axial strain (%) Fig. 2. Stress–strain curves for the tested rock specimens under different confining pressures: (a) Undamaged specimens and (b) thermal-treated specimens under the temperature of 600 °C.
significantly influence the strength and deformation behaviors of the coarse marble which will be discussed in the following two subsections. 3.2. Stress–strain relations and failure modes of rock specimens The tested stress–strain curves under different confining pressures for undamaged and thermal-damaged specimens are presented in Fig. 2a and b, respectively. It is seen that the post-peak behaviors for both kinds of specimens are strongly affected by the confining pressure, showing a similar brittle to ductile transition trend. However, a certain degree of differences exist between the post-peak behaviors of the two kinds of specimens. For undamaged specimen (Fig. 2a), when it is under uniaxial compression, the strength drops rapidly from peak to residual, showing a typical brittle behavior. With increase of the confining pressure (i.e., 5–25 MPa), the curve does not drop down immediately after reaching its peak strength. Instead, the curve remains at the peak value for a while before dropping down, showing a strain-softening manner. The stress–strain curves show a distinct ductile material behavior under high confining pressures (i.e., 30– 40 MPa). For thermal-damaged specimens (Fig. 2b), the ductility of the tested stress–strain curves is much higher than that of undamaged specimens. Under uniaxial compression, the stress– strain curve shows a typical strain-softening behavior. In addition, the ductile plastic deformation behavior can already be detected in the stress–strain curves when the confining pressure exceeds a moderate value (i.e., 20 MPa). The pre-peak deformation behavior is also different between the two kinds of specimens. In the low confining pressures (i.e., 0–
5 MPa), there is strong non-linearity in the initial deformation stage of the stress–strain curve for thermal-damaged specimens. The nonlinear stress–strain behavior in the initial deformation stage has recently been studied,21,22 and can be used to characterize the microcrack density in the rock specimen. It is indicated that a large amount of microcracks reside in the rock specimen by thermal heating, which is consistent with the results from optical microscope observations. However, the non-linearity in the initial deformation stage gradually diminishes with increasing confining pressure. This is due to the fact that the microcracks in the rock specimen are partly closed by the applied confining pressure prior to the application of the axial stress. The test results for two kinds of specimens are summarized in Table 1. It is seen that there is no obvious difference in the Poisson's ratios of the two kinds of specimens. However, the Young's moduli of thermal-damaged specimens are quite lower when compared to that of undamaged specimens. In addition, the failure modes for both kinds of specimens are quite different. For undamaged specimens, the rock specimen fails in a splitting way under uniaxial compression. In moderate confining pressures (i.e., 5–25 MPa), the rock specimens show a single shearing failure. A conjugate shearing or ductile failure can be observed when the specimen is tested under high confining pressures (i.e., 30– 40 MPa). On the other hand, the thermal-damaged specimens fail in a conjugate shear or ductile way in a moderate confining pressure (i.e., 15 MPa or 20 MPa), which is quite different with the failure behavior for undamaged specimens. The results show that the thermal heating can enhance the ductility of rock specimens to a large degree. 3.3. Strength behavior of rock specimens The relationships between the peak strength and the confining pressure for undamaged and thermal-damaged specimens are shown in Fig. 3. It is seen that with increase of the confining pressure, the peak strengths for both kinds of specimens increase. However, the peak strength difference decreases with increasing confining pressure. Due to thermal heating, a lot of microcracks will be generated inside the specimen. The thermal induced microcrack damage can lead to a large strength difference in the low confining pressure. With increasing confining pressure, the microcracks can be partly closed by application of the confining pressure. The effect
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microstructure (i.e., microcracks) that mainly controls the rock strength; however, the confinement will be the key factor that influences the rock strength under high confining pressures. The strength behavior of thermal-damaged rocks had previously been studied by several other researchers.20,23 Variations of peak strength with confining pressure of Wombeyan marble – 196920,24 and Wombeyan marble – 197623 are presented in Fig. 4a and b, respectively. It is seen that the strength behaviors of the two rocks are quite similar with that of the coarse marble studied in this paper, showing a rapid increase in strength of thermal-damaged rock at high confinement.
4. Conclusions
Fig. 3. Variation of the peak strength with confinement for undamaged and damaged rock specimens.
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Wombeyan marble - 1969 150
σ1 (MPa)
120 90 60
Undamaged Thermal undamaged
30 0
0
5
10
15
20
25
30
σ 3 (MPa)
Acknowledgments The authors gratefully appreciate the editors and two anonymous reviewers for their valuable comments and constructive suggestions in improving this study. The financial supports from the National Natural Science Foundation of China (Grant no. 51579189), the National Basic Research Program of China (No. 2011CB013501), the China Postdoctoral Science Foundation (Grant no. 2015M582273), and the Fundamental Research Funds for the Central Universities are gratefully acknowledged.
200
Wombeyan marble - 1976 160
σ1 (MPa)
The temperature is an important factor that influences the mechanical behavior of rocks and hence affects the stability of engineering structures constructed in or on rock. Based on triaxial compression tests on undamaged and thermal-damaged specimens of a coarse marble, the mechanical properties of the two kinds of specimens are investigated and compared. After thermal heating to the temperature of 600 °C, the color of the specimen changes from milk white to dark gray and the longitudinal wave velocity significantly decreases. From optical microscope observations on thin sections of the specimens, it is seen that a large amount of microcracks, especially the grain boundary microcracks, reside in the thermal damaged specimen. The thermal induced microcrack damage has a great influence on the stress–strain relationships and failure modes of the specimens. The ductility of the post-peak stress–strain curves of thermal-damaged specimens is much enhanced when compared to that of undamaged specimens, and strong nonlinearity is detected for thermal-damaged specimens under low confining pressures. In addition, the strength difference is large in low confining pressures for the two kinds of specimens. However, the strength of thermal-damaged specimen increases rapidly with increasing confining pressure, approaching the same strength with undamaged specimen.
120
80 References
40
0
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5
10
15
20
25
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35
σ 3 (MPa) Fig. 4. Variation of peak strengths with confining pressures for (a) Wombeyan marble – 196920,24 and (b) Wombeyan marble – 1976.23
of microcracks on the strength will gradually diminish when the confining pressure increases, resulting in a decreasing strength difference. In other words, under low confining pressures, it is the
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