Elsevier

Engineering Geology

Volume 253, 10 April 2019, Pages 94-110
Engineering Geology

On the tensile mechanical characteristics of fine-grained granite after heating/cooling treatments with different cooling rates

https://doi.org/10.1016/j.enggeo.2019.03.014Get rights and content

Highlights

  • Effect of cooling rate on the tensile mechanical behaviours of granite was investigated.

  • The fracture surface roughness was quantified using a 3D optical profilometer.

  • The relation between the tensile strength and the surface roughness was studied.

Abstract

This study discusses the results from tests performed to investigate the tensile mechanical characteristics of granite after exposure to heating and cooling treatments. The samples, which were previously heated to various elevated temperatures and then cooled at various cooling rates, were tested using the Brazilian disc method. The variations in the P-wave velocities, microscopic cracks, tensile strengths and fracture surface roughnesses of the samples as a result of the various heating/cooling treatments were analysed. The results show that the water-cooled samples exhibited the largest decrease in P-wave velocity and the largest numbers of newly generated cracks (including both inter-granular and intra-granular cracks) on the surface, which indicates that the samples heated and cooled at higher cooling rates were more susceptible to the heating/cooling treatments. The cooling rate had less influence on the tensile strengths of the samples when they were heated to 400 °C or less. Nevertheless, when the heating temperature was 600 °C or greater, the water-cooled samples exhibited the lowest tensile strengths, followed by the air-cooled samples and the oven-cooled samples. After the heating/cooling treatment, the samples with lower tensile strengths had rougher fracture surfaces, which are potentially attributed to the thermally-induced microcracks generated during the heating and cooling process. Finally, the relationship between the tensile strength and fracture surface roughness can be well fitted using an exponential decay function.

Introduction

In many fields of earth science and engineering, such as rock drilling, geothermal energy extraction and deeply buried nuclear waste, the influences of temperature on the physical and mechanical properties of rock cannot be ignored or simply considered (Wawersik and Hannum, 1980; Fox et al., 2013; Weng et al., 2017; Griffiths et al., 2018). The thermal expansions of minerals at elevated temperatures cause a build-up of thermal stresses in the rock matrix and may consequently result in thermal cracking. In this regard, thermally-induced cracks modify the physical and mechanical properties of the rock. For example, thermally induced cracks increase porosity (David et al., 1999; Reuschle et al., 2003; Nasseri et al., 2007; Chaki et al., 2008) and permeability (Chaki et al., 2008; Wang et al., 2013; Ren et al., 2016; Rabczuk and Ren, 2017; Ren et al., 2017; Yang et al., 2017b; Yang and Hu, 2018), and reduce the uniaxial strength (Nasseri et al., 2007; Xu et al., 2009; Yin et al., 2012; Yin et al., 2018a) and elastic modulus of the rock (Brotóns et al., 2013; Huang and Xia, 2015; Fan et al., 2017; Rong et al., 2018). To investigate the temperature-dependent physical and mechanical properties, most of the abovementioned studies were performed by heating the samples to pre-set temperatures and then testing the samples at ambient room temperature (Xu et al., 2009; Yin et al., 2012; Brotóns et al., 2013). Other studies were performed by testing the rock at elevated ambient temperatures, and these studies obtained remarkably distinct results with respect to the mechanical properties and failure modes compared with the results derived at room temperature (Dwivedi et al., 2008; Zhao et al., 2012; Shao et al., 2015; Yin et al., 2016; Kumari et al., 2017b; Wang et al., 2018b).

When subjected to elevated temperatures, rock will fracture when the thermal stresses generated by expansion or contraction of individual grains in contact with other grains reach the tensile or shear strength of the rock (Browning et al., 2016; Peng et al., 2016; Fan et al., 2018; Peng et al., 2018). Two main mechanisms contribute to the generation of thermal stresses: (1) mismatch in thermal expansion coefficients between the various mineral granules (induces inter-granular cracks) and (2) thermal expansion anisotropy within single minerals (induces intra-granular cracks) (Hale and Shakoor, 2003; Browning et al., 2016; Yang et al., 2017a). However, these two mechanisms may not sufficiently illustrate the cracking behaviours of rocks that occur due to thermal shock or large thermal gradients, which are commonly encountered in many deep geological applications, such as the deep geological disposal of nuclear waste and deep geothermal energy exploitation (enhanced geothermal systems, EGS) (Kumari et al., 2017a; Kumari et al., 2018). Thermal shock or large thermal gradients generally result from rapid temperature changes (Yavuz, 2011; Zhao et al., 2016; Zhao et al., 2017), such as the rapid cooling of hot rock. In terms of EGS, water is conventionally used for fracturing and as a heat-carrying media (Shao et al., 2014). Once the cold water is injected into a borehole, the hot rock at the periphery of the borehole profile is subjected to a large cooling rate, whereas the rocks that are distant from the borehole wall experience a much lower cooling rate (Isaka et al., 2018). Therefore, investigations of the cooling rate effect on rock deterioration characteristics are crucial for deep geothermal energy exploitation and other deep geological applications.

To this end, Brotóns et al. (2013) studied the influences of thermal damage and cooling methods (e.g., air-cooling and water-cooling) on the physical properties of calcarenite stone from San Julian. A larger reduction in unconfined compressive strength (UCS) was observed in the water-cooled samples compared to the air-cooled samples. In addition, more works have been conducted concerning the variations in the mechanical properties of rocks (e.g., mainly granite) after heating and various subsequent cooling methods (e.g., cooled in air, oven or water) (dos Santos et al., 2011; Brotóns et al., 2013; Shao et al., 2014; Kumari et al., 2017a; Isaka et al., 2018; Kumari et al., 2018; Rathnaweera et al., 2018; Zhang et al., 2018), and they are summarized in Table 1. Meanwhile, a comprehensive sensitivity analysis has been performed to identify the key factors that have the most influences on material properties, such as the fracture toughness and so on (Vu-Bac et al., 2016; Hamdia et al., 2017; Hamdia et al., 2018). The abovementioned studies all focused on the dependency of mechanical properties, such as the compressive strength, elastic modulus and Poisson's ratio on the heating/cooling treatment. Nevertheless, the tensile behaviours of rock with respect to different heating/cooling treatments are still unclear. In this regard, Zhao et al. (2018) performed Brazilian tensile tests on granites with different grain sizes, which were heated to a maximum temperature of 400 °C and then cooled naturally at atmospheric conditions or in tap water. Their results indicated that the changes in the tensile strength fluctuate with respect to the heating temperature when the heating temperature was less than 300 °C. However, the cooling rate effect on the tensile strength of rock has not been fully understood. As a typical brittle material, rock exhibits completely distinct tensile features from its compressive properties, and in most cases, the tensile behaviours of rock in the field are crucial for rock engineering applications. For example, the surrounding rocks of the injection well of EGS, the fire burning tunnel after rescued with water, and the deep geological disposal of nuclear waste are prone to suffer from spalling or splitting at the adjacent the excavation profile. These failures are mostly dominated by progressive tensile cracking as indicated by Cai et al. (2004) and Martin and Chandler (1994). Therefore, it is necessary to investigate the heating/cooling effects on the tensile mechanical characteristics of rocks to better guarantee the safety and effectiveness of deep geo-engineering applications.

Since granitic rock is the most common host rock for EGS and high-level waste disposal, this paper aims to understand the degradation of granitic rocks induced by various heating/cooling treatments and the respective influence of the degradation on the tensile mechanical behaviours. To this end, the granite samples were heated to different elevated temperatures and then cooled in three ways: oven-cooling, air-cooling and water-cooling. The microscopic features of the heating/cooling induced cracks were observed using optical microscopy, and the tensile strengths of the samples were obtained by Brazilian disc (BD) tests. The fracture surfaces resulting from the Brazilian tests were scanned using a three-dimensional (3D) optical profilometer, and the fracture surface roughness was subsequently quantified. The influences of the cooling rate on the tensile mechanical properties and fracture roughness characteristics of the granite samples were investigated in this study.

Section snippets

Description of the test material

The granites for this study were collected from Rucheng County in Hunan province (China), where hot dry rock (HDR) resources are widely present (see Fig. 1). From the results of preliminary geological explorations, granite is the main host rock, and it is located at depths of 400–600 m. Cylindrical granite samples were cored from an identical block that was quarried in the area. Because the samples were cored from an identical block, the test samples can be prepared with approximately

Effect of the cooling rate on the physical characteristics

Fig. 5 shows the comparisons of the P-wave velocities in the samples before and after heating/cooling treatments. It can be seen that an increase in heating temperature induces an obvious decrease in Vp after the heating/cooling treatment. This trend is consistent in all the samples, irrespective of the cooling method. However, with different cooling rates, the rates of decrease in Vp were distinct (see Fig. 5d). The rate of decrease in Vp for the sample following the CW path is the largest,

Conclusions

In this study, different heating (heating temperatures from 200 °C to 800 °C) and cooling (cooling in an oven, air or water) treatments were first performed on granite samples. The heating/cooling-treated and fresh samples were then subjected to Brazilian disc tests. The roughness of the fracture surface created by the BD test was quantified and then correlated with the tensile strength. The effects of the cooling rates on the tensile mechanical characteristics of granite due to heating/cooling

Acknowledgements

This work was supported by the China Postdoctoral Science Foundation (2017M622524) and the National Natural Science Foundation of China (51774131, 51774130, 51109076). The authors are grateful for this financial support.

References (73)

  • D.B. Fox et al.

    Sustainable heat farming: Modeling extraction and recovery in discretely fractured geothermal reservoirs

    Geothermics

    (2013)
  • K.M. Hamdia et al.

    Sensitivity and uncertainty analysis for flexoelectric nanostructures

    Comput. Methods Appl. Mech. Eng.

    (2018)
  • D.W. Hobbs

    The tensile strength of rocks

    Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

    (1964)
  • S. Huang et al.

    Effect of heat-treatment on the dynamic compressive strength of Longyou sandstone

    Eng. Geol.

    (2015)
  • W.G.P. Kumari et al.

    Temperature-dependent mechanical behaviour of Australian Strathbogie granite with different cooling treatments

    Eng. Geol.

    (2017)
  • W.G.P. Kumari et al.

    Mechanical behaviour of Australian Strathbogie granite under in-situ stress and temperature conditions: an application to geothermal energy extraction

    Geothermics

    (2017)
  • W.G.P. Kumari et al.

    Experimental investigation of quenching effect on mechanical, microstructural and flow characteristics of reservoir rocks: thermal stimulation method for geothermal energy extraction

    J. Pet. Sci. Eng.

    (2018)
  • Y.H. Lee et al.

    The fractal dimension as a measure of the roughness of rock discontinuity profiles

    Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

    (1990)
  • C.D. Martin et al.

    The progressive fracture of Lac Du Bonnet granite

    Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

    (1994)
  • N.O. Myers

    Characterization of surface roughness

    Wear

    (1962)
  • M.H.B. Nasseri et al.

    Coupled evolutions of fracture toughness and elastic wave velocities at high crack density in thermally treated Westerly granite

    Int. J. Rock Mech. Min. Sci.

    (2007)
  • J. Peng et al.

    Comparison of mechanical properties of undamaged and thermal-damaged coarse marbles under triaxial compression

    Int. J. Rock Mech. Min. Sci.

    (2016)
  • T. Rabczuk et al.

    A peridynamics formulation for quasi-static fracture and contact in rock

    Eng. Geol.

    (2017)
  • T.D. Rathnaweera et al.

    Experimental investigation of thermomechanical behaviour of clay-rich sandstone at extreme temperatures followed by cooling treatments

    Int. J. Rock Mech. Min. Sci.

    (2018)
  • H. Ren et al.

    Dual-horizon peridynamics: a stable solution to varying horizons

    Comput. Methods Appl. Mech. Eng.

    (2017)
  • T. Reuschle et al.

    Microstructural control on the elastic properties of thermally cracked granite

    Tectonophysics

    (2003)
  • G. Rong et al.

    Experimental investigation of thermal cycling effect on physical and mechanical properties of bedrocks in geothermal fields

    Appl. Therm. Eng.

    (2018)
  • S. Shao et al.

    Effect of cooling rate on the mechanical behavior of heated Strathbogie granite with different grain sizes

    Int. J. Rock Mech. Min. Sci.

    (2014)
  • S.S. Shao et al.

    Experimental and numerical studies on the mechanical behaviour of Australian Strathbogie granite at high temperatures: an application to geothermal energy

    Geothermics

    (2015)
  • B.S.A. Tatone et al.

    A new 2D discontinuity roughness parameter and its correlation with JRC

    Int. J. Rock Mech. Min. Sci.

    (2010)
  • R. Tse et al.

    Estimating joint roughness coefficients

    Int. J. Rock Mech. Min. Sci.

    (1979)
  • D. Vogler et al.

    A comparison of tensile failure in 3D-printed and natural sandstone

    Eng. Geol.

    (2017)
  • N. Vu-Bac et al.

    A software framework for probabilistic sensitivity analysis for computationally expensive models

    Adv. Eng. Softw.

    (2016)
  • L. Weng et al.

    Rockburst characteristics and numerical simulation based on a strain energy density index: a case study of a roadway in Linglong gold mine, China

    Tunn. Undergr. Space Technol.

    (2017)
  • T.F. Wong et al.

    Thermal expansion of rocks: some measurements at high pressure

    Tectonophysics

    (1979)
  • S.Q. Yang et al.

    An experimental investigation on thermal damage and failure mechanical behavior of granite after exposure to different high temperature treatments

    Geothermics

    (2017)
  • Cited by (163)

    View all citing articles on Scopus
    View full text