Hong Tian

Physical Properties of Sandstones After High Temperature Treatment

Under the influence of high temperatures below the rock melting point, rock micro-structures change significantly (Dwivedi et al. 2008), new micro-cracks are developed, and pre-existing ones extended/widened (Den’gina et al. 1994). Meanwhile, various physical and mineralogical changes take place in the rock matrix. After cooling down to room temperature, thermal-induced changes are irreversible to some extent. Hence, rock physical properties from a macroscopic point of view are temperature-history dependent as they rely on the maximum temperature experienced. Knowledge on this issue is a key factor for successful implementation of modern geotechnical engineering projects, such as nuclear waste storage (Sundberg et al. 2009), underground coal gasification (Roddy and Younger 2010), geological CO2 storage (Rutqvist et al. 2002), geothermal heat extraction (Zhao 2000) and stability analysis of constructions in rocks after exposure to fire (Zhan and Cai 2007). Sandstone is a common sedimentary rock, having broad applications in geotechnical engineering. Therefore, the research on the thermo-physical properties of sandstones is extremely meaningful on a wide range. In this manuscript, an extensive review of international literature, especially of Chinese publications not considered in the English-speaking scientific community so far, covers physical properties such as bulk density, porosity, permeability and compressional wave velocity of sandstones after high temperature treatment. The considered sandstones along with their characteristics are listed in Table 1. The testing procedures of thermal treatment in the references reviewed in this manuscript are identical, taking into account heating the samples at a certain rate and under atmospheric pressure conditions in a furnace until a predetermined temperature is reached. The maximum temperature is maintained for a period (several hours), and then cooled down in the furnace or at ambient conditions. The detailed testing parameters for each reference reviewed are summarized in Table 2.

Mechanical Properties of Sandstones Exposed to High Temperature

Modern rock engineering applications such as deep geological disposal of nuclear waste (Ringwood 1985; Gibb 1999; Hokmark and Claesson 2005; Gibb et al. 2008; Sanchez et al. 2012), geothermal heat (especially of hot dry rock) extraction (Zhao 2000; Ghassemi and Zhou 2011; Feng et al. 2012; Gelet et al. 2012; Cherubini et al. 2013), and underground coal gasification (Burton et al. 2007; Luo et al. 2011; Kempka et al. 2011; Younger 2011; Nakaten et al. 2014) experience high-temperature environments, where rocks generally experience high temperatures up to several hundred degrees Celsius. Consequently, rock behaviors under and after high-temperature conditions are of high interest and still a challenge to scientists and engineers of different disciplines. High temperatures result in thermal stresses and mineral expansion as well as various changes of physical and mineralogical properties within rock bodies (Hajpal and Torok 2004; Tian et al. 2012a, 2013), and thus lead to micro-structure changes and micro-cracks development and extension (Den’gina et al. 1994; Dwivedi et al. 2008). These effects change rock mechanical properties such as elastic modulus and compressive strength under and after high-temperature treatment, from a macro point of view, compared to those at normal temperature. Therefore, corresponding high-temperature mechanical properties of rocks are of high relevance for successful implementation of underground rock engineering projects. Sandstone is a widely distributed sedimentary rock composed mostly of sand-sized minerals or rock grains cemented mainly by siliceous, argillaceous, or calcareous materials. Its mechanical properties depend highly upon the degree of cementation and grain composition which are affected by temperature and burial history. In this manuscript, a comprehensive review of international literature, including Chinese publications not available for the English-speaking scientific community so far, described elastic modulus, compressive and tensile strengths of sandstones under and after high-temperature treatment.

A Modified Mohr-Coulomb Failure Criterion for Intact Granites Exposed to High Temperatures

Rocks often experience high temperatures (several hundred degrees Celsius) due to underground operations, such as deep geological disposal of nuclear waste, geothermal heat extraction, CO2 geological storage and underground coal gasification as well as deep mining. Laboratory studies have shown that mechanical properties such as compressive strength, tensile strength, elastic modulus of rocks such as granite, marble and sandstone are dependent on temperature and temperature-history. Therefore, conventional failure criteria such as the Mohr-Coulomb criterion may not provide a good estimate of rock strength under high temperature conditions. In the present study, a thermo-mechanical modified Mohr-Coulomb failure criterion is proposed based on the extensive review and interpretation of mechanical properties of granites exposed to high temperatures. The deduced criterion takes into consideration the effects of thermal damage and confining conditions on rock strength. A numerical study indicates that the proposed criterion provides a higher quality for depicting rock strength under high temperatures compared to the conventional Mohr-Coulomb criterion. Moreover, according to analyses of the behavior of other rock materials exposed to high temperatures, this criterion is also suitable for other rocks.

High-Temperature Influence on Mechanical Properties of Diorite

Knowledge on rock mechanical behaviors under/after hightemperature conditions is extremely important for projects such as deep geological disposal of nuclear waste (Sellin and Leupin 2013; Verma et al. 2015), hot dry rock (HDR) geothermal energy extraction (Brown et al. 2012; Gelet et al. 2012), and underground coal gasification (Burton et al. 2006; Otto and Kempka 2015), for it can provide a basis for deformation, stability, and safety analyses of the projects. High temperatures can cause thermal expansion of rock-forming minerals, thermal stresses, and chemical reactions in a rock body and thus produces micro-cracks and damages rock microstructures. As a result, rock behaviors exposed to high temperatures may be quite different from those under normal temperature conditions. It has been known that rock strength and deformation modulus generally decrease with increasing temperature, especially beyond a certain temperature (e.g., Dwivedi et al. 2008; Chen et al. 2012; Singh et al. 2015; Tian et al. 2016; Peng et al. 2016; Ding et al. 2016). Therefore, research on high-temperature rock mechanics is important and essential in both theory and practice. Granitic rocks such as granite and diorite are a widely acceptable site for nuclear waste disposal and are also main rock types of HDR reservoir. Up to now, a large amount of experimental research has been performed to investigate the mechanical properties of granite exposed to high temperatures up to 1000 C. Dwivedi et al. (2008) presented deformation modulus, tensile, and compressive strength of granites basically decrease with temperature according to their experiments and literature review. Based on a real-time high-temperature uniaxial compression testing system with strain rates of 0.05 and 0.5 mm/min, Singh et al. (2015) found uniaxial compression strength (UCS) and elastic modulus (E) of granite decrease with temperature from 200 C onward, while Homand-etienne and Houpert (1989), Chen et al. (2012) and Liu and Xu (2014) discovered the threshold temperature is 400 C. They found granite UCS changes slightly from room temperature to 400 C, but dramatically decreases with temperature from 400 C onward, by means of testing on granite sample after high-temperature treatment. Shao et al. (2015) and Yin et al. (2016) observed UCS and E of granites decrease with temperature up to 1000 C. Diorite reserves may be much larger than that of granite in some areas and can be a substitute for granite as the host rock of nuclear waste geological disposal. It is also one type of hot dry rocks. However, limited reference related to the mechanical behavior of diorite exposed to high temperatures is available now. To extend our knowledge on high-temperature influences on mechanical properties of diorite, a series of uniaxial compression tests were performed on diorite samples after thermal treatment up to 1000 C. & Gang Mei gang.mei@cugb.edu.cn

Performance Evaluation of GPU-Accelerated Spatial Interpolation Using Radial Basis Functions for Building Explicit Surfaces

This paper focuses on evaluating the computational performance of parallel spatial interpolation with Radial Basis Functions (RBFs) that is developed by utilizing modern GPUs. The RBFs can be used in spatial interpolation to build explicit surfaces such as Discrete Elevation Models. When interpolating with large-size of data points and interpolated points for building explicit surfaces, the computational cost would be quite expensive. To improve the computational efficiency, we specifically develop a parallel RBF spatial interpolation algorithm on many-core GPUs, and compare it with the parallel version implemented on multi-core CPUs. Five groups of experimental tests are conducted on two machines to evaluate the computational efficiency of the presented GPU-accelerated RBF spatial interpolation algorithm. Experimental results indicate that: in most cases, the parallel RBF interpolation algorithm on many-core GPUs does not have any significant advantages over the parallel version on multi-core CPUs in terms of computational efficiency. This unsatisfied performance of the GPU-accelerated RBF interpolation algorithm is due to: (1) the limited size of global memory residing on the GPU, and (2) the need to solve a system of linear equations in each GPU thread to calculate the weights and prediction value of each interpolated point.

Effects of High Temperature on the Mechanical Properties of Chinese Marble

With the increasing tendency of rock engineering applications to involve high temperatures, such as deep geological disposal of nuclear waste (Salama et al. 2015; Verma et al. 2015; Ye et al. 2016) and exploitation of geothermal resources (Sigurvinssona et al. 2007; Gelet et al. 2013; Zeng et al. 2013), research on the mechanical properties of rocks under/after exposure to high temperature has again attracted great attention. The rock material, a typical porous medium, contains original micro-holes and micro-cracks. Under hightemperature conditions, mineral thermal expansion and reactions occur in a rock, leading to variations in rock physical and mineralogical characteristics (Tian et al. 2012, 2013; Ozguven and Ozcelik 2014), and causing extension of the primary micro-cracks and development of new ones inside the rock body (Ferrero and Marini 2001). Consequently, the rock mechanical properties (e.g., elastic modulus and uniaxial compressive strength) exhibit macroscopic variation with temperature, which has been verified experimentally through measurements of rock samples both under and after exposure to high-temperature conditions (e.g., Ranjith et al. 2012; Brotóns et al. 2013; Yang et al. 2014; Zhang et al. 2016). Among the corresponding test results, extensive literature studies have been conducted on common igneous rocks such as granite (Heuze 1983; Dwivedi et al. 2008) and sedimentary rocks such as sandstone (Tian et al. 2012), providing interesting conclusions and guidance to scholars and engineers. Unfortunately, a similar literature study on metamorphic rocks has not been openly published in English. Marble is a widely distributed metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Its mechanical characteristics are mainly influenced by the original rock mineral composition and the environments experienced during metamorphic processes. In this study, an extensive review of Chinese publications, not considered in the English-speaking scientific community so far, covering the elastic modulus, uniaxial compressive strength, and peak strain of marble under/after exposure to high-temperature conditions and a generalized rule describing the effects of high temperature on marble are presented.

MeshCleaner: A Generic and Straightforward Algorithm for Cleaning Finite Element Meshes

Mesh cleaning is the procedure of removing duplicate nodes, sequencing the indices of remaining nodes, and then updating the mesh connectivity for a topologically invalid Finite Element mesh. To the best of our knowledge, there has been no previously reported work specifically focused on the cleaning of large Finite Element meshes. In this paper we specifically present a generic and straightforward algorithm, MeshCleaner, for cleaning large Finite Element meshes. The presented mesh cleaning algorithm is composed of (1) the stage of compacting and reordering nodes and (2) the stage of updating mesh topology. The basic ideas for performing the above two stages efficiently both in sequential and in parallel are introduced. Furthermore, one serial and two parallel implementations of the algorithm MeshCleaner are developed on multi-core CPU and/or many-core GPU. To evaluate the performance of our algorithm, three groups of experimental tests are conducted. Experimental results indicate that the algorithm MeshCleaner is capable of cleaning large meshes very efficiently, both in sequential and in parallel. The presented mesh cleaning algorithm MeshCleaner is generic, simple, and practical.