P.G. Ranjith

A comparative study of generalized regression neural network approach and adaptive neuro-fuzzy inference systems for prediction of unconfined compressive strength of rocks

The engineering properties of rocks play a significant role in planning and designing of mining and civil engineering projects. A laboratory database of mechanical and engineering properties of rocks is always required for site characterization and mineral exploitation. Due to discontinuous and variable nature of rock masses, it is difficult to obtain all physicomechanical properties of rocks precisely. Prediction of unconfined compressive strength from seismic wave velocities (Compressional wave, Shear wave) and density of rock using generalized regression neural network (GRNN) and adaptive neuro-fuzzy inference systems (ANFIS) can be appropriate and alternate methods to minimize the time and cost of tests. GRNN and ANFIS models were trained with 41 data sets using conjugate gradient descent algorithms and hybrid learning algorithm, respectively. Performance of both the models was examined with 15 testing data sets. In the present study, obtained network performance indices such as correlation coefficient, mean absolute percentage error, root mean square error and variance account for indicate high performance of predictive capability of GRNN system and closer to actual data over the ANFIS.

Energy monitoring and analysis during deformation of bedded-sandstone: use of acoustic emission.

This paper investigates the mechanical behaviour and energy releasing characteristics of bedded-sandstone with bedding layers in different orientations, under uniaxial compression. Cylindrical sandstone specimens (54 mm diameter and 108 mm height) with bedding layers inclined at angles of 10°, 20°, 35°, 55°, and 83° to the minor principal stress direction, were produced to perform a series of Uniaxial Compressive Strength (UCS) tests. One of the two identical sample sets was fully-saturated with water before testing and the other set was tested under dry conditions. An acoustic emission system was employed in all the testing to monitor the acoustic energy release during the whole deformation process of specimens. From the test results, the critical joint orientation was observed as 55° for both dry and saturated samples and the peak-strength losses due to water were 15.56%, 20.06%, 13.5%, 13.2%, and 13.52% for the bedding orientations 10°, 20°, 35°, 55°, and 83°, respectively. The failure mechanisms for the specimens with bedding layers in 10°, 20° orientations showed splitting type failure, while the specimens with bedding layers in 55°, 83° orientations were failed by sliding along a weaker bedding layer. The failure mechanism for the specimens with bedding layers in 35° orientation showed a mixed failure mode of both splitting and sliding types. Analysis of the acoustic energy, captured from the acoustic emission detection system, revealed that the acoustic energy release is considerably higher in dry specimens than that of the saturated specimens at any bedding orientation. In addition, higher energy release was observed for specimens with bedding layers oriented in shallow angles (which were undergoing splitting type failures), whereas specimens with steeply oriented bedding layers (which were undergoing sliding type failures) showed a comparatively less energy release under both dry and saturated conditions. Moreover, a considerable amount of energy dissipation before the ultimate failure was observed for specimens with bedding layers oriented in shallow angles under both dry and saturated conditions. These results confirm that when rock having bedding layers inclined in shallow angles the failures could be more violent and devastative than the failures of rock with steeply oriented bedding layers.

Single phase water flow through rock fractures

Flow analysis plays a major role in various geotechnical applications, and the understanding of flow mechanisms is essential for the development of a hydro-mechanical flow model suitable for underground excavations in rock. Discrete flow analysis through discontinuities is reviewed including empirical and analytical flow models. The influence of external loading on joint deformation and single-phase flow show that the surface roughness and aperture size are the prime factors influencing flow rate. Nevertheless, the idealization of natural fractures as smooth parallel plate joints is still followed in many numerical models, because of the simplicity of the cubic law when applied to fracture networks. A numerical study of water flow through a network of joints employing Universal Distinct Element Code (UDEC) is used to quantify the effects of joint orientation and external stress acting on idealized joints.It is found that, for the same joint spacing, the flow rate into an excavation depends on the boundary block size (Ab) relative to the excavation size (Ae). The inflow becomes excessive if Ab/Ae is less than 4, but becomes very small if Ab/Ae exceeds 8.

Effect of Varied Durations of Thermal Treatment on the Tensile Strength of Red Sandstone

The strength of rocks differs with changes in temperature, pressure, time and fluid interactions, and the presence of joints and the stress history of the rock affect the strength. The cumulative presence of these factors in the underground environment alters the behavior of the rock. Since precise replication of these factors in laboratory-scale models is difficult, it is difficult to observe the change in the strength of rocks. However, it is imperative to study the effect of these properties on the strength behavior of rocks. Rocks have very low tensile strength compared to compressive or shear strength, making it easier for rocks to fail under tensile loading. The success in processes, such as underground coal gasification (UCG), enhanced oil recovery (EOR), underground nuclear waste disposal and geothermal energy, depends principally on the long-term stability of the host rock. In these processes, rocks are subjected to loading and heating. It is therefore vital to study the effect of temperature on tensile strength for designing structures within rock. Gasification of coal, under site conditions, is conducted at temperatures above 700 C. The temperature in the cavity may reach temperatures as high as 1500 C (Burton et al. 2007; Sirdesai et al. 2015). At such high temperatures, the geomechanical properties of the rock undergo large variation (Hajpál and Török 2004; Ranjith et al. 2012; Tian et al. 2012; Wu et al. 2013; Zhang et al. 2009). In conjunction with time, the magnitude of the effect changes and the behavior is altogether different. The peak strength and the load-bearing capacity of the rock change with changes in temperature and time. The host rock in processes such as UCG or nuclear waste disposal is subjected to extended periods of heating. Exposure to high temperatures for an extended duration results in the formation of thermal stresses, thereby initiating expansion as well as accelerating the failure process. Therefore, it is essential to study the effect of extended durations of thermal treatment on the strength of rocks. Extensive research has been conducted to understand the effect of heat treatment on the tensile strength of rocks. Studies of the thermomechanical behavior of igneous rocks such as granites and basalts suggest that the tensile strength of the rocks decreases with the increase in temperature (Dwivedi et al. 2008; Heuze 1983; Homand-Etienne and Houpert 1989; Yin et al. 2015). The persistent decrease in the tensile strength of granites and basalts may be associated with the low porosity and negligible existence of microcracks within these rocks. The grains present in these rocks expand at the onset of heating, resulting in the creation of newmicro-cracks, which causes a reduction in strength. The tensile strength of khondalitic rocks from southern India was found to increase with temperature up to 100 C. Further exposure to higher temperatures causes a sharp decrease & Rajesh Singh georajeshsingh@gmail.com

Deep coal seams as a greener energy source: a review

Today, coal and oil are the main energy sources used in the world. However, these sources will last for only a few decades. Hence, the investigation of possible energy sources to meet this crisis has become a crucial task. Coal bed methane (CBM) is a potential energy source which can be used to fulfil the energy demand. Since the amount of carbon dioxide (CO2) emitted to the atmosphere from the use of CBM is comparatively very low compared to conventional energy sources, it is also a potential mitigation option for global warming.This paper reviews CBM recovery techniques with particular emphasis on CO2-enhanced coal bed methane (CO2-ECBM) recovery. The paper reviews (1) conventional CBM recovery techniques and problems associated with them, (2) CBM production-enhancement methods, including hydro-fracturing and enhanced CBM recovery techniques, such as N2-ECBM and CO2-ECBM, (3) the importance of the CO2-ECBM technique compared to other methods and problems with it, (4) the effect of CO2 injection during the CO2-ECBM process on coal seam permeability and strength and (5) current CO2-ECBM field projects and their progress.Although conventional CBM recovery methods are simple (basically related to the drawdown of the reservoir pressure to release methane from it), they are inefficient for the recovery of a commercially viable amount of methane from coal seams. Therefore, to enhance methane production, several methods are used, such as hydro-fracturing and ECBM (N2-ECBM and CO2-ECBM). The CO2-ECBM process has a number of advantages compared to other methane recovery techniques, as it contributes to the mitigation of the atmospheric CO2 level, is safer and more economical. However, as a result of CO2 injection into the coal seam during the CO2-ECBM process, coal mass permeability and strength may be crucially changed, due to the coal matrix swelling associated with CO2 adsorption into the coal matrix. Both injecting CO2 properties (gas type, CO2 phase and pressure) and coal seam properties (coal rank and temperature) affect this swelling. Although there are many related studies, a number of gaps exist, especially in the area of coal rank and how the effect of other factors varies with the rank of the coal seam. To date, there have been few CO2-ECBM field projects in the world. However, the reduction of CO2 injectability after some time of CO2 injection, due to coal matrix swelling near the well bore, is a common problem in the field. Therefore, various permeability-enhancing techniques, such as hydro-fracturing and injection of an inert gas such as N2 or a mixture of inert gases (N2 + CO2) into the seam to recover the swelled areas are under test in the field.

Mechanical Properties of Reconstituted Australian Black Coal

Coal is usually highly heterogeneous. Great variation in properties can exist among samples obtained even at close proximity within the same seam or within the same core sample. This makes it difficult to establish a correlation between uniaxial compressive strength (UCS) and point load index for coal. To overcome this problem, a method for making reconstituted samples for laboratory tests was developed. Samples were made by compacting particles of crushed coal mixed with cement and water. These samples were allowed to cure for four days. UCS and point load tests were performed to measure the geomechanical properties of the reconstituted coal. After four days curing, the average UCS was found to be approximately 4 MPa . This technical note outlines some experimental results and correlations that were developed to predict the mechanical properties of the reconstituted black coal samples. By reconstituting the samples from crushed coal, it is hoped that the samples will retain the important mechanical and p...

Experimental investigation of geochemical and mineralogical effects of CO2 sequestration on flow characteristics of reservoir rock in deep saline aquifers.

Interactions between injected CO2, brine, and rock during CO2 sequestration in deep saline aquifers alter their natural hydro-mechanical properties, affecting the safety, and efficiency of the sequestration process. This study aims to identify such interaction-induced mineralogical changes in aquifers, and in particular their impact on the reservoir rock’s flow characteristics. Sandstone samples were first exposed for 1.5 years to a mixture of brine and super-critical CO2 (scCO2), then tested to determine their altered geochemical and mineralogical properties. Changes caused uniquely by CO2 were identified by comparison with samples exposed over a similar period to either plain brine or brine saturated with N2. The results show that long-term reaction with CO2 causes a significant pH drop in the saline pore fluid, clearly due to carbonic acid (as dissolved CO2) in the brine. Free H+ ions released into the pore fluid alter the mineralogical structure of the rock formation, through the dissolution of minerals such as calcite, siderite, barite, and quartz. Long-term CO2 injection also creates a significant CO2 drying-out effect and crystals of salt (NaCl) precipitate in the system, further changing the pore structure. Such mineralogical alterations significantly affect the saline aquifer’s permeability, with important practical consequences for the sequestration process.

Evaluation of creep mechanical behavior of deep-buried marble under triaxial cyclic loading

Triaxial compression experiments were carried out on deep-buried marble specimens to investigate their short-term and creep mechanical behavior under cyclic loading. First, based on the results of short-term triaxial experiments, the elastic, plastic, and strength behaviors of marble were analyzed. The results show that for the same confining pressure, the elastic modulus of marble remains basically constant at the lower axial deviatoric level but decreases slowly after yielding strength; in contrast, the plastic modulus reduces rapidly with the increase of axial deviatoric stress. However, the elastic and plastic moduli of the tested marble were quite independent of the confining pressure. The relationship between axial deviatoric stress and plastic deformation of marble can be described well by the interface model. The peak strength of marble under higher stress increases with the confining pressure, which can be better described in accordance with the Mohr–Coulomb criterion. And then, in accordance with the experimental results of marble creep under triaxial cyclic loading, the instant elastic and plastic strains, and the visco-elastic and visco-plastic strains were all separated successfully, which provided a better foundation for constructing a visco-elasto-plastic creep model of rock. The creep strain rate of marble under different deviatoric stresses is analyzed, which shows that the steady-state creep rate of marble increases nonlinearly with the increase of axial deviatoric stress. In the end, the creep mechanical behavior of marble under cyclic loading is theoretically analyzed using the creep model. The results show that Burgers creep model can describe the creep behavior of marble under the loading condition satisfactorily but is inadequate to describe the creep behavior of marble under the unloading condition. Therefore, by adopting the fundamental hypothesis of visco-plastic mechanics, a visco-elasto-plastic creep model of rock material is constructed, which can describe the unloading creep behavior of marble better than Burgers creep model. The creep model curve agrees very well with the experimental results, which verifies the proposed visco-elasto-plastic creep model.

On covert channels between virtual machines

Virtualization technology has become very popular because of better hardware utilization and easy maintenance. However, there are chances for information leakage and possibilities of several covert channels for information flow between the virtual machines. Our work focuses on the experimental study of security threats in virtualization, especially due to covert channels and other forms of information leakage. The existence of data leakage during migration shutdown and destruction of virtual machines, is tested on different hypervisors. For empirically showing the possibility of covert channels between virtual machines, three new network based covert channels are hypothesized and demonstrated through implementation, on different hypervisors. One of the covert channels hypothesized is a TCP/IP steganography based covert channel. Other covert channels are a timing covert channel and a new network covert channel having two pairs of socket programs. We propose a VMM (Virtual Machine Monitor) based network covert channel avoidance mechanism, tackling detection resistant covert channel problems. We also address issue of reducing the possibilities of network based covert channels using VMM-level firewalls. In order to emphasize the importance of addressing the issue of information leakage through virtual machines, we illustrate the simplicity of launching network covert channel based attacks, by demonstrating an attack on a virtual machine using covert channels through implementation.

A Mesostructure-based Damage Model for Thermal Cracking Analysis and Application in Granite at Elevated Temperatures

Thermal stress within rock subjected to thermal load is induced due to the different expansion rates of mineral grains, resulting in the initiation of new inter-granular cracking and failure at elevated temperatures. The heterogeneity resulting from each constituent of rock should be taken into account in the study of rock thermal cracking, which may aid the better understanding of the thermal cracking mechanisms in rock. In this paper, a mesostructure-based numerical model for the analysis of rock thermal cracking is proposed on the basis of elastic damage mechanics and thermal–elastic theory. In the proposed model, digital image processing (DIP) techniques are employed to characterize the morphology of the minerals in the actual rock structure to build a numerical specimen for the rock. In addition, the damage accumulation induced by thermal (T) and mechanical (M) loads is considered to modify the elastic modulus, strength and thermal properties of individual elements with the intensity of damage. The proposed model is implemented in the well-established rock failure process analysis (RFPA) code, and a DIP-based RFPA for the analysis of thermally induced stress and cracking of rock (abbreviated as RFPA-DTM) is developed. The model is then validated by comparing the simulated results with the well-known analytical solutions. Finally, taking an image from a granite specimen as an example, the proposed model is used to study the thermal cracking process of the granite at elevated temperatures and the effects of temperature on the physical–mechanical behaviors of the granite are discussed. It is found that thermal cracks mostly initiate at the location of mineral grain boundaries and propagate along them to form locally closed polygons at the elevated temperatures. Moreover, the effects of temperature on the uniaxial compressive strength and elastic modulus of the granite are quite different. The uniaxial compressive strength decreases consistently with increasing temperature, but there exists a threshold temperature for elastic modulus which starts to decrease as the temperature increases after it exceeds the threshold.