Estimation of joint trace length probability distribution function in igneous, sedimentary, and metamorphic rocks

火成岩、沉积岩及变质岩节理迹长概率分布函数估计

Arabian Journal of Geosciences, June 2014, Volume 7, Issue 6, pp 2353-2361

Abstract: To predict the behavior of structures in and on jointed rock masses, it is necessary to characterize the geomechanical properties of joints and intact rock. Among geometry properties of joints, trace length has a vital importance, because it affects rock mass strength and controls the stability of the rock structures in jointed rock masses. Since joint length has a range of values, it is useful to have an understanding of the distribution of these values in order to predict how the extreme values may be compared to the values obtained from a small sample. For this purpose, three datasets of joint systems from nine exposures of igneous, metamorphic, and sedimentary rocks are studied. Joint trace length is one of the most difficult properties to measure accurately, but it may be possible to record other geometrical properties of exposed joints accurately; thereby, support vector machine (SVM) model is used to predict the joint trace length. SVM is a novel machine learning method, which is a powerful tool used to solve the problem characterized by small sample and non-linearity with a good generalization performance. Consequently, goodness-of-fit (GOF) tests were applied on these data. According to these GOF tests, the lognormal distribution was found to be the best probability distribution function for representing a joint trace length distribution.

Coal Mine Roadway Stability in Soft Rock: A Case Study

煤矿软岩巷道稳定性实例研究

Rock Mechanics and Rock Engineering, November 2014, Volume 47, Issue 6, pp 2225-2238

Abstract: Roadway instability has always been a major concern in deep underground coal mines where the surrounding rock strata and coal seams are weak and the in situ stresses are high. Under the high overburden and tectonic stresses, roadways could collapse or experience excessive deformation, which not only endangers mining personnel but could also reduce the functionality of the roadway and halt production. This paper describes a case study on the stability of roadways in an underground coal mine in Shanxi Province, China. The mine was using a longwall method to extract coal at a depth of approximately 350 m. Both the coal seam and surrounding rock strata were extremely weak and vulnerable to weathering. Large roadway deformation and severe roadway instabilities had been experienced in the past, hence, an investigation of the roadway failure mechanism and new support designs were needed. This study started with an in situ stress measurement programme to determine the stress orientation and magnitude in the mine. It was found that the major horizontal stress was more than twice the vertical stress in the East–West direction, perpendicular to the gateroads of the longwall panel. The high horizontal stresses and low strength of coal and surrounding rock strata were the main causes of roadway instabilities. Detailed numerical modeling was conducted to evaluate the roadway stability and deformation under different roof support scenarios. Based on the modeling results, a new roadway support design was proposed, which included an optimal cable/bolt arrangement, full length grouting, and high pre-tensioning of bolts and cables. It was expected the new design could reduce the roadway deformation by 50 %. A field experiment using the new support design was carried out by the mine in a 100 m long roadway section. Detailed extensometry and stress monitorings were conducted in the experimental roadway section as well as sections using the old support design. The experimental section produced a much better roadway profile than the previous roadway sections. The monitoring data indicated that the roadway deformation in the experimental section was at least 40–50 % less than the previous sections. This case study demonstrated that through careful investigation and optimal support design, roadway stability in soft rock conditions can be significantly improved.

Poromechanics of adsorption-induced swelling in microporous materials: a new poromechanical model taking into account strain effects on adsorption

微孔材料吸附引起的膨胀孔隙介质力学：考虑应变对吸附影响的新孔隙介质力学模型

Continuum Mechanics and Thermodynamics, May 2014

Abstract: A poromechanical model is presented for estimating swelling of nano-porous media fully saturated with a fluid phase. From the Gibbs adsorption isotherm, the effective pore pressure and the volumetric strain are estimated incrementally taking into account the variations of porosity upon swelling and therefore the variations of the poromechanical properties (apparent modulus, Biot coefficient, Biot modulus). Moreover, the interaction between swelling and the adsorption isotherms are examined by proposing a correction to the Gibbs formalism by taking into account the pore volume variation upon swelling. First, comparisons with experimental data found in the literature are performed, and a fair agreement is observed.

Mine Overburden Dump Failure: A Case Study

矿山覆排土（石）场破坏实例研究

Geotechnical and Geological Engineering, April 2014, Volume 32, Issue 2, pp 297-309

Abstract: In any open cast mine, the management of stripped spoil during mining is crucial to the mine’s successful operation. The improper management of the overburden (OB) dump can result in stability issues which may affect safety and production of the mine. Various literatures have reported the failure of open pit dumps and the consequences in loss of life, production and impact on neighbouring amenities. Recently, the failure of an out-of-pit OB dump at an Indian Colliery was reported. The failed OB dump displaced spoil approximately 70 m from the original location of the dump toe and impacted on neighbouring amenities. This paper back analyse material properties and investigates the probable mechanism of this OB failure. Well established tools including limiting equilibrium and continuum numerical methods have been used to understand and identify the failure kinematics of this dump. It has been found that the residual friction angle of the material comprising the dump structural unit dominates stability. Mobilisation of residual strength can occur by operational induced strains and/or the presence of water. The shear strength of the foundation was fully mobilised and provided a slip surface for the overlying dump material. The resultant failure is manifest as a bilinear wedge movement of two ridged blocks defined by linear rupture planes.

Assessments of Strength Anisotropy and Deformation Behavior of Banded Amphibolite Rocks

带状闪石强度各向异性及变形特征估算

Geotechnical and Geological Engineering, April 2014, Volume 32, Issue 2, pp 429-438

Abstract: Assessment of strength anisotropy in transversely isotropic rocks has been one of the most challenging subjects in rock engineering. However, far too little attention has been paid to banded amphibolite rocks. This study aim to evaluate strength and deformation anisotropy behavior of banded amphibolite rocks. The dynamic mechanical tests including ultrasonic pulse test, uniaxial compressive strength, Brazilian test and deformability test were performed on drilled rock samples as a function of foliation plane angle (β = 0°, 30°, 60° and 90°). The results obtained have shown that the dynamic mechanical properties of amphibolite rocks have different values concerning banding plane. Compression and shear waves taken parallel to the foliation plane show highest values than those obtained in the other directions. Under uniaxial test, the banded amphibolite has a U-shaped anisotropy with maximum strength at β = 90° and minimum strength is obtained when β = 30°. Strength anisotropic index ranges between 0.96 and 1.47. It seems that the high range value of anisotropic index is mainly due to slight undulation of foliation planes, that being not perfectly straight. The results of elastic deformation test show that there is no clear dependence on microstructures characteristics of subtype-amphibolite rocks that controlling modulus “shape-anisotropy”. However, in this study, Young modulus values of amphibolite rocks with β follow both types of shape-anisotropy, “U-shape” and “decreased order-shaped”. Thus, this study recommended that further research be undertaken regarding the role of modulus “shape-anisotropy” within the same lithotype.

Overview and modeling of mechanical and thermomechanical impact of underground coal gasification exploitation

地下矿井煤气化开发的力学与热力学影响之回顾与模拟

Mitigation and Adaptation Strategies for Global Change, March 2014

Abstract: From an economic point of view Underground Coal Gasification (UCG) is a promising technology that can be used to reach coal resources that are difficult or expensive to by conventional mining methods. Furthermore, the process addresses safety concerns, by avoiding the presence of workers underground. An optimal UCG process requires the integration of various scientific fields (chemistry, geochemistry, geomechanics) and the demonstration of limited of environmental impacts. This paper focuses on the mechanical component of the UCG operation and its impact on the surrounding environment in terms of stability and land subsidence. The mechanical components are also considered. Underground mining by coal combustion UCG challenges include the mechanical behavior of the site and of stability of the overburden rock layers. By studying the underground reactor, its inlet and outlet, we confirm the key role played by mechanical damage and thermo-mechanical phenomena are identified. Deformation or collapse above the cavity may cause a collapse in the overlying layers or subsidence at the surface level. These phenomena are highly dependent on the thermoporomechanical behavior of the rock surrounding the cavity (the host rocks). Unlike conventional methods, the UCG technology introduces an additional variable into the physical problem: the high temperatures, which evolve with time and space. In this framework, we performed numerical analyses of the coal site that could be exploited using this method. The numerical results presented in this paper are derived from models based on different assumptions describing a raw geological background. Several 3D (3 dimensional) and 2D (2 dimensional, plane) nonlinear finite element modelings are performed based on two methods. The first assumes a rock medium as a perfect thermo-elastoplastic continuum. In the second, in order to simulate large space scale crack propagation explicitly, we develop a method based upon finite element deactivation. This method is built on a finite element mesh refinement and uses Mohr-Coulomb failure criterion. Based on the analysis of the numerical results, we can highlight two main factors influencing the behavior and the mechanical stability of the overburden, and consequently the UCG process evolution. The first is the size of the cavity. This geometrical parameter, which is common to all types of coal exploitation, is best controlled using the classic exploitation method. We show that in the case of UCG, the shape of the cavity and its evolution over time can be modified considerably by the thermomechanical behavior of the host rocks. The second is the presence of a heat source whose location and intensity evolve over time. Even if thermal diffusivity of the rock is low and only a small distance from the coal reactor is thermally affected, we show that the induced mechanical changes extend significantly in the overburden, and that subsidence can therefore be estimated at the surface. We conclude the integration of the mechanical analysis into a risk analysis process mechanical analysis can be integrated in a thorough risk analysis.

In-situ Rock Spalling Strength near Excavation Boundaries

开挖边界附近现场岩石剥落强度

Rock Mechanics and Rock Engineering, March 2014, Volume 47, Issue 2, pp 659-675

Abstract: It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch’s solution (with approximated circular tunnel geometry and hence σ max = 3σ 1 −σ 3) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ max at the excavation boundary. The ratio of σ max /UCS is related to the observed depth of failure and failure initiation occurs when σ max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the “actual” in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic “as-built” excavation boundary condition is honored, the “actual” in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an “apparent” rock strength.

Relationship Between Pre-failure and Post-failure Mechanical Properties of Rock Material of Different Origin

不同来源岩石材料破坏前与破坏后力学特性关系

Rock Mechanics and Rock Engineering, February 2014

Abstract: Under compression, gathering data related to the post-failure part of the stress–strain curve requires stiff servo-controlled testing systems. In unconfined conditions, data related to the post-peak region of the intact rock parameters are not common as pre-peak and peak state parameters of stress–strain behavior. For problems involving rock in the failed state around structures, proper choice of plastic constitutive laws and post-failure parameters is important for the modeling of the failed state. The aim is to relate commonly used intact rock parameters of pre-failure (tangent modulus E i and secant modulus E s) and peak strength (σ ci) states to parameters of the post-failure state under unconfined compression. Post-failure parameters are the drop modulus (D pf), representing the slope of the falling portion in brittle state, residual strength (σ cr), and dilatancy angle (ψ°). Complete stress–strain curves were generated for various intact rock of different origin. Seventy-three post-failure tests were conducted. Samples included in the testing program were chosen to represent rocks of different origin. Specimens of granite, rhyodacite, dunite, quartzite series, glauberite, argillite, marl, and lignite were used in the tests. The results from the pre-failure and peak state testing parts were processed and compared to the post-failure stress–strain parameters. For the estimation of post-failure parameters in terms of the pre-peak and peak states, the functional relations were assessed. It was found that the drop modulus D pf increases with rock strength σ ci, following a power function with an approximate power of two. With an exponential trend, the D pf/E s ratio increases with decreasing E i/σ ci ratio. Relations estimating the residual strength and dilatancy from the pre-peak and peak state parameters are in logarithmic and exponential functional forms, respectively.

Coal waste management practices in the USA: an overview

美国煤渣（煤矸石等）管理实践综述

International Journal of Coal Science & Technology, June 2014, Volume 1, Issue 2, pp 163-176.

Abstract: This paper provides an overview of coal waste management practices with two case studies and an estimate of management cost in 2010 US dollars. Processing of as-mined coal typically results in considerable amount of coarse and fine coal processing wastes because of in-seam and out-of-seam dilution mining. Processing plant clean coal recovery values run typically 50 %–80 %. Trace metals and sulfur may be present in waste materials that may result in leachate water with corrosive characteristics. Water discharges may require special measures such as liner and collection systems, and treatment to neutralize acid drainage and/or water quality for trace elements. The potential for variations in coal waste production and quality depends upon mining or processing, plus the long-term methods of waste placement. The changes in waste generation rates and engineering properties of the coal waste during the life of the facility must be considered. Safe, economical and environmentally acceptable management of coal waste involves consideration of geology, soil and rock mechanics, hydrology, hydraulics, geochemistry, soil science, agronomy and environmental sciences. These support all aspects of the regulatory environment including the design and construction of earth and rock embankments and dams, as well as a wide variety of waste disposal structures. Development of impoundments is critical and require considerations of typical water-impounding dams and additional requirements of coal waste disposal impoundments. The primary purpose of a coal waste disposal facility is to dispose of unusable waste materials from mining. However, at some sites coal waste impoundments serve to provide water storage capacity for processing and flood attenuation.