Deformation analysis of transversely isotropic coal-rock mass with porous and cracks

Coal-rock as a typical sedimentary rock has obvious stratification,namely it has transversely isotropic feature.Meanwhile,deformation leads to coal-rock mass having the characteristics of different porous and crack structures as well as local anisotropy.Equivalent axial and circumferential strain' formulas of the pure coal-rock mass specimen with a single crack were derived through the establishment of equivalent mechanical model of standard cylindrical coal-rock specimen,and have been widely used to a variety of media combined different structures containing multiple cracks.The complete stress strain curve of a real coal-rock specimen was obtained by the CTC test.Additionally,according to the comparison with the theoretical value,the theoretical mechanical model could well explain the deformation characteristics of coal-rock mass and verify its validity.Further,following features were analyzed:strain normalized coefficient and elastic modulus(Poisson's ratio) in vertical and parallel direction to the stratification,stratification angle,porosity,pore radius,normal and tangential stiffness of crack,and the relationship of different crack width with different tangential stiffness of crack.Through the analysis above,it substantiate this claim that the theoretical model with better reliability reflects the transversely isotropic nature of the coal-rock and the local anisotropy caused by the porous and cracks.

Numerical manifold method for thermo-mechanical coupling simulation of fractured rock mass

As a calculation method based on the Galerkin variation,the numerical manifold method(NMM)adopts a double covering system,which can easily deal with discontinuous deformation problems and has a high calculation accuracy.Aiming at the thermo-mechanical(TM)coupling problem of fractured rock masses,this study uses the NMM to simulate the processes of crack initiation and propagation in a rock mass under the influence of temperature field,deduces related system equations,and proposes a penalty function method to deal with boundary conditions.Numerical examples are employed to confirm the effectiveness and high accuracy of this method.By the thermal stress analysis of a thick-walled cylinder(TWC),the simulation of cracking in the TWC under heating and cooling conditions,and the simulation of thermal cracking of the SwedishÄspöPillar Stability Experiment(APSE)rock column,the thermal stress,and TM coupling are obtained.The numerical simulation results are in good agreement with the test data and other numerical results,thus verifying the effectiveness of the NMM in dealing with thermal stress and crack propagation problems of fractured rock masses.

Self-catalyzed Effect and Cracking Risk in Mass Concrete Containing Micro-slag

The main results obtained from the experimental and engineering investigation on the heat evolution and cracking risk of a furnace concrete block were presented. The heat evolution of experimental mortars containing micro-slag under different environmental temperatures was instrumented in order to investigate the self-catalyzed effect, which was discovered in engineering. More-over,the thermal stress of the furnace concrete due to heat temperature rise was calculated to evaluate the cracking risk of mass concrete containing micro-slag due to self-catalyzed effect. The experimental results illustrate that with the development of hydration and initial temperature of mixture, the hydra-tion can be also accelerated and temperature of concrete will be continued to rise, which was the self-catalyzed effect. And the thermal stress due to self-catalyzed effect could not result in the cracking of furnace concrete.

ZONAL DISINTEGRATION MECHANISM OF DEEP CRACK-WEAKENED ROCK MASSES UNDER DYNAMIC UNLOADING

Size and quantity of fractured zone and non-fractured zone are controlled by cracks contained in deep rock masses. Zonal disintegration mechanism is strongly dependent on the interaction among cracks. The strong interaction among cracks is investigated using stress superposition principle and the Chebyshev polynomials expansion of the pseudo-traction. It is found from numerical results that crack nucleation, growth and coalescence lead to failure of deep crack- weakened rock masses. The stress redistribution around the surrounding rock mass induced by unloading excavation is studied. The effect of the excavation time on nucleation, growth, interaction and coalescence of cracks was analyzed. Moreover, the influence of the excavation time on the size and quantity of fractured zone and non-fractured zone was given. When the excavation time is short, zonal disintegration phenomenon may occur in deep rock masses. It is shown from numerical results that the size and quantity of fractured zone increase with decreasing excavation time, and the size and quantity of fractured zone increase with the increasing value of in-situ geostress.

THE CONSTITUTIVE RELATION OF CRACK-WEAKENED ROCK MASSES UNDER AXIAL-DIMENSIONAL UNLOADING

An accurate and efficient numerical method for solving the crack-crack interaction problem is presented. The method is mainly by means of the dislocation model, stress superposition principle and Chebyshev polynomial expansion of the pseudo-traction. This method can be applied to compute the stress intensity factors of multiple kinked cracks and multiple rows of periodic cracks as well as the overall strains of rock masses containing multiple kinked cracks under complex loads. Many complex computational examples are given. The dependence of the crack-crack interaction on the crack configuration, the geometrical and physical parameters, and loads pattern, is investigated. By comparison with numerical results under confining pressure unloading, it is shown that the crack-crack interaction under axial-dimensional unloading is weaker than those under confining pressure unloading. Numerical results for single faults and crossed faults show that the single faults are more unstable than the crossed faults. It is found from numerical results for different crack lengths and different crack spacing that the interaction among kinked cracks decreases with an increase in length of the kinked cracks and the crack spacing under axial-dimensional unloading.

Similarity model tests of movement and deformation of coal-rock mass below stopes

For a study of the movement and deformation of coal-rock mass and low protected seams below a stope,as well as for fracture developments and rules of evolution of permeability,we designed a plane strain model test stand to carry out model tests of similar materials in order to improve the effect of gas drainage from low protected seams and to measure the movement and deformation of coal-rock mass using a method of non-contact close-range photogrammetry.Our results show that 1) using paraffin melting to take the place of coal seam mining can satisfy the mining conditions of a protective seam;2) coal-rock mass under goafs has an upward movement after the protective seam has been mined,causing floor heaving;3) low protected seams become swollen and deformed,providing a good pressure-relief effect and causing the coal-rock mass under both sides of coal pillars to become deformed by compression and 4) the evolution of permeability of low protected seams follows the way of initial values→a slight decrease→a great increase→stability→final decrease.Simultaneously,the coefficient of air permeability increased at a decreasing rate with an increase in interlayer spacing.

Coordination and evolution mechanism of fractal crack network in mining rock mass destruction

Based on the natural characters of stratum, complicated geological mining conditions and the essence of mining rock mass destruction, the complexity of rock mass destruction caused by miningw as analyzed. The inner link between rock mass destruction phenomena caused by mining and nonlinear science was revealed. There are numerous cracks in natural rock mass. The cracks’ distribution is irregular and is of statistical fractal structure. Self-organizational nonlinear evolution of the inner structure flaws leads to the rock mass destruction with external force. The evolution includes single fault’s fractal development, formation and evolution of fractal crack network and coordination of fractal crack network, etc. The law of fractal crack network’s evolution was introduced, at the same time, the coordination of fractal crack network was analyzed. Finally, based on coordination the principal equation of mining-caused subsidence of structural rock mass was established and its steady-state solution and unsteady-state solution were found.

Analysis and Numerical Simulation of Hydrofracture Crack Propagation in Coal-Rock Bed

In underground coal mines,hydrofracture can cause the increase of breathability in the fractured coal bed.When the hydrofracture crack propagates to the interface between the coal bed and the roof-floor stratum,the crack may enter roof-floor lithology,thus posing a limit on the scope of breathability increase and making it difficult to support the roof and floor board for subsequent coal mining.In this work,a two-dimensional model of coal rock bed that contains hydrofracture crack was constructed.Then an investigation that combines the fracture mechanics and the system of flow and solid in rock failure process analysis(RFPA2D-Flow)were carried out to study the failure mechanism at the interface between rocks and coals,and critical water pressure that hydrofracture crack propagates.The results indicated that the main factors that affect the direction of hydrofracture crack propagation are the angle of intersection between coal-rock interface and horizontal section,horizontal crustal stress difference,tension-shear mixed crack fracture toughness in coal-rock interface and differences in elasticity modulus of coal-rock bed.The possibility of crack directly entering coal-rock interface would increase with the increase in angle of intersection or horizontal crustal stress difference.The trend that crack propagates along the coal-rock interface will become stronger with the decrease of the fracture toughness at the coal-rock interface and the increase of the elasticity modulus difference between the coal bed and the roof strata.The results of this study was to put forward a method of controlling hydrofracture crack,optimize the fracturing well location provides a certain theoretical basis.

Experimental Study on Mechanical Characteristics of Cracked Rock Mass Reinforced by Bolting and Grouting

The stress hardening characteristics of the reinforced rock mass in uniaxial compression tests were revealed by means of the experimental study on mechanical characteristics of cracked rock mass reinforced by bolting and grouting. And the load-beating mechanism of the reinforced rock mass was perfectly reflected by the experiment. The results can offer some useful advice for support design and stability analysis of deep drifts in unstable strata.

Equivalent elastic compliance tensor for rock mass with multiple persistent joint sets:Exact derivation via modified crack tensor

Discontinuities constitute an integral part of rock mass and inherently affect its anisotropic deformation behavior.This work focuses on the equivalent elastic deformation of rock mass with multiple persistent joint sets.A new method based on the space geometric and mechanical properties of the modified crack tensor is proposed,providing an analytical solution for the equivalent elastic compliance tensor of rock mass.A series of experiments validate the capability of the compliance tensor to accurately represent the deformation of rock mass with multiple persistent joint sets,based on conditions set by the basic hypothesis.The spatially varying rules of the equivalent elastic parameters of rock mass with a single joint set are analyzed to reveal the universal law of the stratified rock mass.

Numerical simulation on pressure-relief deformation characteristics of underlying coal-rock mass after upper protective layer excavation

Based on the occurrence features of Group B coal-seams at a coal mine in the Huainan coal mining area, the elasto-plastic mechanical damage constitutive functions and numerical model for the protective layer excavation were established. With the UDEC2D computer program, after the upper protective layer was mined, the stress field change trends, crack development, and expansion deformation trends of underlying coal rock seams in the floor of the working face were simulated and analyzed. The simulation results show the stress changes in coal rock seams, the evolution process of pre-cracks during the process of upper protective layer mining, the caved zone and fractured zone of the underlying coal rock seams. At the same time, the results from the actual investigation and analysis of protected layer deformation match the simulation values, which verifies the validity and accuracy of numerical simulation results. The study results have an important guiding significance for gas management in low permeability and high gas coal seams with similar mining conditions.

Application and prospects of 3D printing in physical experiments of rock mass mechanics and engineering:materials,methodologies and models

The existence of joints or other kinds of discontinuities has a dramatic efect on the stability of rock excavations and engineering.As a result,a great challenge in rock mass mechanics testing is to prepare rock or rock-like samples with defects.In recent years,3D printing technology has become a promising tool in the feld of rock mass mechanics and engineering.This study frst reviews and discusses the research status of traditional test methods in rock mass mechanics tests of making rock samples with defects.Then,based on the comprehensive analysis of previous research,the application of 3D printing technology in rock mass mechanics is expounded from the following three aspects.The frst is the printing material.Although there are many materials for 3D printing,it has been found that 3D printing materials that can be used for rock mass mechanics research are very limited.After research,we summarize and evaluate printing material that can be used for rock mass mechanics studies.The second is the printing methodology,which mainly introduces the current application forms of 3D printing technology in rock mass mechanics.This includes printed precise casting molds and one-time printed samples.The last one is the printing model,which includes small-scale samples for mechanical tests and large-scale physical models.Then,the benefts and drawbacks of using 3D printing samples in mechanical tests and the validity of their simulation of real rock are discussed.Compared with traditional rock samples collected in nature or synthetic rock-like samples,the samples made by 3D printing technology have unique advantages,such as higher test repeatability,visualization of rock internal structure and stress distribution.There is thus great potential for the use of 3D printing in the feld of rock mass mechanics.However,3D printing materials also have shortcomings,such as insufcient material strength and accuracy at this stage.Finally,the application prospect of 3D printing technology in rock mass mechanics research is proposed.

Fracture behaviors of columnar jointed rock mass using interface mechanics theorem

For a special geological structure of columnar jointed rock mass(CJRM),its mechanical properties are strongly affected by the columnar joints.To describe the fracture behaviors of CJRM using the basic theories of interface mechanics for composite materials,the interface stresses of the vertical and horizontal joints,which are the two primary joints in the CJRM under triaxial compression,are studied,and their mathematical expressions are derived based on the superposition principle.Based on the obtained interface stresses of the vertical and horizontal joints in the CJRM,the crack initiation of the joint interface in the CJRM is studied using the maximum circumferential stress theory in fracture mechanics.Moreover,based on this investigation,the fracture behaviors of CJRM are analyzed.According to the results of similar material physical model tests for the CJRM,the theoretical study is verified.Finally,the influence of the mechanical parameters of the CJRM on the joint interface stress is discussed comprehensively.

Construction Technical Temperature Cracking for a and Control of Concrete Plate Type Raft Foundation

The mass fiat raft foundation Section is large and Cement is in large quantities, the temperature changes in the larger cement hydration heat of the water releasing, the temperature stress is main reason reducing cracking. According to the basement of a project of Guangzhou large slab raft foundation engineering as an example, Discussing the construction technology measures of early crack of large volume concrete of raft plate in the process of construction,. The results show that it Can effectively prevents the slab foundation structure cracking and achieves good results through controlling Concrete materials and concrete temperature Parameter such as the lifting machine temperature and the pouring temperature and the expansion joint or the whole pouring length.

Spatial Thermal Crack Control in Mass Concrete

The finite element software,MIDAS is used to predict the distribution of temperatures and,analyzes the cracking control methods within a hydrating mass concrete.The temperature control of mass concrete has great significance in assuring the project quality.Adiabatic or semi adiabatic temperature measurement is mostly used for measuring and controlling the temperature fluctuation during construction.The temperature distribution produced by the finite element thermal analysis of the model is used to quantify the maximum allowable internal temperature difference before crack initiation on concrete.This study analyzes the data from one high-rise structure project in Shanghai are used to verify the finite element model developed.Results suggest that reliance on a limiting maximum temperature differential to control cracking in massive concrete applications should be supplemented with a requirement for analysis showing the calculated spatial temperature and stress response to the predicted temperature distribution within the concrete,to ensure that the induced tensile stresses will not exceed the tensile strength of the concrete and so minimize the risk of having thermal cracks at early age.

冰充填裂隙对冻结岩体压缩破坏特征的影响研究

冻结岩体的力学性质是决定冻结地层工程施工与运营安全的核心因素,而冻结岩体的力学性质由裂隙结构和冻结温度等条件决定。本文在常温和冻结条件下开展了含预制裂隙砂岩的力学实验,通过高速摄影技术对试样表面的裂纹起裂和扩展过程进行了监测,基于实验结果着重讨论了冰充填裂隙对冻结岩体压缩破坏特征的影响机制。结果表明:(1)冻结没有改变试样强度随裂隙倾角的变化趋势,随裂隙倾角的增大,试样的强度均呈先减后增的趋势。强度最小值出现在裂隙倾角α=60°时,最大值出现在α=90°时;(2)在冻结状态下,试样的起裂模式随裂隙倾角的增加整体呈现“拉伸起裂→剪切起裂→拉伸起裂”的变化趋势,此外,相同裂隙倾角的试样在冻结状态下起裂的应力水平均高于在常温状态下;(3)冻结裂隙砂岩的力学性质受冻结作用和裂隙倾角双重控制。并进一步分析了冰填充裂隙对冻结岩体压缩破坏特征的影响,认为冰均起到了支撑、填充和黏结作用。在裂隙倾角较小时(0°~30°),裂隙冰的法向压力较高,强化效应主要来源于冰的支撑作用;当裂隙倾角较大(45°~75°)时,裂隙冰上的法向压力较低,强化效应主要来源于冰-岩界面黏结作用;裂隙倾角(近90°)与加载方向平行时,裂隙冰的法向应力变为拉应力,强化效应主要来源于冰-岩界面黏结作用。

基于数字图像相关方法的煤岩表面应变场及裂纹演化特征

基于数字图像相关方法,开展了煤岩体试样单轴压缩破坏的裂纹演化特征及破坏过程研究,分析了试样破坏过程的表面应变场、变形局部化带的拉伸位移演化和错动位移演化特征,精细化分析煤岩体变形破坏过程。研究结果表明:(1)根据试样的应力-应变曲线特征,将试样分为缓慢破坏型和瞬间破坏型。缓慢破坏型试样的破坏过程较为平稳,裂纹宽度较小。瞬间破坏型试样破坏过程较为剧烈,表面裂纹宽度增加,表现出明显的岩爆现象。(2)加载过程中,缓慢破坏型试样先产生应变集中后,裂纹沿变形局部化带扩展延伸并逐渐贯通;瞬间破坏型试样在应力峰值后试样表面瞬间产生宏观裂纹。(3)试样表面应变演化特征与加载过程中应力变化特征对应,反映了裂纹扩展规律。缓慢破坏型试样在加载过程中表面应变变化过程较为缓慢,并在后期呈现波动状态;瞬间破坏型试样在应力峰值点处,拉伸应变瞬间突增达到峰值,导致试样破坏。

破碎岩体隧道洞口开挖诱导滑坡与古滑坡耦合分析

以大栗树高速公路隧道洞口施工为工程背景,研究了在考虑古滑坡因素下洞口工作面边坡失稳及其加固技术。利用钻孔测斜数据(BCD)和Midas GTS折减强度有限元分析模型,分别推测了最危险滑动面,得知两者滑动面位置相似。并采用不平衡推力传递法,基于BCD推断滑动面计算安全系数。以BCD和有限元法的滑动面为基础,将滑动面近似假设为对数螺旋线,考虑地形因素,建立考虑多台阶边坡上限分析模型,研究了不同加固措施对边坡稳定的影响。结果表明,3种方法计算出的边坡安全系数结果相近。

超宽鱼腹式现浇箱梁混凝土裂缝控制

近年来,国内站城综合体TOD开发成为一种新型建筑模式,地铁、市政道路、商业综合体等多种大体量建筑形态交叉同步施工成为工程实践难题。基于此,结合已经实施的成都博览城综合交通枢纽工程,解析福州路桥鱼腹式多幅四箱室闭合型超宽箱梁,采取优化混凝土配合比、U形冷却管布置、后浇带布置、支架沉降控制、箱梁浇筑顺序等5种措施,解决大体积C60梁体混凝土防开裂难题,并取得明显实践成果,获得国家优质工程奖和土木工程詹天佑奖。

地铁车站大体积混凝土裂缝产生原因及解决措施分析

地铁建设近年来流行于各大中城市,地铁车站建设工程大体积混凝土施工比较普遍,在混凝土凝固过程中由于其体积收缩以及各种压力的作用,会造成大量的墙体裂缝;大面积的裂缝会导致建筑渗漏,给地铁车站带来了很大威胁。本文从荷载、位移、温度三个方面分析了地铁车站大体积混凝土结构的开裂成因;根据裂缝的大小、深度、成因等几个角度对裂缝进行了划分;从设计、材料、施工三个角度,对如何防止混凝土开裂进行了探讨。本文研究成果对保障地铁车站大体积混凝土施工质量有一定的参考意义。