基于二阶功准则的岩质滑坡准静态-动态转化数值分析

地质不连续带的破坏往往对岩质边坡的失稳起控制性作用。二阶功破坏准则可以分析除振颤失稳外的所有材料失稳问题,因此,利用该准则分析了岩体不连续带的材料失稳破坏问题。首先从数学上解释了使用二阶功分析二维地质不连续带的优势;而后将二阶功破坏准则引入FLAC有限差分计算软件,并根据相关监测数据对南芬铁矿岩质滑坡进行了二维数值分析。滑坡是一个准静态到动态的转化过程,动能的突然爆发标志着准静态-动态转化的发生。二阶功和二阶动能之间存在直接关系,据此关系估算出的南芬铁矿岩质边坡动能演化曲线可以证明二阶功对岩土材料失稳破坏判别的有效性。     

Micromechanical analysis of failure propagation in frictional granular materials

The extent to which the evolution of instabilities and failure across multiple length scales can be reproduced with the aid of a bifurcation analysis is examined. We adopt an elastoplastic micropolar constitutive model, recently developed for dense cohesionless granular materials within the framework of thermomicromechanics. The internal variables and their evolution laws are conceived from a direct consideration of the dissipative mechanism of force chain buckling. The resulting constitutive law is cast entirely in terms of the particle scale properties. It thus presents a unique opportunity to test the potential of micromechanical continuum formulations to reproduce key stages in the deformation history: the development of material instabilities and failure following an initially homogeneous deformation. Progression of failure, initiating from frictional sliding and rolling at contacts, followed by the buckling of force chains, through to macroscopic strain softening and shear banding, is reproduced. Bifurcation point, marking the onset of shear banding, occurred shortly after the peak stress ratio. A wide range of material parameters was examined to show the effect of particle scale properties on the progression of failure. Model predictions on the thickness and angle of inclination of the shear band and the structural evolution inside the band, namely the latitudinal distribution of particle rotations and the angular distributions of contacts and the normal contact forces, are consistent with observations from numerical simulations and experiments. Copyright © 2009 John Wiley & Sons, Ltd.     

Discussing the definition of the second‐order work for unsaturated soils

The volumetric compaction due to wetting processes is a phenomenon observed quite often in unsaturated soils. Under certain circumstances, saturation events can result into a sudden and unexpected collapse of the system. These phenomena are usually referred to as wetting‐induced collapses, without providing any detailed theoretical justification for this terminology. In order to predict in a general fashion the occurrence of coupled instabilities induced by saturation processes, a generalization of the theoretical approaches usually employed for saturated geomaterials is here provided. More specifically, this paper addresses the problem of hydro‐mechanical instability in unsaturated soils from an energy standpoint. For this purpose, an extension of the definition of the second‐order work is here suggested for the case of unsaturated porous media. On the basis of some examples of numerical simulations of laboratory tests, coupled hydro‐mechanical instabilities are then interpreted in the light of this second‐order energy measure. Finally, the implications of the theoretical results here presented are commented from a constitutive modelling perspective. Two possible alternative approaches to formulate incremental coupled constitutive relations are indeed discussed, showing how the onset of hydro‐mechanical instabilities can be predicted using an extended form of Hill's stability criterion. Copyright © 2010 John Wiley & Sons, Ltd.     

含预制单裂隙花岗岩的真三轴单面临空岩爆试验研究

通过开展含预制单裂隙花岗岩的真三轴单面临空岩爆试验,并利用高速摄像系统和声发射（acoustic emission,简称AE）系统监测岩爆过程,探究了不同产状裂隙岩石的破坏模式、强度变形和声发射演化特性,分析了裂隙产状与岩爆过程及弹射动能之间的关系,对比了含裂隙岩石岩爆发生机制与无裂隙完整岩石的差异。有关力学特性的分析表明,随着裂隙倾角的减小,岩样破坏模式大体呈现由“内剪外劈”向“Z型斜剪”变化的趋势,裂隙对岩石强度的削弱作用不断增大。当裂隙倾角小于30°时,岩石峰值应力普遍仅为完整岩样的一半左右;小倾角裂隙的长度越大,岩样岩板劈裂现象变得显著,形成岩爆坑略微变大,且强度折减幅度越大,峰值轴向应变相应变小;裂隙位置向临空面靠近会加剧岩板的劈裂效应,塑性阶段普遍会产生较大变形并萌生大量裂纹,当裂隙已出露且切断临空面将不易形成岩爆坑。有关岩爆过程及弹射动能的分析表明,随着裂隙倾角的减小,岩样弹射动能呈现先显著降低后小幅回升的变化规律,30°倾角为转变拐点;岩样内部裂隙距临空面越近,裂隙岩样的弹射动能越小;树脂填充裂隙使得岩样弹射动能极大提升,而水泥填充的则无明显提升。有关声发射特征的分析表...     

Characterization of mesoscale instabilities in localized granular shear using digital image correlation

Within shear bands in sands, deformation is largely non-affine, stemming primarily from buckling of well-known force chains and also from vortex-like structures. In the spirit of current trends toward multiscale modeling, understanding the links between these mesoscale deformational entities and corresponding macroscale response will form the basis for the next generation of sand behavioral models and may also aid in efforts to understand jamming–unjamming transitions in dense granular flows in general. Experimental methods to quantify and characterize such subscale kinematics, in particular in real sands, will play critical roles in these efforts. Digital Image Correlation (DIC) is a fast growing experimental technique to nondestructively measure surface displacements from digital images. Here, DIC has been employed to identify and characterize the development of vortex structures inside shear bands formed in dense sands during plane strain compression. A rigorous assessment of the DIC method has been performed, in particular for subscale behavioral characterization in unbonded granular solids, and guidelines are offered for accurate implementation. While DIC systematically overestimates shear band thickness, a methodology has been devised to compensate for this overestimation. Shear band thickness for four different uniform sands were found to range between 6 and 9 grain diameters, and for a well-graded sand between 8 and 9.5 grain diameters. These determinations agree with visual inspections of grain kinematics from the image data, as well as recent theoretical predictions.     

X‐ray tomography of tsunami deposits: Towards a new depositional model of tsunami deposits

X‐ray computed microtomography is used to obtain high resolution imagery of a historical tsunami deposit in Andalusia, Spain (1755 Lisbon tsunami). The technique allows characterization of grain‐size distribution, structures, component analysis and sedimentary fabric of fine‐grained unconsolidated tsunami deposits at resolutions down to particle scale. The results are validated by comparing to data obtained using other techniques such as laser diffraction, anisotropy of magnetic susceptibility and X‐ray microfluorescence on the same deposits. Specific technical details such as sampling, scanning and image processing methods, and further improvements are addressed. The use of X‐ray computed microtomography provides new insights into the stratigraphy of the deposits and gives access to significantly more detailed view of key sedimentary features such as mudlines, rip‐up clasts, crude laminations, convolutions, floating outsized clasts and contacts between successive units. This analysis of the 1755 tsunami deposits using X‐ray computed microtomography allows the proposal of new hypotheses for the sedimentary processes forming tsunami deposits. Deposition by settling is limited and the section analysed here is dominated by a high shear stress leading to the development of traction carpets, with laminated mudlines corresponding to the basal frictional region of these carpets. The onset of the tsunami backwash is marked by a micro‐vortex resembling Kelvin–Helmoltz instabilities.     

深埋洞室开挖瞬态卸荷诱发振动的衰减规律

深埋洞室爆破开挖过程中围岩初始地应力的瞬态卸荷会诱发振动。针对圆形隧洞开挖,分析了地应力瞬态卸荷过程中围岩应变能、动能、径向应力做功3者之间的平衡机制,并采用量纲分析,建立了基于开挖岩体应变能的瞬态卸荷诱发振动衰减公式。结合锦屏二级引水隧洞和瀑布沟地下主厂房爆破开挖实例,开展了瞬态卸荷诱发振动的识别分离,并分析了诱发振动的衰减规律。研究表明：地应力瞬态卸荷诱发振动的峰值与被开挖岩体应变能密度的二次方根成正比,且开挖卸荷体积越大,诱发振动越强烈;与Lu-Hustrulid衰减公式相比,新衰减公式应用范围更广,使用精度更高。     

Penetration test in coarse granular material using Contact Dynamics Method

Field tests are widely used for soil characterization in geotechnical applications in spite of implementation difficulties. The light penetrometer is a well-known testing tool for fine soils, but the physical interpretation of the output data in the case of coarse granular materials is far less evident. Indeed, the data are considerably more sensitive in this case to various parameters such as fabric structure, particle shapes or the applied impact energy. In order to achieve a better understanding of the penetration process into a coarse granular material, a numerical study was performed by means of contact dynamics simulations. The penetration of a moving tip in a sample composed of irregular grain shapes was studied and the influence of the driving velocity and input energy on the penetration strength was analyzed. The results show that the latter grows with both the penetration rate and energy, despite the strong fluctuations occur due to a jamming–unjamming process in which the contact network connectivity evolves intermittently in correlation with the penetration strength. This analysis suggests that the time-averaged data provided by a penetrometer is reliable information from which the bulk strength properties of coarse granular materials can be evaluated.     

Upper and lower tropospheric energetics of standing and transient eddies in wave number domain during summer monsoon of 1991

Kinetic energy exchange equations (Saltzman 1957) in wave number domain are partitioned into standing, transient and standing-transient components following Murakami (1978, 1981). These components are computed for the 1991 summer monsoon using dailyu andv grid point data at 2.5° latitude-longitude interval between the equator and 40°N at 200 hPa and 850 hPa levels for the period June through August. The data are obtained from NCMRWF, New Delhi. The study shows that at 200 hPa wave number 1 over Region 3 (30°N to 40°N), wave number 2 over Region 2 (15°N to 30°N) and wave number 3 over Region 1 (equator to 15°N) dominate the spectrum of transport of momentum and wave to zonal mean flow interaction. Wave number 1 over Region 1 and Region 3 and wave number 2 over Region 2 are the major sources of kinetic energy to other waves via wave-to-wave interaction. At 850 hPa wave number 1 over Region 3 has maximum contribution in the spectrum of transport of momentum and kinetic energy and more than 90% of its contribution is from the standing component. This indicates that standing wave number 1 over Region 3 plays a very important role in the dynamics of monsoon circulation of the lower troposphere. The study further shows that although the circulation patterns at 200 hPa and 850 hPa levels are opposite in character, a number of energy processes exhibit a similar character at these levels. For example, (i) transport of momentum by most of the waves is northward, (ii) small scale eddies intensify northward, (iii) eddies are sources of kinetic energy to zonal mean flow over Region 1 and (iv) standing eddies are sources of kinetic energy to transient eddies. Besides the above similarities some contrasting energy processes are also observed. Over Region 2 and Region 3 standing and transient eddies are sources of kinetic energy to zonal mean flow at 200 hPa, while at 850 hPa the direction of exchange of kinetic energy is opposite i.e. zonal mean flow is a source of kinetic energy to standing as well as transient eddies. L(n) interaction indicates that at 200 hPa waves over R2 maintain waves over R1, while at 850 hPa waves over R1 maintain waves over R2. It has been found that the north-south gradient of zonal mean of zonal wind is the deciding factor of wave to zonal mean flow interaction.     

Kinetic Energy Dissipation and Convergence Criterion of Discontinuous Deformations Analysis (DDA) for Geotechnical Engineering

The discontinuous deformation analysis (DDA) is a numerical method for modeling discontinuous deformation behaviour of jointed rocks. In this paper, two basic problems are discussed related to kinetic energy dissipation and the convergence criterion for the DDA method when it is applied to geotechnical engineering. In view of the fact that the deformation and progressive failure can be treated as a quasi-static process with low kinetic energy dissipation rates, this paper introduces a viscous damping component to absorb discrete blocks’ kinetic energy, establishes the global equations of motion of the discrete block system that take damping effects into account, investigates the energy dissipation mechanism when solving a static or quasi-static problem, and defines the convergence criteria of displacement, kinetic energy and unbalanced force for DDA solutions when the system arrives at a stable state.     

弹脆性圆形煤巷应力跃升及冲击地压预测模型

为预测煤巷冲击地压灾害,将强冲击倾向煤视作理想弹脆性材料,建立圆形煤巷力学模型,分析煤巷围岩的应力和变形能密度分布特征,进而建立煤巷冲击地压预测模型,利用该模型预测冲击地压灾害风险。研究结果表明：弹脆性煤巷围岩的切向应力和变形能密度在弹性区与破坏区界面处发生跃升;煤体强度损伤度和地应力增大,煤巷破坏区半径增大,煤巷围岩切向应力和变形能密度跃升高度增大;切向应力跃升为煤巷失稳破坏提供了力源,变形能密度跃升为冲击地压发生提供了能量源;切向应力和变形能密度跃升高度越大,煤巷越容易失稳和冲击;基于切向应力和变形能密度跃升,建立了煤巷冲击地压解析预测模型,预测结果与现场冲击地压实际情况一致,从而为脆性煤巷冲击地压预测提供了一种新的方法。     

岩爆和冲击地压的差异解析及深部工程地质灾害关键机理问题

岩爆和冲击地压原来在自然界并不存在,完全是由于人类进行深部地下空间利用、深部矿产资源开采等工程建设时诱发产生的,两者同属于最典型的深部工程地质灾害.由于历史原因,岩爆和冲击地压长期存在概念混用的情况.本文比较详细地综述了岩爆和冲击地压领域国内早期的研究历程,系统解析了岩爆和冲击地压之间存在的差异.在研究对象(硬岩和煤的...     

Spectral analysis of monsoonal global energy and enstrophy and their nonlinear exchanges in the zonal wavenumber domain

Vertical distributions of various components of the tropospheric global rotational kinetic energy, enstrophy and available potential energy during July 1979, and the contributions to these from different zonal wave categories were studied. Representative levels in the lower and upper troposphere for the stationary and transient energetics were identified on the basis of different components of energy and enstrophy. The eddy energy and enstrophy contained in different zonal scale components in the lower and upper troposphere were studied to find out the preferred scales for stationary and transient monsoonal motion in the two atmospheric layers. The role of different zonal wave categories in the nonlinear exchanges of energy and enstrophy arising due to stationary-stationary, transient-transient, stationary-transient and observed flow interactions was examined. Stationary and transient global spectra of the aforesaid dynamical variables in terms of the zonal wavenumber(m) with triangular truncation atm = 42 were utilized for this purpose. It was found from the global average kinetic energy in lower and upper troposphere that the global stationary and transient motions were comparable in the lower troposphere while in the upper troposphere stationary motion dominated over the transient motion. The computed zonal and eddy energy confirmed that the stationary motion was predominantly zonal while the transient motion was dominated by eddies. From the time mean nonlinear interaction of kinetic energy (enstrophy) of observed flow it was seen that the long and short waves as well as the zonal flow gained kinetic energy (enstrophy) from medium waves due to nonlinear interactions. The transfer of available potential energy due to nonlinear interaction was down the scale except for short waves in the upper troposphere. The stationary-transient interaction was found to be an important element of the spatial-temporal varying atmospheric flow.     

The Hadean–Archaean transition at 4 Ga: From magma trapping in the mantle to volcanic resurfacing of the Earth

The Hadean–Archaean transition is poorly known because of the dearth of Hadean rocks. A new conceptual model is presented based on variations in mantle potential temperature (T_p) with time. The critical issue is the depth of melting with respect to a negatively buoyant magma sink between 410 and 330 km (14–11 GPa). Hadean plume magmatism begins below the magma sink, leading to generation of a refractory upper mantle reservoir and the minor production of boninite‐like magmas near the surface. With cooling, the onset of melting migrates above the magma sink, a situation likely occurring since 3.9 Ga and corresponding to T_ps of ~1870°C or less. Therefore, a burst of mafic to ultramafic volcanism was produced at 3.9–3.8 Ga. This extensive volcanism may have triggered gravitational instabilities and favoured the recycling of the Hadean crust into the mantle. Results of this model are discussed in the light of existing isotopic data.     

Microseism Induced by Transient Release of In Situ Stress During Deep Rock Mass Excavation by Blasting

During deep rock mass excavation with the method of drill and blast, accompanying the secession of rock fragments and the formation of a new free surface, in situ stress on this boundary is suddenly released within several milliseconds, which is termed the transient release of in situ stress. In this process, enormous strain energy around the excavation face is instantly released in the form of kinetic energy and it inevitably induces microseismic events in surrounding rock masses. Thus, blasting excavation-induced microseismic vibrations in high-stress rock masses are attributed to the combined action of explosion and the transient release of in situ stress. The intensity of stress release-induced microseisms, which depends mainly on the magnitude of the in situ stress and the dimension of the excavation face, is comparable to that of explosion-induced vibrations. With the methods of time–energy density analysis, amplitude spectrum analysis, and finite impulse response (FIR) digital filter, microseismic vibrations induced by the transient release of in situ stress were identified and separated from recorded microseismic signals during a blast of deep rock masses in the Pubugou Hydropower Station. The results show that the low-frequency component in the microseismic records results mainly from the transient release of in situ stress, while the high-frequency component originates primarily from explosion. In addition, a numerical simulation was conducted to demonstrate the occurrence of microseismic events by the transient release of in situ stress, and the results seem to have confirmed fairly well the separated vibrations from microseismic records.     

A stability criterion for elasto‐viscoplastic constitutive relationships

In this paper, the onset of mechanical instability in time‐sensitive elasto‐viscoplastic solids is theoretically analyzed at the constitutive level and associated with the occurrence of ‘spontaneous accelerations’ under stationary external perturbations. For this purpose, a second‐order form of Perzyna's constitutive equations is first derived by time differentiation, and a sufficient stability condition is identified for general mixed loading programs. These loading conditions are in fact the most general in both laboratory tests and real boundary value problems, where a combination of certain stress and strain components is known/prescribed. The theoretical analysis leads to find precise stability limits in terms of material hardening modulus. In the case of constitutive relationships with isotropic strain‐hardening, no instabilities are possible while the hardening modulus is larger than the so‐called ‘controllability modulus’ defined for (inviscid) elasto‐plastic materials. It is also shown that the current stress/strain rate may also directly influence the occurrence of elasto‐viscoplastic instability, which is at variance with elasto‐plastic inviscid media. Copyright © 2015 John Wiley & Sons, Ltd.     

MHD simulations of current-sheet formation over a bipolar active region

We present the results of numerical simulations of the development of a current sheet in the solar corona over a bipolar region during the emergence of two new sunspots arranged collinearly with older spots. Two fronts of increased plasma density form at the boundary of the rising new magnetic flux. One of these is due to the generation of a current sheet, whose magnetic field accumulates energy for a flare. The other front is a branch of the density perturbation, and separates the old and new magnetic fluxes in a region where the magnetic field lines have the same direction on both sides of the boundary. The development of this perturbation is not associated with the energy accumulation in the corona, and hinders observation of the preflare state and complicates analysis of the results. This second front can be interpreted as the eruption of a filament before the onset of the flare. A scheme conservative with respect to magnetic flux was introduced in the Peresvet code that solves the MHD equations, in order to suppress numerical instabilities in regions of large magnetic-field gradients.     

沙坝地形上沿岸流不稳定运动模式变化特性

为研究沙坝地形上沿岸流不稳定运动特征模式变化特性,建立了考虑底摩擦影响的不稳定模型,并用该模型计算了两种沙坝地形(坡度分别为1:40和1:100)上的不稳定特征模式,这些模式内容包括不稳定运动波长、传播速度、不稳定增长率和扰动速度场。将这些结果与忽略底摩擦计算结果进行了比较,分析了底摩擦对不稳定特征模式的影响。上述研究表明沙坝地形上不稳定运动传播速度约为时均沿岸流最大值的0.71~0.79;1:40沙坝地形上产生不稳定运动所用的时间比1:100地形上更短;底部摩擦可以很大程度上抑制沿岸流不稳定运动的产生;一般观测到的(准)韵律形、新月形沙坝都在后剪切区(紧邻时均沿岸流最大值向海方向区域)产生;不稳定运动在沙坝处运动最强,并且坡度越缓,不稳定运动沿岸方向波长越长。     

Regional hazard prediction of rock bursts using microseismic energy attenuation tomography in deep mining

Rock burst prediction is a worldwide challenge that we have long tried to overcome. This study tentatively proposed a method to regionally predict rock burst hazards using microseismic energy attenuation. To verify the feasibility of the proposal, first, the mechanism of microseismic energy propagation and attenuation in rock medium was explored, and dominant attenuation characteristics of microseismic waves were analyzed. Second, a spatial attenuation model of microseismic energy was established, and the average energy attenuation coefficient for each wave path was defined. A 3D seismic energy attenuation inversion algorithm was put forward, and the corresponding computation matrix was developed. Third, a continuous microseismic field investigation was carried out in a deep coal mine. Seismic energy attenuation coefficient was confirmed using the calibrated focus position and energy determination. Based on data discretization processing, energy attenuation inversion and tomography, potential rock burst hazard regions were strictly zoned in mining areas. Finally, regional prediction results obtained from the microseismic energy attenuation were compared with the direct measurement results obtained from the classical drilling dust method to verify the reliability of proposed approach. It turns out that rock burst hazard regions predicted by the microseismic energy attenuation agreed well with the objective hazardous situations. Seismic energy attenuation coefficient is a significant evaluation factor that directly mirrors the inelastic performance of rock medium. Energy attenuation coefficient threshold used for determining the rock burst hazard regions was 3.0 km^?1. Reliability of the seismic energy attenuation inversion and tomography was closely related to the spatial distribution of microseisms in a localized region. The optimum spatial density of microseisms was 0.2 m^?3. Regional rock burst prediction using microseismic energy attenuation is an effective approach for revealing potential hazardous regions in deep mining conditions. This approach improves the pertinence of geological hazard prevention and provides a beforehand reference for targeted hazard management.     

From solid to granular gases: the steady state for granular materials

This paper aims at extending the well‐known critical state concept, associated with quasi‐static conditions, by accounting for the role played by the strain rate when focusing on the steady, simple shear flow of a dry assembly of identical, inelastic, soft spheres. An additional state variable for the system, the granular temperature, is accounted for. The granular temperature is related to the particle velocity fluctuations and measures the agitation of the system. This state variable, as is in the context of kinetic theories of granular gases, is assumed to govern the response of the material at large strain rates and low concentrations. The stresses of the system are associated with enduring, frictional contacts among particles involved in force chains and nearly instantaneous collisions. When the first mechanism prevails, the material behaves like a solid, and constitutive models of soil mechanics hold, whereas when inelastic collisions dominate, the material flows like a granular gas, and kinetic theories apply. Considering a pressure‐imposed flow, at large values of the normal stress and small values of the shear rate, the theory predicts a nonmonotonic shear rate dependence of the stress ratio at the steady state, which is likely to govern the evolution of landslides. Copyright © 2013 John Wiley & Sons, Ltd.