Laboratory experimental study on pumping-induced earth fissures

Hydrogeology Journal - The mechanisms associated with pumping-induced earth fissures have so far been unclear. To gain an insight into the mechanisms, a physical model test was conducted, which...     

Frictional crack initiation and propagation analysis using the numerical manifold method

By employing both a physical mesh and a mathematical mesh to formulate a physical problem, the numerical manifold method (NMM) can lead to a very simple meshing task, which allows directly capturing the discontinuities across the crack surfaces without further incorporating unknowns to the related nodes through enrichment functions. These features enable the NMM to handle complex crack problems. In this study, based on the contact technique of the NMM and the incorporation of the Mohr–Coulomb crack initiation criterion, the effects of the friction and cohesion on the crack growth from a closed flaw (crack) under compression were investigated. A limited number of comparisons between the numerical results and the physical experiments show that with the Mohr–Coulomb crack initiation criterion, the NMM can not only accurately predict the pure tensile or pure shear crack growth, but the NMM can also satisfactorily predict the development of mixed shear–tensile crack types. Using a parametric analysis, the effects of the confining stress, the flaw inclination angle, the flaw friction angle and the material strengths on the crack development (crack initiation stress, crack initiation angle, crack type developed) have been investigated.     

岩石倾斜裂隙与水平裂隙扩展贯通试验及数值模拟研究

采用最新配置的水泥砂浆材料,通过在试件中布置倾斜裂隙和水平裂隙,开展了单轴和双轴条件下的压缩试验,详细分析了不同加载条件下试件裂隙的扩展和贯通规律。试验结果表明:无裂隙内水压时,在单轴压缩条件下倾斜裂隙在扩展贯通水平裂隙后裂纹的扩展路径和方向发生明显改变,试样最终发生劈裂破坏。侧压的增大抑制了倾斜裂隙端部翼裂纹和次生裂纹的萌生,高侧压下翼裂纹没有贯穿水平裂隙,试样破坏模式变成了剪切破坏。水力耦合作用下,单轴压缩时倾斜裂隙翼裂纹尖端的最大主应力分布范围随着内水压的增大逐渐变小,翼裂纹的起裂应力、起裂角和峰值强度逐渐降低,试样发生劈裂破坏。通过数值模拟得到的裂纹扩展贯通规律与试验结果具有良好的一致性,开展了不同内水压时倾斜裂隙扩展贯通无水压的水平裂隙的数值分析,随着内水压的增加倾斜裂隙首先萌生共面裂纹随后产生翼裂纹,翼裂纹扩展贯通水平裂隙时扩展路径同样会发生改变。水力耦合作用下侧压也会抑制翼裂纹的萌生,随着内水压的增大减弱了侧压对倾斜裂隙翼裂纹的抑制作用。     

地裂缝灾害研究现状与展望

1927年美国首先发现地裂缝以来，各国学者对地裂缝的发生、发展进行了细致的研究。美国学者首先从地震研究入手，认为地裂缝的形成受构造因素影响。后逐渐加强研究地下水超采对地裂缝的影响，形成了构造与地下水开采复合成因观点。我国学者重点对汾渭地堑地裂缝成因问题进行了较系统的研究，提出几种成因机理模型，并用GIS技术建立简单的预测预报系统。文章在详细介绍国内、外对地裂缝成因机理和水文特性的研究以及预测、预报工作的同时，对今后研究趋势进行了展望。目前，地裂缝的研究正从以往的定性静态描述转向以过程为目标的精确定量动力学研究。利用相似材料模拟地裂缝的形成与发展，以非线性理论（如分形）对地裂缝系统进行定量的数学描述，建立准确的预测、预报模型（如人工神经网络模型），并与GIS相结合，建立完善的预测、预报系统，减少地裂缝灾害对环境的影响，进而对它加以利用，是今后地裂缝研究的主要方向。     

Subduction initiation and ophiolite crust: new insights from IODP drilling

International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.     

Influence of crack surface friction on crack initiation and propagation: A numerical investigation based on extended finite element method

Rock strengths are directly influenced by the open or closed flaws widely distributed in rock masses. Extensive studies have been conducted on the propagations of open flaws in rocks. However, few concerns are paid on the propagation of closed flaws, the influence of the surface friction on the initiation and propagation of closed flaws should be investigated systematically. In present article, the crack initiation and propagation in rock like material subjected to compressive loads have been investigated. The effects of crack surface friction on crack initiation and propagation have been quantified with the help from extended finite element method which is efficient and accurate. Based on the analysis on stress distribution and propagation patterns, following results are obtained: Firstly, minor effects are exerted by crack surface friction on the stress distribution around the flaws when the flaws inclination angle is 30° and 45°. However, as the inclination angle increases to 60°, the effects are much more significant. Secondly, as the inclination angle ranges from 30° to 60°, the most favorable angle for crack propagation is 45°. Thirdly, the initiation location and angle of the wing cracks will not be influenced by the frictions. However, the propagation length will be greatly influenced by the friction and the inclination angle.     

无锡光明村地裂缝研究

根据差异性地面沉降是引发地裂缝的基本原理对光明村地裂缝进行了实证.通过对地质构造和水文地质条件分析比较,认为该地裂缝与江阴南部地裂缝同属一条地裂缝带,在发生的时空关系上具有高相关性.地下水超采是产生地裂缝的主要原因,光明村与地下水位埋深超过40 m的河塘镇存在密切的水力联系.近期的实测数据表明,光明村地裂缝仍处于活动中,最大差异沉降量达5 mm/a.光明村地裂缝的稳定与否主要取决于江阴南部水文地质条件的改善程度.     

Rotation of porphyroblasts in non-coaxial deformation: insights from computer simulations

Porphyroblast inclusion trails have the potential to provide critical information about tectonometamorphic events. Recently, however, traditional interpretations of inclusion trails have been called into question by the suggestions that porphyroblasts do not rotate during non-coaxial deformation and that apparent spiral inclusion trails can be generated in coaxial deformation. We present a new computer model that simulates inclusion trail development. Model results suggest: (1) that the extent of porphyroblast rotation is controlled by conditions at the porphyroblast-matrix boundary; (2) that curved inclusion trails may develop in unrotated porphyroblasts; (3) that classic "snowball" inclusion trails are most simply explained by rotational growth histories; and (4) that some of the observations used to support the view that porphyroblasts do not rotate (e.g. weakly sigmoidal inclusion trails, apparent truncations of inclusion trails) can be accounted for by variations in the growth rate of rotating porphyroblasts.     

Diopside-bearing EL6 EET 90102: insights from rare earth element distributions

EET 90102 is the first known diopside-bearing EL6 chondrite. Diopside occurs in most aubrites and is occasionally found as rare small grains in unequilibrated enstatite chondrites, but is unknown from equilibrated enstatite chondrites. We have carried out a study of the rare earth element (REE) distributions in EET 90102, with a specific emphasis on diopside, in order to better understand its origin in this meteorite. We also present data for Ca-rich pyroxenes from two unequilibrated (EH3) enstatite chondrites for comparison.Our data show that diopside and other silicates in EET 90102 exhibit volatility-related anomalies indicative of formation under highly reducing conditions. Such anomalies have not previously been observed in EL6 chondrites, although they are common in unequilibrated enstatite chondrites. Diopside in EET 90102 probably formed by metamorphic equilibration of enstatite and oldhamite. The REE compositions of some grains, in particular the presence of positive Yb anomalies, indicate that they inherited their REE characteristics largely from CaS. Other grains have REE patterns that are more consistent with a derivation of diopside primarily from enstatite.In contrast to other EL6 chondrites, which experienced slow cooling, EET 90102 was quenched from high metamorphic temperatures. Thus, there may have been insufficient time to completely homogenize diopside REE compositions.The presence of diopside in EET 90102 simplifies one outstanding problem of aubrite formation. Melting of a diopside-bearing enstatite chondrite protolith provides a source for the abundant diopside in aubrites without requiring the oxidation of oldhamite, as suggested by previous research.     

关于地裂缝地质灾害的优势面分析

地裂缝的成因可以分为天然构造地裂、人为诱发地裂和复合成因地裂、地震断裂,抽水不均匀地面沉降伴生地裂和渗透变形地裂是最主要的地裂缝类型。地裂缝以构造地裂和构造地裂基础上由人为活动诱发现象为主,用优势面观点和方法分析和研究是可行的。根据优势面的原理,提出防灾和减灾的思路,指出地裂灾害预测和预警系统研究对分析、评估和减灾是最有效的手段。防治地裂缝的原则是针对性治理和顺应自然取代对抗,对策是注重避让与选址、引调地表水、做好结构设计和地基处理、制定相应的防灾区域管理法规。     

火山与深空深地深海探测密切相关

在已知的太阳系行星及其卫星中几乎都发现有火山。火山作用贯穿于地球及其他星球的整个时空，在它们形成演化中扮演重要角色，是星球的灵魂。地球系统科学和深空、深地、深海探测是当今科技发展战略的重要组成部分和世界科技竞争的制高点，这些重大基础理论和战略需求的研究都离不开火山，无论是上天、入地、下海都要面对火山。火山是探索深地、深海的窗口，是通往宇宙的桥梁。目前人类能探知的地下深度仅约12 km，且只有一个点，而火山却能把数十千米，乃至几百千米深的物质带到地表，覆盖了地球和其他星球的大部分表面，这些火山产物是反演星球内部物质组成、结构和动力过程的重要抓手，对揭示行星差异演化和运行规律具有重要意义。此外，火山作用对气候、环境、资源、生物等都会产生重大影响，是构建地球系统科学的天然纽带，加强火山研究是大科学需要，势在必行。     

Controls on solid-phase inclusion during porphyroblast growth: insights from the Barrovian sequence (Scottish Dalradian)

A series of Barrovian sequence samples ranging from garnet to sillimanite zones were investigated to infer their porphyroblast-forming reactions and mineral inclusion histories. Quartz is overgrown and partly consumed during garnet formation and remains as inclusion-rich layers in porphyroblasts of the garnet zone. Staurolite crystals in the staurolite zone display sharp transitions between inclusion-rich and inclusion-free areas, suggesting two stages of growth with a different role of quartz in each. The inclusion-rich domains formed similarly to those in garnet by simple overgrowth and resorption of matrix minerals, with thermodynamic constraints suggesting that this staurolite-forming reaction required the presence of chloritoid that is now absent from the examined samples. The participation of garnet was limited in staurolite formation, with chloritoid breakdown supplying sufficient material to form the large amounts (c. 25 vol%) of staurolite found in the rock. This reaction produces an excess of SiO₂, which leaves the crystal domain as SiO_2aq and thus caused the formation of the inclusion-free areas in the staurolite and precipitation of quartz in the matrix. In the sillimanite zone, staurolite is consumed forming new garnet. The newly formed garnet has less quartz inclusions than its core due to a proportionally greater consumption of quartz by the second garnet-forming reaction than by the initial, garnet-grade reactions. Textural and thermodynamic data both suggest that inclusions in these porphyroblasts represent leftovers of a preferentially overgrown matrix than co-products of the porphyroblast-forming reaction.     

基于GA-ANN的苏锡常地裂缝危险性评价

文章以苏锡常地区地裂缝危险性评价为例,利用遗传算法(GA)对人工神经网络(ANN)进行改进,先用GA优化BP网络初始权重,再用BP算法修改网络权重,实现不同尺度的同步调整。选择30点的不同地质条件组成样本对所建模型进行训练,评价指标包括:基岩埋深、基岩起伏度、地下水位、地面沉降梯度、含水层导水系数和粘性土层厚度。经过500次GA迭代,得到苏锡常地区地裂缝ANN模型的最佳权重组合,该耦合模型能对全区地裂缝地质条件进行正确分类,精度接近1‰。     

Controls on the geometry of tails around rigid circular inclusions: insights from analogue modelling in simple shear

We used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems. Results show that: (1) σ inclusions can develop in both slipping and non-slipping modes, but δ-inclusions only form in the latter. (2) σ inclusions develop when the mantle production rate is constant and the mantle is transected by the separatrix. δ inclusions form when the mantle is initially outside the separatrix and later comes inside this line. (3) In the slipping mode, the wedge-shaped tail of σ-inclusions always has a straight external surface parallel to the shear plane, whereas in the non-slipping mode the external surface is curved inwards (external embayments). (4) Together with earlier theoretical results, σ- and δ inclusions always show stair-stepping of tails when embedded in a viscous matrix under homogeneous simple shear deformation. (5) Maximum stair-stepping occurs in the slipping mode and is at least equal to the inclusion diameter. If our models bear significant similarity to nature, then (i) the straight or curved character of σ-inclusions could mean that they had, respectively, a slipping or non-slipping interface with the surrounding recrystallized matrix, and (ii) δ-inclusions may result from shear deformation under retrogressive metamorphic conditions in thrust systems.     

黑方台黄土泥流滑坡及发生机制研究

黑方台因农业灌溉触发了大量泥流滑坡。为了充分反映黑方台黄土泥流滑坡发育特征,首先基于IKONOS影像对黑方台泥流滑坡进行了遥感解译,随后又开展了详细野外调查工作。结果表明,黄土泥流滑坡呈流态化运移特征,滑距可达300 m以上,是黑方台危害最为严重的一类滑坡。钻探和塬边露头测量显示,研究区黄土底部黏土高程西高东低。地下水数值模拟结果显示,研究区地下水总体上由台塬西部流向地势较低的东部台缘,使得该处地下水抬升幅度要高于西部,从而造成泥流滑坡集中分布于东部台缘。为充分反映该条件下黄土泥流滑坡的破坏机制,开展了室内饱和原状黄土应力路径试验,分析研究了黄土泥流滑坡液化机制     

Three-dimensional numerical study on the failure characteristics of intermittent fissures under compressive-shear loads

A 3-D conjugated bond-pair-based peridynamic model is developed to comprehensively investigate failure characteristics of rock-like materials with intermittent fissures in the compressive-shear loading tests. Rock-like specimens containing one single central fissure are first simulated. Numerical results indicate that the 3-D conjugated bond-pair-based peridynamic model can faithfully reproduce failure characteristics of rock-like materials under compressive-shear loads. Then, the failure characteristics of rock-like specimens containing two parallel central intermittent fissures are numerically investigated. Effects of fissure inclination angle, fissure ligament length and rock bridge angle on fracturing behaviors, such as crack coalescence patterns, are also studied as well as crack initiation stress and coalescence stress.

     

Stress transfer in earthquakes, hazard estimation and ensemble forecasting: Inferences from numerical simulations

Observations indicate that earthquake faults occur in topologically complex, multi-scale networks driven by plate tectonic forces. We present realistic numerical simulations, involving data-mining, pattern recognition, theoretical analyses and ensemble forecasting techniques, to understand how the observable space–time earthquake patterns are related to the fundamentally inaccessible and unobservable dynamics. Numerical simulations can also help us to understand how the different scales involved in earthquake physics interact and influence the resulting dynamics. Our simulations indicate that elastic interactions (stress transfer) combined with the nonlinearity in the frictional failure threshold law lead to the self-organization of the statistical dynamics, producing 1) statistical distributions for magnitudes and frequencies of earthquakes that have characteristics similar to those possessed by the Gutenberg–Richter magnitude–frequency distributions observed in nature; and 2) clear examples of stress transfer among fault activity described by stress shadows, in which an earthquake on one group of faults reduces the Coulomb failure stress on other faults, thereby delaying activity on those faults. In this paper, we describe the current state of modeling and simulation efforts for Virtual California, a model for all the major active strike slip faults in California. Noting that the Working Group on California Earthquake Probabilities (WGCEP) uses statistical distributions to produce earthquake forecast probabilities, we demonstrate that Virtual California provides a powerful tool for testing the applicability and reliability of the WGCEP statistical methods. Furthermore, we show how the simulations can be used to develop statistical earthquake forecasting techniques that are complementary to the methods used by the WGCEP, but improve upon those methods in a number of important ways. In doing so, we distinguish between the “official” forecasts of the WGCEP, and the “research-quality” forecasts that we discuss here. Finally, we provide a brief discussion of future problems and issues related to the development of ensemble earthquake hazard estimation and forecasting techniques.     

Strike-slip fault bridge fluid pumping mechanism: insights from field-based palaeostress analysis and numerical modelling

We present a finite-element study of stress perturbation in evolving compressive and extensional strike-slip fault bridges. The results are compared with a fracture study of a compressive bridge at St Donats, UK. Horizontally interbedded calcareous mudstone and bioclastic calcilutite at St Donats have a distinct vertical permeability anisotropy. This sedimentary sequence behaves as a set of horizontal aquifers. The fluid flow in these aquifers is sensitive to mean stress gradients. Paleostress analysis of field fracture data, verified by finite-element modelling, indicates a rotation of σ₁ towards parallelism with boundary faults inside the growing compressive bridge. Boundary faults and bridge faults recorded numerous fluid flow events. The modelled mean stress pattern shows a regional maximum within the bridge and local maxima/minima pairs at boundary fault tips.Finite-element modelling of an extensional bridge indicates that σ₃ rotates towards parallelism with boundary faults. The mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged. The stress patterns are reestablished by each stress build-up preceding the rupturation of the boundary faults throughout the development stages of strike-slip fault bridges. Mean stress gradients developed pre-failure control the fluid flow in fractures of the strike-slip fault system at and after the end of each stress build-up and the fluid flow in boundary faults post-failure. Fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge. Rupture and sealing of the main bounding-faults is a step-wise process that opens and closes fluid conduits between areas with different pressures.     

Fracture initiation, growth and effect on stress field: a numerical investigation

A numerical investigation was made of the relationships between fracture initiation, growth, stress field and boundary conditions. Two-dimensional plane strain continuum models were used in which fractures appeared as zones of strain localization developed through application of a strain softening Mohr–Coulomb constitutive model. R and R′ fractures developed first, followed by Y fractures at larger strains. The models showed that equal development of conjugate R and R′ fractures is easily changed to favor one or the other set by minor variations in model initial conditions. Strength loss in fractures caused stress field rotations in regions bounded by fractures, altering the orientation of subsequent fractures. The amount and sense of stress field rotation is dependent on the strength loss during displacement on the fractures, the orientation of fractures, and on the boundary conditions. Y oriented fractures could be explained on the basis of a Mohr–Coulomb failure criterion provided that stress field rotation is accounted for. Monitoring of fracture slip activity showed that, under conditions of constant boundary velocity, slip was discontinuous in time, alternating on fractures throughout the model.     

Earth fissures in Jiangsu Province,China and geological investigation of Hetang earth fissure

Earth fissures are a geohazard in Jiangsu Province, China. They can be caused by earthquakes and active faults, underground mining, groundwater extraction and landslides. In order to establish a provincial rehabilitation plan in Jiangsu, a range of monitoring programs, surveys, geological investigations and modeling have been implemented or planned. One of the focuses of the project is the land subsidence and earth fissures caused by excessive groundwater withdrawal in Suzhou, Wuxi and Changzhou (Su-Xi-Chang) area, southern Jiangsu Province. Hetang earth fissure within the Su-Xi-Chang area was first reported in 1995 and a series of investigation has been conducted since then. The site investigations and geophysical survey in 1997 have recognized the causative factor as the excessive groundwater drawdown coupled by the underlying bedrock ridge. An open trench excavation in 2007 and a plane strain analysis suggest that Hetang earth fissures may have cracked from the bedrock ridge to ground surface. Geological drilling in 2007 has further confirmed the existence and configuration of the ridge and extracted soil samples for laboratory tests to obtain soil parameters for numerical analyses and modeling of land subsidence and earth fissures in the Su-Xi-Chang area, Jiangsu, China. The laboratory tests are currently in progress and the result of numerical analyses and modeling is expected to be presented in the near future.