The influence of rock anisotropy on stress measurements by overcoring techniques

SummaryThe Influence of Rock Anisotropy on Stress Measurements by Overcoring Techniques A medium is anisotropic if its properties vary with direction. This is the general characteristic of many rocks, for example, schists, slates, gneisses, phyllites and other metamorphic rocks. Bedded and regularly jointed rocks also display anisotropic behavior.This paper is concerned with the influence of rock anisotropy on in-situ stress measurements. It is limited, to stress measurements by overcoring techniques for which strains and displacements are recorded either on the walls of a pilot hole at the end of one or several boreholes or within instrumented solid or hollow inclusions perfectly bonded to the surface of the pilot hole. The rock is described as homogeneous, continuous, anisotropic and linearly elastic.The following questions are answered with special emphasis on rocks that can be classed as transversely isotropic or orthotropic: the number of independent measurements obtainable in a single borehole; the number of boreholes required to determine the in-situ stress field; the influence of rock anisotropy on these numbers; the influence of the anisotropy type and the error involved by neglecting rock anisotropy.With 5 Figures     

Improvement in the interpretation of stress measurements by use of the overcoring method: development of a new approach

Determination of the state of the initial stresses in a rock formation is still one of the most important and complex problems in geotechnics. The over-coring method of measuring the stresses is one of the measurement methods most frequently used in geotechnics. It has been shown to be highly suitable for measuring the stresses in “hard and sound” rocks. However, applied to rocks which have a partialy elastic behaviour the method comes up against many problems the biggest of which are: (1) the non-elastic behaviour of the rock; and (2) the presence of natural fractures or those caused by the overcoring operation. These problems make it impossible to obtain correct deformation measurements which can be interpreted in terms of stresses. To overcome these problems, a new method has been developed based on the use of the deformations recorded at the start of the overcoring tests this method is the subject of the present article.     

Stress concentration factors for the flat borehole end for use in rock stress measurements

Uniaxial loading tests on blocks and cylinders of different materials, as well as a photoelastic model study, are described. From these tests the stress concentration factors around the flat end of a borehole, drilled into each block or model, are determined. It is shown that one of the stress concentration factors is related to Poisson's ratio of the material in which the borehole is drilled.     

岩石空心圆柱扭剪试验系统研制

针对深部工程岩体开挖时围岩内部应力状态复杂变化的特点,研制了可实现主应力量值变化和应力主轴旋转等复杂应力路径的岩石空心圆柱扭剪试验系统,可用于开展复杂应力作用下岩石的强度、变形和破坏形态的室内试验研究。介绍了该试验系统的基本结构、工作原理及技术参数。该试验系统具有如下特点：（1）通过利用空心圆柱试样与上、下压头的特点,克服了岩石试样扭矩施加的技术困难,将施加轴向力和扭矩的两个加载轴合二为一,彻底解决了轴向力加载与扭矩施加机构之间相互影响的技术难题。（2）通过采用新型自平衡加载三轴室,实现了轴向力、扭矩、内围压和外围压4种荷载的独立施加和控制。（3）采用应力与流量的加载控制方式,并通过软件集成,实现了主应力量值改变和应力主轴旋转耦合应力路径的准确模拟以及加载过程的稳定控制。（4）该试验系统结构简单,功能多样,操作方便。采用该试验系统进行了初步试验,验证了试验系统的可行性与实用性,表明该试验系统具有实现应力主轴旋转及主应力量值改变的复杂应力路径的功能。     

岩石空心圆柱扭剪仪试验能力

针对自行研制的岩石空心圆柱扭剪仪可独立控制4个加载参数（轴力、扭力、内围压和外围压）的功能,通过数学和力学分析,系统地整理出几种易于实现且符合实际工程的应力路径及加载方式：在轴力与内围压满足一定关系的前提下,可以获取岩石的抗拉强度,提供一种新型测量岩石抗拉强度的方法;通过控制内外围压及轴力,可以进行常规三轴试验与真三轴试验,克服了现阶段岩石真三轴试验中试验装置复杂、试样加载面摩擦大的缺点;在轴力与内、外围压分别满足一定关系时,可以分别实现平均应力p不变与中主应力系数b不变的应力路径,用于研究应力主轴旋转对岩石力学性质的影响。上述应力路径的实现对岩石力学性质的研究以及现阶段岩石室内试验的发展具有重要意义。     

不同压裂介质影响下绝对应力测值的试验研究

为研究不同压裂介质影响下的绝对应力测值,利用大尺寸真三轴模拟水压致裂试验系统,用清水及密度分别为1.1、1.2、1.3、1.4 g/cm3的泥浆介质对400 mm×400 mm×400 mm的花岗岩试样进行了水压致裂室内试验,成功获取了连续的压力-时间曲线。利用6种方法识别压力-时间曲线的闭合压力点,与实加最小水平主应力（ 5 MPa）对比,得出单切线等4种方法较为符合实际情况;在考虑试验系统柔度影响前提下,对比了实测和理论重张压力;最后分析了在持续泵压作用下岩石裂缝延伸的压力。试验结果表明,密度为1.1、1.2 g/cm3的泥浆介质对压力特征参数的取值影响较小,误差不超过1 MPa,但密度为1.3、1.4 g/cm3的泥浆介质对特征参数值影响很大。试验结果对水压致裂地应力测量技术在不同压裂介质影响下的适用性有重要的指导意义。     

基于湿度应力场理论的膨胀岩弹塑性本构关系

由于含水率的变化使膨胀岩的弹性模量、泊松比和屈服极限等力学性能都将发生变化,从而引起膨胀应力、塑性流动和随湿度场而变化的屈服准则等都相互耦合在一起,因此,提出了基于湿度应力场理论的膨胀岩的弹塑性本构模型,并运用参变量变分原理建立了处理这一类问题的数值变分原理及其相应的有限元形式。     

Controlled failure of hollow rock cylinders in uniaxial compression

SummaryControlled Failure of Hollow Rock Cylinders in Uniaxial Compression Detailed consideration is given to the behavior of hollow rock cylinders loaded in uniaxial compression. Elastic stress distributions calculated by the finite element method show that radial stresses are considerably lower in hollow than in solid cylinders, and that the uniformity of the stresses can be improved considerably by using loading platens having the same cross-section as the specimen. Servo-controlled uniaxial compression tests carried out on solid and thick-walled cylinders of white Tennessee marble show no essential differences in the behavior of the two specimen types with similar strengths and fracture phenomena being observed. In tests carried out in servo-controlled, stiff, and conventional machines, the progressive formation of large numbers of short subaxial cracks is followed by the development of macrofractures such as slabbing and shearing well past the peak of the stress-strain curve.

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ZusammenfassungDer gesteuerte Bruch hohler Gesteinszylinder unter einachsigem Druck Das Verhalten hohler Gesteinszylinder unter einachsigem Druck ist eingehend beschrieben. Die Methode der endlichen Elemente ermöglicht die Berechnung der elastischen Spannungsverteilung und zeigt, daß die radialen Spannungen im Hohlzylinder bedeutend geringer sind als im massiven. Der Gebrauch von Druckplatten vom gleichen Querschnitt wie dem der Probe verbessert die Gleichförmigkeit der Spannungen wesentlich.Servo-gesteuerte einachsige Druckversuche an dichtwandigen hohlen und an massiven Proben von weißem Tennessee-Marmor lassen keine wesentlichen Unterschiede zwischen dem Verhalten der beiden Probenarten erkennen. Ähnliche Festigkeiten und Brucherscheinungen sind im hohlen und massiven Zylinder beobachtet worden. In Versuchen mit servo-gesteuerten starren und in konventionellen Maschinen folgt der progressiven Bildung einer großen Anzahl kurzer, etwa axialer Risse die Entstehung von größeren Brucherscheinungen, wie z. B. Abschalungen und Scherbrüche, und zwar deutlich nach dem Scheitelpunkt der Arbeitslinie.

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RésuméFracturation contrôlée de cylindres rocheux creux en compression uniaxiale Une étude détaillée du comportement de cylindres rocheux creux soumis à une compression uniaxiale est faite ici. La répartition des contraintes en milieu élastique déterminée par la méthode des éléments finis a montré que les contraintes radiales étaient beaucoup plus petites dans les cylindres creux que dans les cylindres pleins et que l'uniformité des contraintes pouvait être considérablement améliorée en utilisant des platines de mise en charge ayant la même section que l'éprouvette.Des essais de compression simple effectués avec une presse asservie sur des cylindres de marbre du Tennessee soit pleins soit à paroi épaisse ne montrent aucune différence notable quand au comportement des deux types d'éprouvettes. La résistance à la rupture et les phénomènes de fracturation se sont avérés comparables dans les deux cas. Au cours des essais effectués avec une presse raide asservie et une presse classique, la formation progressive d'un grand nombre de courtes fissures sub axiales est suivie par le développement de macrofractures telles qu'écaillage et cisaillements une fois dépassé le pic de la courbe effort déformation.

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With 11 Figures     

基于损伤理论的圆形巷道围岩应力场分析

由于巷道掘进破坏了原岩地应力场,使围岩应力重新分布,应力集中区的高应力造成巷道变形破坏.特别是布置在高地应力区的巷道变形破坏更加严重.因此必须合理准确地计算巷道围岩应力场,为巷道变形破坏计算及稳定性分析提供理论依据.岩体内部存在微裂纹,其发生、扩展、并合,决定了岩石材料的宏观力学性能,因此巷道围岩应力场的计算应该考虑岩石材料的损伤特性.本文基于损伤理论计算得到的不同地应力情况下的圆形巷道围岩应力场,计算结果与实际情况符合得较好.     

Tectonic stresses on the boundary of the Aegean domain: “in situ” measurements by overcoring

Direct “in situ” measurements of the horizontal stresses on the border of the Aegean domain, obtained by overcoring in boreholes a few meters deep, revealed the various characteristics of plate boundaries: continent—continent collision, subduction, inter-arc basin, etc.     

岩体结构均质区划分法的适用性研究

岩体结构均质区划分是结构面统计与建模的基础。实践中以主控要素作指标、以最优划法为依据,更易获客观真实划分结果。针对国内核废料处置库北山预选区岩体结构面,开展了测窗范围截取、取样偏差及块网变异修正前处理,编程实现了多种方法下的均质区划分,初步探究各法在北山的判别力、适用性、局限性。结果表明：针对北山预选区而言,关联系数法和Mahtab法适用性较弱,Miller法最强;而Miller法中又以“34格大面积块”的划分效果最佳。将此最优划法应用至北山芨芨槽块段,希望能给结构面网络建模及后续渗流计算提供有益参考。     

Contribution on methodology in determination of rock pressure from inclusions in quartz

An investigation of the temperatures and pressures at which carbon dioxide undergoes partial homogenization turns out to be a much simpler indicator of the PTV environment in which crystallization occurred than the photographic geometricmethods used previously. All differences between values obtained by various methods turn out to be less than the allowable random error. — M.E. Burgunker     

Local measurements of the polar deformation response in a hollow cylinder apparatus

This paper describes the set-up, calibration and performance of the local instrumentation that measures the deformation response of the test-specimen in a new hollow cylinder apparatus (HCA). Customised, single- and double-axis inclinometers measure the axial and twist deformations of the outer specimen wall surface. Proximity transducers in the cell chamber and the specimen bore cavity measure the radial wall‐surface displacements and precision gearing allows the relocation of the transducers from outside the pressure-cell. The inclinometers were modified (reduced gauge of 45 mm) to suit the smaller-sized test specimen and the more compact pressure cell of the HCA. The instruments were calibrated using an optical table and laser distance-measurement system to achieve the necessary accuracy for strain measurements over the pseudo-elastic range. The calibration in water data for the proximity transducers was non-linear and strongly influenced by the specimen wall curvature. Equations are presented to compute the true torsional shear strain response from the measured inclinometer tilt angle, specimen dimensions and deformation response. External measurements include significant errors due to apparatus compliance, specimen end-restraint and bedding effects, inaccuracies in measuring volume changes and the fact that the deformations are calculated assuming that the entire specimen deforms as a right cylinder.     

岩石全应力-应变试验与岩爆理论研究

在高应力地区的地下工程中,岩爆现象日益受到重视.由于岩体自身的复杂性,岩爆的理论研究进展缓慢.用岩石刚性伺服试验机开展岩石力学试验,可得到完整的岩石全应力-应变曲线,对于揭示岩石强度、变形特征具有实际的意义,从而成为人们研究岩爆机理的一种首选方法.     

Plastic stress relaxation around solid inclusions in pyrope

Misfit between primary (syngenetic) inclusions and pyrope host is usually due to differences in the coefficients of thermal expansion and compressibilities. It is shown that the misfit stresses are relieved by the generation and rearrangement of dislocations. Zones of plastic flow have been recognized around inclusions in pyropes from Bohemian garnet peridotites and in kimberlite pyropes from South Africa and Yakutia. The extent of plastic yield is determined by the history of the pyrope in the ductile regime. Implications for using the piezothermometric method of Rosenfeld and Chase (1961) which is based upon the elastic strain, are discussed.Publications No. 38 in the Norwegian Geotraverse Project.     

Limits of stress measurements in the Earth's crust

Summary Limits of Stress Measurements in the Earth's Crust. Accurate measurements of stress in the Earth's crust cannot be performed at deep levels with currently available recording procedures. The maximum stress recordable is about 100MPa, and the maximum depth 1000–1200 m.

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Zusammenfassung Grenzen der Bestimmbarkeit der Spannungen in der Erdkruste. In großen Tiefen kann mit derzeit verfügbaren Untersuchungsmethoden die tatsächliche Spannung in der Erdkruste nicht bestimmt werden. Die größte meßbare Spannung beträgt ungefähr 100 MPa, die größte Tiefe 1000–1200 m.

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Résumé Limites de la possibilité de la détermination des contraintes dans l'écorce terrestre. Il est impossible de faire des déterminations exactes des contraintes dans l'écorce terrestre en utilisant les méthodes courantes. La contrainte la plus grande qui peut être mesurée est à peu près 100 MPa et la profondeur maximale 1000–1200 m.

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A lecture delivered at the Joint General Assemblies of the IASPEI and IAVCEI in Durham, August 1977.

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With 4 Figures     

The concept of residual stress in rock

The purpose of this paper is to clarify the concept of residual stress in rock. Residual stress is stress near a point in a body subjected to zero external tractions and to zero temperature gradients, excluding body forces. Thus, residual stresses can develop in rock if there are local phase transformations, inelastic strains, or differences in thermal or elastic properties. In these cases, residual stresses can result from changes in temperature, applied stress or configuration of the body.Analysis of residual stresses at the scale of mineral grains within a polycrystalline aggregate such as rock is virtually intractable. One can, however, obtain important insights into residual stresses within bodies with widely spaced sources of residual stress, such as inclusions, and within bodies comprised of multilayers. The analyses indicate that patterns of residual stress in rock can be expected to be extremely complicated. For example, study of residual stresses in a body containing a circular inclusion indicates that:

1. (1) There is a single state of residual stress within an inclusion but the state within the surrounding medium is variable. Thus, values of residual stress within rocks reported in the literature generally are of minor value because the sizes and shapes of the sources and the positions of the measurements relative to the positions of sources of residual stresses in the bodies have not been determined.

2. (2) Residual stresses within an inclusion can be tensile or compressive, even though the applied stresses were compressive, depending upon the source of residual stress.

3. (3) The magnitudes and orientations of residual stresses in an isolated body of rock containing one or more inclusions depends upon the size and shape of the body. The same general conclusions are derived from an analysis of residual stresses in a simple multilayered body.

4. (4) In addition, however, the anisotropy of a multilayered body tends to cause principal residual stresses to parallel the layers rather than to parallel the applied stresses that were responsible for inducing the residual stresses. Thus, without identifying the sources of residual stresses in a body, one cannot infer the directions of principal tectonic stresses that might have been responsible for the residual stresses.

Comparison of the theoretical results with measurements of change of residual stress in blocks of granite, with maximum dimensions of 2.5 m in the field and 0.2 m in the lab oratory, suggests that sources of residual stress are inhomogeneous elements or elements of inelastic deformation within the blocks that are smaller than the blocks themselves, but larger than individual mineral grains. The sources of residual stress are unknown in these granites.     

冲击地压矿井的围岩与支护统一吸能防冲理论

针对冲击地压过程的能量传递与耗散,基于块系围岩与支护系统动力模型,研究冲击扰动在岩体中传播时,围岩与支护特性对能量传递与耗散的影响,通过岩体和支护中的阻尼吸能作用对支护端岩块的动能变化进行分析。经分析表明:支护端岩块的冲击能量是可控的,当增大围岩中局部岩块间的阻尼作用时,支护端岩块的动能明显下降,有效地吸收了冲击能量,同时局部岩块间的阻尼分散作用在相邻两个块体间的吸能效果相比于集中作用时更好;当增大支护中的阻尼作用时,相比于同样的围岩中阻尼变化,其吸收冲击能量的效果较弱。当围岩与支护中的阻尼共同作用进行吸能防冲时,应以岩体主动吸能为主,支护被动吸能为辅进行统一吸能防冲,由此提出围岩与支护统一吸能防冲理论,为吸收冲击能量防御冲击地压动力灾害提供思路。     

Tectonophysical implications of rock stress determinations

Knowledge of spatial variations (concerning both magnitude and orientation) and temporal variations of the stress tensor at various points in the earth's crust is crucial to our understanding of the fundamental nature of tectogenesis, The means by which this data can be acquired forms the principal subject of a newly developed field of endeavor concerned with thein- situ measurement of stress. This field of research is expanding, with rapid advances in development of techniques, numbers of personnel, and acquisition of field data. Although primarily motivated by engineering and mining operations (because of the immediate applicability of this data to subsurface ground control), the geologic implications — thus far largely ignored — appear enormous. Determinations of stress have been made in every continent. The purpose of this paper is to emphasize the geological (and geophysical) relevance of current stress measurement work, which imposes conditions with which adequate theories of tectogenesis must comply.

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Zusammenfassung Die Kenntnis räumlicher Veränderungen (sowohl hinsichtlich der Größe wie der Richtung) und zeitlicher Veränderungen des stress tensor an verschiedenen Punkten der Erdkruste ist entscheidend für unser Verständnis der Grundzüge der Tektogenese. Die Mittel, mit deren Hilfe diese Daten gewonnen werden können, bilden den Hauptgegenstand eines neu entwickelten Tätigkeitsbereiches, der sich mitin situ-Messungen des stress befaßt. Mit dem schnellen Fortschritt der technischen Entwicklung, steigendem Personalstand und zunehmendem Anfall von Geländeergebnissen dehnt sich dieses Forschungsgebiet immer weiter aus. Obwohl zunächst Ingenieur- und Bergbautätigkeit (wegen der unmittelbaren Anwendbarkeit dieser Daten auf die Untertagekontrolle) Ausgangspunkt waren, erscheinen uns die — bisher weitgehend außer acht gelassenen — geologischen Folgerungen sehr erheblich. Stress-Bestimmungen werden auf jedem Kontinent angestellt. Der Zweck dieser Arbeit ist es, die geologische (und geophysikalische) Relevanz der gegenwärtigen Stress-Messungen hervorzuheben, mit deren Ergebnissen entsprechende Theorien der Tektogenese in Einklang stehen müssen.

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Résumé La connaissance des variations de l'espace (concernant aussi bien la magnitude que l'orientation) et des variations temporelles du «stress tensor» à des points différents de la croûte de la terre est décisive à notre entendement de la nature fondamentale de la tectogenèse. Les moyens par lesquels ces dates peuvent être obtenues, sont le sujet principal d'une sphère d'activité récemment développée, qui s'occupe des mesuragesin situ du stress. Ce domaine de recherches s'élargi avec le développement rapide des techniques, nombre du personnel, et acquisition des dates de champs. Bienque primairement motivé par des opérations ingénieures et minières (à cause de l'applicabilité immédiate de ces dates au contrôle du sous-sol), les conclusions géologiques — largement ignorées jusqu'à présent — apparaîssent énormes. Des déterminations de stress ont été exécutées dans tous les continents. Le but de ce travail est donc de souligner l'importance géologique (et géophysicale) des travaux de mesurage de stress en cours, imposant des conditions avec lesquelles des théories adéquates de tectogenèse doivent être mises d'accord.

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