Quantitative Parameters for Rock Joint Surface Roughness

Summary The morphologies of two artificial granite joints (sanded and hammered surfaces), one artificial regularly undulated joint and one natural schist joint, were studied. The sanded and hammered granite joints underwent 5 cycles of direct shear under 3 normal stress levels ranging between 0.3–4 MPa. The regularly undulated joint underwent 10 cycles of shear under 6 normal stress levels ranging between 0.5–5 MPa and the natural schist replicas underwent a monotonous shear under 5 normal stress levels ranging between 0.4–2.4 MPa. In order to characterize the morphology of the sheared joints, a laser sensor profilometer was used to perform surface data measurements prior to and after each shear test. Rather than describing the morphology of the joints from the single profiles, our characterization is based on a simultaneous analysis of all the surface profiles. Roughness was viewed as a combination of a primary roughness and a secondary roughness. The surface angularity was quantified by defining its three-dimensional mean angle, θ_s, and the parameter Z2_s. The surface anisotropy and the secondary roughness were respectively quantified by the degree of apparent anisotropy, k _a, and the surface relative roughness coefficient, R _s. The surface sinuosity was quantified by the surface tortuosity coefficient, T _s.  Comparison between the means of the classical linear parameters and those proposed shows that linear parameters underestimate the morphological characteristics of the joint surfaces. As a result, the proposed bi-dimensional and tri-dimensional parameters better describe the evolution of the joints initial roughness during the course of shearing.     

A New Method for In-situ Non-contact Roughness Measurement of Large Rock Fracture Surfaces

Summary This paper presents a new method for in-situ non-contact measurements of fracture roughness by using a total station (TS). The TS is a non-reflector geodetic instrument usually used for measuring control points in surveying and mapping. By using a special-developed program, the TS can be used as a point-sensor laser scanner to scan a defined area of the fracture surface automatically, in field or in laboratory, at a distance away from the target surface. A large fracture surface can be automatically scanned with a constant interval of the sampling points, both within a defined area or along a cross-section of the exposed rock face. To quantify fracture roughness at different scales and obtain different densities of the scanned points, the point interval can be selected with the minimum interval of 1 mm. A local Cartesian co-ordinate system needs to be established first by the TS in front of the target rock face to define the true North or link the measurements to a known spatial co-ordinate system for both quantitative and spatial analysis of fracture roughness. To validate the method, fracture roughness data recorded with a non-reflector TS was compared with the data captured by a high-accuracy 3D-laser scanner. Results of this study revealed that both primary roughness and waviness of fracture surfaces can be quantified by the TS in the same accuracy level as that of the high accuracy laser scanner. Roughness of a natural fracture surface can be sampled without physical contact in a maximum distance of tens of meters from the rock faces. Received May 24, 2001; accepted July 24, 2002; Published online November 19, 2002     

Fracture Toughness and Fracture Roughness in Anisotropic Granitic Rocks

In this paper we present an experimental approach aimed at assessing the correlation between fracture toughness (K _IC) and fracture roughness of two granitic rocks (Barre and Stanstead granites) exhibiting significant fracture toughness anisotropy. Roughness values have been estimated for fractured surfaces obtained from Chevron Cracked Notch Brazilian Disc samples failed under mode I along three orthogonal planes with respect to their microstructural fabrics. There exists a clear correlation between roughness and toughness within each rock examined along the three planes. Specific orientation of micro-crack alignment could result in preferred out-of-plane propagation of the test-crack irrespective of grain-size distribution. These experimental observations reinforce the hypothesis of the existence of a link among pre-existing petrofabric anisotropy, fracture toughness, fracture roughness, and the evolution and extent of the associated induced fractures within the process zone of granitic rocks along specific directions. This study also highlights the need for employment of pre-failure and advanced post-failure diagnostic techniques in quantifying these inter-relationships.     

青海木里三露天天然气水合物钻孔岩心构造裂隙特征

通过对三露天井田天然气水合物钻孔岩心观察与统计分析,从线密度、倾角、充填程度等方面对裂隙的特征进行讨论,明确研究区天然气水合物赋存类型以及构造裂隙的基本特征,最后讨论了裂隙对天然气水合物形成的作用。结果表明：井田内 天然气水合物绝大多数赋存于粉砂岩、暗色泥岩、油页岩中,其中62.5%赋存于构造裂隙中;研究区构造裂隙以低倾角(占50.37%)、未被充填裂隙为主(占73.68%),且水合物上覆地层裂隙 密度均不同程度减小。研究区裂隙的发育特征对天然气水合物成藏起到重要的作用：研究区73.68%的裂隙为未充填,为烃类气体的运移和聚集提供了空间;50.37%的裂隙为低倾角,垂直方 向上的渗透率值较小,不同层位的连通性差,极大地限制了烃类气体向上扩散,使其得以聚集保存;最后,裂隙发育程度的降低,同样抑制了烃类气体向上扩散,起到了相对“盖层”的作用 。     

Quantification of Aperture and Relations Between Aperture,Normal Stress and Fluid Flow for Natural Single Rock Fractures

Accurate quantification of rock fracture aperture is important in investigating hydro-mechanical properties of rock fractures. Liquefied wood’s metal was used successfully to determine the spatial distribution of aperture with normal stress for natural single rock fractures. A modified 3D box counting method is developed and applied to quantify the spatial variation of rock fracture aperture with normal stress. New functional relations are developed for the following list: (a) Aperture fractal dimension versus effective normal stress; (b) Aperture fractal dimension versus mean aperture; (c) Fluid flow rate per unit hydraulic gradient per unit width versus mean aperture; (d) Fluid flow rate per unit hydraulic gradient per unit width versus aperture fractal dimension. The aperture fractal dimension was found to be a better parameter than mean aperture to correlate to fluid flow rate of natural single rock fractures. A highly refined variogram technique is used to investigate possible existence of aperture anisotropy. It was observed that the scale dependent fractal parameter, K _v, plays a more prominent role than the fractal dimension, D _a1d, on determining the anisotropy pattern of aperture data. A combined factor that represents both D _a1d and K _v, D _a1d × K _v, is suggested to capture the aperture anisotropy.     

岩体裂隙等效水力隙宽的统计确定方法

针对天然裂隙岩体等效水力隙宽确定的复杂性,利用平行板水流的立方定律及流量等效原理,推导出光滑平行板单裂隙的等效水力隙宽公式,同时对隙宽分布的频率特性进行了探讨,认为非均匀隙宽单裂隙的等效水力隙宽可近似取40%频率的隙宽,或取隙宽的几何平均值与调和平均值的算术平均值.     

Asperity Height and Aperture of an Artificial Tensile Fracture of Metric Size

Summary  A tensile fracture of about 1 m in length was created by indenting wedges in a block of granite, and the heights of the two fracture surfaces were measured using a large, non-contact surface profile measurement system with a laser profilometer to determine the aperture distribution of the fracture. Based on the measured data, the frequency characteristics of the asperity heights, the initial aperture (the aperture when the surfaces are in contact at a single point), and the size effect on the statistical properties were analyzed. The results can be summarized as follows:

Size and Geometry Effects on Rock Fracture Toughness: Mode I Fracture

In this paper, the effects of specimen size and geometry on the apparent mode I fracture toughness (K _c) of an Iranian white marble (Neyriz) are studied. A number of fracture tests were conducted on center-cracked circular disk (CCCD) specimens with different radii to investigate the size effects on K _c. The experimental results demonstrate that the apparent fracture toughness increases in bigger specimens. In order to explain the experimental results, the modified maximum tangential stress (MMTS) criterion is used, where higher order terms of the Williams’ series expansion are included in the maximum tangential stress criterion. It is shown that the MMTS criterion provides good estimates for the apparent fracture toughness of Neyriz marble, obtained from fracture tests of edge-cracked triangular specimens. It is, therefore, concluded that the proposed criterion is able to account for the size and geometry effects on the fracture resistance of rocks simultaneously.     

An Investigation of Discontinuity Roughness Scale Dependency Using High-Resolution Surface Measurements

The influence of roughness on the hydro-mechanical behavior of rock discontinuities has long been recognized. As a result, several definitions and measures of roughness have been developed. According to the ISRM (Int J Rock Mech Min Sci Geomech Abstr 15(6):319–368, 1978), discontinuity roughness comprises large-scale (waviness) and small-scale (unevenness) components. However, the division between these scales is not clear and most investigations of surface roughness have been restricted to small fracture surfaces (<1 m²). Hence, the large-scale components of roughness are often neglected. Furthermore, these investigations typically define roughness using two-dimensional profiles rather than three-dimensional surfaces, which can lead to biased estimates of roughness. These limitations have led to some contradictory findings regarding roughness scale dependency (scale effects). This paper aims to provide some explanation of these contradictory findings. Through the in situ digitization and analysis of two adjacent large-scale (~2 × 3 m² and ~2 × 2 m²) migmatitic-gneiss fracture surfaces, the influence of sample size on roughness estimates are investigated. In addition, the influence of measurement resolution on roughness estimates is investigated by digitizing small-scale (100 × 100 mm²) samples from the same fracture with varying resolution. The findings show roughness to increase as a function of the sampling window size, in contrast to what is commonly assumed. That is, the combined waviness and unevenness of a discontinuity relative to its mean plane increases with scale. Compared to the sampling window size, the resolution of surface measurements is shown to have a far greater influence on roughness estimates. This influence of measurement resolution may explain some of the contradictory roughness scale relationships that have been published previously. It is important to note that the observed decrease in shear strength with increasing scale, as observed in many prior studies, is not being questioned; rather, a clarification of the role of roughness in this phenomenon is sought.     

Observing Deformation and Fracture of Rock with Speckle Patterns

The high resolution technique of electronic speckle pattern interferometry (ESPI) can be very useful in determining deformation of laboratory specimens and identifying initiation of failure. The in-plane ESPI theory is described and the fringe pattern of the processed ESPI image is analyzed to determine deformation and crack opening displacement. Fringes on the ESPI image represent lines of equal intensity, which relate to surface displacement. An ESPI system was constructed and calibrated for measuring uni-directional displacements. Several types of the experiments, such as uniaxial compression and fracture testing, were conducted to demonstrate the utility of ESPI.     

Development and Application of Rock Fracture Propagation Numerical Simulation System

Geotechnical and Geological Engineering - The mechanism of rock fracture evolution under loading has always been the focus of rock mechanics community. In this paper, to realize the development of...     

Statistical Aspects of Microheterogeneous Rock Fracture: Observations and Modeling

Rocks and other geomaterials are heterogeneous materials, with a well-recognized hierarchy of defects from micro-heterogeneities on the grain level to a large-scale network of cracks and layering structures. Their nature create a challenge for determining macroscopic properties, particularly for properties that are scale dependent, complicating both the property measurement and its appropriate application in modeling. This paper discusses the concept of a “representative volume”, which is commonly used in modeling microheterogeneous but statistically homogeneous material by an effective homogeneous continuum. The foundation of this concept is presented, along with its limitations in dealing with properties like strength and fracture toughness that exhibit a scale effect. This limitation is illustrated with a study of brittle fracture of a concrete where it is considered a model for statistically homogeneous rock. The study includes determining a scaling rule for the scale effect in fracture toughness, and shows that the fracture of brittle materials like rocks and concrete appears in the form of highly tortuous, stochastic paths. This reflects a complex interaction between a crack and pre-existing as well as newly formed micro-defects controlled by chance, and results in a large scatter of all fracture-related parameters. This behavior suggests a synthesis of fracture mechanics with probability and statistics, and so a brief exposition of statistical fracture mechanics (SFM) that addresses the statistical aspects of fracture is also presented. SFM is a formalism that combines fracture mechanics methods with probability theory and serves as the basis for an adequate modeling of brittle fracture.     

基岩裂隙渗透性的野外研究

本文着重探讨了基岩裂隙的野外测量及渗透张量的计算方法，并给出了作者在湖南辰溪煤矿的实测计算结果及在指导采煤中的应用。     

岩石断裂的研究现状浅析

经典断裂力学理论的研究已经取得很大的进展,然而由于岩石材料的性质,该理论在岩石材料研究方面遇到了局限,岩石断裂研究具有其特殊性。在参考相关文献的基础上,简要介绍了常用的岩石断裂准则,以及岩石断裂研究新情况。