John A. Hudson

ESTIMATION OF DISCONTINUITY SPACING AND TRACE LENGTH USING SCANLINE SURVEYS

Abstract Knowledge of the spacing and size of discontinuities in a rock mass is of considerable importance for the prediction of rock behaviour. The characteristics of discontinuities can be estimated using scanline surveys but the precision of the estimates must be obtained and the bias caused by linear sampling must be eliminated before they can validly be used. Initially, an expression is presented which gives the degree of confidence that can be assigned to the measured mean discontinuity spacing. A reduced form of this expression is obtained for cases where the discontinuity spacings follow the negative exponential distribution. The precision of discontinuity frequency and RQD estimates is also explained. The distribution of trace lengths produced by the intersection of planar discontinuities with a planar rock face is used to determine the distribution of trace lengths, the distribution of semi-trace lengths and the distribution of censored semi-trace lengths intersected by a randomly located scanline. Comparison of the actual and sampled distributions demonstrates the bias introduced by scanline sampling of trace lengths. Relations between the distributions can be used to produce analytical or graphical methods of estimating mean trace length from censored measurements at exposures of limited extent.

Numerical methods in rock mechanics

The purpose of this CivilZone review paper is to present the techniques, advances, problems and likely future development directions in numerical modelling for rock mechanics and rock engineering. ...

Discontinuities and rock mass geometry

Abstract Variation in discontinuity frequency as a function of scanline orientation in a plane is studied for rock masses containing sets of discontinuities. The spacing values between discontinuity intersection points that can occur along such scanlines are considered in order to develop a probability density distribution of block lengths. The ideas are extended to block area and volume distributions synthesized from the products of discontinuity spacing values along two and three axes respectively. Such distributions are also considered for rock masses were each discontinuity occurs with a random orientation. Histograms of discontinuity spacing and block area values compiled from measurements made on a variety of rock exposure from Pre-Cambrian to Jurassic are in general agreement with the theoretical distributions. The results are presented in dimensionless form and an explanation of the procedure for determining the scaling factors that allow computation of block area and volume distributions for a particular rock mass is included.

A comprehensive method of rock mass characterization for indicating natural slope instability

Abstract The purpose of this paper is to present an application of the Rock Engineering Systems (RES) methodology for developing a new rock mass classification, sensitive to large-scale instability in natural slopes and suitable for indicating critical sites. In the area under study, located in the Italian Central Alps, and in most natural slopes, analysis is complicated by lack of data, geological complexity, scale of the instability phenomena and the high number of interacting factors. In order to be able to have a structured approach to such complexity, a comprehensive method based on the RES approach was adopted. This is an objective-based approach which allows the utilization of all the information related to the project, tailoring the classification to the actual circumstances. Hence those parameters which are particularly active at the site can be evaluated and the importance of their interactions established. Following this approach, 19 parameters relating to the general environment and to the rock mass characteristics are considered. Their reciprocal causes and effects are analysed in order to weight each parameter according to its degree of interactivity in the system. The final definition of the rock mass instability index takes into account the variability of the parameter values for different slopes in assigning ratings to different classes of parameter values. In parallel, a predictability rating is computed, according to the presence in the field of a number of ‘indicators of instability’. Both indices allow discrimination of critical slopes, and are in good agreement with field evidence demonstrating the value of the approach.

Coupled T–H–M issues relating to radioactive waste repository design and performance

Abstract In this paper, coupled thermo-hydro-mechanical (THM) issues relating to nuclear waste repository design and performance are reviewed. Concise statements, that were developed from DECOVALEX discussions, on the current state-of-knowledge are presented. Section 1 describes the THM background and the interface with performance assessment (PA). The role of THM issues in the overall repository design context is amplified in Section 2, which includes a review of the processes in terms of repository excavation, operation and post-closure stages. It is important to understand the overall context, the detailed THM issues, the associated modelling and how these issues will be resolved in the wider framework. Also, because uncoupled and coupled numerical codes have been used for this subject, there is discussion in Section 3 on the nature of the codes and how the content of the codes can be audited. To what extent does a particular code capture the essence of the problem in hand? Consideration is also given to the associated question of code selection and the future of numerical codes. The state-of-knowledge statements are presented in Section 4 under 11 headings which follow the repository design sequence. The overview conclusion is that “A predictive THM capability is required to support repository design because precedent practice information is insufficient. Many aspects of THM processes and modelling are now well understood and there is a variety of numerical codes available to provide solutions for different host rock and repository conditions. However, modelling all the THM mechanisms in space and time is extremely complex and simplifications will have to be made — if only because it is not possible to obtain all the necessary detailed supporting information. Therefore, an important step is to clarify the THM modelling requirement within the PA context. This will help to indicate the complexity of THM modelling required and hence the models, mechanisms, type of computing, supporting data, laboratory and in situ testing, etc. required. An associated transparent and open audit trail should be developed.” We also include comments from reviewers and highlight four outstanding issues which are currently being studied in the DECOVALEX III programme.

The fully-coupled model for rock engineering systems

Abstract This paper introduces the fully-coupled model (FCM). The model is developed from rock engineering systems (RES) concepts and graph theory. The basic device used in the RES approach is the interaction matrix: for any rock mechanics or rock engineering problem, the relevant state variables of the system are listed along the leading diagonal of the matrix—and the relations between each pair of variables, i.e. the binary mechanisms, are identified in the off-diagonal boxes. The interaction matrix can contain any number of state variables, depending on the engineering objective and level of analysis required. The FCM considers the interaction matrix as a mechanism network. Graph theory is used to assess the contributions of all the mechanisms in all the pathways, a key feature being the identification of mechanism feedback loops and their stability. The (uncoupled) binary interaction matrix is thus transformed into a (fully-coupled) global interaction matrix. The system outputs for any problem being modelled are then obtained from the system inputs via the global interaction matrix. Advantages of the FCM in the context of rock engineering are that the primary state variables of the system, the binary interactions, all the mechanism pathways, each step in the coupling process, and the consequential evolution of the matrix values can be clearly identified. The FCM has been developed initially with linear mechanisms. Five numerical examples of the linear FCM are given: simultaneous equations, spring structures, a pressurized thick cylinder, a six-variable dam system and a nine-variable pressure tunnel system. Using these examples, we demonstrate that the output resulting from any input perturbations can be rapidly assessed with the FCM. We also indicate how the FCM can be extended to include non-linear mechanisms.

A new approach to studying complete rock engineering problems

Abstract With the increasing sophistication of our interpretative techniques, numerical analysis and rock characterization schemes, it is becoming more important to base rock engineering design and the associated site investigation, construction and monitoring procedures on a coherent structural understanding of the complete rock engineering problem. This includes not only the primary mechanisms and parameters, but also the interactions between them. This paper outlines a novel approach to listing and presenting the rock mechanics and rock engineering parameters so that all the interactive or coupled mechanisms and terms can be identified. At its lowest level, this provides a useful checklist for a project; at higher levels, it can provide the logic behind the entire design procedures.

In Situ rock stresses and their measurement in the U.K.—Part I. The current state of knowledge

Abstract There is increasing awareness of the significance of the natural in situ rock stress field in engineering design. Following an explanation of the reason why in situ stress is so important, the fundamental nature of the stress state in rocks is described. Overlain on this basic understanding are many complicating factors, such as the effects of inhomogeneity, discontinuities and the residual stress component. The effects of these complicating factors are also described. Finally, given this background to the nature of in situ rock stress, the known state of stress in rocks in the U.K. is outlined. This Part I paper contains an overview of stress in general and the stress fields in the U.K. The Part II paper following describes the stress measurement methods that have been used and the specific results obtained in the U.K. rock stress measurement programme.

Engineering Rock Mechanics: Part 2. Illustrative Worked Examples

It contains illustrative worked examples of engineering rock mechanics in action as the subject applies to civil, mining, petroleum and environmental engineering. The book covers the necessary understanding and the key techniques supporting the rock engineering design of structural foundations, dams, rock slopes, wellbores, tunnels, caverns, hydroelectric schemes and mines. There is a question and worked answer presentation with the question and answer sets collated into twenty chapters which match the subject matter of the first book. Contents: Preface Units and Symbols Part A: Illustrative Worked Examples Questions and Answers Introduction Geological setting Stress In situ rock stress Strain and the theory of elasticity Intact rock: deformability, strength and failure Fractures and hemispherical projection Rock masses: deformability, strength, failure and Permeability Anisotropy and inhomogeneity Testing techniques Rock mass classification Rock dynamics and time dependency Rock mechanics interactions and rock engineering systems Excavation principles Rock reinforcement and rock support Foundations and slopes instability mechanisms Design of surface excavations Underground excavation instability mechanisms Design of underground excavations

ISRM Suggested Method for Rock Fractures Observations Using a Borehole Digital Optical Televiewer

Fractures in rock masses are important for the study of a whole range of rock mechanics and rock engineering issues including evaluation of the rock mass geometry, analysis of the Excavation Damaged Zone (EDZ), understanding the rock mass behaviour and response to excavation, numerical analyses, and reinforcement/support design. A digital borehole camera records a continuous, magnetically orientated digital 360 colour image of the borehole wall, making it possible to directly observe lithological changes in the rock mass and its contained fractures (Paillet et al. 1990; Pusch 1998). Fractures display sinusoidal curves on the flattened image, enabling the strike and dip of the fractures to be determined directly from the images orientated to North (Kamewada et al. 1989; Wang et al. 2002; Williams and Johnson 2004). The technology has been widely applied in geological exploration, especially in petroleum (Maddox 1998; Tague 1999; Palmer and Sparks 1991), mining (Gochioco et al. 2002; Deltombe and Schepers 2000), Glacier (Engelhardt et al. 1978), geotechnical and environmental engineering (Lau et al. 1987; Miyakawa et al. 2000; Lahti 2004; Cunningham 2004; Cunningham et al. 2004; Schepers et al. 2001; Roberson and Hubbard 2010; Uchita and Harada 1993; Li et al. 2012a). It has also been used to observe crack development and fracture evolution around underground excavations, contributing to the establishment of the EDZ characteristics (Li et al. 2012a, b; Yuji 1983). There are two main types of digital borehole camera used: the first is a digital optical televiewer, such as the OPTV (Optical Televiewer), OTV (Optical Televiewer) and OBI-40 (Slimhole Optical Televiewer) (Williams and Johnson 2004; Lahti 2004; Cunningham 2004; Cunningham et al. 2004; Schepers et al. 2001; Roberson and Hubbard 2010); the other is a digital panoramic borehole camera, such as the DIPS (Borehole Image Processing System) and DPBCS (Digital Panoramic Borehole Camera System) (Wang et al. 2002; Wang and Law 2005; Williams and Johnson 2004; Uchita and Harada 1993; Li et al. 2012a). The main parameters of these two kinds of camera are listed in Table 1. The first digital camera was developed as a stand-alone system in 1987 (Williams and Johnson 2004). Since then the tool has gradually become a standard tool. Although there are different types of digital camera system, the basic principle, components and operations of these test systems are almost the same. Thus, this Suggested Method describes the observation of fractures in a rock mass and the identification of EDZ. The apparatus and operating procedure are presented together with the possible ways of reporting the results. The recommendations are supported by case example data.