Engineering Rock Mass Classification

Tunnelling, Foundations and Landslides

R K Goel, Bhawani Singh

• Publisher: Butterworth-Heinemann
• Year: 2011
• ISBN:
  9780123858795
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Rock mass classification methods are commonly used at the preliminary design stages of a construction project when there is very little information. It forms the bases for design and estimation of the required amount and type of rock support and groundwater control measures. Encompassing nearly all aspects of rock mass classifications in detail, Civil Engineering Rock Mass Classification: Tunnelling, Foundations and Landsides provides construction engineers and managers with extensive practical knowledge which is time-tested in the projects in Himalaya and other parts of the world in complex geological conditions.

Rock mass classification is an essential element of feasibility studies for any near surface construction project prior to any excavation or disturbances made to earth. Written by an author team with over 50 years of experience in some of the most difficult mining regions of the world, Civil Engineering Rock Mass Classification: Tunnelling, Foundations and Landsides provides construction engineers, construction managers and mining engineers with the tools and methods to gather geotechnical data, either from rock cuts, drifts or core, and process the information for subsequent analysis. The goal is to use effective mapping techniques to obtain data can be used as input for any of the established rock classification systems. The book covers all of the commonly used classification methods including: Barton's Q and Q' systems, Bieniawski's RMR, Laubscher's MRMR and Hoek's and GSI systems. With this book in hand, engineers will be able to gather geotechnical data, either from rock cuts, drifts or core, and process the information for subsequent analysis. Rich with international case studies and worked out equations, the focus of the book is on the practical gathering information for purposes of analysis and design.

• Identify the most significant parameters influencing the behaviour of a rock mass
• Divide a particular rock mass formulation into groups of similar behaviour, rock mass classes of varying quality
• Provide a basis of understanding the characteristics of each rock mass class
• Relate the experience of rock conditions at one site to the conditions and experience encountered at others
• Derive quantitative data and guidelines for engineering design
• Provide common basis for communication between engineers and geologists

Chapter 1. Philosophy of Engineering Classifications

When you can measure what you are speaking about, and express it in numbers, you know something about it, but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts, advanced to the stage of science.

Lord Kelvin

Rock mass classifications form the backbone of the empirical design approach and are widely employed in rock engineering. Among other approaches, the empirical approach, based on rock mass classifications, is the most popular because of its simplicity and ability to manage uncertainties. The geological and geotechnical uncertainties can be tackled effectively using proper classifications. In any engineering classification system, a minimum rating is assigned to the poorest rock mass and the maximum rating to the best rock mass. Thus, every parameter of a classification plays a more dominant role as overall rating decreases. This book presents an integrated system of classifications and their applications for tunnels, foundations, and landslides in light of the field research conducted in India and Europe over the last three decades. Extensive tables and correlations are included for characterization of rock masses to be used in the available software. It is shown that the engineering rock mass classification is an amazingly successful approach.

Keywords: Engineering rock mass classification; Geological maps; Philosophy; Present practice; Uncertainties

The classification

The science of classification is called “taxonomy”; it deals with the theoretical aspects of classification, including its basis, principles, procedures, and rules. Knowledge tested in projects is called the “practical knowledge.” Surprisingly the rating and ranking systems have become popular in every part of life in the twenty-first century.

Rock mass classifications form the backbone of the empirical design approach and are widely employed in rock engineering. Engineering rock mass classifications have recently been quite popular and are used in feasibility designs. When used correctly, a rock mass classification can be a powerful tool in these designs. On many projects the classification approach is the only practical basis for the design of complex underground structures. The Gjovik Underground Ice Hockey Stadium in Norway was designed by the classification approach.

Engineering rock mass classification systems have been widely used with great success in Austria, South Africa, the United States, Europe, and India for the following reasons:

1. They provide better communication between planners, geologists, designers, contractors, and engineers.

2. An engineer's observations, experience, and judgment are correlated and consolidated more effectively by an engineering (quantitative) classification system.

3. Engineers prefer numbers in place of descriptions; hence, an engineering classification system has considerable application in an overall assessment of the rock quality.

4. The classification approach helps in the organization of knowledge and is amazingly successful.

5. An ideal application of engineering rock mass classification occurs in the planning of hydroelectric projects, tunnels, caverns, bridges, silos, building complexes, hill roads, rail tunnels, and so forth.

The classification system, in the last 60 years of its development, has been cognizant of the new advances in rock support technology starting from steel rib supports to the latest supporting techniques such as rock bolts and steel fiber reinforced shotcrete (SFRS).

Philosophy of classification system

In any engineering classification system, the minimum rating is called “poor rock mass” and the maximum rating is called “excellent rock mass.” Thus, every parameter of a classification plays a more dominant role as overall rating decreases, and many classifications are accurate in both excellent and poor rock conditions. Reliability may decrease for medium rock conditions. No single classification is valid for assessment of all rock parameters. Selection of a classification for estimating a rock parameter is, therefore, based on experience. The objective should be to classify the undisturbed rock mass beyond excavated faces. Precaution should be taken to avoid the double-accounting of joint parameters in the classification and in the analysis. Thus, joint orientation and water seepage pressure should not be considered in the classification if these are accounted for in the analysis.

It is necessary to account for fuzzy variation of rock parameters after allowing for uncertainty; thus, it is better to assign a range of ratings for each parameter. There can be a wide variation in the engineering classifications at a location. When designing a project, the average of rock mass ratings (RMR) and geological strength index (GSI) should be considered in the design of support systems. For rock mass quality (Q), a geometric mean of the minimum and the maximum values should also be considered in the design.

A rigorous classification system may become more reliable if uncertain parameters are dropped and considered indirectly. An easy system's approach (Hudson, 1992) is very interesting and tries to sequence dominant parameters at a site (see Chapter 27). This classification is a holistic (whole) approach, considering all parameters.

Hoek and Brown (1997) realized that a classification system must be non-linear to classify poor rock masses realistically. In other words, the reduction in strength parameters with classification should be non-linear, unlike RMR in which strength parameters decrease linearly with decreasing RMR. (Mehrotra, 1993, found that strength parameters decrease non-linearly with RMR for dry rock masses.) More research is needed on the non-linear correlations for rock parameters and rock mass characterization.

Sound engineering judgment evolves out of long-term, hard work in the field.

Need for engineering geological map

Nature tends to be heterogeneous, which makes it easy to predict its weakest link. More attention should be focused on the weak zones (joints, shear zones, fault zones, etc.) in the rock mass that may cause wedge failures and/or toppling. Rock failure is localized and three dimensional in heterogeneous rock mass and not planar, as in homogeneous rock mass.

First, a geological map on macro-scale (1:50,000) should be prepared before tunneling or laying foundations. Then an engineering geological map on micro-scale (1:1000) should be prepared soon after excavation. This map should highlight geological details for an excavation and support system. These include Q, RMR, all the shear zones, faults, dip and dip directions of all joint sets (discontinuities), highest ground water table (GWT), and so forth along tunnel alignment. The engineering geological map helps civil engineers immensely. Such detailed maps prepared based on thorough investigation are important for tunnel excavations. If an engineering geological map is not prepared then the use of a tunnel boring machine (TBM) is not advisable, because the TBM may get stuck in the weak zones, as experienced in Himalayan tunneling. An Iraqi proverb eloquently illustrates this idea:

Ask 100 questions, but do not make a single mistake.

Management of uncertainties

Empirical, numerical, or analytical and observational approaches are various tools for engineering designs. The empirical approach, based on rock mass classifications, is the most popular because of its simplicity and ability to manage uncertainties. Geological and geotechnical uncertainties can be tackled effectively using proper classifications. Moreover, this approach allows designers to make on-the-spot decisions regarding supporting measures if there is a sudden change in the geology. The analytical approach, on the other hand, is based on assumptions and obtaining correct values of input parameters. This approach is both time-consuming and expensive. The observational approach, as the name indicates, is based on monitoring the efficiency of the support system.

Classifications are likely to be invalid in areas where there is damage due to blasting and weathering such as in cold regions, during cloudbursts, and under oceans. If the rock has extraordinary geological occurrence (EGO) problems, then these should be solved under the guidance of national and international experts.

According to Fairhurst (1993), designers should develop design solutions and design strategies so that support systems are ductile and robust, that is, able to perform adequately even in unknown geological conditions. For example, shotcreted and reinforced rock arch is a robust support system. The Norwegian Method of Tunneling (NMT) after 30 years, has evolved into a successful strategy that can be adopted for tunnel supporting in widely different rock conditions.

Present-day practice

Present-day practice is a combination of all of the previously described approaches. This is basically a “design as you go” approach. Experience led to the following strategy of refinement in the design of support systems.

1. In feasibility studies, empirical correlations may be used for estimating rock parameters.

2. At the design stage, in situ tests should be conducted for major projects to determine the actual rock parameters. It is suggested that in situ triaxial tests (with σ₁, σ₂, and σ₃ applied on sides of the cube of rock mass) should be conducted extensively, because σ₂ is found to affect both the strength and deformation modulus of rock masses in tunnels. This is the motivation for research, and its presentation in this book is likely to prove an urgent need for in situ polyaxial tests.

3. At the initial construction stage, instrumentation should be carried out in drifts, caverns, intersections, and other important locations with the objective of acquiring field data on displacements both on the supported excavated surfaces and within the rock mass. Instrumentation is also essential for monitoring construction quality. Experien...

Table of contents

• Cover image
• Table of Contents
• Front matter
• Copyright
• Dedication
• Preface
• Acknowledgments
• Chapter 1. Philosophy of Engineering Classifications
• Chapter 2. Shear Zone Treatment in Tunnels and Foundations
• Chapter 3. Rock Material
• Chapter 4. Rock Quality Designation
• Chapter 5. Terzaghi's Rock Load Theory
• Chapter 6. Rock Mass Rating
• Chapter 7. Tunneling Hazards
• Chapter 8. Rock Mass Quality Q-System
• Chapter 9. Rock Mass Number
• Chapter 10. Rock Mass Index
• Chapter 11. Rate of Tunneling
• Chapter 12. Support System in Caverns
• Chapter 13. Strength Enhancement of Rock Mass in Tunnels
• Chapter 14. Rock Mass Quality for Open Tunnel Boring Machines
• Chapter 15. Strength of Discontinuities
• Chapter 16. Shear Strength of Rock Masses in Slopes
• Chapter 17. Types of Failures of Rock and Soil Slopes
• Chapter 18. Slope Mass Rating
• Chapter 19. Landslide Hazard Zonation
• Chapter 20. Allowable Bearing Pressure for Shallow Foundations
• Chapter 21. Method of Excavation
• Chapter 22. Rock Drillability
• Chapter 23. Permeability and Groutability
• Chapter 24. Gouge Material
• Chapter 25. Engineering Properties of Hard Rock Masses
• Chapter 26. Geological Strength Index
• Chapter 27. Evaluation of Critical Rock Parameters
• Chapter 28. In Situ Stresses
• Appendix I. Shear and Normal Stiffness of Rock Joints
• Appendix II. Bond Shear Strength of Grouted Bolts
• Index