Ground Mechanics in Hard Rock Mining

M.L. Jeremic

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000151466
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This book illustrates the advantages of the application of ground mechanic concepts to hard rock mining. It examines the ground stability for various mine layouts using specific case histories. The book deals with ground support, reinforcement, and stabilization of mining structures.

CHAPTER 1

Concept of stress and strain

Analysis of the nature of stress can be achieved by mathematical and analytical theories which deal with the description of the state of stress throughout the body. The concept of force, stress, normal stress and shear stress is based on the physical definitions and the relations between these quantities which are mathematically formulated. These theoretical principles can be applied to any body composed of a continuous distribution of matter. In some instances, restrictions should be made as to the nature of the material i.e. rock.

In order to evaluate the rock behaviour in the underground mining it is necessary to understand the meaning of the terms stress and strain and how these are connected in the constitutive laws. It will then be easier to understand the various relationships which govern the deformations which are produced from given stress or loading conditions. It should be pointed out that stress and strain are defined independently from the material properties and therefore apply to all types of rock material.

1.1 PRINCIPLES OF STRESS

When rocks are subjected to stresses, several states of stress could be detected as individually described under this subheading.

To discuss the stresses from a theoretical point of view, it is necessary to return to fundamentals and explain the nature of the stresses in relation to the rock.

The stress is considered in a very simplified context, so that understanding of this phenomenon cannot be a problem. A full discussion of stress theory is found in several books on Rock Mechanics.^1,2,3

Finally it should be understood that the virgin stress in rock mass is caused by the overburden load or geological structure, whose state can be physically defined and mathematically formulated.

1.1.1 Definition of the stresses

In order to clarify the stress phenomena, it is necessary to give a principal definition of the forces as follows:¹

(a) Body force is produced without physical contact with other bodies (i.e. gravitation, magnetism, lithostatic, inertia, etc.). It is defined as a force per unit volume which depends on the volume and specific gravity of rock mass. The body force of volume AN has the resultant forces which act along respective principal coordinate systems (X,Y,Z).

Figure 1.1.1. Surface body forces and internal forces

(b) Surface force is produced by physical contact with other bodies. When a body is subjected to a system of forces the resultant and the moment of which are both nil, it suffers no accelerations.¹

(c) Internal forces are induced by the forces on the body. They maintain equilibrium and prevent the collapse of the body. Internal stresses can be defined by analyzing the forces upon arbitrary virtual plane which divide the solid body into two parts which are in the state of equilibrium (Figure 1.1.1). By considering a small portion of the plane of area ΔA, and force ΔF acts upon the plane, the value of ΔF will vary with the area ΔA, but the state of stress at that point is constant and is defined as:

This is the definition of internal stress, which also could be decomposed in the components of respective principal co-ordinate systems (X, Y, Z). It should be understood that the force is a vector quantity defined by magnitude and direction, but the stress is a tensor quantity and is not only defined by its magnitude and direction but also by the direction of the plane upon which it acts.

1.1.2 Stress state at a point in plane

The state of stress at a point considers a small element enclosing the point. If the body is in equilibrium, the stresses are balanced by equal and opposite forces across the element. The components of the traction on the X plane are σ_x normal to the X plane and τ_xy along it. The first subscript defines the plane upon which the stress acts and the second subscript defines the direction in which the stress acts. Because the normal st...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1. Concept of Stress and Strain
• 2. Deformations of Rocks
• 3. Failure of Rocks
• 4. Strength of Rocks
• 5. Monitoring of Rock Structures
• 6. Ground Conditions
• 7. Ground Stresses
• 8. Mine Pillar Structures
• 9. Open Stoping
• 10. Artificially Supported Stope Structure
• 11. Caving and Drawing Methods
• 12. Combined Open/Filled Structures
• 13. External Ground Support
• 14. Internal Ground Support
• 15. Ground Stabilization
• Subject Index