Rock Mech. Topic 1

• Rock mechanics


  The theoretical and applied science of the mechanical behavior or rock; it is that branch of mechanics concerned with the response of rock to the force fields of its physical environment
• Rock

  • Naturally occurring and coherent aggregate of one or more minerals
  • Constitutes the basic unit of which the solid Earth is composed
  • Typically form recognizable and mappable volumes

  
• Mechanics


  • Science concerned with the motion of bodies under the action forces, including the special case in which a body remains at rest
  • Of concern in the problem of motion are the forces that bodies exert on one another
• Rock mechanics deals with the response of rock to an applied disturbance, which is considered here as an engineering, man-induced disturbance
• Rock mechanics is an interdisciplinary field concerned with the study of the mechanical behavior or rock masses, and more appropriately, with the response of rock to the forces resulting from its physical environment
• Rock mechanics deals with the properties of rock and the special methodology required for design of rock-related components engineering schemes
• Rock mass


  Rock with discontinuities, alterations, water inflow, etc.
• Force fields


  Original/in-situ or induced (due to mining or tunneling)
• Rock mechanics common problems


  • How will rock react when put to man's use?
  • What is the bearing capacity of a rock on its surface and at various depth to carry various loads?
  • What is the shear strength of rock?
  • How will rock perform under dynamic loads?
  • What is the effect of earthquakes on a rock-foundation system?
  • What is the modulus of elasticity of rocks?
  • What is the Poisson's ratio of rocks?
  • What are the effects of rock defects (jointing, bedding planes, schistosity, fissures, cavities, and other discontinuities) on its strength properties?
• Most rocks are discontinuous (contain fractures, faults, bedding planes) or more general term as "discontinuity" planes that divide rocks into block of rock bounded by discontinuities

  
• Rock mass


  The whole array of rock blocks & discontinuity planes
• Ground


  Any natural material present at the site where engineering structures are laid, divided into soil and rock
• Soil


  Consists of loose particles not cemented together
• Objectives of rock mechanics


  • Perform engineering rock surveys
  • Develop rational rock sampling, identification and classification methods
  • Apply the knowledge or rock mechanics for solution of practical engineering problems
  • Study the physical, mechanical, elastic, inelastic, plastic & rheological properties of rocks
  • The ultimate understanding of the strength of the rock and its performance in-situ
• Major significance of rock mechanics in mining practice


  • To reduce, avoid or eliminate mine hazards thus ensuring mine safety
  • To reduce mining pre-construction, operational and post-operation costs
  • To ensure Return of Investment (ROI) and profits
• Strength of materials


  A load is applied to a stress-free material, and so the imparted stresses and strains in it are analyzed
• Rock mechanics


  • The rock in-situ is initially subjected to initial and primary stress field
  • Upon introducing an underground opening in the rock mass, a new stress condition in rock is brought about namely: stress is released locally resulting in a corresponding unloading deformations
• The basic difference between Strength of Materials and Rock Mechanics is the changed stress conditions and deformations in the body of the material brought about by an excavated underground opening
• Importance of study of rock strength


  • Mine stability- in open pit slopes, benches and surface mine structures; in underground openings such as roof and walls of mine tunnels, adits, shaft, drift, x-cut, etc.
  • Cost Analysis
  • Mine Safety
  • Production Efficiency
• Rock as an engineering material


  • In Civil Engineering: Rock will be used either as building material so the structure will be made of rock. A structure will be built on the rock or a structure will be built in the rock. Rock is removed to form a structure, ie. Excavation of rock is removed to forma a structure.
  • In Mining Engineering: Rock is excavated to obtain the mineral present in it.
• Geological properties are important in rock excavation
• Stress release


  Stress is released locally resulting in a corresponding unloading deformations
• Difference between Strength of Materials and Rock Mechanics


  Changed stress conditions and deformations in the body of the material brought about by an excavated underground opening
• Importance of study of rock strength


  • Mine stability- in open pit slopes, benches and surface mine structures; in underground openings such as roof and walls of mine tunnels, adits, shaft, drift, x-cut, etc.
  • Cost Analysis
  • Mine Safety
  • Production Efficiency
• Rock in Civil Engineering


  • Rock will be used either as building material so the structure will be made of rock
  • A structure will be built on the rock
  • A structure will be built in the rock
  • Rock is removed to form a structure, ie. Excavation of rock is removed to forma a structure
• Rock in Mining Engineering


  • Rock is excavated to obtain the mineral present in it
• Discontinuities


  • They dominate the modulus, strength and permeability values
  • They often have a significant effect on the transmission and relief of stress (and hence the stress distribution) in the rock, and thus on the excavation and support methods required
  • Geometrical and mechanical characteristics need to be understood and evaluated using standard methods of description

  
• Characteristics of discontinuities


  • Orientation
  • Spacing
  • Persistence
  • Roughness
  • Wall strength
  • Aperture
  • Filling
  • Seepage
  • Number of sets
  • Block size
• An open discontinuity causes the rock to have zero tensile strength at that location; and, similarly, the presence of discontinuities significantly reduces both the compressive and shear stiffnesses of a rock mass
• Ideal mechanical behavior of solids


  • Ideally homogenous
  • Ideally continuous
  • Ideally isotropic (nondirectional in properties)
  • Ideally linear
  • Ideally elastic
• Actual mechanical behavior of rocks


  • Heterogenous
  • Discontinuous
  • Anisotropic
  • Nonlinear
  • Inelastic

  
• Discontinuities are the weak points of rock
• Presence of water seepages in discontinuities
• Rock mass with discontinuities


  • Weaker than the intact rock because shear and tensile strength of discontinuities are lower than those of the intact material
  • More deformable than intact rock
  • Tensile strength is low and for many rock masses zero
  • Porosity is higher due to the storage capacity and the permeability is often higher due to the conductivity via the discontinuities
  • Leads to an anisotropic behavior of rock mass and all rock mass properties, such as deformability, permeability, etc.
  • Movement along discontinuities are plastic rather than elastic
  • Strength is dependent on shape & size of rock mass, environment, amount and orientation of discontinuities, and the intact rock strength
• There is a great variety of joints, from small cracks to long shears of seams
• The reason for the strength behavior of rock and other brittle materials can be sought in the presence of microscopic cracks and flaws in the base material that is in itself strong
• Any deformation of sufficient magnitude will lead to local cracking or development of microscopic flaws, pores or weakened regions
• In tension, microcracks gown, join and ultimately lead to fracture at a low load
• In compression, the friction on such microcracks that are stressed in shear leads to increased strength. With confinement, the crack growth is further restricted and friction is increased
• With a sufficiently high hydrostatic pressure, the deformation of the wear part of the aggregate grains will be plastic, while the hard grains still only deform elastically