Geotechnics and Soil/Rock Mechanics

• Geotechnics applies scientific methods and engineering principles to understand and utilize Earth's crust materials for engineering solutions.
• This encompasses soil and rock mechanics, geology, geophysics, and hydrology and is practiced by engineering geologists and geotechnical engineers.

Subdivisions of Geotechnics

• Soil mechanics studies the physical properties of soil, directly impacting structure design.
• Rock mechanics focuses on the theoretical and applied science of rock and rock mass behavior, and related methodologies for engineering design.

Geotechnics Application in Mining

• Geotechnics is applied in the prediction, prevention, and mitigation of slope collapse in open-pit mines.
• It supports the design and performance prediction of earthen structures like dams, waste/ore stockpiles.
• It informs the selection of ground support systems for underground development headings.
• It defines the type of underground mining method to be employed.

Theories in Rock Mechanics

• Rock mechanic studies stresses and deformation in solid materials, leading to two adapted theories.
• Theory of Elasticity states rocks have elastic properties where strains from external loads are instantaneously and totally recoverable upon removal; a perfectly elastic material completely recovers when loads are removed.
• Theory of Plasticity states rocks have plastic properties where strains from external loads are permanent upon removal.

Rock Mass

• Rock mass consists of intact rock but is more commonly formed from an array of intact rock blocks with boundaries formed by discontinuities.

Rock Mass Components

• Intact rock is blocks of rock without mechanical discontinuities, possessing tensile strength.
• Discontinuities are planes or surfaces marking changes in physical/chemical rock characteristics, acting as planes of weakness like bedding planes, joints, fractures, or faults.
• The existence of discontinuities in a rock mass makes rock mechanics a unique subject.
• A "discontinuity” denotes separation in rock continuum having effectively zero tensile strength.
• Stability is proportional to 1 / the number of discontinuities and also proportional to 1 / engineering dimension.

Geometrical Properties of Discontinuity

• Rock mass geometry includes spacing, frequency, orientation (dip direction/dip angle), persistence (size and shape), roughness, aperture, clustering, and block size.
• No standardized method exists for measuring/characterizing rock structure geometry.
• Emphasis and accuracy of specified separate parameters depend on engineering objectives.

Rock Properties

• Rock varies according to scale as a solid material.
• Rock is often heterogeneous, discontinuous, and anisotropic.

Physical Rock Properties

• Rocks containing quartz as a binder are strongest.
• Rocks with a clayey binder are the weakest.
• Structure applies to well-pronounced macroscopic features of rock, signifying special features like the position of joints, fractures, folds, and faults.
• Texture refers to the arrangement of grains or particles on a freshly exposed rock surface easily seen by the naked eye.

Specific Gravity

• Specific gravity is the ratio of a substance's density to a standard, usually water.
• Formula: SG = ρrock / ρwater

Unit Weight

• Unit weight (Y) of soil and rock above the groundwater table is expressed as the ratio of total weight of rock (or soil) in air (W) to its total volume (V), including voids.
• Formula: Y = Weight / Volume
• Rocks with heavy minerals have higher unit weights
• Usually, igneous and metamorphic rocks have greater unit weights than sedimentary rocks.
• The more porous a rock, the less its unit weight.

Porosity

• Porosity refers to rock voids, pores, and fractures, leading to nonuniform structures and textures, diminishing the rock's mechanical strength and other properties.
• Formula: Porosity (Pt) = (Pore Volume (Vp) / Total Volume (Vt)) * 100%

Void Ratio

• Void Ratio is the ratio of the volume of voids to the volume of solids and is expressed in decimal fractions.
• Formula: Void ratio, e = Volume of voids / Volume of solids = Vν / V

Moisture Content

• Moisture Content is the weight of water present in a rock expressed as a percentage of the rock's oven-dry weight.
• Formula: MC = (w-d)/w * 100, where w = wet weight and d = weight after drying

Degree of Saturation

• The degree of saturation is the ratio of the volume of water to the volume of voids
• Formula: S = Vw/Vv

Permeability

• Permeability is the property of porous material that permits the passage or seepages of fluids.

Thermal Proterties

• An increase in temperature lowers rock strength and increases ductility.
• The geothermal gradient is 25C – 30C/km of depth.
• Changes in rock temperature may bring about "Rock Exfoliation."

Mechanical Properties

• Hardness is the resistance to abrasion.

Macro Hardness Tests

• This includes the Rockwell Hardness Test, is a method consisting of indenting the test material with a diamond cone indenter.
• The Brinell hardness test, consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg.
• The Vickers hardness test consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf.

Micro Hardness Tests

• This refers to static indentations with loads not exceeding 1 kgf.
• The indenter is either the Vickers diamond pyramid or the Knoop elongated diamond pyramid.
• The Knoop hardness number (KHN) is the ratio of the load applied to the indenter, P (kgf) to the unrecovered projected area A (mm2).

Durability

• Durability is the ability of rock to withstand weathering or external pressure.

Elasticity

• Elasticity is the ability of rock to regain its original form after an external load is removed.

Plasticity

• Plasticity is a the character of rock that changes shape in any direction without rupture under a stress.
• It is characterized by the existence of a yield point beyond which permanent strains appear, where plastic deformation is the permanent deformation after complete unloading.

Stress vs. Strain

• Deformation; change in volume or shape of a rock body.
• Strain
• Deformation is the character of rock that changes from its original form or volume or rock specimen by tectonic forces.

Stress

• The force that acts on a rock unit to change its shape and/or its volume, causing strain or deformation.
• Types of directed stress include compression, tension, shear.
• Compression is action of coincident oppositely directed forces acting towards each other.
• Tension is actions of coincident oppositely directed forces acting away from each other.
• Shear is actions of coincident oppositely directed forces acting parallel to each other across a surface.

Strength

• Strength refers to the ability of an object to resist deformation.
• Compressive strength is the capacity of a material to withstand axially directed pushing forces.
• Tensile strength measures the force required to pull something to the point where it breaks.

Strain

• Strain is any change in original shape or size of an object in response to stress.

Kinds of Deformation

• Elastic vs plastic
• Brittle vs ductile

Elastic Deformation

• It is a temporary change in shape or size that is recovered when the deforming force is removed.
• Hooke's Law: strain in a solid is proportional to the applied stress within the elastic limit

Ductile (Plastic) Deformation

• It is a permanent change in shape or size that is not recovered when stress is removed.
• This occurs by the slippage of atoms or small groups of atoms past each other in the deforming material without loss of cohesion.

Brittle Deformation

• This is the loss of cohesion of a body under the influence of deforming stress.
• Usually occurs along sub-planar surfaces that separate zones of coherent material.

Factors that Affect Deformation

• These include temperature, pressure, strain rate, and rock type.
• Rock faulting or folding depends on variation of these factors.
• Rocks are elastic and brittle near the earth's surface.
• They are more plastic and ductile deeper in the crust due to increasing temperature and pressure.

Strength

• Strength is the ability of an object to resist externally applied loads.
• Compressive strength is the capacity of a rock to withstand loads tending to reduce size.
• “The compressive strength of rock decreases with the increase in rock porosity, viz volume of voids.

Tensile Strength

• The capacity of a rock to withstand loads tending to elongate.
• Tensile strength of rocks is important as is analyzing rock strength and stability of roofs and domes of U/G openings.

Shear Strength

• Shear strength is a material's ability to resist forces causing its internal structure to slide against itself.
• Shear strength is relevant in the stability of U/G openings and assessing the degree of stabilization in rock slopes.