Lecture 7 Mechanical Properties of Rocks

Topics
Rock properties: mass density, porosity, and permeability
Stress
Mohr's circle
Strain
Elasticity of rocks
Rock properties: strength
Engineering classification of intact rocks
Rock mass properties

Mechanics refers to the response of materials to applied loads. For engineering interest, Earth materials can be divided as rocks, soils, and fluid. Rocks are important building materials and they provide foundations to many engineering structure. This lecture deals with mechanical properties of rocks. Derivations and examples will be given during the lecture.

Rock properties: density, porosity, and permeability

specific gravity: the ratio between the mass and that of equal volume of water (i.e. the ratio of mass density and water density).

unit weight gamma=(specific gravity)x(unit weight of water)
    unit weight of water= 62.4 pcf (lbs/ft3)
    for most rocks, gamma = 120 to 200.

porosity n: measurement of the relative amount of void space (containing liquids and or gases).
    porosity=(void space)/(total volume)

permeability: measurement of the rate at which fluids will flow through a saturated materials. We will discuss the measurements of permeability later in the lecture of Groundwater.

Stress

Stress is force per unit area acting on a plane at any point within a material. There are three types of stresses:

compressive stress: equal forces that act towards a point from opposite directions
tensile stress: equal forces that pull away from each other.
shear stress: equal forces that act in opposite directions but
offset from each other to act as a couple.

Principal stresses (chap 8, p.133)

On any plane within a solid, there are stresses acting normal to the plane (either compressional or tensional, called normal stresses) and shear stresses acting parallel to the plane. At any point within a solid, it is possible to find three mutually perpendicular principal stresses which are maximum, intermediate, and minimum. On the planes perpendicular to the principal stresses (called principal planes), there are not shear stresses.

Mohr's circle (chap 8, p.134)

Suppose we wish to measure stresses (both normal and shear) acting on any given plane besides the principal stresses. In general, this is a three dimensional problem and can be done using mathematical tensors and vectors.

In a special case where we can assume that the intermediate and minimum stresses are equal (for example below the ground surface), we can work in two dimensions. Mohr's circle provides a simple, graphical method to find the normal and shear stresses on inclined planes from principal planes using the maximum and minimum principal stresses.

Strain

The application of stress to a material causes it to deform. The amount of deformation is called strain.

axial strain: deformation along the direction of loading dL/L.
lateral strain: the lateral extension perpendicular to the direction of loading, dB/B.
Poisson's ratio = (lateral strain)/(axial strain).

Elasticity of rocks

Some of the deformation of a rock under stress will be recovered when the load is removed. The recoverable deformation is called elastic and the nonrecoverable part is called plastic deformation. Plastic behavior involves continuous deformation after some critical value of stress has been reached.

Commonly, the elastic deformation of rock is directly proportional to the applied load. The ratio of the stress and the strain is called modulus of elasticity.

Rock properties: strength

Rock strength indicates the level of stress needed to cause failure.

compressive strength is the compressive stress required to break a rock sample. The unit is pounds per square inch (psi) or newtons per square meter (pascals).

unconfined (uniaxial) compression test:
the rock sample is unconfined at its side while the load is applied vertically until failure occurs. In this case, the compressive strength is called unconfined compressive strength (uniaxial compressive strength).

confined compress test:
For design of underground structure (such as tunnels, mining, waste repository), we need to take into account of the confining pressure at depth. This is done at laboratory by so-called triaxial compression test. The failure curve constructed using Mohr's circle after a series of tests gives the shear strength (cohesion) and internal friction (angle of shearing resistance) of the rock (or soil) sample. This will be further discussed on Mohr-Coulomb failure criterion in the next lecture on Soil Mechanics.

Engineering classification of intact rocks

The engineering classification of intact rocks is based on the uniaxial compressive strength and the modulus of elasticity, developed by Deere and Miller. Intact rock is internally continuous, intact, and free from weakness planes such as jointing, bedding, and shearing.

Rocks are subdivided into five strength categories: A through E for very high to very level of strength.

Rock classification also involves the modulus of elasticity. More specifically, the modulus ratio is used, which is the ratio of the modulus of elasticity to the unconfined compressive strength. Three modulus ratio categories are H (high) for >500, M (medium) for 200-500, and L (low) for <200.

Rock mass properties

The strength and deformation properties of intact rocks cannot be directly applied to the overall rock mass in the field situation. The strength and behavior of a rock mass are largely controlled by the nature of its discontinuities or weakness planes. Discontinuities act to lower the strength of the rock mass. The rock mass tends to fail along existing weakness planes rather than develop new fracture within intact solid rocks.

Examples of rock mass discontinuities include:
sedimentary: bedding planes, sedimentary structure (mud cracks, ripple marks, cross beds, etc.)
structural: faults, joints, fissures
metamorphic: foliation
igneous: cooling joints, flow contacts, intrusive contacts, dikes, sills