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The main mechanical components in rock mass
Intact rock; defined as rock containing no significant fractures
Rock Mass; consists of intact rock and discontinuities
Elementary Rock Types
- products of the metamorphism which transforms naturally occurred rocks by temperature, pressure and chemical process.
- Marble (fine-coarse grained), Slate, micro-fine grained, laminated), Schist (coarse grained), Gneiss (altered granite)
- solidification of cooled magma. Intrusive (plutonic), below surface; Extrusive (volcanic), on surface.
- Granite (igneous intrusive rock), basalt (grey to black extrusive volcanic rock), quartz (lattice of silica (SiO2) tetrahedra)
- consolidation of sediments
- 75-80% of the Earth's land area. As sediment deposition builds up, the overburden pressure squeezes the sediment into layered solids in a process known as lithification (rock formation). Sedimentary rocks are laid down in layers called beds or strata.
- Conglomerate (large grains in clay matrix), sandstone (Medium round grains in calcite matrix -composed of quartz and/or feldspar), shale (harder laminated)
- Also, chalk & limestone
Important geological factors in rock masses:
- Intact rock: Intact rock is defined as rock containing no significant fractures.
- Discontinuities: is a general term denoting any separation in a rock mass with low tensile strength.
- In-situ stress: is pre-existing state of tress in the rock.
- Pore water: Pore fluids existing in rock masses.
- Time influence : Time component affects engineering performance of rock structure.
The most useful description of the mechanical behaviour of intact rock is...
- the stress-strain curve.
Rock discontinuities/Rock structure
- •Faults – discontinuities on which shear displacement has taken place
- •Beddings – divide sedimentary rocks into beds of strata
- •Joints – breaks of geological origin along which there has been no displacement
- Normal (extension)
- Reverse (compression)
- Ripple-marked beds
- Fossil shell beds
- Uniaxial Compressive Strength:
- measures the stress at which a sample will break in unconfined compression
- A stress-strain plot will have a characteristic shape. Initially it will be linear as the sample shortens and swells and then curved as permanent distortion occurs.
- •Young’s modulus (E)
- •Poisson’s ratio (n)
- •Unconfined compressive strength (UCS or sigmac)
Description of Discontinuities
- •Orientation – is described by the dip and dip direction of the line of steepest declination in the plane of discontinuity (stereographic projection)
- •Spacing – refers to the average spacing between adjacent discontinuities
- •Persistence – The discontinuity trace length
- •Roughness – The inherent surface roughness relative to plane of discontinuity
- •Aperture – The perpendicular distance between adjacent rock walls
- •Filling – The material that separates the adjacent rock walls
- (p43, p60 week 2 lecture notes)
Strength of jointed rock masses (Hoek’s 1983 Rankine Lecture)
- Single joint
- Sparsely jointed
- Densely jointed
- Compacted rockfill