Geomechanics 1: Week 4
Rock Mass Classification
Rock behaviour-scale effect
§ Intact rock: This is the smallest element of rock block not cut by any fracture
§ Rock mass: In-situ rock together with its discontinuities and weathering profile
Intact rock
Physical properties
§
§ Mineral, colour, texture etc.
§ Mechanical properties
UCS, Point Load Index, Modulus of elasticity, peak strain, Poisson's ratio, Residual strength
Why do you classify rock mass?
Ø To assess excavation difficulties
Ø To check stability of rock slopes
Ø To design tunnels
Ø To determine load supporting capacity
Ø To determine water transmitting capacity
Ø To design underground and open cut mining
Ø To store energy in underground caverns
Rock mass strength
Strength depends on:
– Material properties
• Intact rock strength
• Modulus of elasticity
– Joint properties
• Fracture spacing, orientation
• Infill material (eg. clay, silt)
, • Fracture roughness (eg. smooth, rough)
• Fracture condition (eg. tight or loose)
– Wet or dry condition
Site investigation-drilling
To determine the quality of rocks.
Drilling methods are:
(1) Conventional method (drill strings lowered up to the core depth, rock core brought out
by bringing full length of the string, inefficient)
(2) Wireline driling method (most common and efficient), drill strings remain within the
hole while rock samples recovered from the bottom of the hole by bringing out the inner
core barrel.
RQD = Rock Quality Designation
= % of rock recovered as sound lengths which are 100mm or more in length.
What it counts?
• Degree of fracturing of the rock mass
• Do not include strength or joint properties
• Partial representation of rock mass quality?
, Weathering classifications
Rock strength-Field estimation
Degree of fragmentation
Intact rock strength
§ Unconfined compressive strength (UCS or qu)
§ Point load index test - UCS = 24 Is(50)
Rock Mass Classification
Rock behaviour-scale effect
§ Intact rock: This is the smallest element of rock block not cut by any fracture
§ Rock mass: In-situ rock together with its discontinuities and weathering profile
Intact rock
Physical properties
§
§ Mineral, colour, texture etc.
§ Mechanical properties
UCS, Point Load Index, Modulus of elasticity, peak strain, Poisson's ratio, Residual strength
Why do you classify rock mass?
Ø To assess excavation difficulties
Ø To check stability of rock slopes
Ø To design tunnels
Ø To determine load supporting capacity
Ø To determine water transmitting capacity
Ø To design underground and open cut mining
Ø To store energy in underground caverns
Rock mass strength
Strength depends on:
– Material properties
• Intact rock strength
• Modulus of elasticity
– Joint properties
• Fracture spacing, orientation
• Infill material (eg. clay, silt)
, • Fracture roughness (eg. smooth, rough)
• Fracture condition (eg. tight or loose)
– Wet or dry condition
Site investigation-drilling
To determine the quality of rocks.
Drilling methods are:
(1) Conventional method (drill strings lowered up to the core depth, rock core brought out
by bringing full length of the string, inefficient)
(2) Wireline driling method (most common and efficient), drill strings remain within the
hole while rock samples recovered from the bottom of the hole by bringing out the inner
core barrel.
RQD = Rock Quality Designation
= % of rock recovered as sound lengths which are 100mm or more in length.
What it counts?
• Degree of fracturing of the rock mass
• Do not include strength or joint properties
• Partial representation of rock mass quality?
, Weathering classifications
Rock strength-Field estimation
Degree of fragmentation
Intact rock strength
§ Unconfined compressive strength (UCS or qu)
§ Point load index test - UCS = 24 Is(50)
