Rock mass strength analysis Flashcards
What are rock bursts?
Explosive failures of rock which occur when very high stress concentrations are induced around underground openings
What are the key factors that control rock mass strength?
Schistosity
Fracture densities
Fracture orientation
Fracture types
What important factors are considered by rock mass strength?
Fracture densities, oreintation and type
Why is siesmic refraction done?
field experiment for rock strength - Seismic velocity is controlled by the bulk modulus of the rock, a function of rock mass strength provides an estimate of the rippability of the rock
What does seismic velocity measure?
The compressibility and stiffness of rock mass expressed as rippability (by digger)
clear correlation between siemsic velocity and rippability
How do rock fractures reduce rock strength?
Allow inelastic deformation
What is the classification of fractures?
Microfractures (spacing 1 mm – 1 cm)
Joints (spacing 1cm – 1 m)
Faults (spacing generally > 1 m)
What will rock mass strength be like compared to intact rock strength?
1/5 to 1/10
How is fracture density assesed?
RQD - Rock Quality Designation (intactness of borehole core)
What is the RQD equation?
Σ(core lengths > 10 cm) x (100/borehole length)
What is the max RQD and the value for a sound rock?
Max= 100
Sound = >70
what does RQD not account for?
discontinuity orientation infill, weathering etc
How does fracture/ clevage orientation affect slope stability?
fractures dipping into excavation more likel to fail then those dipping away
What orientations of bedding are most stable?
Horizontal bedding most stable then vertical least stable is dipping
How does fracture type affect strength?
‘Rough’ (clean, tight) fractures barely affect shear strength, but gaps with/without weak infilling material (especially clay gouge) reduce
What are slickenslides?
smooth, polished surfaces on rocks that are created by friction and abrasion between two blocks of rock moving past each other along a fault
What are the different ways faults can be seen in core?
Core disking (locked in stress released)
Intersecting joints
Micro-defect density
What are the 2 main systems for rock mass classification?
Geomechanics system (RMR - rock mass strength, +)
Norwegian Q system(*)
What does the geomechanics system consider?
UCS
RQD
Mean fracture spacing
Fracture condtions
Groundwater state
Future orientation
What does the Norweigian Q system consider?
RQD
Joint set number
Joint roughness factor
Joint alteration and clay infillings
Joint water inflow/ pressure
Stress reduction factor
What does the strength of jointed rock depend on?
properties of the intact rock pieces and also upon the freedom of these pieces to slide and rotate under different stress conditions
What is the freedom of rock pieces controlled by?
geometrical shape of the intact rock pieces
WHat does Hoek browns Geological strength index not consider?
ANisotropic rock
What is the range for the geological stress index (GSI)?
~100 very strong
0 weak
