Rock mass strength analysis

Question 1

What are rock bursts?

Answer 1

Explosive failures of rock which occur when very high stress concentrations are induced around underground openings

Question 2

What are the key factors that control rock mass strength?

Answer 2

Schistosity
Fracture densities
Fracture orientation
Fracture types

Question 3

What important factors are considered by rock mass strength?

Answer 3

Fracture densities, oreintation and type

Question 4

Why is siesmic refraction done?

Answer 4

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

Question 5

What does seismic velocity measure?

Answer 5

The compressibility and stiffness of rock mass expressed as rippability (by digger)

clear correlation between siemsic velocity and rippability

Question 6

How do rock fractures reduce rock strength?

Answer 6

Allow inelastic deformation

Question 7

What is the classification of fractures?

Answer 7

Microfractures (spacing 1 mm – 1 cm)
Joints (spacing 1cm – 1 m)
Faults (spacing generally > 1 m)

Question 8

What will rock mass strength be like compared to intact rock strength?

Answer 8

1/5 to 1/10

Question 9

How is fracture density assesed?

Answer 9

RQD - Rock Quality Designation (intactness of borehole core)

Question 10

What is the RQD equation?

Answer 10

Σ(core lengths > 10 cm) x (100/borehole length)

Question 11

What is the max RQD and the value for a sound rock?

Answer 11

Max= 100
Sound = >70

Question 12

what does RQD not account for?

Answer 12

discontinuity orientation infill, weathering etc

Question 13

How does fracture/ clevage orientation affect slope stability?

Answer 13

fractures dipping into excavation more likel to fail then those dipping away

Question 14

What orientations of bedding are most stable?

Answer 14

Horizontal bedding most stable then vertical least stable is dipping

Question 15

How does fracture type affect strength?

Answer 15

‘Rough’ (clean, tight) fractures barely affect shear strength, but gaps with/without weak infilling material (especially clay gouge) reduce

Question 16

What are slickenslides?

Answer 16

smooth, polished surfaces on rocks that are created by friction and abrasion between two blocks of rock moving past each other along a fault

Question 17

What are the different ways faults can be seen in core?

Answer 17

Core disking (locked in stress released)
Intersecting joints
Micro-defect density

Question 18

What are the 2 main systems for rock mass classification?

Answer 18

Geomechanics system (RMR - rock mass strength, +)
Norwegian Q system(*)

Question 19

What does the geomechanics system consider?

Answer 19

UCS
RQD
Mean fracture spacing
Fracture condtions
Groundwater state
Future orientation

Question 20

What does the Norweigian Q system consider?

Answer 20

RQD
Joint set number
Joint roughness factor
Joint alteration and clay infillings
Joint water inflow/ pressure
Stress reduction factor

Question 21

What does the strength of jointed rock depend on?

Answer 21

properties of the intact rock pieces and also upon the freedom of these pieces to slide and rotate under different stress conditions

Question 22

What is the freedom of rock pieces controlled by?

Answer 22

geometrical shape of the intact rock pieces

Question 23

WHat does Hoek browns Geological strength index not consider?

Answer 23

ANisotropic rock

Question 24

What is the range for the geological stress index (GSI)?

Answer 24

~100 very strong
0 weak