Rock strength and strength of rock mass

Question 1

rock strength

Answer 1

rocks ability to resist stress without failure

Question 2

what does mechanical strength depend on? x3

Answer 2

mineral composition, arrangement of minerals, any cracks being present

Question 3

confining pressure

Answer 3

the combined lithostatic and hydrostatic pressure. At depth, all principle stresses are equal

Question 4

what does confining pressure from above do to rock?

Answer 4

change its volume

Question 5

how will incompetent and competent rocks behave under stress?

Answer 5

incompetent- ductile

competent- brittle

Question 6

How do you test rock strength?

Answer 6

uniaxial compressive strength test- cylinder core samples placed in hydraulic jaws, load is slowly increased until it fails at peak strength. Called uniaxial because principle stress is almost entirely down cylinder

Question 7

what does rock density depend on? x3

Answer 7

its constituent minerals, voids in rock or water saturation

Question 8

rock density equation

Answer 8

% of mineral in rock x density of mineral

Question 9

what are failures at a discontinuities usually a reaction to?

Answer 9

shear stress

Question 10

What happens at discontinuities as stress increases?

Answer 10

there’ll be some elastic behaviour until the point where material holding discontinuity together fails

Question 11

what can cause an increase in stress at a discontinuity? x3

Answer 11

roughness of the surface and asperities being forced over each other
crystals fracturing
combination of the 2

Question 12

what does the strength of a jointed rock mass depend on? x4

Answer 12

strength of rock, type, orientation and spacing of joints

Question 13

name 2 types of joints

Answer 13

tensional

shear

Question 14

What makes a jointed rock mass stronger? x3

Answer 14

joints infilled with secondary mineral
joints narrow or tight
fewer joints per unit length

Question 15

what happens when water enters fractures?

Answer 15

hydrostatic pressure acts against area of joint surface, decreasing frictional forces

Question 16

unloading joints

Answer 16

internal stresses due to burial are released when rock is excavated or exposed by erosion or uplift= dangerous sets of joints parallel to surface. May need grouting to improve stability

Question 17

what are the hazards of faults? x3

Answer 17

may contain gouge- likely to have clay content= lose strength when saturated
allow passage or water= dissolution+ removal of fine minerals
different rock types on either side= different properties

Question 18

What does a bedding plane mark? x2

Answer 18

time deposition temporarily ceased- fine material settle out= weak layer especially when wet
change in rock type= change in properties= concentration of stresses at boundary

Question 19

Why is severe folding a problem?

Answer 19

rapid changes in orientation and jointing

Question 20

How can weathering effect rocks?

Answer 20

change properties of sediment and alter rock strength