Rock Mechanics Quiz 2

Question 1

Explain how the presence of discontinuity affect rock mass.

Answer 1

Due to the presence of discontinuities, the rock mass is weaker than the intact rock specimen, showing lower strength and stiffness.

Question 2

Explain the effects of water to the geological discontinuities.

Answer 2

Water reduces the friction along the discontinuities, and the increased pore water pressure reduces the effective stresses and hence the shear strength.

Question 3

Define discontinuity.

Answer 3

Discontinuity is a generic term used to describe a fault, joint, bedding plane, foliation, cleavage or schistosity.

Question 4

Define fault.

Answer 4

Fault is a planar fracture along which noticeable movement has taken place.

Question 5

Define joints.

Answer 5

Joints are filled or unfilled fractures within the rock mass that do not show any sign of relative movement.

Question 6

Define bedding planes.

Answer 6

Bedding planes are formed when the sediments are deposited in the rock formation process, creating planes of weakness, which are not necessarily horizontal.

Question 7

Define foliation.

Answer 7

Foliation occurs in metamorphic rocks where the rock-forming minerals exhibit platy structure or banding, thus developing planes of weakness.

Question 8

Define cleavage.

Answer 8

Cleavages are planes of weaknesses that occur often as parallel layers and are formed in a metamorphic process.

Question 9

Define schistosity.

Answer 9

Schistosity is a type of cleavage seen in metamorphic rocks such as schists and phyllites, where the rocks tend to split along parallel planes of weakness.

Question 10

Define orientation.

Answer 10

Orientation of the discontinuity, measured by the dip and dip direction.

Question 11

Define spacing.

Answer 11

Spacing is the perpendicular distance between two adjacent discontinuities of the same set.

Question 12

Define persistence.

Answer 12

Persistence is a measure of the extent to which the discontinuity extends into the rock.

Question 13

Define roughness.

Answer 13

Roughness of a rock joint refers to the large-scale surface undulations observed over several meters and the small-scale unevenness of the two sides relative to the mean plane, observed over several centimeters.

Question 14

Define wall strength.

Answer 14

Wall strength refers to the compressive strength of the rock that makes up the walls of the discontinuity.

Question 15

Define aperture.

Answer 15

Aperture is the perpendicular distance between the two adjacent rock walls of an open discontinuity where the space is filled by air or water.

Question 16

Define filling.

Answer 16

Filling is the term used to describe the material that occupies the space between the adjacent rock walls of a discontinuity.

Question 17

Define seepage.

Answer 17

Seepage occurs mainly through discontinuities, as the permeability of the intact rock is generally very low.

Question 18

Define “number of joint sets.

Answer 18

Number of joint sets determines the ability of the rock mass to deform without actually undergoing any failure within the intact rock.

Question 19

Define block size.

Answer 19

in a rock mass depends on the number of discontinuity sets, spacing and persistence that separates the blocks.

Question 20

The rock mass is described by one of the following adjectives reflecting the block size and shape (ISRM, 1978).

Answer 20

• Massive – few joints or very wide spacing
• Blocky – approximately equidimensional
• Tabular – one dimension considerably smaller than the other two
• Columnar – one dimension considerably larger than the other two
• Irregular – wide variations of block size and shape
• Crushed – heavily jointed to sugar cubes

Question 21

Define stand-up time.

Answer 21

stand-up time is the time that an open excavation can stand unsupported before it caves in.

Question 22

Define joint alteration.

Answer 22

Joint alteration number is a measure of the degree of alteration of the joint wall or infill material, which is quantified in terms of residual friction angle ϕr.

Question 23

Define active stress.

Answer 23

‘active stress’ term. It is well known that water can reduce the effective normal stress (σ′), which in turn reduces the shear strength.

Question 24

Explain the effect of discontinuity in a rock mass.

Answer 24

Due to the presence of discontinuities, the rock mass is weaker than the intact rock specimen, showing lower strength and stiffness

Question 24

GEOLOGICAL STRENGTH INDEX FOR JOINTED ROCKS

Answer 24

INTACT OR MASSIVE – intact rock specimens or massive in situ rock with few widely spaced discontinuities.
BLOCKY – well interlocked undisturbed rock mass consisting of cubical blocks formed by three intersecting discontinuity sets.
VERY BLOCKY – interlocked, partially disturbed mass with multi-faceted angular blocks formed by 4 or more joint sets.
BLOCKY/DISTURBED/ SEAMY – olded with angular blocks formed by many intersecting discontinuity sets. Persistence of bedding planes or schistosity.
DISINTEGRATED – poorly interlocked, heavily broken rock mass with mixture of angular and rounded rock pieces.
LAMINATED/SHEARED – lack of blockiness due to close spacing of weak schistosity or shear planes.

Question 25

Define strong rock.

Answer 25

Specimen requires more than one blow by geological hammer to fracture it.

Question 26

Define Extremely strong rock.

Answer 26

Specimen can only be
chipped by a geological
hammer.

Question 27

What are the parameters of rock mass rating?

Answer 27

It is a rating out of a maximum of one hundred, based on the first
five parameters such as the strength of intact rock, RQD, mean spacing of the discontinuities, condition of discontinuities, groundwater conditions, orientation of discontinuities. The ratings of the first five factors are added to make up the RMR, which lies in the range of 0–100. The last one is the orientation of discontinuities, an adjustment to the RMR considering how favourable or unfavourable the joint orientations are with
respect to the project.

Question 28

What are the two major parameters of GSI?

Answer 28

The two major parameters are (1) surface condition of the discontinuity and (2) interlocking among the rock blocks.

Question 29

Rock mass classification are based on what?

Answer 29

The rock mass is classified based on intact rock properties (intact rock strength), joint characteristics (e.g. spacing and roughness) and the boundary conditions (stress field and water).