Rock Mechanics Quiz 3

Question 1

Define Rock slopes.

Answer 1

Rock slopes either occur naturally or are engineered by people as products of excavations to create space for buildings, highways and railway tracks, powerhouses, dams and mine pits.

Question 2

What is the main purpose of slope stability analysis?

Answer 2

In most civil and mining engineering projects, the main purpose of slope stability analysis is to contribute to the safe and economic design of rock slopes.

Question 3

Define plane failure and illustrate it.

Answer 3

Sliding without rotation
along a face; single or
multiple blocks

Question 4

Define Wedge failure and illustrate it.

Answer 4

Sliding without rotation on two nonparallel
planes, parallel to their line of intersection;
single or multiple blocks

Question 5

Define Circular failure and illustrate it.

Answer 5

Sliding on a cylindrical
face

Question 6

Define Toppling failure and illustrate it.

Answer 6

Forward rotation about
an edge/base; single or
multiple blocks

Question 7

Describe a large rock slope and single sliding mass in relation to mode of failure.

Answer 7

If a rock slope is large and embraces a mix of rock types and structures, more than one of the basic failure modes may be
expected. On the contrary, within a single sliding mass, it is not unreasonable to find more than one of the basic failure modes at the site.

Question 8

Discuss stability of rock slopes.

Answer 8

The stability of rock slopes is greatly controlled by the shear strength along the joints and interfaces between the unstable rock block/wedge and intact rock, as well as by the geometric interaction of jointing and bedding patterns in the rock mass constituting the slope.

Question 9

Challenges in determining the magnitude of the available shear strength.

Answer 9

The magnitude of the available shear strength along joints and interfaces is very difficult to determine due to the inherent variability of the material and the difficulties associated with sampling and laboratory testing.

Question 10

Enumerate the factors that directly or indirectly influence the
strength along the joints.

Answer 10

1. The planarity and smoothness of the joint’s surfaces. A smooth planar surface will have a lower strength than an irregular and rough surface.
2. The inclination of the discontinuity plane with respect to the slope.
3. The openness of the discontinuity, which can range from a small fissure to a readily visible joint
4. The extent of the weathering along the surfaces and the possible infill of the joint with weaker material such as clays and calcareous materials. A calcareous infill may potentially increase the strength of the joint, whereas a soft clay infill may reduce the strength of the joint to
   the same level as the clay material itself.

Question 11

What are calcareous materials?

Answer 11

Calcareous Materials are compounds of calcium and magnesium.

Question 12

Define piezometer.

Answer 12

Piezometer is used to measure underground water pressure. It converts water pressure to a frequency signal via a diaphragm and a tensioned steel wire.

Question 13

Role of Limit equilibrium methods in slope stability analysis.

Answer 13

Limit equilibrium methods have been useful in developing the fundamental understanding of rock slope stability analysis
for simple modes of failure.

Question 14

Role of Numerical methods in slope stability analysis.

Answer 14

Numerical methods help analyze the rock slopes, especially failing in a combination of basic modes and other known failure modes such as erosion, slumping, block torsion, and etc.

Question 15

Define factor of safety (FS) against sliding of a rock block

Answer 15

Factor of Safety (FS) against sliding
of a rock block is defined as FS=Fr/Fi where Fr is the total force available to resist the sliding of rock block and the Fi is the total force tending to induce sliding.

Question 16

Discuss limiting equilibrium and stable slopes.

Answer 16

For a slope on the point
of failure, a condition of limiting equilibrium exists in which Fr = Fi, and thus FS = 1. For stable slopes, Fr > Fi, and therefore FS > 1. In practice, rock slopes with FS = 1.3 to 1.5 are considered to be stable.

Question 17

Define angle of internal friction.

Answer 17

angle of internal friction is the slope of a linear representation of the shear strength of earth materials.

Question 18

Define friction.

Answer 18

Friction is the force that resists motion when the surface of one object comes in contact with the surface of another.

Question 19

factors/physical situations can be present at many field sites (Factors of Safety section)

Answer 19

1. Tension crack in slope with no water
2. Tension crack in rock slope filled with water partially or fully
3. Seepage pressure at the joint or bedding plane
4. Surcharge at the top of the slope
5. Horizontal and vertical seismic loads
6. Stabilising force through reinforcing system such as rock bolts, anchors and cables

Question 20

Define surcharge.

Answer 20

Surcharge is the additional load on the soil due to any overlying structure or any moving object.

Question 21

Define rock bolting.

Answer 21

Rock bolting is the systematic reinforcement and anchorage of rock slopes by the insertion and grouting of steel bars into holes predrilled into the more or less fractured rock mass, improving its stability.

Question 22

Define uplift force.

Answer 22

Uplift force is the upward pressure applied to a structure in the ground that has the potential to raise the structure relative to its surroundings.

Question 23

Two key observations in Variation of factor of safety (FS) with vertical seismic coefficient kv.

Answer 23

1. With an increase in kv in the downward direction, FS decreases almost linearly, but it increases as kv increases in the upward
   direction.
2. FS is greater than unity for any value of kh less than 0.25, and it is higher for smaller values of kh, which is an expected observation.

Question 24

What are the different mechanism of toppling failure?

Answer 24

1. Block toppling takes place in a hard rock mass when individual blocks or columns are composed of two normal joint sets, with the main set dipping steeply into the slope face.
2. Flexural toppling occurs when continuous columns of rock dipping steeply towards the slope break in flexure and tilt forward.
3. Block–flexural toppling is a complex mechanism characterised by pseudocontinuous flexure along blocks that are divided by a number of cross-joints.

Question 25

Explain the illustration of rock slopes made through excavations for highways.

Answer 25

It illustrates how site selection for a highway project can result in stable and unstable slopes. The excavated slope should be created on the hillside only where rock strata dip away from the excavation.

Question 26

Enumerate slope stabilisation methods.

Answer 26

1. Geometrical methods, in which the geometry of the slope is changed. This include slope flattening, removal of part of the soil/rock or load from the top of the slope, construction of pressure berms at the toe, terracing, replacement of slipped material by free draining material and recompaction of slip debris. 2. Hydrological methods, in which the groundwater table is lowered or the water content of the soil/rock is reduced. This include the installation of surface and subsurface drains, inverted filters and thermal methods (ground freezing and heating methods). 3. Chemical and mechanical methods, in which the shear strength of the sliding soil/rock mass is increased or the external force causing the slope failure is reduced. This include grouting, construction of restraining structures, gabion structures, crib walls, embankment piles, lime and cement columns, ground anchors, rock bolting, soil nailing and root piles, earth reinforcement and plantation of grasses and shrubs.

Question 27

What is rock reinforcement?

Answer 27

Rock reinforcement is a method of adding strength to the rock in order to prevent failure.

Question 28

What are the most useful forms of reinforcement and how is it installed?

Answer 28

The most useful forms of reinforcement are rock bolts and anchors, which can be used on both natural and engineered slopes to prevent blocks of rock from falling/sliding away from the main mass when isolated by discontinuity planes. They are installed in such a way that the axial load in the bolt/anchor increases the effective stress at depth in soil and rock, thus improving the strength, and a component of the anchor force may also act to reduce destabilising forces and moments.

Question 29

What is the purpose of steel rods?

Answer 29

Steel rods, known as dowels, are sometimes installed and grouted into the jointed rock to act as reinforcement. Dowels are not stressed during the installation process unlike rock bolts and anchors.

Question 30

How is shotcrete applied?

Answer 30

Shotcrete is a pneumatically applied, fine aggregate mortar that is usually placed in a 50–100-mm layer, and is often reinforced for improved tensile and shear strength. It is generally applied along with drain holes as a surface protection layer to the excavated rock slope face before its significant deformation or alteration, in order to provide a high strength, vary rapidly.

Question 31

What is the purpose of shotcrete?

Answer 31

The shotcrete effectively controls the fall of loose small rock blocks, but it provides little support against basic modes of slope failure discussed in the previous sections.

Question 32

What is grouting?

Answer 32

Grouting is a technique of injecting a fluid grout into the rock mass to replace all air or water present in its fissures and cracks.

Question 33

Enumerate the common stabilisation techniques used in practice.

Answer 33

1. Flattening of overburden slope 2. Trimming of unstable rock blocks 3. Scaling of small loose materials/blocks 4. Construction of drains and drain holes 5. Use of dowels 6. Installation of rock anchor to prevent sliding along discontinuity plane 7. Rock bolting to strengthen the jointed rock mass 8. Construction of concrete or masonry walls with weep-holes 9. Construction of rock trap ditches at the toe of the slopes 10. Providing rock catch fences/walls along the slope to make the adjacent areas safe for public use

Question 34

What is active measures? Give example.

Answer 34

Active measure prevent the detachment of rock blocks from their original position. Example, rock bolts and anchors.

Question 35

What is passive measure? Give example.

Answer 35

Walls, ditches, catch fences, rock sheds, tunnels and so on are passive measures as they do not directly interfere in the process of rock detachment, but control the dynamic effects of moving/falling blocks.

Question 36

When selecting a stabilisation technique or a combination of techniques, what it is considered?

Answer 36

The selection of a stabilisation technique or a combination of techniques requires consideration of geotechnical, construction, and environmental aspects. The selection is also greatly controlled by the level of stabilisation and its design life, and finally the cost. If the stabilisation work has to be effective for a longer period, the initial cost of stabilisation may be higher.

Question 37

Define RMR.

Answer 37

RMR or Rock Mass Rating is a system utilized to rate the quality of a rock mass, following the parameters which are the strength of intact rock, rock quality designation, condition of the discontinuity, mean spacing of the discontinuity, groundwater condition, as well as the orientation of the discontinuity. Each of which has a corresponding value or score, wherein the first five parameters has a total score of 100, and the last one is an adjustment to the RMR considering how favourable or unfavourable the joint orientations are with respect to the project.

Question 38

Define slaking.

Answer 38

Slaking is the process in which earth materials disintegrate and crumble when exposed to moisture.

Question 39

Define berm.

Answer 39

Any level strip of ground at the summit or sides, or along the base, of a slope.

Question 40

Define berm slope.

Answer 40

berm slope means the angle, measured in degrees between the horizontal and an imaginary line joining the bench toe and crest

Question 41

Define berm height.

Answer 41

Bench height is a term used to refer the vertical height of a single bench.

Question 42

What is natural slope?

Answer 42

Natural slopes are formed due to physical processes that include plate tectonics and weathering/erosion of rock masses that result in material deposition.

Question 43

What is engineered slope?

Answer 43

Engineered slopes are constructed slopes that undergo specific design and stabilization measures to ensure stability.