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GLGY202 Applications of Geoscience > Engineering Considerations > Flashcards

Engineering Considerations Flashcards

1
Q

Structures designed and built by civil engineers lie on or below the Earth’s surface. The properties of:

A

rocks and soils and processes that alter them determine the stability of those structures

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2
Q

Altering processes include:

A

weathering, deformation, earthquakes, volcanoes, heavy precipitation, etc.

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3
Q

Rock mechanics

A

The study of the properties and mechanical behaviour of rock materials in response to the forces acting on them within their physical environment

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4
Q

Why was rock mechanics created?

A

Because underground engineering projects (i.e digging a tunnel) needed to know when and if rock was going to fail

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5
Q

What projects are important for using rock mechanics?

A

Projects where the rock is the structure or supports a structure

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6
Q

Mass Wasting

A

The downslope movement of earth materials due to gravity

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7
Q

Mass wasting events are classified based on:

A
  • Type of movement (flow, slide, fall)
  • Type of material (rock or sediment)
  • Velocity
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8
Q

Falls

A

Freefalls of earth materials

  • Rocks are loosened by: root growth, frost wedging, heavy precipitation, etc.
  • Velocity: extremely rapid
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9
Q

Materials classify the type of fall:

A

Rock = rockfall
Fine-grained soil = earthfall
Coarse-grained soil = debrisfall

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10
Q

Slides/landslides

A

Coherent masses of earth material slide down slope along a failure surface called a slide plane within well-defined boundaries (debris flows, in contrast, flow over the land surface)

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11
Q

Slumps

A

Slow slope failures along a curved slide surface

  • Blocks rotate during failure (rotational slide)
  • Usually occurs in homogeneous substrate as opposed to strongly stratified (layers) and lithified (stuck together) rock masses
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12
Q

Scarp

A

Steep scar on the undistributed side of the failure, the zone of detachment

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13
Q

Block Glides

A 
Occur when coherent masses of rock or sediment move along planar sliding surfaces
Failure planes can be:
- Sedimentary bedding planes
- Metamorphic foliation planes
- Faults
- Fractures
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14
Q

Flows

A

Mass movements of unconsolidated material move over land

- Fluid-like behaviour

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15
Q

Flows are caused by:

A
  • Rainfall
  • Steep slopes
  • Lack of vegetation
  • Presence of loose soil and debris
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16
Q

Creep

A

Imperceptibly slow downslope movement of rock and soil particles near the ground surface

  • Appears to be continuous but is the result of numerous minute, discrete downslope movement
  • Rate depends on the steepness of the slope, water content, type of sediment, and vegetation
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17
Q

How does vegetation affect the rate of soil creep?

A

Roots anchor sediment in place and take up water content -> slows it down

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18
Q

Effects of soil creep: objects resting on top of the soil are

A

created by it as it descends down the slope

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19
Q

Fast flows

A

Dense mixtures of sediment and water

  • Rock avalanche (rock fragments)
  • Debris flow (coarse sediment)
  • Mudflow (mud, can transition into debris flow)
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20
Q

In unsaturated sediments, water tension pulls grains:

A

towards each other

  • water in some pore spaces bind particles
  • some pore spaces are filled with air
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21
Q

In saturated sediments, pore pressure

A

pushes grains apart

- water between all particles keeps them apart and allows them to flow

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22
Q

Debris Flows

A
  • Behave like a fluid and can flow very fast (10m/s or more)
  • Most dangerous of all mass movements
  • Occur when heavy rainfall, snowmelt, or dam-burst water mixes with loose soil and rock on a slope surface
  • Often get funnelled into channel and deposited on valley floor
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23
Q

Problems with the choice of site for the dam reservoir:

A
  • Canyon was steep-sided, river had undercut its banks, limestone rocks of canyon walls were interbedded with the slippery clays, which inclined towards the axis of the canyon
  • Saturation of clays reduced their internal strength allowing for slip
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24
Q

A rock mass is a large body of rock

A
  • Generally, rock masses are broken up by discontinuities (planes of weakness) that divide it into smaller blocks of intact rock (unbroken rock between discontinuities, although microscopic discontinuities may exist)
  • This gives rock a discontinuous and anisotropic character, meaning it has different properties in different directions
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25
Q

Types of discontinuities

A
  1. Bedding planes (sedimentary rocks)
  2. Joints/cracks (breaks without displacement)
  3. Faults (breaks with displacement)
  4. Foliation (metamorphic rocks)
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26
Q

Fundamental question for civil/geotechnical engineers:

A

Will the combination of initial stresses and stresses induced by construction and operation of an engineering structure produce rock failure and what will be the extent of the failure zone?

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27
Q

To answer this question, you need to know:

A
  1. How strong the rock mass is, and

2. Strength of any discontinuities

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28
Q

Strength of the rock mass depends on:

A
  1. Strength of the intact rock, and

2. Strength of any discontinuities

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29
Q

Strength of the intact rock:

A
  • Under increasing stress, a rock mass will go through many changes in shape and/or volume until it breaks
  • Change in shape and/or volume of a rock caused by stress = strain
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30
Q

Unconfined compressive strength tests show that rocks can exhibit elastic, ductile, or brittle behaviours

A

Ductile deformation: uniform deformation along a broad band without loss of cohesion (stays together)
Brittle fracture: sudden loss of cohesion along discrete planes (breaks apart)

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31
Q

Measuring Intact Rock Strength

A

Rock strength: the max amount of stress you can apply to a rock before it breaks
- A cylinder of rock is removed from the ground (so no confining pressure) and a unidirectional force is applied axially until the rock breaks
- The material’s strength is calculated by dividing the max load at failure by the cross-sectional area of the sample
o = F/A
- Uniaxial compressive strength is the basis for classification in rock mechanics

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32
Q

What makes igneous rocks stronger than sedimentary rocks?

A

Interlocking crystal structure

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33
Q

What makes sedimentary rocks stronger than soils?

A

Sedimentary rocks are consolidated and soils are unconsolidated

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34
Q

What controls rock strength?

A
  1. Rock type
  2. Confining pressure
  3. Water
  4. Amount and duration of stress
  5. Weathering
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35
Q

Rock type is based on:

A
  • Mineral composition (i.e felsic)
  • Texture (crystalline or clastic)
  • Structures (foliation, bedding, folds)
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36
Q

3 Rock Types

A

Sedimentary: clastic, chemical, or biogenic
Igneous: intrusive and extrusive
Metamorphic: foliated and non-foliated

37
Q

Different rocks have different strengths. The properties that determine

A

rock type also determine how strong it will be

38
Q

Rock strength is controlled by physical properties:

A
  • Mineral composition
  • Texture
  • Structures (discontinuities)
39
Q

Intact, unweathered rock strength depends on:

A
  • Mineral assemblage: framework silicates (quartz) are stronger than sheet silicates (mica)
  • Texture: crystalline textures are stronger than clastic textures with cement
40
Q

The strongest rocks tend to be igneous and

A

some metamorphic rocks due to their interlocking textures of strong minerals

41
Q

Extrusive igneous rocks are generally stronger than intrusives because

A

igneous rocks with fine-grained, interlock crystalline textures tend to have the strongest UCS (unconfined compressive strength) and can maintain the tallest cliffs

42
Q

The strength of clastic rocks depends on the development and type of cementation

A
  • Mudstone and shales are weaker than sandstones due to van der wals bonding (weak residual attraction) between clay minerals
43
Q

Confining Pressure

A
  • Rocks that are buried deeply experience more confining pressure (equal compression in all directions)
  • Confining pressure increases rock strength
44
Q

Water

A
  • In unsaturated sediments and fractured rocks, surface tension helps to hold the sediment or rock together
  • Under saturated conditions, water experiences confining stress and pushes back equally in all directions: pore pressure
  • This counteracts confining pressure, reducing the strength of rock
45
Q

Water (on rock strength)

A
  • In unsaturated sediments and fractured rocks, surface tension helps to hold the sediment or rock together
  • Under saturated conditions, water experiences confining stress and pushes back equally in all directions: pore pressure
  • This counteracts confining pressure, reducing the strength of rock
46
Q

Amount and duration of stress

A
  • Stress applied over a long period of time means rocks are more likely to deform plastically
47
Q

Weathering: reduces rock strength

A
  • Includes both chemical and mechanical processes that break down rocks
  • Increases porosity and permeability in a rock creating more S.A that can be subjected to weathering
  • Eventually weathering will turn a rock (strong) into sediment/soil (weak)
48
Q

How do discontinuities affect rock mass strength?

A

Rock masses are more likely to fail along discontinuities than in intact rock, therefore discontinuities control deformation and failure process in the rock mass

49
Q

Failure along discontinuities

A

Shear strength of a discontinuity
A: rock is trimmed, cut, and fixed in a shear box with mortar or resin. The discontinuity is aligned coincident where the 2 halves of the apparatus meet
B: normal stress is imposed on rock through a jack or pneumatic press. Increasing amounts of shear stress are applied on the apparatus until the rock sample ruptures

50
Q

What controls the strength of discontinuities?

A
  1. Surface roughness
  2. Joint width
  3. Extent of weathering in fracture planes
  4. Water
  5. Continuity
  6. Spacing
  7. Orientation
51
Q

Surface roughness

A

Rough surfaces act to increase the coefficient of friction (helps prevent rock from sliding down no problemo)

52
Q

Joint width

A

Hairline or healed fractures are stronger than gapped fractures/joints

53
Q

Extent of weathering in fracture planes

A

Weathered fracture planes tend to be weaker than fresh fracture planes (strength of soils is low compared to rocks)

54
Q

Water (on discontinuities)

A
  • Wet fractures: water acts to reduce the coefficient of friction (helps it move)
  • Flowing fractures: elevated pore pressure acts to lift and reduce effective stress
55
Q

Continuity

A

Short discontinuous surface transfer some of the stress to intact rock and may not weaken the rock mass significantly

56
Q

Spacing

A

Closely spaced joints reduce rock strength tremendously

57
Q

Orientation

A
  • The steeper the dip of the plane of weakness, the more likely rocks will fail
  • Do joints or joint intersections daylight the slope? (daylighting is fracture surface that dips in the same direction as the slope of the outcrop)
58
Q

Flawless rock does not exist

A

All rock masses contain weaknesses that weaken with time (weathering) and may fail if disturbed (cutting a slope or tunnelling) or triggered (weather event)

59
Q

The geometry, architecture, and strength of discontinuities determine:

A

how the rock will behave if disturbed

60
Q

What is the factor of safety?

A

The main force responsible for mass movement is gravity

  • On a flat surface, gravity acts downward
  • As long as the material remains on the flat surface, it will not move downward
  • Unless the ground becomes weak or unstable, a rock is not going anywhere due to gravity
61
Q

Slope failure often occurs in rock masses when slabs of rock slide down a dipping discontinuity under the force of gravity

A

On a dipping plane, the force of gravity is still straight down even though the block slides downslope at an angle

62
Q

For a dipping plane, the force of gravity (Fg) can be resolved into 2 components:

A

The normal force (Fn): perpendicular to the dipping plane

The driving force (Fs): parallel to the dipping plane

63
Q

The normal force helps hold the rock in place whereas the driving force causes

A

shear stress parallel to the dipping plane that drives the rock down slope

64
Q

How to calculate Fg?

A 
F = ma
Fg = mg
(m = mass (kg) measured or estimated, g = acceleration due to gravity = 9.81m/s^2)

Fg = mg = m x 9.81m/s^2

m = (l x w x h x density) of moving block

65
Q

How to calculate Fs?

A

Dip = angle from horizontal down to the inclined plane
Y = gamma = angle
Trig rules: SOH CAH TOA

SinY = opp/hyp
SinY = Fs/mg
Fs = mg x sinY
66
Q

If a rock is at rest on a dipping plane, there is another force that is acting against the driving force to keep that rock from sliding, which is

A

the resisting force

67
Q

How to solve for Fn?

A

Trig rules: SOH CAH TOA
cosY = adj/hyp
cosY = Fn/mg
Fn = cosY x mg

68
Q

But the normal force is not the only component of the resisting force

A

Frictional resistance and cohesion also prevent the block from sliding down slope

69
Q

Resisting force =

A

umg x cosY + ACo

u = coefficient of friction
A = area of base of sliding block
Co = cohesion
70
Q

If Co = 0, then resisting force =

A

umg x cosY

71
Q

Cohesion is

A

a material’s tendency to stick together

72
Q

Cohesion is higher in

A

clayey sediment than in sandy sediments. Unlike sand, clay can be molded and shaped bc it has cohesion (due to electrostatic forces)

  • Crystalline rocks have v high cohesion bc of their interlocking textures
  • Sand has no cohesion
  • Cohesion in sandstone is created by cements which bind the sand together
73
Q

Discontinuities may or may not have cohesion depending on:

A

whether or not it has been filled with cement

- A fault or fracture filled with cement is healed (also called vein deposits)

74
Q

Sand can form a pile because

A

there is internal friction between the grains

- The more compacted the sand is, the stronger it becomes

75
Q

The coefficient of friction is related to the

A

material type and frictional interaction between grains

76
Q

To evaluate whether or not a rock mass will fail, we need to evaluate the factor of safety

A

FoS = resisting forces/driving forces

  • FoS < 1 means the rock is unstable and will slide
  • FoS = 1 means the rock is at equilibrium
  • FoS > 1 means the rock is stable and will not move

Civil engineering projects require a FoS of 2 or 3

77
Q

Consolidated rocks will move downslope when

A

the driving forces become greater than the resisting forces
- As slope angle increases, driving forces increase while resisting forces decrease. Therefore, steep slopes are more likely to fail than shallow slopes

78
Q

The role of water in rocks:

A
  • Reduces FoS by lowering resisting force
  • In rocks, water can dissolve cements (lower cohesion)
  • Can reduce internal friction if there is enough water present
  • Can produce clays through chemical weathering (clays take on water, reducing cohesion. wet clays are generally weaker than dry clays tho)
79
Q

Factor of Safety =

A

sum of resisting forces/sum of driving forces

= (umg x cosY + ACo)/mgsinY

80
Q

How to increase FoS:

A
  • Increase the coefficient of friction
  • Increase cohesion
  • Decrease slope angle
  • Add more resisting forces (bolts, backfill)
  • Drain water
81
Q

In summary, the geological factor determining the strength of rock masses are:

A
  • Strength of intact rock (based on rock type)

- Strength of discontinuities (based on roughness, spacing, fluids, etc.)

82
Q

Rocks may fail when:

A
  • The rocks were stable, but became unstable over time due to weathering of minerals into clay
  • There are major changes in the state of stress (eg earthquakes, mass wasting, excavating, building)
  • There are major changes in hydrologic conditions (e.g intense precipitation, flooding) that triggers failure
83
Q

Rock Mass Strength can be evaluated by assessing the nature of the rock mass:

A

RQD: Rock Quality Designation
RMR: Rock Mass Rating

84
Q

Rock Quality Designation

A

RQD describes the mechanical quality of rock recovered when taking a core

  • During cutting, a core tends to break at discontinuities
  • Weak rock masses tend to break into small pieces
  • Strong rock tend to remain intact and whole

RQD = (sum of length of core pieces >10cm/total length of core)x 100

85
Q

Rock Mass Rating

A

RMR provides a semi-quantitative measure of the strength or stability of the rock mass based on 6 parameters

86
Q

What are the 6 parameters of RMR?

A
  • UCS (unconfined compressive strength)
  • RQD index
  • Spacing of discontinuities
  • Condition of discontinuities
  • Orientation of discontinuities
  • Groundwater conditions
87
Q

Each RMR parameter is assigned a value (low = bad, high = good)

A
  • Values are added together to give a rating of 100 (VALUES ARE SUBJECTIVE TO INDIVIDUAL WHO MAKES ASSESSMENT)
  • Low ratings represent ~cohesionless, v weak rock mass
  • High ratings represent a v strong rock mass
88
Q

By quantifying the strength of the rock mass, you can determine:

A
  • If the rock will support the structure
  • How much support to add to the rock to make it support the structure
  • How long the rock will last before failing (probability, not reality)

GLGY202 Applications of Geoscience (9 decks)

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