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
2
Q
Altering processes include:
A
weathering, deformation, earthquakes, volcanoes, heavy precipitation, etc.
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
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
5
Q
What projects are important for using rock mechanics?
A
Projects where the rock is the structure or supports a structure
6
Q
Mass Wasting
A
The downslope movement of earth materials due to gravity
7
Q
Mass wasting events are classified based on:
A
- Type of movement (flow, slide, fall)
- Type of material (rock or sediment)
- Velocity
8
Q
Falls
A
Freefalls of earth materials
- Rocks are loosened by: root growth, frost wedging, heavy precipitation, etc.
- Velocity: extremely rapid
9
Q
Materials classify the type of fall:
A
Rock = rockfall
Fine-grained soil = earthfall
Coarse-grained soil = debrisfall
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)
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
12
Q
Scarp
A
Steep scar on the undistributed side of the failure, the zone of detachment
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
14
Q
Flows
A
Mass movements of unconsolidated material move over land
- Fluid-like behaviour
15
Q
Flows are caused by:
A
- Rainfall
- Steep slopes
- Lack of vegetation
- Presence of loose soil and debris
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
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
18
Q
Effects of soil creep: objects resting on top of the soil are
A
created by it as it descends down the slope
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)
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
21
Q
In saturated sediments, pore pressure
A
pushes grains apart
- water between all particles keeps them apart and allows them to flow
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
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
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
25
Types of discontinuities
1. Bedding planes (sedimentary rocks)
2. Joints/cracks (breaks without displacement)
3. Faults (breaks with displacement)
4. Foliation (metamorphic rocks)
26
Fundamental question for civil/geotechnical engineers:
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?
27
To answer this question, you need to know:
1. How strong the rock mass is, and
| 2. Strength of any discontinuities
28
Strength of the rock mass depends on:
1. Strength of the intact rock, and
| 2. Strength of any discontinuities
29
Strength of the intact rock:
- 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
30
Unconfined compressive strength tests show that rocks can exhibit elastic, ductile, or brittle behaviours
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)
31
Measuring Intact Rock Strength
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
32
What makes igneous rocks stronger than sedimentary rocks?
Interlocking crystal structure
33
What makes sedimentary rocks stronger than soils?
Sedimentary rocks are consolidated and soils are unconsolidated
34
What controls rock strength?
1. Rock type
2. Confining pressure
3. Water
4. Amount and duration of stress
5. Weathering
35
Rock type is based on:
- Mineral composition (i.e felsic)
- Texture (crystalline or clastic)
- Structures (foliation, bedding, folds)
36
3 Rock Types
Sedimentary: clastic, chemical, or biogenic
Igneous: intrusive and extrusive
Metamorphic: foliated and non-foliated
37
Different rocks have different strengths. The properties that determine
rock type also determine how strong it will be
38
Rock strength is controlled by physical properties:
- Mineral composition
- Texture
- Structures (discontinuities)
39
Intact, unweathered rock strength depends on:
- Mineral assemblage: framework silicates (quartz) are stronger than sheet silicates (mica)
- Texture: crystalline textures are stronger than clastic textures with cement
40
The strongest rocks tend to be igneous and
some metamorphic rocks due to their interlocking textures of strong minerals
41
Extrusive igneous rocks are generally stronger than intrusives because
igneous rocks with fine-grained, interlock crystalline textures tend to have the strongest UCS (unconfined compressive strength) and can maintain the tallest cliffs
42
The strength of clastic rocks depends on the development and type of cementation
- Mudstone and shales are weaker than sandstones due to van der wals bonding (weak residual attraction) between clay minerals
43
Confining Pressure
- Rocks that are buried deeply experience more confining pressure (equal compression in all directions)
- Confining pressure increases rock strength
44
Water
- 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
Water (on rock strength)
- 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
Amount and duration of stress
- Stress applied over a long period of time means rocks are more likely to deform plastically
47
Weathering: reduces rock strength
- 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
How do discontinuities affect rock mass strength?
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
Failure along discontinuities
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
What controls the strength of discontinuities?
1. Surface roughness
2. Joint width
3. Extent of weathering in fracture planes
4. Water
5. Continuity
6. Spacing
7. Orientation
51
Surface roughness
Rough surfaces act to increase the coefficient of friction (helps prevent rock from sliding down no problemo)
52
Joint width
Hairline or healed fractures are stronger than gapped fractures/joints
53
Extent of weathering in fracture planes
Weathered fracture planes tend to be weaker than fresh fracture planes (strength of soils is low compared to rocks)
54
Water (on discontinuities)
- 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
Continuity
Short discontinuous surface transfer some of the stress to intact rock and may not weaken the rock mass significantly
56
Spacing
Closely spaced joints reduce rock strength tremendously
57
Orientation
- 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
Flawless rock does not exist
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
The geometry, architecture, and strength of discontinuities determine:
how the rock will behave if disturbed
60
What is the factor of safety?
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
Slope failure often occurs in rock masses when slabs of rock slide down a dipping discontinuity under the force of gravity
On a dipping plane, the force of gravity is still straight down even though the block slides downslope at an angle
62
For a dipping plane, the force of gravity (Fg) can be resolved into 2 components:
The normal force (Fn): perpendicular to the dipping plane
| The driving force (Fs): parallel to the dipping plane
63
The normal force helps hold the rock in place whereas the driving force causes
shear stress parallel to the dipping plane that drives the rock down slope
64
How to calculate Fg?
```
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
How to calculate Fs?
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
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
the resisting force
67
How to solve for Fn?
Trig rules: SOH CAH TOA
cosY = adj/hyp
cosY = Fn/mg
Fn = cosY x mg
68
But the normal force is not the only component of the resisting force
Frictional resistance and cohesion also prevent the block from sliding down slope
69
Resisting force =
umg x cosY + ACo
```
u = coefficient of friction
A = area of base of sliding block
Co = cohesion
```
70
If Co = 0, then resisting force =
umg x cosY
71
Cohesion is
a material's tendency to stick together
72
Cohesion is higher in
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
Discontinuities may or may not have cohesion depending on:
whether or not it has been filled with cement
| - A fault or fracture filled with cement is healed (also called vein deposits)
74
Sand can form a pile because
there is internal friction between the grains
| - The more compacted the sand is, the stronger it becomes
75
The coefficient of friction is related to the
material type and frictional interaction between grains
76
To evaluate whether or not a rock mass will fail, we need to evaluate the factor of safety
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
Consolidated rocks will move downslope when
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
The role of water in rocks:
- 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
Factor of Safety =
sum of resisting forces/sum of driving forces
| = (umg x cosY + ACo)/mgsinY
80
How to increase FoS:
- Increase the coefficient of friction
- Increase cohesion
- Decrease slope angle
- Add more resisting forces (bolts, backfill)
- Drain water
81
In summary, the geological factor determining the strength of rock masses are:
- Strength of intact rock (based on rock type)
| - Strength of discontinuities (based on roughness, spacing, fluids, etc.)
82
Rocks may fail when:
- 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
Rock Mass Strength can be evaluated by assessing the nature of the rock mass:
RQD: Rock Quality Designation
RMR: Rock Mass Rating
84
Rock Quality Designation
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
Rock Mass Rating
RMR provides a semi-quantitative measure of the strength or stability of the rock mass based on 6 parameters
86
What are the 6 parameters of RMR?
- UCS (unconfined compressive strength)
- RQD index
- Spacing of discontinuities
- Condition of discontinuities
- Orientation of discontinuities
- Groundwater conditions
87
Each RMR parameter is assigned a value (low = bad, high = good)
- 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
By quantifying the strength of the rock mass, you can determine:
- 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)
