Quiz 3 Topic 4 Flashcards
What determines stability of structures built by engineers?
the properties of rocks and soils and the
processes that alter them determine the stability of those
structures
Define rock mechanics?
is the study of the properties and mechanical
behavior of rock materials in response to the forces acting on
them within their physical environment
Why was rock mechanics created?
Created because underground engineering projects and mining
activities (e.g. excavating tunnels) needed to know when and if
rock was going to fail
What type of projects is rock mechanics important in?
Important for every project where the rock is the structure or supports a structure
Is Mass wasting a landslide?
No, landslide is type of mass wasting
What are mass wasting events classified by?
The type of movement (flow, slide, fall)
The type of material (rock/sediment)
and the velocity
What is the def of mass wasting?
Mass wasting is the downslope movement of earth materials due to gravity.
What are falls?
Type of mass wasting- is
Free fall of earth
materials
* Rocks loosened by:
* Root growth
* Frost wedging
* Heavy precipitation
* Etc…
is very fast
Define slides/landslides?
Coherent masses of earth material slide down slope along a failure
surface called a slide plane within well-defined boundaries.
What are slumps and glides? What are the diff between them?
Are both types of slides (have a well defined plane where a whole chunk of material moves in one slide)
Slumps are a rotational slide that have a curved failure surface
Glides- Have planar failure surfaces
What faster slumps or glides?
glides are rapid slumps are slow
Define slumps?
are slow slope failures along a curved slide surface.
* Blocks rotate backwards during failure
* Usually occurs in homogeneous substrate, as opposed to strongly
stratified (layered) and lithified (stuck together) rock masses.
Name parts of a slump?
Head of slump where failure starts is crown, bottom is toe
define scarp?
a steep
scar on the
undisturbed side
of the failure. The
zone of
detachment.
What are block glides? Are they faster or slower than slumps?
occur when coherent masses of rock or sediment
move along planar sliding surfaces, are faster than slumps
What can be the plane of weakness in slumps?
Failure planes can be sedimentary bedding planes, metamorphic foliation planes, faults, and fractures
What are flows?
are mass movements of unconsolidated (loose) material that move overland
Why type of behaviour do flows exhibit?
fluid like behaviour
What are flows caused by?
Rainfall, steep slopes, no vegetation, presence of loose soil and debris
What is an example of a slow flow?
creep
What is creep and hows it caused?
is slow downslope movement of rock and soil particles near the ground surface, looks continuous but isnt.
This caused by freeze thaw cycles and thermal cycles and wet and dry cycles.
At night it freezes water so it expands, in day it thaws and gravity pulls it downwards, leads to creep.
What does the rate of creep depend on?
Rate depends on the
steepness of slope, water
content, type of
sediment, and
vegetation
What are some effects of soil creep?
Objects that are resting on top of the soil are carried by it as it descends down the slope, for ex with trees the tops of the root are curved due to creep but the bottoms are straight so the tree will be curved as the bottom but grow straight.
What are the three fast flows?
rock avalanches, debris flows, mudflows
What do fast flows material consist of?
Dense mixtures of sediment and water
What do rock avalanches material that flows consist of?
rock fragments, usually have a big chunk of free falling rock that slams into something and creates small pieces of rock (here there is NO water)!
What type of sediment does debris flow contain?
debris flow contains coarse sediment- you have a range of sediments including sediment that are very big and very small
What do mudflows consist of?
mud
What are lahars?
Is slurry of water and volcanic ash going downstream
How are flows created out of sediment?
most sediments have a small film of water that strengthens the sediment due to water tension pulling it inwards, however if too much water, the sediment becomes saturated and the pressure of the sediment gets exerted on the water which responds with an equal force keeping sediment apart and allowing it to flow.
What are the characteristics of debris flow?
behave like a fluid
and can flow very fast (10 m/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 sloping surface
* Often get funneled into channel
and deposited on a valley floor
What are the two jobs done in an engineering project?
Engineering the structure to make sure it’s secure and engineering the structure in a place that is secure
Why was building a reservoir in vaiont valley a bad idea?
The canyon was steep sided and showed a syncline, limestone were dipping towards reservoir and the water interacted with the limestone which caused dissolving (got undercutting of limestone where it contacted water), the beds of rock had clay in them which got submerged inot water as syncline so slope is towards resevoir these sweeled and caused limestone to dissolves and just slip into the resevoir
Define a rock mass?
is a large body of rock
What are rock masses broken up by?
discontinuities (for ex: planes of weakness that intersect- which divide into blocks of intact rock
What properties do discontinuities give rocks?
gives it an anisotropic character (if you apply stress to rock in one direction it may fail but in another direction it won’t fail) meaning it has different properties in different directions
What are the four types of discontinuities?
Bedding planes
joints/cracks
Faults (breaks with displacement)
Foliation (metamorphic rocks)- have this happen through pressure, get minerals perpendicular to the direction of stress which causes a plane of weakness (a foliation)
What is the fundamental question that needs to be answered by engineers?
Will the combination of initial stresses and stresses induced by the construction and
operation of an engineering structure produce rock failure and what will be the extent
of the failure zone?
To answer the fundamental question what two things do you need to know?
You need to know how strong the rock mass is and how stressors are redistributed due to interactions between the rock mass and the structure
What does the strength of the rock mass depend on?
the strength of the intact rock, and
The strength of any discontinuities
define strain
Change in the shape and/or volume of a rock
caused by stress is called strain
What are three stages of strain?
elastic deformation- fully reversible
Ductile deformation- no fracture but is fully irreversible, will maintain warped shape
Brittle deformation- is where fracture occurs as stress is too much
what stage of strain has stress proportional to strain?
elastic deformation
What’s the diff between stress and strain
stress is force applied, strain is deformation that occurs
What tests show the three different stages of strain
the unconfined compressive strength tests
Define rock strength?
the maximum amount of
stress you can apply to a rock before it
fractures.
How do you measure rock strength?
Through an unconfined compressive strength test:
here a piece of rock is removed from ground and an unidirectional force is applied axially until the rock breaks.
The rock strength is calculated by dividing the maximum load at failure by the cross-sectional area of the sample
What is the basis of rock mechanics?
Uniaxial compressive strength is the basis
for classification in rock mechanics.
What is ultimate strength?
The amount of strain the rock can go under until it reaches rupture (brittle deformation), this is the number used to compare rock strength
What rocks are defined as strong?
igneous and metamorphic rocks are very strong
What rocks are medium rocks?
sedimentary rocks- like siliciclastic/carbonate rocks
What are the weakest rocks?
soils, clay rocks as no crystalline structures holding them together
What makes igneous rocks stronger than sedimentary rocks?
the intercrystallien structures in them make them really strong
What makes sedimentary rocks stronger than soils?
are clastics and carbonates, have grains of stuff connected by cement/sediment- gives variation in composition which make sit stronger than soil
Why is marble weaker than igneous rocks?
for ex: marble- a metamorphic rock, the crystals are large but it’s weaker than igneous rock, why? this is because calcite isn’t strong compared to quartz in igneous rocks
Why are soils so weak?
They are unconsolidated- so therefore only held together by cohesion- more clay means stronger soil. NO cement or crystalline texture so weaker than igneous and sedimentary rocks
What controls rock strength?
The rock type (igneous, metamorphic, sedimentary)
the rock type is based on Mineral composition (e.g. felsic)
* Texture (crystalline or clastic)
* Structures (foliation, bedding, folds)
What is unweathered rock strength based on?
mineral assemblage and texture
What are the strongest rocks?
igneous and metamorphic rocks
What is stronger extrusive or intrusive rocks?
extrusive igneous rocks as fineley crystalline rocks have more cohesive forces- so basalt better than granite for cliffs
What does the strength of clastic rocks depend on?
the strength of clastic rocks depends on the developement and type of cementation
What is stronger quartz clastic rock or calcite one?
quartz clastic rock
Why are mudstones, shales, and clays weaker than sandstones?
because mudstones, shales, and clays only have van der walls forces that hold it together, sandstone actually has some intercrystalline forces
What is the second thing that affects rock strength?
Confining Pressure
* Rocks that are buried deeply experience more confining pressure
(equal compression in all directions)
* Confining pressure increases rock
strength
What is tuff?
volcano clastic rocks
What is the third thing that affects strength of rocks?
Water, as if saturated you get an opposite pressure of pore pressure which undoes confinging pressure which makes weak rocks.
During UCS- water is never present
What is the fourth thing that affects rock strength?
Amount and duration of stress
Stress applied over a long period of time means rocks more
likely deform plastically
What is the fifth thing that weakens rocks?
weathering, Increases porosity and permeability in a rock, creating more surface area that can
be subjected to weathering
* Eventually, weathering will turn a rock (strong) into a sediment/soil (weak)
weathering also erodes layers of rocks reducing unconfined pressure whihc causes rocks to bounce back and crack causing more discontinuties making it a weaker rock.
A rock mass is only as strong as?
Your discontinuities
What is the sheer strength?
is tested via a sheer force test
Apply sheer parallel to discontinuity and apply normal force (perpendicular) and ultimate strength of discontinuity is when the rocks slip. Need to do a diff orientation for testing each discontinuity.
What controls the strength of discontinuities?
surface roughness, joint width, Extent of weathering in fracture planes, Water, Continuity, Spacing, Orientation,
How does surface roughness effect the strength of discontinuities?
rough surfaces act to increase coefficient of friction, make sit stronger
How does joint width affect discontinuity strength?
Hairline fractures lead to much stronger rocks thna gapped joints.
How does weathering effect discontinutity strength?
weathered fracture planes tend to be
weaker than fresh fracture planes as they produce soil in the fractures. (Strength of soils is low compared to rocks). Roots make rocks stronger if soils but roots growing in crack of rock will make it weaker
How does water effect the strength of discontinuties?
Water reduces friction, wet fractures are therefore weaker than dry, flowing fractures- have high pore pressure which make it more prone to failure also can saturate joints and increae weathering
How does continuity affect strength of discontinuties?
shorth length continuous are stronger that long ones a sheet of rock failure is smaller
How does spacing affect discontinuity strength?
Less psace betwene cracks are worse than more psace
How does orientation affect the strength of discontunites?
Daylighting: occurs when discont is in same orientation as slope of surface, this is very bad when it happens weakens discont. stepper plane of weakness the more likely the rocks will fail.
What does RQD stand for?
Rock Quality Designation
What does RMR stand for?
Rock Mass Rating
