Module 7 Flashcards
Stress
Stress=F/A; Force over Area
When talking about weight
F=ma=ρVg
ρ is density, or mass per unit volume
V is volume. ρV is mass
g is the acceleration of gravity
V=Az
z=height
F=ρAzg/A
F=ρzg
Stress=ρzg/A
Over a single unit area, Area=1
Stress=ρz*g (per unit area)
Effective Stress
Equation, importance
Effective Stress=Total Stress-Pore Pressure
Provides the resistance to sliding, defines shear stress in soil, it opposes compression.
Pore Pressure
ρ hg=Pore Pressure
water
h is a one dimensional distance from bottom of unit area to where water ends.
Pore pressure is a force in all directions applied by the water
Notice density is pertaining to water, which is an inverse to the density of the porous rock.
PHOTO IN FOLDER
Strain: What is it, equation
PIC IN FOLDER
Strain is a measure of the deformation as a result of stress
Strain=Δz/z
Types of Strain
Elastic, Plastic, Brittle
Elastically strained material bounces back to original after stress
Plastically strained material doesn’t bounce back fully, but partially
Brittle doesn’t bounce back :(
Imagine a rubber band returning to original state after stretching (elastic), rubber band retaining some stretch as a result of stretching (plastic), or a rubber band snapping (brittle).
Synclines and anticlines are examples of elastic stress, elastic waves are a result of elastic stress being released in an earthquake, or sound waves being made by a hammer.
Rock Mechanics: What is it about
How does rock behave under stress? How will the dam of a reservoir react to the stress of all the water?
3 characteristics of rock are considered in this domain:
1. Intact strength, or Unconfined Compact Strength (UCS)
2. Degree of fracturing, or Rock Quality Designation (RQD)
3. Pore Pressure of groundwater within fractures
Rock Mass Rating (RMR) and Rock Rating
Rock Rating is a scare from 1 to 5
Rock Mechanics: Intact Strength
Intact strength, or Unconfined Compact Strength (UCS)
Determined by rock type and degree of weathering. Tested by subjecting a core of rock to pressure until it breaks.
>50 MPa is strong rock
50>x>15 MPa is moderately strong rock
15>x>0 MPa is weak rock
Rock Mechanics: Degree of Fracturing
Degree of fracturing, or Rock Quality Designation (RQD)
Function of the fracture… number, orientation and condition.
Calculated as the Sum of the length of all fractures longer than 10 cm (4 inches) divided by the total length, which is typically 2 meters.
>75% is good rock, 75>x>50% is fair rock, 50%> is poor rock
Rock Mechanics: Pore Pressure
Pore Pressure of groundwater within fractures
This is derived from rate of seepage. Dry, moist, wet, flowing
Soil Mechanics: What is it about
It is about soil, which is anything you can pick up with a bulldozer.
It concerns a soils…
1. behavior when under stress (shear)
2. properties as a construction material
3. When it fails under shear or high pore pressure conditions (liquefaction)
Soil properties
Grain size distribution. This indicates whether a soil is cohesionless (gravels/sand) or cohesive (silt/clay)
Cohesion is the electrostatic tension between particles. More cohesion means they stick together more.
Properties of sand/gravel as a construction material
Can be densely compacted (because pore pressure is low), has high friction angles (steep=still stable), and has minimal compression.
Exception is if sand is clean and loose, under which conditions it is susceptible to liquefaction.
Properties of clay/silt as a construction material
Cannot be densely compacted (because pore pressure is high), has high friction angles (never stable), Responds elastically to shear stress (susceptible to failure). High compressibility and subsidence.
when looking at Plasticity index vs Liquid limit graph, describe where different soil classification are in the 4 quadrants, going clockwise from bottom left
ML, CL, CH, MH/OH
Picture in folder
Order of Soil classification
GW GP GM GC Well sorted, poor, silty, clay
SW SP SM SC well sorted, poor, silty, clay
<12% fines are first 2 in each line
Problem Soils
Montmorillonite- Sodium; Expands with hydration, shrinks without. Popcorn texture.
Sensitive clays with high Liquidity index
Pyrite bearing soils
Organic, compressible soils-carbon oxidizes, soil collapses over time
Clean, loose sands-susceptible to liquefaction
Hydrocollapsible soils (loess)
Liquidity index and Plastic index
LL-water content needed for soil to go from plastic state to liquid state
PI- water content threshold going from plastic state to solid state
Optimum moisture content
compaction is suboptimal when dry or flooded. There is a sweet spot of hydration to maximize soil compaction.
Common subsidence situations
Limestone=sinkhole- Below sediment, limestone bedrock can have cavities as a result of dissolution. The sediment could progressively trickle into the cavity, resulting in an eventual collapse of an structure on top into a sinkhole.
Differential compaction-grabens have varying thickness across the landform. This thickness relates to compaction, and thus variable compaction should be expected to cause issues.
Slope stability
We calculate the factor of safety of a slope by comparing the resisting and driving forces for a mass wasting event (landslide).
Factor of Safety=Resisting Force/Driving Force
Stable if FS>1
Unstable if FS<1
Resisting F=cohesionlength+ wcos(α)tan(φ)
Driving F=wsin(α)
Approaches to preparing for earthquakes at a large scale (across a state)
Deterministic- Go scenario by scenario. For one earthquake, see what areas are effected and delegate tasks to geographically close areas that aren’t as effected.
Probabilistic- Experts decide probability an earthquake erupts within 30 years and uses a computer algorithm to see what areas will likely need help in next 30 years.
Solutions to areas susceptible to earthquake damage-what makes em dangerous?
Loose fine sand, little clay. Shallow (<30 ft) water table, strong cyclical
seismic shaking.
Solutions
1. Densification of soil (compact it in place, pound it)
2. Dewatering the soil (liquefaction less likely)
3. Mixing clay with sands (expensive)
