MODULE 5 Flashcards
Soil Mechanics Triangle
- Ground Profile
- Appropriate Model
- Soil Behaviour
Types of Ground Failure
- volcanic eruptions
- earthquakes and liquefaction
- landslides
- expansive soils
- subsidence
- erosion (and deposition)
Volcanic Hazards engineering
- generally not a surprise
- active volcanic sites known and monitored
- engineered solutions expensive and dont always work so best mitigation is avoidance
Volcanic Hazards
- noxious gases, floods, tsunami, atmospheric shock waves
- crop damage, livestock poisoning, water contamination, famine
Earthquake Hazards
- little knowledge of earthquake effects (learning constantly)
- effects can be mitigated by engineering design, however not always economic
Liquefaction Requirements
- strong earthquake motion
- loose, granular soil
- shallow groundwater
3 Layers of Earth
- core (inner and outer)
- mantle
- crust
lithosphere
continental crust - thicker but less dense
oceanic crust - thinner and more dense
- where it all happens
Plate Movement
- Divergent Boundary
- Convergent Boundary
- Transform Boundary
Divergent Boundary
plates move away from each other - small earthquakes
Convergent Boundary
plates forced together and under each other - large earthquakes (North and South Islands)
Transform Boundary
plates slide past each other (central New Zealand)
New Zealand Plates
North: Pacific under Australian
South: Australian under Pacific
Centre: transform fault to account for differences
Geological Processes of Christchurch
- glaciers, rivers, sea, volcanoes, earthquakes
Basic Rock Types
- igneous
- metamorphic
- sedimentary
Igneous
- crystals
- isotropic (randomly arranged)
- strong
- good aggregate
Metamorphic
- crystals
- anisotropic (aligned)
- variable strength
- poor aggregate
Sedimentary
- particles
- anisotropic (macro scale)
- isotropic (micro scale)
- variable strength
- variable aggregate quality
extrusive
cools quickly (fine crystals)
intrusive
cools slowly (coarse crystals)
basic
dark colour
acidic
lighter colour
increasing metamorphism
- rock becomes increasingly crystalline
- crystals become larger
- crystals align
- eventually different minerals segregate into bands
Earth’s External Processes
- Weathering
- Erosion
- Mass Wasting
Erosion
physical removal of material by mobile agents such as water, wind, ice or gravity
Mass Wasting
transfer of rock and soil downslope under the influence of gravity
Weathering
- Physical Weathering - breaking of rocks into smaller pieces without changing their composition
- Chemical Weathering - breaks down rock components and changes its mineral and chemical composition
Types of Physical Weathering
- frost wedging
- unloading
- thermal expansion
- biological activity
Weathering Susceptibility
- original rocks
- dry climate (cold or hot) likely to experience mechanical weathering
- wet climate (especially hot) likely to experience chemical weathering (fast) - cold temps = slower
- surface area, fracture density
Residual Soil
(in situ)
- forms in place on bedrock, no input of outside material
Transported Soil
- forms an unconsolidated material transported to site (not local bedrock)
Weathering of Pyrite
- very reactive in O2
- frequently formed in coal and shale deposits
- oxidation leads to volume increase and pyrite heave
Mitigation for Pyrite Heave
- excavate with minimum disturbance to rock (shattering of bedrock provides easy entry of air)
- protect exposed surfaces with concrete grout or asphalt coating
- insulate basement floor
- avoid building on fractured shale
- avoid use of pyritic material as fill
- removal of pyrite fill
Transportation agents
- water (fluvial = river ; marine = sea)
- ice (glacial)
- wind
- gravity
with increasing transport distance:
- particles become smaller
- particles become more rounded (spherical)
- particle size distribution becomes more uniform (poorly graded)
Soil characteristics determined by:
- source material
- transport mechanism
- transport distance
- energy of system
- depositional environment
Peat
- high water content
- low bearing capacity
- low shear strength
- highly compressible
- oxidise
- flammable
avulsion
sudden change of channel position
braided river
- multiple streams (braids) divided by bars composed of either sand or gravel
- often extremely dynamic (bars may change position on a day to day basis)
Problems with Braided Rivers
- bank erosion
- loss of land
- bridge destabilisation
- flooding
Major hazards in fluvial soils
- peat
- scour
- channel migration
Engineering Hazards (glacial soil)
- Loess: collapsible, high permeability and subject to internal corrosion
- Varved clays (marine): sensitive and prone to liquefaction and flow
glaciofluvial deposits
sediments formed in association with glacial meltwater
varved clays
fine grained in cold months ; coarse grained in warm months
- form layers ; hazard from spontaneous liquefaction (high w/c, low strength)
Ice Transport
- ice flows as a result of gravity (laminar flow)
- transport distance does NOT increase roundness or decrease grain size
- can transport very large grains (>10 m)
- grinding at base of (warm) ice produces rock flour
abrasion
ice flowing over rock/soil, grinding it smaller and smaller
- leads to formation of rock flour
- can open rock fractures
- warm-based ice tends to be more erosive or abrasive
Till
glacial deposit that is angular and gap-graded
Marine environment - nearshore/shallow water
highest energy, coarse material
Marine environment - wave dominated shorelines
high energy, sand and gravel
Marine environment - estuaries and lagoons
low energy, silt and clay
Marine environment - red/dead seas
lowest energy, evaporation and deposition of salts
Deep Sea sedimentation - Slumps
sediment transport by mass with little deformation or folding of layers
Deep Sea sedimentation - Slurries
debris flows and mud flows, destroy any previous bedding layer
- turbidity currents
- deep sea canyons form by these processes
Deep Sea sedimentation - Ice Rafting
- polar latitudes, debris melting from icebergs
- glacial marine sediment
Turbidity Currents
- slopes above 2o considered relatively steep and worried about slips/failure and stability
- tends to be channelised flow
- as gradient decreases, sediment begins to deposit
- turbidites: what remains after deposition from turbidity currents (well stratified according to grain size)
Mass Wasting
- all processes that bring about slow or rapid downslope movement of soil and rock as a result of gravity
- due to slope angle, slope position and amount of water
- occurs when shear stress is greater than shear strength
Classification of Landslides
- Falls - immediate separation of falling material from parent rock or soil mass
- Slide - moving material remains in contact and movement takes place along discrete shear surfaces
- Flows - material becomes disaggregated and movement occurs without necessarily forming discrete shear surfaces
Landslide Stabilization
- Drainage
- Improving Slope Material
- Modifying Slope Profile
- Supporting or Anchoring the Slide Mass
