T013 Foundation types

Types of foundation
1. Shallow
  1. Spread foundation
  2. mat/ raft foundation
  3. used: when stable soil of good bearing capacity is near the surface.
    1. foundations r placed directly below and transfer the bdg load directly 2 the supporting soil
2. Deep
  1. piling
  2. piled walls
  3. Diaphragham walls
  4. cassions
  5. used: when the underlying soil has unstable bearing capcacity
    1. Foundations extend to transfer load to more app bearing stratum rick or dense soils
3. Factor 2 consider when selecting foundations
  1. pattern & size of building load
  2. Subsurface & g.water conditions
  3. impact on adj properties
  4. BR req.
  5. construction method & risk
Advantages of shallow foundations
1. affordable cost
2. simple construction
3. doesn't need expertise
shallow foundation
1. spread footings
  1. an enlargement @ bottom of column/ bearing wall - spreads the load over large soil area
  2. footing must be placed below the depth of frost penetration layer to avoid ground heave
    1. frostline = 305mm
  3. common as cheap & ease of construction
  4. used in small- medium sized structures
  5. Shape and sizes of spread footings
    1. Square/ Pad
      1. individual spread footing supporting columns & piers
      2. use 1:2:4 concrete mix + reinforcement
      3. reinforcement resists/carries tension loads
      4. types
        mass concrete 4 steel columns
        reinforced concrete with slopping upper face
        plain reinforced concrete
        stepped reinforced concrete
        accommodate sloping gradient + maintain required depth
    2. Rectangular
      1. used when obstructions prevent using pads
      2. use when large movement r present
    3. circle spread
      1. used: flag poles; power lines etc
    4. combined
      1. support more than one column; maybe support interior loads as well as perimeter
      2. useful when columns r 2 close 2gether 2 have own footings
    5. continuous spread
      1. continuous spread footings of foundation walls
    6. ring spread
      1. continuos footing in a circle
      2. commonly used to support wall above ground or storage tanks
2. lowest part of shallow foundation are spread footings. ext laterally 2 distribute load an area so that it doesn't exceed the bearing capacity of soil
3. Raft fondations
  1. the bdg is essentially put on one large footing
  2. flat concrete slab - heavily reinforced
    1. may b stiffened with ribs/ beams
  3. used: when bearing capacity of soil is low compared to building loads
    1. rule of thumb: if footing are going to spread the area of 50% of bdg footprint ; its economical 2 have a raft
  4. advantage: reduce differential settlement as concrete resists the movement
  5. Used: when soil is prone 2 diff settlement - the strenght of the raft will subdue issues
    1. e.g. expansive soils create diff settlement
  6. used: structural loads r erractic and create diff settlement
  7. used: when lateral loads are not uniformly distributed
  8. used: when uplift leads r more than footings can cope with
  9. when bottom of structure is below the g.water table - e.g. hydrostatic uplift
Deep foundations
1. ext through unsuitable soil 2 transfer bdg load to more appropriate bearing stratum
2. piles
  1. what is it?
    1. a slender structural member either concrete/steel/wood
    2. installed in ground to transfer bdg loads
  2. when are they used?
    1. when soil nr surface doesn't have enough bearing capacity 2 support bdg
    2. settlement of soil exceeds tolerable limits
    3. diff settlement - soil variability/ non-uniform structural loads
  3. transfer of load in 2 ways:
    1. end bearing pile
      1. transmit load to firm layer of grounds e.g. rock; gravel; v. dense sand
    2. friction pile
      1. tranfer load to penetrable soil by means of friction/ cohesion between soil and embedded surface of pile
  4. Types of piles
    1. concrete
      1. in-situ
        form: driving cylindrical steel shells in 2 ground at desired depths
        steel shell doesn't contribute to transfer cap.
        steel shell: open a hole to facilitate construction of concrete pile like formwork
        vigilant QM & construction practice 2 ensure integrity
        cavity filled with concrete
        Advantages
        sustain hard driving
        suits 4 marine environ
        easily inspected
        length can b changes
      2. pre-cast
        square/octagonol/circular x-sections
        fab off-site & pre-stressed
        problems with transporting some lenghts
        higher capacity than timber
      3. drilled shafts
    2. steel
      1. shapes
        H piles
        rolled steel sections
        H: sometimes encase in concrete 2 pt at below water table
        cylindrical
        tapered
        used when req penetrate boulder/ rock
      2. welded sections up 2 70m
      3. needs treatment from corrosion
    3. timber
      1. oldest form of pile
      2. length dependent on tree size
      3. usually used as frictional pile
      4. can have a steel shoe and drive band 2 prevent spliting
      5. susceptible to rot
      6. cheaper but low capacity
    4. composite
      1. constructed of 2 materials e.g. timber & concrete upper so 2 prevent the portion above water from deteriorating
  5. when to choose which type of pile?
    1. pile type avilable
    2. Locvation & type of structure (magnitude of load)
    3. g. conditions (soil type)
    4. cost
    5. durability
  6. Pile Construction
    1. Displacement
      1. soil is displace radially & virtically as pile shaft is driven or jack into the ground
      2. drop hammer/ jack
      3. types of displacement piles
        pre-formed displacement piles
        Precast concrete/ steel
        driven & insitu displacement
        2 forms
        driving a temp steel casing with enclosed end 2 form void and fill with concrete. steel removed
        the same as above but the steel tube is left in place
        Helical insitu
        created using a special type of auger
        the soil is compacted as the auger is screwed into the ground
        the auger has a hollow stem which concrete can b pumped. the auger is slowly unscrewed leaving in-stiu pile in place
      4. method of installation
        drop hammer/weight
        common with displacement
        diesel hammer
        best 4 driving into granular non cohesive soils
        vibration hammer
        jacking
        vibration free/ minimal noise
    2. Non-displacement
      1. soil is removed leaving a hole 2 b filled with concrete or pre-cast concrete piles dropped in 2 hole & grouted in
      2. Auger drilled
      3. types
        small dia. in-situ
        large in-situ
        partially preformed
        grout/concrete intruded piles
3. piers
  1. its a vertical bridge support
  2. a foundation 4 carrying very heavy structural load
  3. things to consider
    1. driling through wet /caving soils need 2 use temp steel casing. Use tremie & pump to remove water & place concrete. more exp and large dia. hole
    2. reinforcement : hard 2 get to depth - use large bars vs. more bars
4. cassions
  1. what is it?
    1. prefab hollow box thats sunk into the ground & filled with concrete = foundation
    2. common bridge/pier work
    3. cassions can be floated to a location to be sunk in place
    4. created by aurguing a deep hole in the ground; re-bar and concrete poured; normally bell shaped
5. compensated foudation

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