T013 Foundation types

T013 Foundation types

1. Types of foundation

   • Shallow
     • Spread foundation
     • mat/ raft foundation
     • used: when stable soil of good bearing capacity is near the surface.
       • foundations r placed directly below and transfer the bdg load directly 2 the supporting soil
   • Deep
     • piling
     • piled walls
     • Diaphragham walls
     • cassions
     • used: when the underlying soil has unstable bearing capcacity
       • Foundations extend to transfer load to more app bearing stratum rick or dense soils
   • Factor 2 consider when selecting foundations
     • pattern & size of building load
     • Subsurface & g.water conditions
     • impact on adj properties
     • BR req.
     • construction method & risk

2. Advantages of shallow foundations

   • affordable cost
   • simple construction
   • doesn't need expertise

3. shallow foundation

   • spread footings
     • an enlargement @ bottom of column/ bearing wall - spreads the load over large soil area
     • footing must be placed below the depth of frost penetration layer to avoid ground heave
       • frostline = 305mm
     • common as cheap & ease of construction
     • used in small- medium sized structures
     • Shape and sizes of spread footings
       • Square/ Pad
         • individual spread footing supporting columns & piers
         • use 1:2:4 concrete mix + reinforcement
         • reinforcement resists/carries tension loads
         • types
           • mass concrete 4 steel columns
           • reinforced concrete with slopping upper face
           • plain reinforced concrete
           • stepped reinforced concrete
             • accommodate sloping gradient + maintain required depth
       • Rectangular
         • used when obstructions prevent using pads
         • use when large movement r present
       • circle spread
         • used: flag poles; power lines etc
       • combined
         • support more than one column; maybe support interior loads as well as perimeter
         • useful when columns r 2 close 2gether 2 have own footings
       • continuous spread
         • continuous spread footings of foundation walls
       • ring spread
         • continuos footing in a circle
         • commonly used to support wall above ground or storage tanks
   • 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
   • Raft fondations
     • the bdg is essentially put on one large footing
     • flat concrete slab - heavily reinforced
       • may b stiffened with ribs/ beams
     • used: when bearing capacity of soil is low compared to building loads
       • rule of thumb: if footing are going to spread the area of 50% of bdg footprint ; its economical 2 have a raft
     • advantage: reduce differential settlement as concrete resists the movement
     • Used: when soil is prone 2 diff settlement - the strenght of the raft will subdue issues
       • e.g. expansive soils create diff settlement
     • used: structural loads r erractic and create diff settlement
     • used: when lateral loads are not uniformly distributed
     • used: when uplift leads r more than footings can cope with
     • when bottom of structure is below the g.water table - e.g. hydrostatic uplift

4. Deep foundations

   • ext through unsuitable soil 2 transfer bdg load to more appropriate bearing stratum
   • piles
     • what is it?
       • a slender structural member either concrete/steel/wood
       • installed in ground to transfer bdg loads
     • when are they used?
       • when soil nr surface doesn't have enough bearing capacity 2 support bdg
       • settlement of soil exceeds tolerable limits
       • diff settlement - soil variability/ non-uniform structural loads
     • transfer of load in 2 ways:
       • end bearing pile
         • transmit load to firm layer of grounds e.g. rock; gravel; v. dense sand
       • friction pile
         • tranfer load to penetrable soil by means of friction/ cohesion between soil and embedded surface of pile
     • Types of piles
       • concrete
         • 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
         • pre-cast
           • square/octagonol/circular x-sections
           • fab off-site & pre-stressed
           • problems with transporting some lenghts
           • higher capacity than timber
         • drilled shafts
       • steel
         • 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
         • welded sections up 2 70m
         • needs treatment from corrosion
       • timber
         • oldest form of pile
         • length dependent on tree size
         • usually used as frictional pile
         • can have a steel shoe and drive band 2 prevent spliting
         • susceptible to rot
         • cheaper but low capacity
       • composite
         • constructed of 2 materials e.g. timber & concrete upper so 2 prevent the portion above water from deteriorating
     • when to choose which type of pile?
       • pile type avilable
       • Locvation & type of structure (magnitude of load)
       • g. conditions (soil type)
       • cost
       • durability
     • Pile Construction
       • Displacement
         • soil is displace radially & virtically as pile shaft is driven or jack into the ground
         • drop hammer/ jack
         • 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
         • 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
       • Non-displacement
         • soil is removed leaving a hole 2 b filled with concrete or pre-cast concrete piles dropped in 2 hole & grouted in
         • Auger drilled
         • types
           • small dia. in-situ
           • large in-situ
           • partially preformed
           • grout/concrete intruded piles
   • piers
     • its a vertical bridge support
     • a foundation 4 carrying very heavy structural load
     • things to consider
       • 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
       • reinforcement : hard 2 get to depth - use large bars vs. more bars
   • cassions
     • what is it?
       • prefab hollow box thats sunk into the ground & filled with concrete = foundation
       • common bridge/pier work
       • cassions can be floated to a location to be sunk in place
       • created by aurguing a deep hole in the ground; re-bar and concrete poured; normally bell shaped
   • compensated foudation