Foundations

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Foundations

  1. 1. Foundations and basements Things must go down before they go up
  2. 2. Why does it matter what’s underneath?• Buildings are very, very heavy – An estimate for a “typical” house in the USA is 320,000lb = 143 tons• The weight of a building increases during course of construction• The weight of a building varies as it is used• The ground beneath must support this weight without moving
  3. 3. Subsoil and bedrock• If you go deep enough , you will hit bedrock, but you rarely build directly on it• Most buildings are founded on undisturbed subsoil• Never build on topsoil or peat• Types of undisturbed subsoil – Gravel – Sand – Clay – All of the above
  4. 4. Gravel• Must be firm, natural bed.• Can be very strong if undisturbed• Usually occur in flood plane areas
  5. 5. Sand• Finer than gravel• Must be undisturbed natural bed• Can be extremely strong if sand cannot be pushed sideways• “Running sand” which is full of moving water, is very unsuitable to build on.
  6. 6. Clay• Very common, extremely fine grained powdered rock• Can be very strong• Can be “shrinkable”, changing volume with moisture content• Can contain aggressive chemicals
  7. 7. Peat• Peat is soft, organic soil• Never build on organic soils• Peat can occur beneath layers of otherwise stable soil• Only a borehole down to bedrock will determine conclusively that there are no organic sub-soils
  8. 8. Names and terminology• Foundation – Any sub-structure hidden in the ground• Footing – Strip or pad foundations just below walls and columns• Pile – Column-like foundations going deep into the subsoil• Raft – Wide, thin foundation spreading the weight of the building over the whole of its plan area• Basement – Underground room with walls and floors forming the foundations of the building above
  9. 9. Footings, more than just the bottom of a wall • Basic principles of a “footing”, a wide base to a wall, (or a pad under a column): – Calculate the weight of the building – Establish the pressure the subsoil can support by testing – Calculate the area needed to distribute the weight of the building at less than that pressure into the subsoil • Pressure at base of plain wall, 2t/m2 • Pressure at base of 1m wide footingGround level 0.25t/m2 • Load bearing strength of sub soil may be 1 t/m2
  10. 10. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete wall backfill 1.5 –2m
  11. 11. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench wall backfill 1.5 –2m
  12. 12. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench wall•Little concrete backfill 1.5 –2m
  13. 13. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench wall•Little concrete•Brick layer must backfillwork in trench 1.5 –2m
  14. 14. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench wall•Little concrete•Brick layer must backfillwork in trench•Lots of fill 1.5 –2m
  15. 15. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench •Narrow trench wall•Little concrete•Brick layer must backfillwork in trench•Lots of fill 1.5 –2m
  16. 16. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench •Narrow trench wall•Little concrete •Lots of concrete•Brick layer must backfillwork in trench•Lots of fill 1.5 –2m
  17. 17. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench •Narrow trench wall•Little concrete •Lots of concrete•Brick layer must •Brick layer works backfillwork in trench on surface•Lots of fill 1.5 –2m
  18. 18. Shallow strip vs. deep strip foundationsShallow strip: Deep strip, subsoil trench fill concrete•Wide trench •Narrow trench wall•Little concrete •Lots of concrete•Brick layer must •Brick layer works backfillwork in trench on surface•Lots of fill •Little fill 1.5 –2m
  19. 19. Digging foundationsIn reality, digging foundationtrenches is a dirty, difficult anddangerous job.
  20. 20. Comparison of footings• Soft, non-self supporting soils: wide strip footing best• Firm self supporting soils: always use deep strip/trench fill• Depth of foundation the same for both: down to below level of frost and water effects, where soil is strong enough to bear loads• Maximum practical depth 2 m for footings
  21. 21. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil concrete wall backfilltopsoil
  22. 22. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil concrete wall backfilltopsoil
  23. 23. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil •Shallow excavation concrete wall backfilltopsoil
  24. 24. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil •Shallow excavation concrete •Lots of reinforced concrete wall backfilltopsoil
  25. 25. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil •Shallow excavation concrete •Lots of reinforced concrete wall •Little or no fill •Walls built on raft backfill •Raft forms the ground floor structuretopsoil
  26. 26. Basement retaining wallsBasement “foundation”: supports buildings and sides
  27. 27. Basement retaining wallsBasement “foundation”: supports buildings and sides •Deep excavation: needs support
  28. 28. Basement retaining wallsBasement “foundation”: supports buildings and sides •Deep excavation: needs support •Raft forms the basement floor structure
  29. 29. Basement retaining wallsBasement “foundation”: supports buildings and sides •Deep excavation: needs support •Raft forms the basement floor structure •Lots of reinforced concrete •Basement walls must be reinforced and water proof
  30. 30. Basement retaining wallsBasement “foundation”: supports buildings and sides •Deep excavation: needs support •Lots of reinforced concrete •Raft forms the basement floor structure •Basement walls must be reinforced and water proof •Ground floor suspended over basement
  31. 31. Basement retaining wallsBasement “foundation”: supports buildings and sides •Deep excavation: needs support •Lots of reinforced concrete •Raft forms the basement floor structure •Basement walls must be reinforced and water proof •Ground floor suspended over basement •External walls built on basement walls
  32. 32. Pile foundations: when you have to go deepGround beam:reinforced concrete,supported by thepiles, not the ground subsoilPile: concretesteel, wallconcreteor timber backfill weak fill
  33. 33. Pile foundations: when you have to go deepGround beam:reinforced concrete,supported by thepiles, not the ground subsoilPile: concretesteel, wallconcreteor timber backfill weak fill
  34. 34. Pile foundations: when you have to go deepGround beam:reinforced concrete,supported by thepiles, not the ground subsoilPile: concretesteel, wallconcreteor timber backfill weak fill
  35. 35. Pile foundations: when you have to go deepGround beam:reinforced concrete,supported by thepiles, not the ground subsoilPile: concretesteel, wallconcreteor timber backfill weak fill
  36. 36. Pile foundations: when you have to go deepGround beam:reinforced concrete,supported by thepiles, not the ground subsoilPile: concretesteel, wallconcreteor timber backfill weak fill
  37. 37. Pile foundations: when you have to go deepGround beam:reinforced concrete,supported by thepiles, not the ground subsoilPile: concretesteel, wallconcreteor timber backfill weak fill
  38. 38. Pile foundations: when you have to go deepGround beam:reinforced concrete,supported by thepiles, not the ground subsoilPile: concretesteel, wallconcreteor timber backfill weak fill
  39. 39. Types of pileEnd bearing: Friction: loads Bored pile Driven pileloads to the to the sidesbottom
  40. 40. Types of pileEnd bearing: Friction: loads Bored pile Driven pileloads to the to the sidesbottom
  41. 41. Types of pileEnd bearing: Friction: loads Bored pile Driven pileloads to the to the sidesbottom
  42. 42. Types of pileEnd bearing: Friction: loads Bored pile Driven pileloads to the to the sidesbottom
  43. 43. Piling rigs Pile driver: hammers preformed piles directly into the sub soilBored piling rig:large augerscrewed into Pin piles, small scalesub soil to driven piles; steelcreate deep tubes hammered inhole for pile and filled with concrete
  44. 44. They all use concrete?• All of these techniques refer to this material• It is one of the fundamental construction materials, but usually not well understood• This is why you need to know all about concrete

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