Foundations and basements

1. Foundations and basements Things must go down before they go up

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. 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. Gravel • Must be firm, natural bed. • Can be very strong if undisturbed • Usually occur in flood plane areas

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. 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. 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. 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. 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 footing 0.25t/m2 Load bearing strength of sub soil may be 1 t/m2 Footings, more than just the bottom of a wall Ground level

10. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete wall backfill 1.5 –2m

11. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench wall backfill 1.5 –2m

12. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench • Little concrete wall backfill 1.5 –2m

13. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench • Little concrete • Brick layer must work in trench wall backfill 1.5 –2m

14. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench • Little concrete • Brick layer must work in trench • Lots of fill wall backfill 1.5 –2m

15. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench • Little concrete • Brick layer must work in trench • Lots of fill • Narrow trench wall backfill 1.5 –2m

16. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench • Little concrete • Brick layer must work in trench • Lots of fill • Narrow trench • Lots of concrete wall backfill 1.5 –2m

17. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench • Little concrete • Brick layer must work in trench • Lots of fill • Narrow trench • Lots of concrete • Brick layer works on surface wall backfill 1.5 –2m

18. Shallow strip vs. deep strip foundations Deep strip, trench fill Shallow strip: subsoil concrete • Wide trench • Little concrete • Brick layer must work in trench • Lots of fill • Narrow trench • Lots of concrete • Brick layer works on surface • Little fill wall backfill 1.5 –2m

19. Digging foundations In reality, digging foundation trenches is a dirty, difficult and dangerous job.

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. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil concrete wall backfill topsoil

22. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil concrete wall backfill topsoil

23. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil • Shallow excavation concrete wall backfill topsoil

24. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil • Shallow excavation • Lots of reinforced concrete concrete wall backfill topsoil

25. Raft foundations Raft foundation: stable but weak sub-soil near the surface subsoil • Shallow excavation • Lots of reinforced concrete • Little or no fill • Walls built on raft • Raft forms the ground floor structure concrete wall backfill topsoil

26. Basement retaining walls Basement “foundation”: supports buildings and sides

27. Basement retaining walls Basement “foundation”: supports buildings and sides • Deep excavation: needs support

28. Basement retaining walls Basement “foundation”: supports buildings and sides • Deep excavation: needs support • Raft forms the basement floor structure

29. Basement retaining walls Basement “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. Basement retaining walls Basement “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. Basement retaining walls Basement “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. Pile foundations: when you have to go deep Ground beam: reinforced concrete, supported by the piles, not the ground subsoil concrete Pile: steel, concrete or timber wall backfill weak fill

33. Pile foundations: when you have to go deep Ground beam: reinforced concrete, supported by the piles, not the ground subsoil concrete Pile: steel, concrete or timber wall backfill weak fill

34. Pile foundations: when you have to go deep Ground beam: reinforced concrete, supported by the piles, not the ground subsoil concrete Pile: steel, concrete or timber wall backfill weak fill

35. Pile foundations: when you have to go deep Ground beam: reinforced concrete, supported by the piles, not the ground subsoil concrete Pile: steel, concrete or timber wall backfill weak fill

36. Pile foundations: when you have to go deep Ground beam: reinforced concrete, supported by the piles, not the ground subsoil concrete Pile: steel, concrete or timber wall backfill weak fill

37. Pile foundations: when you have to go deep Ground beam: reinforced concrete, supported by the piles, not the ground subsoil concrete Pile: steel, concrete or timber wall backfill weak fill

38. Pile foundations: when you have to go deep Ground beam: reinforced concrete, supported by the piles, not the ground subsoil concrete Pile: steel, concrete or timber wall backfill weak fill

39. Types of pile End bearing: loads to the bottom Driven pile Friction: loads to the sides Bored pile

40. Types of pile End bearing: loads to the bottom Driven pile Friction: loads to the sides Bored pile

41. Types of pile End bearing: loads to the bottom Driven pile Friction: loads to the sides Bored pile

42. Types of pile End bearing: loads to the bottom Driven pile Friction: loads to the sides Bored pile

43. Piling rigs Pile driver: hammers preformed piles directly into the sub soil Bored piling rig: large auger screwed into sub soil to create deep hole for pile Pin piles, small scale driven piles; steel tubes hammered in and filled with concrete