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. 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 footing
Ground level 0.25t/m2
• Load bearing
strength of sub soil
may be 1 t/m2

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

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

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

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

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

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

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

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

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

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
structure
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
Pile: concrete
steel, wall
concrete
or timber backfill
weak fill

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

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

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

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

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

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

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

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

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

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

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

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