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Foundations and basements
Things must go down before they go up
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
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
Gravel
• Must be firm, natural bed.• Can be very strong if undisturbed• Usually occur in flood plane areas
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.
Clay
• Very common, extremely fine grained powdered rock
• Can be very strong• Can be “shrinkable”,
changing volume with moisture content
• Can contain aggressive chemicals
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
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
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 0.25t/m2
• Load bearing strength of sub soil may be 1 t/m2
Ground level
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• Wide trench
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• Wide trench• Little concrete
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• Wide trench• Little concrete• Brick layer
must work in trench
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• Wide trench• Little concrete• Brick layer
must work in trench
• Lots of fill
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• Wide trench• Little concrete• Brick layer
must work in trench
• Lots of fill
• Narrow trench
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• Wide trench• Little concrete• Brick layer
must work in trench
• Lots of fill
• Narrow trench• Lots of concrete
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• Wide trench• Little concrete• Brick layer
must work in trench
• Lots of fill
• Narrow trench• Lots of concrete• Brick layer
works on surface
1.5 –2m
Shallow strip: Deep strip, trench fill
Shallow strip vs. deep strip foundations
concrete
subsoil
wall
backfill
• 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
1.5 –2m
Shallow strip: Deep strip, trench fill
Digging foundations
In reality, digging foundation trenches is a dirty, difficult and dangerous job.
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
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
concrete
subsoil
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
concrete
subsoil
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
• Shallow excavation concrete
subsoil
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
• Shallow excavation• Lots of reinforced concrete
concrete
subsoil
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
• Shallow excavation• Lots of reinforced concrete• Little or no fill• Walls built on raft• Raft forms the ground floor
structure
concrete
subsoil
wall
backfill
topsoil
Basement retaining walls
Basement “foundation”: supports buildings and sides
Basement retaining walls
Basement “foundation”: supports buildings and sides• Deep excavation: needs support
Basement retaining walls
Basement “foundation”: supports buildings and sides• Deep excavation: needs support• Raft forms the basement floor structure
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
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
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
Pile foundations: when you have to go deep
concrete
subsoil
wall
backfillweak fill
Pile: steel, concrete or timber
Ground beam: reinforced concrete, supported by the piles, not the ground
Pile foundations: when you have to go deep
concrete
subsoil
wall
backfillweak fill
Pile: steel, concrete or timber
Ground beam: reinforced concrete, supported by the piles, not the ground
Pile foundations: when you have to go deep
concrete
subsoil
wall
backfillweak fill
Pile: steel, concrete or timber
Ground beam: reinforced concrete, supported by the piles, not the ground
Pile foundations: when you have to go deep
concrete
subsoil
wall
backfillweak fill
Pile: steel, concrete or timber
Ground beam: reinforced concrete, supported by the piles, not the ground
Pile foundations: when you have to go deep
concrete
subsoil
wall
backfillweak fill
Pile: steel, concrete or timber
Ground beam: reinforced concrete, supported by the piles, not the ground
Pile foundations: when you have to go deep
concrete
subsoil
wall
backfillweak fill
Pile: steel, concrete or timber
Ground beam: reinforced concrete, supported by the piles, not the ground
Pile foundations: when you have to go deep
concrete
subsoil
wall
backfillweak fill
Pile: steel, concrete or timber
Ground beam: reinforced concrete, supported by the piles, not the ground
Types of pile
End bearing: loads to the bottom
Friction: loads to the sides
Bored pile Driven pile
Types of pile
End bearing: loads to the bottom
Friction: loads to the sides
Bored pile Driven pile
Types of pile
End bearing: loads to the bottom
Friction: loads to the sides
Bored pile Driven pile
Types of pile
End bearing: loads to the bottom
Friction: loads to the sides
Bored pile Driven pile
Piling rigs
Bored piling rig: large auger screwed into sub soil to create deep hole for pile
Pile driver: hammers preformed piles directly into the sub soil
Pin piles, small scale driven piles; steel tubes hammered in and filled with concrete