Transcript Foundation 1

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
Ground level
• 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
Shallow strip vs. deep strip foundations
Shallow strip:
Deep strip,
trench fill
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
Deep strip,
trench fill
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
•Little concrete
Deep strip,
trench fill
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
•Little concrete
•Brick layer must
work in trench
Deep strip,
trench fill
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
•Little concrete
•Brick layer must
work in trench
•Lots of fill
Deep strip,
trench fill
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
•Little concrete
•Brick layer must
work in trench
•Lots of fill
Deep strip,
trench fill
•Narrow trench
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
•Little concrete
•Brick layer must
work in trench
•Lots of fill
Deep strip,
trench fill
•Narrow trench
•Lots of concrete
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
•Little concrete
•Brick layer must
work in trench
•Lots of fill
Deep strip,
trench fill
•Narrow trench
•Lots of concrete
•Brick layer works
on surface
subsoil
concrete
wall
backfill
1.5 –2m
Shallow strip vs. deep strip foundations
Shallow strip:
•Wide trench
•Little concrete
•Brick layer must
work in trench
•Lots of fill
Deep strip,
trench fill
•Narrow trench
•Lots of concrete
•Brick layer works
on surface
•Little fill
subsoil
concrete
wall
backfill
1.5 –2m
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
subsoil
concrete
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
subsoil
concrete
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
subsoil
•Shallow excavation
concrete
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
subsoil
•Shallow excavation
•Lots of reinforced concrete
concrete
wall
backfill
topsoil
Raft foundations
Raft foundation: stable but weak sub-soil near the surface
subsoil
topsoil
•Shallow excavation
•Lots of reinforced concrete
•Little or no fill
•Walls built on raft
•Raft forms the ground floor
structure
concrete
wall
backfill
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
Ground beam:
reinforced concrete,
supported by the
piles, not the ground
Pile:
steel,
concrete
or timber
subsoil
concrete
wall
backfill
weak fill
Pile foundations: when you have to go deep
Ground beam:
reinforced concrete,
supported by the
piles, not the ground
Pile:
steel,
concrete
or timber
subsoil
concrete
wall
backfill
weak fill
Pile foundations: when you have to go deep
Ground beam:
reinforced concrete,
supported by the
piles, not the ground
Pile:
steel,
concrete
or timber
subsoil
concrete
wall
backfill
weak fill
Pile foundations: when you have to go deep
Ground beam:
reinforced concrete,
supported by the
piles, not the ground
Pile:
steel,
concrete
or timber
subsoil
concrete
wall
backfill
weak fill
Pile foundations: when you have to go deep
Ground beam:
reinforced concrete,
supported by the
piles, not the ground
Pile:
steel,
concrete
or timber
subsoil
concrete
wall
backfill
weak fill
Pile foundations: when you have to go deep
Ground beam:
reinforced concrete,
supported by the
piles, not the ground
Pile:
steel,
concrete
or timber
subsoil
concrete
wall
backfill
weak fill
Pile foundations: when you have to go deep
Ground beam:
reinforced concrete,
supported by the
piles, not the ground
Pile:
steel,
concrete
or timber
subsoil
concrete
wall
backfill
weak fill
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
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