foundations

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


concrete

subsoil

wall

backfill

• Wide trench

1.5 –2m



concrete

subsoil

wall

backfill

• Wide trench• Little concrete

1.5 –2m



concrete

subsoil

wall

backfill

• Wide trench• Little concrete• Brick layer

must work in trench

1.5 –2m



concrete

subsoil

wall

backfill


must work in trench

• Lots of fill

1.5 –2m



concrete

subsoil

wall

backfill


must work in trench

• Lots of fill

• Narrow trench

1.5 –2m



concrete

subsoil

wall

backfill


must work in trench

• Lots of fill

• Narrow trench• Lots of concrete

1.5 –2m



concrete

subsoil

wall

backfill


must work in trench

• Lots of fill

• Narrow trench• Lots of concrete• Brick layer

works on surface

1.5 –2m



concrete

subsoil

wall

backfill


must work in trench

• Lots of fill

• Narrow trench• Lots of concrete• Brick layer

works on surface

• Little fill

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

concrete

subsoil

wall

backfill

topsoil

Raft foundations


• Shallow excavation concrete

subsoil

wall

backfill

topsoil

Raft foundations


• Shallow excavation• Lots of reinforced concrete

concrete

subsoil

wall

backfill

topsoil

Raft foundations


• 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 “foundation”: supports buildings and sides• Deep excavation: needs support


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


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 “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 “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

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

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

foundations

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