Bearing Capacity - Foundation Types

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Bearing Capacity - Types of foundation in relation to soil type & condition

Strip Foundations Strip foundations are generally used for load bearing walls or similar types of construction.

They can be used on most types of soils such as firm to stiff clays and medium to dense granular soils.

Un-reinforced strip foundations

General practice for the most heavily loaded wall of a two-storey dwelling house

Load per meter run is about 50kN

Taking an overall width of 450mm this would give an average bearing capacity of 110kN/m2

For this value the FOS (factor of safety) on the ultimate bearing capacity of a stiff clay or dense sand may be 4 - 5.

The maximum safe bearing capacity of a soil with good supporting value is not always utilised. For this reason depending on soil and site conditions, a narrow strip foundation may be used.

Points to take into consideration when deciding whether or not to use a narrow strip foundation:

Economics (cost of materials/labour)

Excavation type (hand or machine)

Type of soil

Stability of soil

Shrinkage and swelling of soil type

On sloping ground the strip foundation need not be on the same level - it could be stepped.

Reinforced strip foundations

Usually has a cost advantage over un-reinforced concrete where weak and heavy wall loading require a wide strip at relatively shallow depth

reinforcement in the form of longitudinal bars is desirable to bridge over local weak or hard spots, or when there are variable load conditions

Pad Foundations Pad foundations are generally used for columns and piers.

The methods of design of reinforced and unreinforced pad foundations are similar to those of strip foundations. The method of construction, and the foundation dimensions etc will also be governed to some extent by the soil conditions and type.

For a series of columns the continuous pad, or beam construction may be a more viable option both economically and practically

Raft foundations Raft foundations are commonly used in soils with a low bearing capacity where the foundation pressure must be spread over a large area. They are also used on soils where the compressibility varies - the partial rigidity of a still slab and beam construction is utilised to bridge over areas of more compressible soil. This minimises differential settlement of the foundation.

Raft foundations are used particularly in mining areas where foundations are susceptible to subsidence and differential settlement due to old mine workings and shafts.

Plain Slab rafts

Used where large settlements are not anticipated and a high degree of stiffness is not required

Construction consists of a mesh reinforcement in the top and bottom of the slab. This resists bending moments that may form within the slab due to ground movements

The slab is usually constructed below ground level

In soils susceptible to frost, the slab is usually at least 450 - 500mm below ground level and possibly down to a depth of 750mm.

Stiffened edged slab rafts

suitable for foundations of single or two storey buildings on weak compressible soil such as peaty clays or very loose granular fill

the ground floor slab is an integral part of the raft and stepping down of the peripheral part of the slab

Slab and beam raft

Used as a foundation for heavy buildings where stiffness is the principle requirement to avoid excessive distortion

This distortion may be caused as a result of load variations or compressibility of the soil

Deep raft foundations - typically used for basement or cellular/buoyancy raft foundations.

Pile foundations

Piles are relatively long, slender members used to transmit foundation load through soil strata of low bearing capacity to deeper soil or rock strata having higher bearing capacity. They are also used in normal ground conditions to resist uplift forces or in poor soil conditions to resist horizontal loads.

Piles are a convenient method of foundation construction for works over water such as jetties, bridges and piers.

Some typical foundation problems where a pile system might provide a solution are:

Where a soil layer with adequate bearing capacity is too deep to use conventional footings

Where the soil layers immediately beneath a structure are soft or poorly compacted

for structures transmitting very high concentrated loads

for structures transmitting significant horizontal or inclined loads

Piles are classified into two types:

Driven or displacement piles - which are usually pre-formed then driven or hammered into the ground

Bored or replacement piles - where a hole is bored into the ground, and the pile is then formed within the hole, usually from reinforced concrete.

Another way to classify piles is in terms of the way their load carrying capacity is derived:

End bearing piles - the pile acts as a laterally restrained column, with the load being transmitted to the base of the pile and resisted by the soil/rock at base level

Friction piles - the load is transmitted to the soil through frictional resistance along the shaft of the pile. Pure friction piles tend to be quite long as the load carrying capacity is a function of the shaft area in contact with the soil.

In the majority of cases for friction piles, the load carrying capacity is also dependant on the end bearing, as well as shaft resistance, and so may be designed for both to be utilised.

Other types of pile foundations are tension piles (to resist overturning of tall buildings) or piles with enlarged bases to carry high loads in end bearing.