Module 2: Foundations & Building Structure Strip, raft, pad & beam, pile — and what they do

You can't diagnose foundation movement without knowing how the foundation works. The four common types and the soils they suit.

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CORE Subsidence · Module 2 of 6

Content

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Subsidence Repairs

Once the cause of damage is confirmed as subsidence and any mitigation works are completed (drain repair, tree management works, etc.) the building needs to be repaired:-

Substructure Repairs – If mitigation works cannot achieve ground stability, substructure reinforcement is required. Methods include:-

Traditional Underpinning – constructing a new mass concrete foundation at a stable depth

Pad & Beam – Excavating intermittent concrete “pads” down to firm ground and constructing a continuous concrete beam across the pads.

Piling – using an auger to form a pile down to a designated depth and placing reinforcement and concrete into the augured hole

Resin Injection – Used to stabilise ground and lift structures

Superstructure Repairs – Once ground stability is achieved repairs the cracks in the building will need repairing. The type of masonry will dictate the repair method. Crack stitching methods may also be used

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Foundations

  • Strip Foundation
  • Raft Foundations
  • Pad and Beam Foundation
  • Piling

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Foundation

Concrete Strip Footings

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Trainer note:

The purpose of a foundation is to transmit the load onto the ground safely. A heavy load will need a wider footing to stop it sinking into soft ground (that’s why excavators have wide caterpillar tracks). A wide footing exerts a reduced pressure.

Concrete Strip Footings - a 150 mm – 250 mm thick concrete strip laid at the bottom of a trench forms an effective foundation. The width of the strip is increased in softer ground. The strip can be reinforced with steel bars to span over ‘soft spots’ of earth.

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Trench Fill

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Trainer note:

Trench Fill - modern foundations are deeper (typically 900 mm-1500 mm) than, say, pre-war houses (typically 250 mm-600 mm). Builders found it quicker and cheaper to simply fill the foundation trench with mass concrete rather than dig a trench, lay a strip footing and build up the substructure wall.

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Raft Foundations - in very weak ground it may be necessary to spread the weight of the house over as big an area as possible, i.e. a reinforced concrete slab the size of the house.

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Raft Foundation

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Trainer note:

Raft Foundations - in very weak ground it may be necessary to spread the weight of the house over as big an area as possible, i.e. a reinforced concrete slab the size of the house. The raft must be designed to be stiff enough not to break if non uniform settlement occurs. It is common for concrete slabs to be built without reinforcement (eg. small garages). These are better described as “oversite” concrete slabs than true rafts.

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Pad and Beam

Pad and Beam Foundations - where the good ground is deep (typically 2m or 3m or more) it is not economic to dig trenches. Deep trench fill can be affected by earth pressure pushing against its sides (lateral earth pressure). The solution is to dig deep pits every 3 m or 4 m, fill them with concrete and span beams between the filled pits to support the walls above. This form of construction has however generally been superseded by the use of piling in place of the pads as enhanced piling machinery and techniques have developed.

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Trainer note:

Pad and Beam Foundations - where the good ground is deep (typically 2 m or 3 m or more) it is not economic to dig trenches. Deep trench fill can be affected by earth pressure pushing against its sides (lateral earth pressure). The solution is to dig deep pits every 3 m or 4 m, fill them with concrete and span beams between the filled pits to support the walls above. This form of construction has however generally been superseded by the use of piling in place of the pads as enhanced piling machinery and techniques have developed.

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Piling

A steel case is hammered into the ground until a predetermined ‘set’ - i.e. when the casing refuses to move a measured amount when the driving weight hits it. The casing is filled with steel bars and concrete to form the pile. Typical usage of a driven pile is in loose soils or fill (not suitable for desiccated clays).

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Underpinning

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Foundation Carrying Load - Rock

Rock has a high bearing capacity and can take direct wall load with no need for a wide footing

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Foundation Carrying Load - Clay

Clay will have a medium to high bearing capacity and walls will be carried on a strip footing

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Foundation Carrying Load – Soft Clay, Silts

Soft clays, silts etc. will have a low bearing capacity and the wall loads will be spread by a raft foundation

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Foundation Carrying Load – Fill, Peat

Fill, Peat Etc. is very weak and will have no bearing capacity. Even a raft might sink. Piles take the wall to stronger ground

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Solid Floor Construction

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Suspended Floor Construction

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Trainer note:

Suspended floors

• Timber - Probably the most commonly used, spanning back many hundreds of years. The form of construction comprises timber joists, generally spanning the shortest distance, wall to wall, supporting floorboards (or in more recent times chipboard) spanning in the opposite direction. Suspended timber ground floors are generally supported off a series of dwarf walls (known as sleeper walls) reducing the span of the floor joists where the sleeper walls should be taken to separate foundations.

Knowledge check

Pass mark: 80%. You'll get immediate feedback with the correct answer + rationale on each question. Wrong answers can be retaken without penalty.

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