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About bored piers

There are good reasons why developers generally steer clear of 'tricky' sites. Potentially exorbitant groundwork costs, coupled with prolonged periods of uncertainty, can rapidly transform a viable project into a daring gamble. However, what might appear to be a 'problem plot' can sometimes turn out to be a blessing in disguise for self builders willing to take on technical challenges.

The primary role of foundations is to anchor the building to good bearing ground (in other words, ground capable of supporting the building). Even relatively lightweight structures, such as timber frame houses, need to be securely 'fixed in place' to resist ground movement.

The trouble is, conventional trench foundations start to become uneconomic below about 2m deep. So on sites where stable ground is in short supply, you're likely to need something a bit more sophisticated to protect your home from the ravages of nature. This normally means consulting a structural engineer at the design stage. The need for 'special' foundations can also extend the time taken for groundworks from three or four days for a conventional build, to perhaps two or three weeks - more than doubling your total costs for this stage of the project.

What Constitutes a Tricky Plot?

The three main causes of escalating groundworks are sloping sites, the close proximity and/or presence of trees, and 'bad ground';

  • Sloping Sites
  • Presence of Trees
  • Bad Ground

Bored Piers

Also called caissons, drilled shafts, drilled piers, cast-in-drilled-hole piles (CIDH piles) or cast-in-situ piles, a borehole is drilled into the ground, then concrete (and often some sort of reinforcing) is placed into the borehole to form the pile. Rotary boring techniques allow larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Construction methods depend on the geology of the site; in particular, whether boring is to be undertaken in 'dry' ground conditions or through water-saturated strata. Casing is often used when the sides of the borehole are likely to slough off before concrete is poured.

A machine similar to a large posthole digger drills out the pier holes. The drilling rig can also penetrate to depths of three metres or more without any effort or problem in free standing soil. There is minimal disturbance to adjoining structures.

For end-bearing piles, drilling continues until the borehole has extended a sufficient depth (socketing) into a sufficiently strong layer. Depending on site geology, this can be a rock layer, or hardpan, or other dense, strong layers. Both the diameter of the pile and the depth of the pile are highly specific to the ground conditions, loading conditions, and nature of the project. Pile depths may vary substantially across a project if the bearing layer is not level. Drilled piles can be tested using a variety of methods to verify the pile integrity during installation.

Bored Piers are the ideal foundation solution for 'Bad Ground' environments!

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Different types Some types of concrete slabs may be more suitable to a particular site and climate zone than others. Slab-on-ground Slab-on-ground is the most common and has two variants: conventional slabs with deep excavated beams and waffle pod slabs, which sit near ground level and have a grid of expanded polystyrene foam pods as void formers creating a maze of beams in between. Conventional slabs can be insulated beneath the broad floor panels; waffle pods are by definition insulated beneath. Both may benefit from slab edge insulation. Suspended slab Suspended slabs are formed and poured in situ, with either removable or 'lost' non-loadbearing formwork, or permanent formwork which forms part of the reinforcement. Precast slab Precast slabs are manufactured off site and craned into place, either in finished form or with an additional thin pour of concrete over the top. They can be made from conventional or post-tensioned reinforced concrete, or from autoclaved aerated concrete (AAC) (see Autoclaved aerated concrete).