Bored piers; Also, commonly referred to as bored piles, drilled piers, drilled shafts, caissons, Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles, are a non-displacement deep foundation method whereby deep holes are drilled in the earth for constructing concrete cylindrical foundations mainly to transfer the load of a building to the subsurface layers of the earth at different depths.
Bored piers are commonly used as a foundation solution when the ground conditions are considered fair to good (where there is no threat of shallow water table, where soils are cohesive, where rock and spoil removal isn’t a problematic or costly exercise etc.). They are generally required to support structures where there are weak soils near the surface, or when loads are very high. In addition to supporting structures, bored piers are used to anchor buildings or substantial complexes against uplift forces and to assist structures in resisting lateral and overturning forces.
- A drilling method is determined.
The drilling method will depend on the soil profile. Whilst a good soil investigation report will help decide on the drilling technology it is the piling contractors experience which will determine the geologically specific tooling best suited to minimize disturbance of the surrounding soil and deliver the most cost-effective outcome through increased penetration rates.
- A void is formed by boring before a pier is produced.
A conventional auger is attached to a shaft and rotated under pressure to dig into the soil. When the auger flights are filled with soil, the auger is raised above the ground and emptied to form a shaft (drilled shaft).
- Expecting the unexpected
When we encounter rock it’s not until a borehole is begun that we can properly assess the density and consistency of the formation to then be able to work out the rate of penetration and select the tooling most suitable to form the shaft.
- Once the shaft has been drilled, a pier casing may need to be installed.
We can construct bored piers both cased and un-cased and can also offer the use of temporary pile liners where necessary. A pier casing is often required in more challenging conditions where granular soils or groundwater are present. The casing provides superior resistance to soil collapse in such unstable conditions.
- Next a large diameter concrete cylinder is constructed
This is done by placing reinforcing steel and fresh concrete into the drilled shaft.
The difference between a driven pile foundation and bored pier foundation lies in the method of construction.
Bored pier foundations are typically poured in place and transfer the load only through bearing, while driven piles are driven straight in and transfer the load through friction and/or bearing.
Generally, bored pier foundations are shallower in depth than the driven pile foundation.
Bored pier foundations are preferred in a location where the top strata consist of decomposed rock overlying a strata of sound rock. In such a condition, it becomes difficult to drive the bearing piles through decomposed rock. In the case of stiff clays, which offer large resistance to the driving of a bearing pile, bored pier foundation can be conveniently constructed.
Several considerations need to be factored in when we do a job costing:
– Drilling method required
– Type and size of plant and tooling required
– Project Brief:
Geotechnical report specifying subsurface conditions
Number of holes, depth and diameter of holes required
Cased or uncased piers?
Geotechnical reports, commonly known as soil reports, are fundamental in assessing subsurface conditions and in making recommendations on foundation design parameters. Recommendations and opinions in such reports are based on the interpretation of borehole logs at point locations and information from published geological maps.
Under the Building Act 1993 protection work is work that must be undertaken [by an owner who is proposing building work] to protect an adjoining property of a building site from proposed building works. It can be done on an adjoining property or on the building site where the building work is occurring, or both. An example of protection work is the shoring up of the adjoining property (which may include retaining walls and bored piers).
For more info visit the Victorian Building Authority website on the link here.