Also called caissons, drilled shafts, drilled piers, Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles. Rotary boring techniques offer 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-logged but stable strata - i.e. 'wet boring'.
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. Typical socket depths are equal to the diameter of the pile in hard rock layers and 2.5 times the diameter of the pile in soft rock layers.[citation needed]
'Dry' boring methods employ the use of a temporary casing to seal the pile bore through water-bearing or unstable strata overlying suitable stable material. Upon reaching the design depth, a reinforcing cage is introduced, concrete is poured in the borehole and brought up to the required level. The casing can be withdrawn or left in situ.
'Wet' boring also employs a temporary casing through unstable ground and is used when the pile bore cannot be sealed against water ingress. Boring is then undertaken using a digging bucket to drill through the underlying soils to design depth. The reinforcing cage is lowered into the bore and concrete is placed by tremie pipe, following which, extraction of the temporary casing takes place.
In some cases there may be a need to employ drilling fluids (such as bentonite suspension) in order to maintain a stable shaft. Rotary auger piles are available in diameters from 300 mm to 2400 mm or even larger and using these techniques, pile lengths of beyond 50 metres can be achieved.
A common mode of failure for drilled piles is formation of a reduced section due to the collapse of the walls of the shaft during installation, reducing the pile capacity below applied loads. Drilled piles can be tested using a variety of methods to verify the pile integrity during installation.
[edit] Under reamed piles
Underreamed piles have mechanically formed enlarged bases that have been as much as 6 m in diameter. The form is that of an inverted cone and can only be formed in stable soils. The larger base diameter allows greater bearing capacity than a straight-shaft pile.
[edit] Augercast pile
An augercast pile, often known as a CFA pile, is formed by drilling into the ground with a hollow stemmed continuous flight auger to the required depth or degree of resistance. No casing is required. A cement grout mix is then pumped down the stem of the auger. While the cement grout is pumped, the auger is slowly withdrawn, lifting the soil on the flights. A shaft of fluid cement grout is formed to ground level. Reinforcement placed by hand is normally limited to 6 metres in depth. Longer reinforcement cages can be installed by a vibrator, or placed prior to pumping cement grout if appropriate specialized drilling equipment is used.
Augercast piles cause minimal disturbance, and are often used for noise and environmentally sensitive sites. Augercast piles are not generally suited for use in contaminated soils, due to expensive waste disposal costs. In ground containing obstructions or cobbles and boulders, augercast piles are less suitable as damage can occur to the auger. An alternative to augercast piles in contaminated soils areas would be CMC ground improvement (Controlled Modulus Column- Developed by Menard ) in which a hollow stemmed displacement auger is used to drill the elements to the required depth. This process minimizes spoils and is usually used for warehouses, residential buildings, Bridge abutments, highway embankments and petrochemical plants.[citation needed]
[edit] Pier and grade beam foundation
In most drilled pier foundations, the piers are connected with grade beams - concrete beams at grade (also referred to as 'ground' beams) - and the structure is constructed to bear on the grade beams, sometimes with heavy column loads bearing directly on the piers. In some residential construction, the piers are extended above the ground level and wood beams bearing on the piers are used to support the structure. This type of foundation results in a crawl space underneath the building in which wiring and duct work can be laid during construction or remodelling.