Reinforcement for Bored Piles

Piles subject to tension or lateral forces and eccentric loading, require suitable reinforcement designed to accommodate these forces. Nominal reinforcement for piles in compression only would be four 12 mm bars for a 400 mm pile to five 16 mm bars for a 550 mm pile. A special cage of 5 mm steel, or hoops of flat steel, are employed as lateral ties. The assembled cage should be sufficiently strong to sustain lifting and lowering into the pile bore without permanent distortion or displacement of bars. Bars should not be assembled too densely that concrete aggregate cannot pass freely between them.

Piles loaded in compression need reinforcement of the shaft to a depth of about 2 m greater than the depth of temporary casing. This is to prevent the tendency of concrete lifting, when pulling the casing.

In longer piles overlapping the reinforcements are very frequent. However, in this case aggregates can assemble at the joints and prevent concrete from flowing towards the borehole walls and result in a defective pile shaft. Over designing of steel reinforcement in a bored pile can lead to practical difficulties in constructing the pile shaft. Where large steel cross-sections are unavoidable, consideration can be given to staggering the joints, although this can be difficult to arrange in prefabricated cages. If bar connectors are used, staggering is not a major problem as the lengths involved are shorter. However, these are designed for use either in tension or alternatively in compression, and it is not generally possible to predetermine the orientation of the cage in the pile bore. Mild steel bars can be butt-joined by welding. Provided high-quality welds are used, this can be an effective solution.

Steel concentration can be adjusted depending on the stress pattern. Similarly a reduced density can b used in the plane of the neutral axis.