Chapter 4 case study 2: formation of piles, driven piles
Here we see a number of square section pre-cast concrete piles awaiting installation. They are driven into place in a pattern that reflects the layout of the new dwelling. The piles seenin situ extend out of the ground by differing amounts. This is a potential problem with pre-formed piles, and the final length must be adjusted by trimming on site.
In situ formed piles
In this case the ground conditions posed problems as there was a layer of made-up ground over the bearing strata, which was at a depth of around 1.5–2 metres. Since this is an isolated instance on the site, which is otherwise suitable for strip foundations, the cost of bringing a piling rig to site was uneconomic. Hence the innovative solution here is to form piles in situ using precast manhole units as permanent formwork.
Continuous flight auger piles (CFA)
The sequence of operations involved in the installation of CFA piles is fast and efficient. The centre of the auger is hollow, allowing concrete to be pumped through it as the auger is extracted from the ground. This means that there is no need to support the sides of the pile cavity during the installation, as the process does not create a void in the ground. Following extraction of the auger the reinforcement cage is lowered into the wet concrete to complete the pile.
Here we see the CFA rig in operation with the end of the auger visible. The reinforcement cage is inserted into the new pile after the concrete is poured.
The use of continuous flight auger (CFA) has increased the speed of pile installation and therefore the costs associated with piling have been reduced greatly. CFA piling is sometimes referred to as grout injection. Here we see a typical CFA rig being used to form piles on a site that is relatively close to other existing buildings. Unlike some other forms of piling the CFA method does not result in excessive vibration, which could cause damage to nearby buildings.