Bored pile technology dates back to the beginning of the last century. Nowadays, with increasingly powerful equipment available and borehole walls supported by bentonite or polymer slurry, this technology can be used to construct foundation piles up to 4 m in diameter and 100 m deep.
Bored piles are constructed in the following stages:
• the soil is removed using drilling tools chosen according to the particular characteristics of the ground;
• the slurry is "regenerated" by removing the coarsest material of the soil (this process is called "desanding");
• a steel reinforcement cage is inserted in the borehole;
• the borehole is filled with concrete.
To prevent the soil near the surface from collapsing, it is established practice to first insert a co-axial casing with a diameter slightly larger than the diameter of the tool.
The actual length of the casing depends on the characteristics of the soil encountered in the first couple of metres of soil excavated.
This temporary casing is usually driven into the ground by using the rotary head of the drilling rig or a hydraulic vibratory hammer connected to a service crane.