Bored piles is a type of foundation structure made of reinforced concrete. It is used to carry heavy loads by transmitting the load to a stable soil strata. As the name suggest, bored piles is constructed by first drilling a hole in the ground until a competent load bearing layer is reached. Once this achieved, a reinforcement steel cage is lowered into the drilled hole and the hole is filled with concrete. Thus due to the method of construction of bored piles, it is also known as cast in place piles.
Bored piles varies in diameter and depth. The dimension can varies from 450mm to 2000mm. The diameter or depth of the bored piles are determined by the designers who will size the bored piles according to the load requirement and as well as the soil condition of the site.
Bored piles is widely used and they can be constructed in most soil condition and over water. Due to their flexibility they are widely used in deep foundation for high rise buildings, jetties, bridge foundation and can even be used as vertical retaining structures like a retaining wall or sheet piles wall. In this case the bored piles is known as contiguous bored pile wall.
Bored piles is designed either as a point bearing piles or friction piles. If competent load bearing layer like bed rock is present, then the bored piles will be designed as an end-bearing pile. This means that the load carrying capacity of the piles is mainly derived from the bearing capacity of the rock layer at the toe of the pile.
On the other hand bored piles which are designed as friction piles the load carrying capacity of the piles is derived mainly from the skin friction between the pile surface and the surrounding soil. The designers may some time design bored piles as a combination of skin friction and end-bearing piles.
Bored piles can be constructed as a single pile or a in a pile groups. The can also be inclined to a certain angle. Angle bored piles also known as raked piles are found in structures that requires resistance to horizontal load like in a retaining wall or bridge and piers foundation. However the construction of angle bored piles poses some construction challenges. When bored piles are constructed close to one another or overlapping slightly, this is known as contiguous bored piles wall or secant piles wall.
Below are some simplify diagrams of bored piles uses:-
Single Bored Piles
Group of Bored Piles
Inclined or Raked Piles below Retaining Wall
Contiguous Bored Piles Wall
In the next article we look at how bored piles are constructed.