Understanding Foundation Drilling and Some of the Different Methods Used

Posted on: 22 May 2020

Foundation drilling is usually associated with deep foundations, which are required if the soil near the surface is found to be weak or compressible. Such soil cannot withstand the load of an entire building and, therefore, can compromise its integrity or stability, especially if there is flooding or an earthquake.

What happens is that heavy drilling pieces of equipment are used to drill foundation holes that go past the loose, soft and compressible soil to the hard soil and rock found way deep into the earth's surface. The most common foundations drilled are known as drilled shaft foundations, where after the shaft is drilled, reinforced steel rebar is lowered into the hole and concrete poured into it. These shafts can be as deep as 90 metres (300 feet) and as wide as 9 metres (30 feet). Here's what you need to know about the drilling methods used:

Kelly Drilling

This is the most common method used because it is suitable for most types of soil and rocks. The Kelly drilling method utilises a Kelly bar, after which the soil displaced is conveyed through buckets, core barrels and augers to the surface.

Continuous Flight Auger Drilling

This method is not suitable for most soil and rock types, meaning that soil and rock properties have to meet a certain requirement for continuous flight auger drilling to be possible. It is a drilling method used for either predrilling or if you need to install cast-in-place piles. When drilling, displaced soil/drill cuttings are conveyed to the surface through a continuous flight auger. You can already tell that this method is used if you need to install cast-in-place piles.

Double Rotary Drilling

This method is the same as the continuous flight auger drilling method; the only difference is that the auger is located inside a casing. Just like the continuous flight auger drilling method, double rotary drilling is mainly used to install cast-in-place piles and also predrilling.

Full Displacement Drilling

The full displacement drill method cannot be used for predrilling; it is only used for cast-in-place piles. It differs from the above two methods in that instead of the displaced soil being conveyed with a continuous auger, a smooth casing fitted with a displacement body is used.

Down-the-Hole Drilling

This method is used for very hard soil or rocks. A hammer is attached to the drill rod, which through compressed air is activated on the soil and rocks underground. It rotates as it hits and loosens soil and rocks. Loosened soil or soil cuttings are then conveyed upwards through a flushing current.


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