Foundation drilling is, in essence, using big machines to put big holes in the ground.
Most residential homes have a reinforced, concrete slab foundation poured on top of the ground. Though some settling and shifting will occur, it is not enough to send the walls tumbling down. However, huge structures like bridges and office buildings, need foundations that can handle some movement and are strong enough to carry the weight of the building itself and what’s on or inside of it. To do this, large construction projects use foundation drilling rigs to create drilled shafts. Also known as drilled piers, caissons or bored piles, drilled shafts are constructed by pouring concrete into a drilled hole. The load is supported mainly through skin friction where the subsurface soil surrounding the shaft holds it in place. End-bearing shafts carry the weight load on the base of the shaft. This type of drilled shaft is used less often.
The type of rig used depends on several factors. How deep and what the diameter of the holes will be, obviously. But, the type of soil, where the project is located (in a city or out in the boonies), and how much room there is to maneuver are just some of the considerations. The project requirements will also dictate which foundation drilling method is used.
These are the most common drilling methods:
Kelly Drilling
Kelly drilling is a dry rotary drilling method. It is used to make bored piles with a large diameter, typically over 500mm. It’s is popular because it works for most types of rock and soil. A key part of this method is the telescopic drill rod – also known as the Kelly bar.
Continuous Flight Auger Drilling
Continuous flight auger drilling is another dry rotary method. It uses rotation of a continuous flight auger to displace soil. The auger needs to be the same length as the hole that will be drilled. This makes it a good solution for predrilling.
Double Rotary Drilling
Double rotary drilling combines continuous flight auger drilling with a continuous casing. This method is used for challenging soil conditions or sensitive environments. Drill cuttings are ejected from an opening in the top of the auger.
Full Displacement Drilling
Full displacement drilling is used for cast-in-place piles. The surrounding soil is only displaced and virtually no drill cuttings are brought to the surface. A smooth casing is used and fitted with a displacement body at the lower end.
Grab Drilling
Grab drilling is the oldest of the dry drilling methods. Soil is loosened by cutting or impact driving. This method is typically used for well drilling or cast-in-place piles.
Reverse Circulation Drilling
Reverse circulation drilling can produce boreholes of about 3.2m diameter. The method is useful in challenging soil conditions such as karst. This method uses the mammoth pump principle. The flushing drill fluid rises inside the drill rod and brings the drill cuttings to the surface. This technique can be used for soil or rock drilling.
Down-the-Hole Drilling
For very hard rock or to break up boulders, the down-the-hole drilling method is used. A hammer is mounted on the drill bit at the end of the drill rod. Compressed air is used to pneumatically break and remove the rock or boulders. The bit breaks up the rock by rotating and impacting at the same time. The compressed air moves the loosened drill cuttings to the surface.
No matter the method, there is a foundation drilling rig available to put those big holes in the ground, in the right place, at the right depth.
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