Deep foundations – commercial
Commercial buildings often use deep pile foundations because it is necessary to transfer the heavy loads from the superstructure to solid bearing. The skin friction of deep piles also resists uplift from lateral loads on the structure. Some piles are designed with features to increase their uplift performance when required.
However, deep foundations can be susceptible to some secondary seismic effects, such as lateral spread, differential settlement and deep or severe liquefaction. Liquefaction weakens the skin friction and reduces the foundation’s capacity to resist uplift forces.
There are several types of deep foundation that can provide resistance to seismic loads in commercial buildings.
Bored piles, sometimes called drilled piles, piers or caissons, are a class of deep pile foundation. They distribute the loads from the structure, including those from seismic forces, vertically through a large depth of soil or to a deeper stratum layer such as rock. They are often used to support tall high-rise structures with large vertical loads.
Bored pile foundations must be drilled or augered before the piles can be inserted or cast in place into the ground. In some cases, the pile shaft is designed with a thread or auger included and are intended to be partially or completely self-drilling. Most bored piles require the pile hole to be drilled as the first step in the process.
Because they are drilled and not hammered into place, bored piles are suitable for vibration-sensitive sites. They generally do not suffer from penetration issues like driven piles and can usually extend to much greater depth. They can also be designed with a much larger diameter and with greater skin friction.
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A belled pile foundation is a type of bored pile foundation. It consists of a series of steel or precast concrete shafts with enlarged bearing areas at the bottom that resemble a large cone or bell shape. The cavity for the bell is created either using a special auger or by the pile itself as it is inserted into the ground.
The enlarged lower face of the bell increases the bearing strength of the pile, while the upper bell resists uplift forces.
Belled pile foundations are suitable for sites where there are firm soils but no solid bearing strata such as rock. They are used in commercial construction, although they are more commonly used overseas than in New Zealand.
A continuous flight auger pile, or CFA, foundation is a type of bored pile foundation that consists of a series of concrete piles cast into vertical shafts.
A CFA pile is created by using a continuous flight auger to excavate a deep vertical shaft and then pumping concrete below the auger as it is withdrawn. While the concrete is still wet, the pile is strengthened with a column of steel reinforcing inserted into the wet concrete.
CFA pile foundations are limited by the diameter, depth and penetration capabilities of the auger. However, they can be used in very wet or granular ground conditions without needing to support the walls of the drill hole.
This video provides a good explanation of the process used to construct a CFA foundation.
A screw pile foundation is a type of bored foundation that consists of a series of hollow steel shafts tipped with a helical thread that are screwed vertically into the target strata. The steel shafts provide a very ductile characteristic, and in some designs, the hollow tubes are filled with concrete to add stiffness to the foundation.
The shaft of a screw pile has a relatively small diameter and provides little skin friction when compared to other types of bored piles. However, this transfers the majority of the structural load to the helical threads, which typically provide significant end-bearing strength and uplift resistance.
Screw piles are susceptible to liquefaction if the bearing helices lie within liquefiable soil or within material supported by liquefiable soil, because this adds vertical load to the piles when liquefaction occurs. Therefore, if the pile is passing through liquefiable soil, there should be no helices over that depth of the pile.