There are extreme situations where even a self-tapping screw pile cannot be installed without pre-drilling (or sometimes chiselling). These include a layer of asphalt, debris from an old foundation, large broken bricks, concrete pavement, baked screenings or slag, frozen backfill or sand and gravel mix, and finally, twenty centimeters of ice on a body of water or in a ditch. Clearly, in such cases, the pile needs some assistance, at least during the initial installation phase.
But such challenging conditions account for no more than 5%
of the total number of pile foundations installed. For the remaining majority,
pre-drilling not only won't help, it will actually harm them. Here's why.
Decreased stability
For a pile to maintain verticality and withstand horizontal
loads, its shaft must rest on the ground. It's best if the soil at the screw
location hasn't been disturbed (preferably since the last tectonic shift or the
most recent glacial retreat). A self-tapping pile will expand and further
compact the surrounding soil when screwed in. It will also withstand the
tilting and rocking forces that will inevitably affect the structure installed
on it.
However, even for a bladed pile, which will loosen the
surrounding soil when screwed in, it's best to avoid pre-drilling or any soil
excavation. Whether it can be screwed to the required depth in a given location
without this is another matter.
The pile's service
life is reduced.
The denser the soil around the pile, the less oxygen
penetrates the metal. The slower the corrosion process, and therefore, the
longer the pile's lifespan.
Don't rely too much on the trendy coatings that supposedly
protect the pile foundation from corrosion for 70-100 years. During
installation, they will inevitably be torn off the metal by rocks or debris.
By reducing the soil density, you are making it easier for
oxygen to reach the exposed metal, thereby undermining the reliability and
durability of the pile foundation.
Installation is delayed and becomes more
expensive
Drilling holes is a separate procedure that requires
additional time and costs money. Furthermore, this defeats the purpose of screw
pile technology, which is designed to quickly, inexpensively, and without
unnecessary hassle provide a reliable foundation for a structure in virtually
any soil and on any terrain.
Frost Heave
A loss of soil density surrounding the pile leads to an
increase in the localized freezing depth at the pile's location. This means
longer piles are required to prevent frost from reaching the helix or blade,
preventing frozen soil from catching on them and dragging them upward.
Another side effect of the heave hole is the accumulation of
liquid around the pile. This is not dangerous for well-drained soils, but in
dense clay, a pre-drilled hole will collect moisture like a funnel. The pile
may also suffer from corrosion, but this is not the main concern. Water around
the pile shaft can freeze solidly at subzero temperatures. If the pile is not
sufficiently loaded and the ice's force on the pile shaft is greater than the
sum of the forces holding the pile in the ground and the compressive load from
above, the pile can be lifted out of the ground by a considerable amount by
this ice protrusion.
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