AlexH15 said:
I assume there has to be some limit, but I cant find any specifications for this.
There won't be a specification, there are too many variables, but there are some general guidelines:
It's common to predrill through fills and embankments so that actual driving begins at natural ground elevation.
On projects where pile driving vibration may damage nearby existing structures and piles are point bearing, it's common to predrill to nearly full depth. This minimizes the time needed for actual driving.
If there is hard material or rubble at a shallow depth, predrilling through this layer (then stopping) allows the pile to then be driven to obtain sufficient embedment.
If an oversized hole is predrilled, the hole is to be backfilled with compacted granular material (exactly how to ensure this is accomplished on a deep predrilled hole mystifies me).
A predrilled hole is never the full depth of the piling. The objective is to drive the pile at least some distance... not to just "drop" a pile in a hole and backfill it.
A predrilled hole smaller than the pile can be used to speed up the driving process. How deep and what diameter is a project-by-project joint decision by the Engineer and Contractor.
Predrilling is typical for displacement piling (wood & concrete) not steel piling.
There are plenty of situations where predrilling is not appropriate and should not be allowed.
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