Michael Abbott - Engineering Design Manager
We hear incorrect assumptions, statements and perceptions
about screw piles a lot. The mythbusting
series of posts will hopefully educate you on the intricacies of screw pile
design and dispel some of those myths as loose accusations.
We have identified recently that screw piles have not been
considered at the early design phase on account of an assumption that screw
piles would not be able to provide sufficient lateral support for the
structure. On several occasions now we
have subsequently investigated screw pile options for those structures and
determined that they could, in fact, provide the required lateral support. Furthermore, the use of screw piles would
have made it possible for the structural designers to lighten the structure
overall. The client would have been able
to get the benefit of a faster, cheaper and cleaner piling solution as well as
save cost in the superstructure. How?
Let’s consider some lateral design basics:
- When you try to push an object through a soil medium it offers resistance to that pushing force. Generally, the more rigid and larger the surface area of the object, the more resistance to the push it will offer.
- A stronger soil will provide more resistance than a weaker soil profile.
- Resistance to lateral load is generated both structurally and geotechnically. Geotechnical capacity is generated from the pile moving relative to the surrounding soil, structural capacity is gained from the fact that the steel shaft of the pile doesn’t naturally want to bend and, when within its elastic range, wants to spring back to its original straight condition.
- The geometry of the pile relative to structure, ie raked pile, can result in lateral restraint.
Given the above, I’m not going to argue that screw piles
could compete with the lateral capacities of large diameter bored piles, as the
effective surface area of the screw pile is less. However, what I challenge designers to do is
think about how the structure could be made more laterally efficient (see tips
below), which will bring the overall cost of the project down, and will often
get the lateral loads into a range where screw piles are an option.
Piletech use a range of pipe sizes. Larger and stiffer pipe sections can provide
greater lateral support. We have
successfully installed 406mmÆ
and 457mmÆ CHS
piles to achieve better lateral performance.
With these larger pipe sizes we have been able to offer as much as 500kN
lateral resistance. We also have some
tricks up our sleeve to get even more capacity when required [see previous blog post here]. We have completed dozens of structures around
New Zealand now where our screw piles, in conjunction with foundation, are
providing the necessary lateral capacity.
Figure 1: Example of lateral load test in progress
So, how can you drive efficiency into your structure through
using screw pile foundations:
- Consider passive resistance of the structures pile caps or ground beams in conjunction with the lateral capacity of the piles.
- Consider the ductility of your structure including the piles, we can provide the lateral stiffness (kN/mm) of the screw piles to the structural designers. There is potential to increase building period and reduce spectral shape factor.
- Use fixity of the screw pile into the foundation beams to maximize structural capacity of the pile.
- Where appropriate, consider raked piles to provide lateral support.
- Lighten up your structure as much as you can.
Lateral design forms part of Piletech’s design package. We use the software L-Pile to model the
lateral response to shear loading and, where beneficial, back up that design
with lateral load testing.