Wednesday, 9 December 2015

Myth 3: Screw Piles Can’t Provide Any Lateral Restraint

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.