Tuesday, 20 October 2015

Myth 2: Screw piles disturb the ground during installation adversely affecting tension capacity

Mike Abbott - Engineering Manager Piletech

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. 

When I started working with screw piles 8 years ago I too thought “there must be disturbance of the ground with that helix being installed, mustn’t there?”  I’ll jump straight into the answer… “It could, but not with an experienced screw piling specialist”.  Key reasons why not are:
  • True helix – pile manufacture
  • Empirical evidence
  •  Soil displacement during installation
  • Installation considerations
A screw pile, if manufactured correctly, should have a ‘true helix’.  You can refer to Piletech’s "True or Screwed" on this issue.  The true helix is manufactured with a constant change in pitch over its 360o travel around the pile, and always extends perfectly perpendicular to the pile shaft.  The true helix threads through the soil profile, much like a wood screw, rather than auguring or disturbing it. Manufacturing a true helix is more than an “art”, and requires precision engineering equipment, and experience.  It also requires more than “trust” that the manufacturer will supply a product that conforms to the designer’s specifications. Piletech have sole-supplier agreements with our helix manufacturer that extend over 15 years, for which QA has been consistently outstanding.

“That’s great in theory, but so what?” I hear the sceptics say.  It stands to reason that if there was considerable ground disturbance in the material where the helix had travelled you could expect to see an initial ‘take up’ of pile deflection, under tension loading, at the toe of the pile in this weakened material.  The tension load test results curve would show a sudden rise at the initial low loads and then stiffen up as the ground compressed above the helix…right?  Well, Piletech have performed over 500 load tests over the past 17 years that we’ve been operating.  Of these load tests, more than 150 have been conducted in tension (pull up).  Very few of the load tests (less than 4%) display this phenomenon.  Almost all tests show instant tension take-up at the helix, because the true helix doesn't actually disturb the ground.  Because Piletech undertake regular load tests, we conform to not only "best practice" but international standards.  We know how our piles perform on each project.

Screw piles are a displacement pile with respect to the pile shaft area.  Piletech install an end cap in the base of each pile, which pushes the ground material out around the pile shaft during the installation process.  As such, close to the pile shaft you might actually expect to see a densification of the material.  Piletech has carried out some field testing where shear vanes were performed prior to the installation of a screw pile.  The shear vanes were then repeated in the path of the helix.  The results showed a reduction only in the top 0.5m of installation after which slight improvements to pre-installation shear strength were measured.  The disturbance in the upper 0.5m is not usually an issue given that it is typically removed in the formation of the pile caps or ground beams.
That is not to say that you cannot get disturbance if the helix is manufactured poorly or if site conditions result in a pile not being installed ‘on pitch’.  A pile being installed on pitch means that for each revolution of the pile it will progress downwards by the pitch of the helix.  If, for some reason, the pile starts progressing at less than a helix pitch for each revolution, such as an abrupt change in material properties, then there may be some ground disturbance.  This should be monitored by the pile installer. 

To show I’m not just making it up I have attached a load test curve showing the displacement of a pile on loading.  I have chosen a test performed on a shallow pile to remove the argument that skin friction is masking the effect of initial helix movement.

So – does a screw pile disturb the ground during installation adversely effecting tension capacity?? … “It could, but not with an experienced screw piling specialist”.