James Wood - Piletech Manager
Do you know if your helix is true?
The concept of a true helix is essential to the performance and repeatability of screw pile systems. It allows designers and constructors to predict how a given pile will perform and deliver this during installation.
So what is a ‘True Helix’ and what’s the big deal if it’s
not true? A true helix is defined as
having perfect symmetry: a uniform pitch throughout the 360 degree revolution
and the leading and trailing edges are parallel to each other, much like the
thread on a screw.
 |
| A true helix on the left and a 'duck-bill' helix on the right |
A true helix minimises ground disturbance and produces the
lowest and most consistent torque application. The helix serves two purposes: installation
and load bearing. As it is rotated, the leading/cutting edge of the true helix cuts
through the soil, allowing the top surface of the helix to “pull” the pile
downwards. For every revolution, the pile should penetrate the ground by the
same amount as the pitch of the helix.
If the pitch is not constant the helix disturbs more ground,
creating voids above and below the flight as it rotates. This requires more
torque, increasing the stress placed on the pipe to penetrate to a given depth.
Not a good outcome when you’re encroaching on the shaft’s torque capacity,
having not reached the target founding layer.
If piles are carrying tension loads, an undisturbed soil
column is even more important. A false helix will tender to ‘auger’ the soil
column above and the pile’s tension capacity is significantly reduced.
Design and Pile Sign Off
The correlation between the driving torque and inferred
ground strength is essential to the sign off process of screw piles. A large and accurate
database of load testing information can offer significant savings and
confidence by enabling a refined design, which delivers obvious economic
benefits. Our database of 15 years of sustained static load testing is based on
the constant of the true helix.
Conversely, feedback from false helices can vary
significantly and adds a variable to the database. This either drives the
design towards conservatism and higher costs or, through lack of awareness, causes
inconsistent or over-estimated capacities between various sites or from pile to
pile.
As with all things screw pile, there is not an industry
standard that can be referred to. However, a number of useful documents exist.
So - do you know if your helix is true?