Sunday, 22 September 2013

True or Screwed?

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 pile has benefits in being easier to assemble and minimising the gap between helix and shaft, reducing the chance of defective workmanship: a quality pile. However, the majority of the value comes in the installation and capacity of the pile.

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?