The Golden Nugget: Why is our load test database so valuable?

Briar Fleming - New Business Engineer

From the outside it might seem like screw piling is reasonably straight forward.  Anyone who has tried it will attest to the fact that it is nowhere near as simple as it looks, and one of the key influencing factors occurs before you even arrive on site – the design of the piles needs to balance load carrying capacity (make the piles bigger and stronger!) versus being able to actually get them in the ground (make the piles smaller and more penetrable!).  This skill is something our specialist design engineers have honed over the many years we’ve been operating in this one field – and are now at a point where they’ve almost seen it all, having even installed screw piles into a coral reef in the Maldives. 

For clients we haven’t worked with before, load testing is an area that can cause confusion and we have written a post on load testing helping to de-mystify the various reasons why we sometimes suggest doing a load test (as it allows a more refined design meaning smaller or fewer piles on the project, thereby bringing the cost of the main piling works down), and other times why the risk profile dictates that a load test is highly recommended (the designer will want to prove a bearing capacity of the ground and the corresponding deflection of the pile).

However whether your project has load testing or not, by working with Piletech you are benefiting from something no other screw pile design company in New Zealand has.  It’s a tech-y name but it is an absolute golden nugget of our business and it is closely guarded.  It is our 17 year old Load Test Database.

To put it very simply, when designing piles you need to take into account the strength of the ground, the strength of the pile, and a factor that correlates the ground strength to the torque required to install the pile.  This ensures the pile won’t break or refuse prematurely whilst being installed to design ground strength.  An example of this correlation is shown in the graph at the end of this post, taken from the upcoming IPENZ Practice Note on screw piles.  This factor is variable and is dependent on the screw pile configuration, depth, and soil characteristics. When Piletech was being established in New Zealand (coming from operating in Australia), one of our five key mandates for best practice was to complete a load test for every project to obtain this correlation factor.  Now, having been in this business for so long, this database of over 1000 load test results often means we can determine this factor without needing to complete a load test. The years and years of experience installing piles gives the designers and an additional nuance giving the piles the best ability to penetrate through the ground. 

Again, it’s the balance between making the pile strong enough, whilst also able to penetrate that is one of the key areas that takes screw piling from being pure science, into the area of art.  This very valuable load test database cumulated over 17 years sits behind every design we undertake and gives the client confidence they are getting a cost effective and trustworthy foundation solution for their project – we’re not called the screw pile specialists for nothing.

Installation vs Capacity: The balancing act of screw pile design

Ryan Fleming - Site Engineer

Piles are used where the ground in the near surface is poor and will not support the weight of a structure. On many sites an intermediate dense stratum must be penetrated by the piles in order to reach a more consistent founding layer. Screw piles are typically more sensitive to this than other types of piles; the screw pile designer must walk a tight rope between installation and bearing capacity, whilst providing a cost effective, low risk solution.

When designing a screw pile, we need to consider the amount of torque required to achieve the design load with a specific helix size as well as the torque needed to install the helix to the correct founding depth. As screw piles are self-tapping, when the density of the ground increases, so does the amount of torque required to install the pile. Even though the torque required to achieve the design load in the founding layer may be low, the section size of the shaft may need to be increased to provide enough torque capacity to penetrate through a dense intermediate layer.

As a screw pile designer it is critical to understand the relationship between the permanent performance requirements and the installation process. However, these are not the only inputs into defining the shaft and helix ratio. Others include:

• Composite moment capacity
• Lateral loading / spread
• Corrosion

These various aspects should all be considered to develop a robust piling solution; the probability of success is improved with good geotechnical information and specific experience – get it wrong and the piles will fail to penetrate during installation, causing significant delays to the project.

Walking the tight rope of screw pile design is a real balancing act – remember, it’s a long way down……