HAMMERED??? WHY NOT SCREW?
by Rodney Appleby, New Business Manager
PART II: The stuff you can’t touch…
Often in the feasibility stage the pros and cons of bored piles vs timber piles vs UC piles vs pad foundation get weighed up and compared. Ultimately, the decisions we make have to work on a technical level… and then be economically viable.
Piling can literally be as easy as drilling a hole and filling it with concrete! But the times it’s not that easy (as in 99% of the time), if you’ve not done your homework, and you chose the wrong technique, you will be riding the horse of pain off into a lonely sunset.
Let’s say you had decided driven UC piles were the way to go… But did you consider screw piles? And why, or when, would a screw pile be your best option?
Are there tension loads? With a UC – forget about it! Screw piles can embed themselves into hard layers, and, with a big helix at the base, generate their capacity through end bearing. This allows us to generate much higher loads, especially in tension.
End-bearing or skin friction? UC’s have a very small area on which to bear, so it needs to be “rock solid”. Skin friction as a means of generating capacity is less reliable and can require horrendously deep piles to achieve the desired loads, particularly if there are liquefiable layers.
This is where a screw pile comes into it’s own! Is there an intermediate hard layer? Often this layer may not be enough to provide adequate end bearing using a UC – but with a significantly larger bearing area that a large diameter helix offers – suddenly you may be able to halve the length of the piles – thereby reducing material cost as well as installation time.
And if the soil is absolute poop then think about multiple helices. We’ve put up to five 900Æ helices on a pile shaft to help found our piles at a shallower level. You can’t do that with a UC!
Screw piles = potential to found on intermediate layers = multiple helices = massive cost savings.
A typical driven UC specification will require a percentage of piles to be Pile Driver Analyser tested – otherwise known as PDA testing. This costs money and it takes time, and a lot of lazy contractors will tag out of it, stating they’ll check their pile capacities with the Hiley Formula…. see “rough guess for pile capacity calculation!” The designer re-asserts PDA is required… see time delay… see variation.
Sometimes soils work in mysterious ways, and pile heave is not uncommon. Did you also make sure that the contractor re-hit his piles 24hrs after achieving the set?
Screw piles can factor the cost of a static load test at the start of a project to give everyone certainty. We also have strong correlations between the torque applied to a pile and it’s pile capacity. Piletech record torque readings for every pile, which are reviewed by our Chartered Professional Engineers.
Screw pile testing is completed upfront or during the project without delays – no hidden extras.
NOISE & VIBRATION:
This really is a no brainer. If you’re driving UC’s you’ll be hearing the “ping” for miles and feel the shudder beneath your feet…. This could not only result in complaints from neighbours but maybe even a few cracks pop up that the neighbour “never noticed in my house before”. Dilapidation surveys can cost around $1-2k per house. Hopefully the complaints don’t temporarily shut the site down.
Watch out the Contractor doesn’t charge extra to reduce noise because of a methodology change!
Is your project in a school? Kindergarten? Hospital? Oil & Gas? Screw piles are the pile of choice in the electricity world because vibration monitors placed on adjacent transformers don’t know we’re there!
Screw piles = low noise = next to no vibration = no dilapidation surveys = no noise complaints = reduced risks to your project…. AND NO HEADACHES!
Did you ever hear the story of a contractor who drove UC’sthat lost their verticality? the UC essentially followed a “U” shape, and came back up, across the road – and pushed up a car!!! The ability to control inclination, correct it, and monitor it is not easy, nor cheap.
Conversely, our record screw pile is to a depth of 48m. We’d go deeper – but we hit the hard stuff and didn’t need to. We regularly go 40+ metres without issues. The true-helix keeps the pile on course, and with an open pipe you can tell if our pile lost verticality. Good luck with that on a UC!
Screw piles = better quality control.
So hopefully by now you’re starting to get to thinking that you’ve got nothing to lose by asking Piletech to give a free Rough Order Cost to see if they’re within the ball park of your current bored pile design.
FYI: Make sure you read “Hammered? Why Not Screw? Part I:The stuff you can touch…”