WHAT'S THE MAXIMUM DEPTH OF A SCREWPILE?
We often have a quiz in our presentations where many design consultants are dumbfounded that screw piles can go deeper than 12m.... CODSWALLOP!!!!!
A once good-looking man (who has spent too much time on construction sites and has probably forgotten more than most of us will ever learn) recently asked, "Would there be length restrictions for a screw pile, and if so, what would be an indicative maximum depth to reach a solid base? Would the equipment drive head torque values restrict the length of a screw pile apart from the obvious answer: use bigger equipment?"
Great question - to which we answer:
Our people have proven screw piles can be installed to at least 52m below ground level. They only stopped there because that was the founding rock the pile was designed to bed into.
We regularly install piles (50%+ of our projects) deeper at/around 30m-40m with no problemos! Absolutely business as usual. Easy-peasy.
Could screwpiles go deeper? Yes!!! Absolutely yes! Just like a driven tube (casing) for a bored pile, or a UC for a driven UC can go to depths of 100m+, a screw pile could too. There is no difference!
The benefit of a screwpile is that you could at least screw it in - check your verticality, and keep splicing more pipe, and keep checking verticality. Good luck doing that with a UC! And if you lose verticality, it could be unscrewed easily and gently corrected... A lot harder with other methods.
At some point, there might be too much skin friction from the shaft of the pile.... Interestingly, screw piles are typically designed in "end-bearing" (load transferred through the helix), but this would add up over a long length - and so the designer of the pile would also need to "design for installation". This is often overlooked (!!!), and the danger is that at some point you'll crimple the pipe when more torque is applied than the pipe can handle.
That's why, in certain situations, we design a "punchy" pile design... This is where a smaller diameter is used (reduces the area that creates friction) but we beef up the pipe thickness (go from 6mm to 9mm, or 9mm to 12mm, or even 12mm to 15mm) so that significantly more torque can be applied through the pipe. We also might adjust the helix diameter/thickness too - all in our endeavours to give the operator an easier install.
Because the pipe can take more torque, we'll mobilise our 49T excavators with big powerheads (mustering 500kNm of torque!) - so we can use the jandal if we need to! (Note, not many pipes can handle that torque in our field!!!)
In summary, going deeper is fundamentally dependent on:
the ground conditions: Where is the found rock? How much skin friction?
the designer ensuring piles have been designed for installation.
the plant having the power!!!
Hope you've all learned something more you can soon forget too.
