By Rodney Appleby, New Business Manager
PART I: The stuff you can see and 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 (aka 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 bored piles were the way to go… But did you consider screw piles? And why, or when, would a screw pile be your best option?
Blaringly obvious – concrete and some rebar will always be cheaper than the high cost of steel casings. But as we all know, the cost of a cake is more than just flour, water, sugar and eggs.
There’s more to cost of a pile than just materials…..
THE DIRT & CASINGS:
One of the largest risks to a project is always in the work in the ground. So you’ve got to make sure you’ve done your homework on the Geotech reports. It may be a hard cost to swallow up front with no return on your money but money spent now will save more later.
In my earlier days I learned a rough rule of thumb: If the “N” value of the SPT test is less than 15 it will probably collapse… if the “N” value of the SPT test is above 20 it will probably hold up. Also facto the soil types (eg. sands=uncohesive vs clays (very cohesive), and where the water table is.
If a ground collapse is possible – then you need temporary casings. Trying to “get away with it” will condemn the piles to death as collapse of the pile bore will mix dirt with concrete around your rebar.
“Joe-blow-contractor” can install 6-8m piles with his pendulum auger, and a small vibro attachment. Beyond this the ability of a simple excavator to remove (and not “rip”!) the casings out becomes harder. Now you need a crawler crane, with a vibro hammer – and you’re costs now start to rise significantly!
FYI: permanently cased bored piles have more steel than a screw pile – so must be more expensive.
The crapper the soil, and the deeper they go – the more likely screw piles may be your best bet!
BIGGER PLANT = BIGGER $$$:
A crawler crane and vibro-hammer will:
· Cost between $10k-$30k to mobilise
· Take 1 day to mobilise and 1 day to demobilise.
· Cost around $2-6k per day more than typical screw piling plant (considering all site plant and labour).
· Reduce the area available on site, so no room to “swing the arms” safely, and thus…..
· Drop piling productivity. We regularly install 10-20 piles a day to 24m. Bored piling would be lucky to install 4 piles per day.
o Note: each day more = another $3-6k the client will have to pay for.
· Increase risk… Contractors will now add a risk contingency sum of an extra 2-10% mark-up.
A bored piling operation typically costs more per day than screw piling, with more “one-off” costs.
Crawler cranes, drilling rigs, excavators, vibro hammers, site offices, foreman’s containers, temporary casings, reinforcing cages…. Bentonite/polymer tanks?? Spoil trucks coming/going…. Concrete trucks coming/going. Tremmie pouring pile.
Now combine a main contractor starting the pile caps to reduce the overall foundations programme!
The key to quick piling is being the only contractor on site… It’s not only commercially astute, but more importantly it’s safer!
Screw piling has smaller plant, less materials/deliveries, no spoil removal and less concrete trucks.
Screw piling plant is smaller, quicker, nimbler, easier, and safer!!!
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.
Tune in soon for “Bored? Why not Screw? Part II: The stuff you can’t touch.”