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?
MATERIALS:
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.
THE FOOTPRINT
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.”
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