BORED??? WHY NOT SCREW? Part II

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 (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?

DESIGN: 

This is one of the FIRST areas to assess whether screw piles are an option.  If they can take the loads on the building, then it’s definitely worth further investigation.

We have load tested our screw piles to achieve:

·         Over 4000kN axial

·         Over 3,250kN tension, and

·         Up to 400kN for lateral loads… (with a shear key this can rise to over 650kN..)

Higher loads still!….. Just install 2 for 1 screw:bored piles… Or 3:1… Often it will still be cheaper! 

Screw pile designs are taking much bigger loads now.  Ask the experts…

PROGRAMME:

I’ve completed a rough programme graph comparison below between a screw pile operation and a LDA (Large Diameter Auger) bored pile… it’s dependent on ground conditions, access, and plant mobility.  I’ve assumed the bottom 2m is into competent material, and that access is relatively easy, and includes mobilisation time.

I think the graph speaks for itself.

So a longer programme will effect pricing in two ways:

·         The cost of all piling plant and labour on a bored piling job is typically $3-4k/day more than that of a screw piling project… So every extra day really hurts the bottom line.

·         P&G costs go up.

Screw piling is much much quicker – and this results in cost savings!

ENVIRONMENTAL & TMP:

Pile arisings, spoil, dirt, crap… Whatever you call it, it needs to get off site.  Erosion and sediment control is a major with wet surfaces.  It gets into drains and tracked out on to the road. Silt fences, wheel washing (man + waterblaster), and traffic management all cost more money.

Contaminated spoil… New Work Safe H&S rules state the client, consultant and contractor need to be actively managing this. Tip fees, additional PPE, handling, cartage all cost more money.

If you don’t manage these the council shut your site down, with fines and even convictions!

Screw piles = no spoil = no ground water = no silt controls = no potential environmental incident.

Screw piles = no spoil = no unforeseen contamination variations = no H&S incidents.

Screw piles = less supply trucks + no spoil = no wheel-washing = less TMP $$ & no potential incident

NOISE & VIBRATION:

Large casings require vibro-hammers. They’re very noisy and can often be felt hundreds of metres away from the site.  This could not only result in complaints from neighbours but may be even a few cracks that neighbour “never noticed 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!

Screw piling is often the pile of choice in the electricity world because vibration monitors placed on the transformer never know we were there!

Screw piles = low noise = next to no vibration = no dilapidation surveys = no noise complaints = reduced risks to your project.

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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 “Bored? Why Not Screw? Part I: The stuff youcan touch”