Wednesday, 25 January 2017

CAN’T SEE WOOD FOR THE TREES? WHY NOT SCREW?

by Rodney Appleby, New Business Manager

Often in the feasibility stage the pros and cons of bored piles vs timber piles vs UC piles vs pad
foundations 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 decided timber piles (both driven and concrete encased) were the way to go… They’re generally very cost effective, and relatively simple to install.  But did you consider screw piles?  And why, or when, would a screw pile be a superior option to timber? 

MATERIAL COST: 
On the face of it, and in 95% of cases, there is no contest when comparing $/kg of timber to steel.  So how can a screw pile compete?  Well, there’s more to bread than just flour, yeast, and water.
To make up for the imbalance in material cost – you need to think smart!

DESIGN LOADS:
A timber pile is very limited!  Typically loads of 100-200kN in compression, and dodgy in tension where you would rely on skin friction and then get a very sudden failure. Comparatively a screw pile can take over 4,000kN in compression, and over 3,000kN in tension. 
The structural properties of steel over timber are huge!  Combine this with a big helix sitting at the toe of our piles (and up to 5 of them on 1 pile!) and it’s clear this is a key point of difference!  SCREW PILES CAN TAKE MUCH MUCH BIGGER LOADS.
Now you can re-design a two timber piles for one screw pile ratio solution (2 for 1), 3:1, or 4:1.  We’ve seen as much as a 6:1 ratio alternative.  It also introduces another benefit, as less piles means less pile caps – which in turn drives the cost down.  Now the screw pile solution can provide an overall benefit compared to timber.
Bigger loads per pile = less piles = less pile caps = less cost

PLANT, NOISE & VIBRATION: 
The plant to install screw piles is very similar compared to that of a timber pile.  Large excavators ensure that the plant to install either solution is rather compact and nimble. However the drop-hammer attachments for timber piles provides more noise and vibration to operate. CLUNK! CLUNK! CLUNK!  All project long!  You’ll really feel that vibration particularly as the pile hits the hard layer.  Vibrator attachments for temporary casings also create additional noise and vibration.
Compare this to installing a screw pile – where there is literally more noise and vibration when tracking to the pile position than the installation process itself.  This is why screw piles are a preferred solution to electricity, oil and gas industries – where sensitive and very expensive plant cannot afford the risk of any vibratory damage.
Screw Piles = less noise and vibration (and risk) per day timber.

ENVIRONMENTAL & TMP:
Often contractors will pre-drill a starter hole to help stand the timber pole upright.  If so, make sure you’ve allowed to handle the spoil, and cart it off site.  Erosion and sediment control is a major with wet surfaces.  It gets tracked out on to the road, and into drains. Silt fences, wheel washing (man + waterblaster), and traffic management isn’t cheap over the life of a project.
Contaminated spoil? New Work Safe H&S rules require the client, consultant and contractor to actively manage this risk. Tip fees, additional PPE, handling, cartage must now be budgeted for.
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 trucks + no spoil = no wheel-washing = less TMP $$ & no potential incident

DESIGN & TESTING:
The Capacity check via Hiley Formula???  See also “rough guess!” Even if you stipulate PDA testing most piling contractors will tag out of it – so they can have the “lowest possible cost”.  Check your standard specification – pile heave is not uncommon, so make sure you check 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 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.

DEPTH & DESIGN CHANGE:
Ideally a timber pile is less than 10m.  After 10m (to a maximum of 18m) you require a splice joint – and you’re now introducing additional cost, and diminished design capability. 
Piletech have achieved a max depth of 48.5m – stopping only because we hit the founding layer. When you splice a screw pile there is no diminishing the design capability.  Infact – the deeper you go – the more cost effective screw piling becomes! 
Let’s say you’re borelogs weren’t completely representative across your site.  It’s inexpensive to weld or cut a screw pile and adjust pile length – which cannot be said when changing design to go deeper with timber.
And if the soil is absolute rubbish then screw piles can add multiple helices.  We’ve placed five 900Æ helices on a pile shaft to found our piles at shallower levels.  You can’t do that with timber!
Screw piles = potential to found on intermediate layers = multiple helices = massive cost savings.
Screw piles = easy to redesign and go deeper when the geotech changes.

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 my driven UC pile design. 


Check out “Hammered? Why not Screw?” & “Bored? Why not screw?” for a review of bored piles and driven UC piles.

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