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.

The Golden Nugget: Why is our load test database so valuable?

Briar Fleming - New Business Engineer

From the outside it might seem like screw piling is reasonably straight forward.  Anyone who has tried it will attest to the fact that it is nowhere near as simple as it looks, and one of the key influencing factors occurs before you even arrive on site – the design of the piles needs to balance load carrying capacity (make the piles bigger and stronger!) versus being able to actually get them in the ground (make the piles smaller and more penetrable!).  This skill is something our specialist design engineers have honed over the many years we’ve been operating in this one field – and are now at a point where they’ve almost seen it all, having even installed screw piles into a coral reef in the Maldives. 

For clients we haven’t worked with before, load testing is an area that can cause confusion and we have written a post on load testing helping to de-mystify the various reasons why we sometimes suggest doing a load test (as it allows a more refined design meaning smaller or fewer piles on the project, thereby bringing the cost of the main piling works down), and other times why the risk profile dictates that a load test is highly recommended (the designer will want to prove a bearing capacity of the ground and the corresponding deflection of the pile).

However whether your project has load testing or not, by working with Piletech you are benefiting from something no other screw pile design company in New Zealand has.  It’s a tech-y name but it is an absolute golden nugget of our business and it is closely guarded.  It is our 17 year old Load Test Database.

To put it very simply, when designing piles you need to take into account the strength of the ground, the strength of the pile, and a factor that correlates the ground strength to the torque required to install the pile.  This ensures the pile won’t break or refuse prematurely whilst being installed to design ground strength.  An example of this correlation is shown in the graph at the end of this post, taken from the upcoming IPENZ Practice Note on screw piles.  This factor is variable and is dependent on the screw pile configuration, depth, and soil characteristics. When Piletech was being established in New Zealand (coming from operating in Australia), one of our five key mandates for best practice was to complete a load test for every project to obtain this correlation factor.  Now, having been in this business for so long, this database of over 1000 load test results often means we can determine this factor without needing to complete a load test. The years and years of experience installing piles gives the designers and an additional nuance giving the piles the best ability to penetrate through the ground. 

Again, it’s the balance between making the pile strong enough, whilst also able to penetrate that is one of the key areas that takes screw piling from being pure science, into the area of art.  This very valuable load test database cumulated over 17 years sits behind every design we undertake and gives the client confidence they are getting a cost effective and trustworthy foundation solution for their project – we’re not called the screw pile specialists for nothing.

Mythbusters - applications of screw piles

Mike Abbott - Engineering Manager Piletech

This post is the first in a series of mythbusting queries about screw piles.  We hear incorrect assumptions, statements and perceptions about screw piles a lot.  The mythbusting series of posts will hopefully dispel some of those myths as false accusations.

For many years people in the construction industry have viewed screw piles as petite little piles that only go under residential or light commercial structures.  10 years ago I may have been inclined to agree with that sentiment, but a lot has happened in that time to the point where we have just completed a tension load test on a screw pile to 3250kN; that’s 325 tonnes.

So what has had to change to achieve huge capacity improvements?  The answer is simpler than you might think; nothing really.  Our design philosophies haven’t changed, we have merely up-sized each component in the system.  Larger excavators driving larger power-heads, driving larger diameter and thicker walled pipe driving larger diameter helices into denser ground.  The sum of all of those equates to pile capacities in excess of 4000kN in compression and 3250kN in tension.

Whilst Piletech are not in the business to make and break records, we expect that this will be a world record for tension capacity of a screw pile.  Perhaps if anyone out there has information to the contrary they can let us know.  We would like some further motivation to test to greater heights.