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.

HAMMERED??? WHY NOT SCREW? Part 2

HAMMERED??? WHY NOT SCREW?

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 (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 had decided driven UC 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: 

Are there tension loads?  With a UC – forget about it!  Screw piles can embed themselves into hard layers, and, with a big helix at the base, generate their capacity through end bearing.  This allows us to generate much higher loads, especially in tension.

End-bearing or skin friction?  UC’s have a very small area on which to bear, so it needs to be “rock solid”.  Skin friction as a means of generating capacity is less reliable and can require horrendously deep piles to achieve the desired loads, particularly if there are liquefiable layers. 

This is where a screw pile comes into it’s own!  Is there an intermediate hard layer? Often this layer may not be enough to provide adequate end bearing using a UC – but with a significantly larger bearing area that a large diameter helix offers – suddenly you may be able to halve the length of the piles – thereby reducing material cost as well as installation time.

And if the soil is absolute poop then think about multiple helices.  We’ve put up to five 900Æ helices on a pile shaft to help found our piles at a shallower level.  You can’t do that with a UC!

Screw piles = potential to found on intermediate layers = multiple helices = massive cost savings.

TESTING:

A typical driven UC specification will require a percentage of piles to be Pile Driver Analyser tested – otherwise known as PDA testing. This costs money and it takes time, and a lot of lazy contractors will tag out of it, stating they’ll check their pile capacities with the Hiley Formula…. see “rough guess for pile capacity calculation!”  The designer re-asserts PDA is required… see time delay… see variation.

Sometimes soils work in mysterious ways, and pile heave is not uncommon. Did you also make sure that 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 it’s 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.

NOISE & VIBRATION:

This really is a no brainer.  If you’re driving UC’s you’ll be hearing the “ping” for miles and feel the shudder beneath your feet…. This could not only result in complaints from neighbours but maybe even a few cracks pop up that the neighbour “never noticed in my house 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!

Is your project in a school? Kindergarten? Hospital? Oil & Gas? Screw piles are the pile of choice in the electricity world because vibration monitors placed on adjacent transformers don’t know we’re there! 

Screw piles = low noise = next to no vibration = no dilapidation surveys = no noise complaints = reduced risks to your project…. AND NO HEADACHES!

QUALITY:

Did you ever hear the story of a contractor who drove UC’sthat lost their verticality? the UC essentially followed a “U” shape, and came back up, across the road – and pushed up a car!!!  The ability to control inclination, correct it, and monitor it is not easy, nor cheap.

Conversely, our record screw pile is to a depth of 48m.  We’d go deeper – but we hit the hard stuff and didn’t need to.  We regularly go 40+ metres without issues.  The true-helix keeps the pile on course, and with an open pipe you can tell if our pile lost verticality.  Good luck with that on a UC!

Screw piles = better quality control.

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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 “Hammered? Why Not Screw? Part I:The stuff you can touch…”

HAMMERED??? WHY NOT SCREW? Part 1

HAMMERED??? WHY NOT SCREW?

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 (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 had decided driven UC piles were the way to go…  But did you consider screw piles?  And why, or when, would a screw pile be a superior option?

MATERIALS: 

A pipe compared to UC’s ($/m) – it’s relatively similar. 

The key ingredient here is lead time!  If you’re doing a big job, to keep the costs down, you’ll have to order from China, Indonesia, Korea… wherever… but the Contractor will always tag a 3 month lead time before they can start.  If it’s a smaller job, then they might buy it from Fletcher EasySteel off the shelf, but then you’ll be paying a much higher rate.

Technically, the same thing applies to screw pile pipe…. unless you’re Piletech… Piletech holds between $2M-$3M worth of stock (both pipe and plate) in our yards so that we can turn on a dime, and get your project started – whilst keeping costs low because we bought in bulk some time ago.

Piletech Screw piles = less lead time + cost savings.

BIGGER PLANT = BIGGER $$$: 

For small piles the plant will be similar.  But deeper piles with larger loads require larger drop hammers and leader frames, or vibro-hammers, and a crawler crane.  This means:

·         More upfront costs – as crawler cranes cost between $10k-$30k to mobilise,

·         Take 1 day to mobilise and 1 day to demobilise,

·         Cost around $2-4k 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…..

·         Piling productivity will drop. We regularly install 10-20 piles a day to 24m, and would estimate being 20-30% quicker than similar length driven UC’s.

o   Note: each day more = another $2-4k the client will have to pay for.

Cranes and leaders typically costs more per day than screw piling, with more “one-off” costs.

THE FOOTPRINT

Crawler cranes and leaders?  On a small site even turning becomes an issue.

Screw piling plant is smaller, quicker, nimbler, easier, and safer!!!

ENVIRONMENTAL & TMP:

Often contractors will pre-drill a starter hole to help stand the UC’s 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 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 trucks + no spoil = no wheel-washing = less TMP $$ & no potential incident

CONNECTION DETAILS:

Another potential hidden killer.  Piling contractors often tag out of cutting the piles to height, and then welding nelson studs, or top plates on.  At $200-$500 per pile – this will chew a hole in your profit if not allowed for. 

Screw piles typically have a few bits of rebar coming out of the pile which are concreted in.  No sweat, and allowed for in our costs.

Make sure you allow for connection costs when comparing apples and pears.

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. 

Tune in soon for “Hammered? Why not Screw? Part II:  The stuff you can’t touch...”

Myth 2: Screw piles disturb the ground during installation adversely affecting tension capacity

Mike Abbott - Engineering Manager Piletech

We hear incorrect assumptions, statements and perceptions about screw piles a lot.  The mythbusting series of posts will hopefully educate you on the intricacies of screw pile design and dispel some of those myths as loose accusations. 

When I started working with screw piles 8 years ago I too thought “there must be disturbance of the ground with that helix being installed, mustn’t there?”  I’ll jump straight into the answer… “It could, but not with an experienced screw piling specialist”.  Key reasons why not are:

• True helix – pile manufacture
• Empirical evidence
•  Soil displacement during installation
• Installation considerations

A screw pile, if manufactured correctly, should have a ‘true helix’.  You can refer to Piletech’s "True or Screwed" on this issue.  The true helix is manufactured with a constant change in pitch over its 360^o travel around the pile, and always extends perfectly perpendicular to the pile shaft.  The true helix threads through the soil profile, much like a wood screw, rather than auguring or disturbing it. Manufacturing a true helix is more than an “art”, and requires precision engineering equipment, and experience.  It also requires more than “trust” that the manufacturer will supply a product that conforms to the designer’s specifications. Piletech have sole-supplier agreements with our helix manufacturer that extend over 15 years, for which QA has been consistently outstanding.

“That’s great in theory, but so what?” I hear the sceptics say.  It stands to reason that if there was considerable ground disturbance in the material where the helix had travelled you could expect to see an initial ‘take up’ of pile deflection, under tension loading, at the toe of the pile in this weakened material.  The tension load test results curve would show a sudden rise at the initial low loads and then stiffen up as the ground compressed above the helix…right?  Well, Piletech have performed over 500 load tests over the past 17 years that we’ve been operating.  Of these load tests, more than 150 have been conducted in tension (pull up).  Very few of the load tests (less than 4%) display this phenomenon.  Almost all tests show instant tension take-up at the helix, because the true helix doesn't actually disturb the ground.  Because Piletech undertake regular load tests, we conform to not only "best practice" but international standards.  We know how our piles perform on each project.

Screw piles are a displacement pile with respect to the pile shaft area.  Piletech install an end cap in the base of each pile, which pushes the ground material out around the pile shaft during the installation process.  As such, close to the pile shaft you might actually expect to see a densification of the material.  Piletech has carried out some field testing where shear vanes were performed prior to the installation of a screw pile.  The shear vanes were then repeated in the path of the helix.  The results showed a reduction only in the top 0.5m of installation after which slight improvements to pre-installation shear strength were measured.  The disturbance in the upper 0.5m is not usually an issue given that it is typically removed in the formation of the pile caps or ground beams.

That is not to say that you cannot get disturbance if the helix is manufactured poorly or if site conditions result in a pile not being installed ‘on pitch’.  A pile being installed on pitch means that for each revolution of the pile it will progress downwards by the pitch of the helix.  If, for some reason, the pile starts progressing at less than a helix pitch for each revolution, such as an abrupt change in material properties, then there may be some ground disturbance.  This should be monitored by the pile installer. 

To show I’m not just making it up I have attached a load test curve showing the displacement of a pile on loading.  I have chosen a test performed on a shallow pile to remove the argument that skin friction is masking the effect of initial helix movement.

So – does a screw pile disturb the ground during installation adversely effecting tension capacity?? … “It could, but not with an experienced screw piling specialist”.

Pulling in the same direction

James Wood - Piletech Manager

Over the years we have worked for many clients with a variety of strategies and cultures for procuring their trades; from the hard money, nail the sub-contractor through to collaboration and early specialist involvement or nominated sub-contracts.

However, all 1000 plus of these screw pile specific projects have been lump sums - they provide only so much room to drive collaboration and best-for-project outcomes.

2015 is seeing a new opportunity for Piletech with us being part of an Alliance Contract.
An Alliance Contract can be defined as an agreement between two or more parties to achieve agreed outcomes on the basis of sharing risk and reward.

In this model we are open book on cost and work towards an agreed target outturn costs (TOC), where all parties take a pain gain share in the final outcome. In essence, you sink or swim together, which ensures that all parties do what is best for the success of the project.

The transparency of the contract and the trust and collaboration that this affords, has allowed our specialist knowledge to be leveraged to best effect. The key benefits are:

• Coordinated Design - achieved through early involvement, which has reduced the geotechnical risk and ensures all design drivers are identified and managed upfront;
• Optimised Budget – early testing on site gained early, detailed understanding of the ground. The TOC was then developed which was assessed by an independent Engineer;
• Efficient Procurement - materials are procured specifically for project, saving significant $ through optimising the supply chain. Steel will arrive just in time (JIT);
• Early detailed Methodology – planned as part of the TOC, and optimised in parallel with the supply chain.

The project requires significant screw piling, certainly the largest scope in New Zealand and approaching some of the largest we have seen globally. This model may not be practical for smaller projects with clearer initial scope and risks, it will certainly provide us with experiences and lessons that we can leverage into the traditional business.

It’s an interesting journey we are embarking on that already has already seen many positives - We look forward to the lessons we will learn.