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.

-

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...”

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.

-

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”

Myth 3: Screw Piles Can’t Provide Any Lateral Restraint

Michael Abbott - Engineering Design Manager

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. 

We have identified recently that screw piles have not been considered at the early design phase on account of an assumption that screw piles would not be able to provide sufficient lateral support for the structure.  On several occasions now we have subsequently investigated screw pile options for those structures and determined that they could, in fact, provide the required lateral support.  Furthermore, the use of screw piles would have made it possible for the structural designers to lighten the structure overall.  The client would have been able to get the benefit of a faster, cheaper and cleaner piling solution as well as save cost in the superstructure.  How?

Let’s consider some lateral design basics:

• When you try to push an object through a soil medium it offers resistance to that pushing force.  Generally, the more rigid and larger the surface area of the object, the more resistance to the push it will offer.
•  A stronger soil will provide more resistance than a weaker soil profile.
• Resistance to lateral load is generated both structurally and geotechnically.  Geotechnical capacity is generated from the pile moving relative to the surrounding soil, structural capacity is gained from the fact that the steel shaft of the pile doesn’t naturally want to bend and, when within its elastic range, wants to spring back to its original straight condition.
• The geometry of the pile relative to structure, ie raked pile, can result in lateral restraint.

Given the above, I’m not going to argue that screw piles could compete with the lateral capacities of large diameter bored piles, as the effective surface area of the screw pile is less.  However, what I challenge designers to do is think about how the structure could be made more laterally efficient (see tips below), which will bring the overall cost of the project down, and will often get the lateral loads into a range where screw piles are an option.

Piletech use a range of pipe sizes.  Larger and stiffer pipe sections can provide greater lateral support.  We have successfully installed 406mmÆ and 457mmÆ CHS piles to achieve better lateral performance.  With these larger pipe sizes we have been able to offer as much as 500kN lateral resistance.  We also have some tricks up our sleeve to get even more capacity when required [see previous blog post here].  We have completed dozens of structures around New Zealand now where our screw piles, in conjunction with foundation, are providing the necessary lateral capacity.

Figure 1: Example of lateral load test in progress

So, how can you drive efficiency into your structure through using screw pile foundations:

• Consider passive resistance of the structures pile caps or ground beams in conjunction with the lateral capacity of the piles.
• Consider the ductility of your structure including the piles, we can provide the lateral stiffness (kN/mm) of the screw piles to the structural designers.  There is potential to increase building period and reduce spectral shape factor.
• Use fixity of the screw pile into the foundation beams to maximize structural capacity of the pile.
• Where appropriate, consider raked piles to provide lateral support.
• Lighten up your structure as much as you can.

Lateral design forms part of Piletech’s design package.  We use the software L-Pile to model the lateral response to shear loading and, where beneficial, back up that design with lateral load testing.