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...”

Stripping Down Corrosion

By Sam Parkin, New Business Engineer

One of the most common queries we get from structural engineers is how steel screw piles are designed to deal with corrosion (rusting).  While there are numerous ways of dealing with corrosion, Piletech typically recommend dealing with corrosion by the most powerful tool at our disposal: DESIGN.

There are three typical methods used for corrosion protection:

1.  Coating systems (including galvanising)

2. Cathodic protection

3. Sacrificial section area

In our experience galvanising and other coating systems are problematic as they get compromised and scratched off the pile during the installation process, or before they’re even being unloaded from the truck.  They also have a limited design life. Even worse, galvanising and coating systems can also promote localised corrosion intensity if a crack in the coatings forms. We’ve also seen systems such as Denso tape or HDPE sleeves – these options not only increase costs but increase programme and can cause H&S issues during installation. 

Cathodic protection involves setting up an electrochemical cell with the pile becoming the cathode and a sacrificial anode. This method of preventing corrosion is troublesome as the anode has a limited lifecycle and as such this method presents the need to continuously maintain the system over the design lifecycle of the piles. Initial setup costs for cathodic protection is also comparatively high.

Piletech now believe the best way to ensure quality assurance of the corrosion protection methodology is to simply allow sacrificial wall thickness to achieve the required design life for the piles.  It’s also cheaper, quicker, requires less QA, safer…. And it’s a lot easier – done with the flick of a pen before mobilising to site.

To calculate the required section thickness of the shaft and the helix you must first determine the design life of the piles, look at the surface area affected, and then estimate the rate at which the section will corrode. The design life is generally specified by the Structural Engineer; for permanent structures this is typically 50 or 100 years depending on the importance level of the structure.

With regard to the surface area considered, Piletech allow for corrosion to the outside face of the CHS shaft of a screw pile and to both faces of the pile helices. We don’t typically include for internal corrosion because the pile is concrete filled with a base cap to prevent soil entering into the pile thus eliminating any free oxygen required to cause corrosion.

The corrosion rate is either determined by soil testing on site, or it can be referenced in either AS2159:2009, HERA Design and Construct Bulletin No. 46 (reissued as Bulletin No. 62), NZS3404:2009 and/or The New Zealand Building Code.

The New Zealand Building Code verification method B1/VM4 states that the amount of section area deducted needs to take account of the aggressiveness of the soil and that further guidance can be found in AS2159:2009 Section 6.5 or the HERA Design and Construction Bulletin No. 46.

AS2159:2009 calls for a uniform corrosion allowance (mm/yr) which is dependent on the exposure classification. The exposure conditions depends on the PH, Chloride levels, Resistivity of the soil and the soil condition (high or low permeability soil). Using AS2159:2009 the steel sections of our piles typically fall into the non-aggressive (less than 0.01mm/year) or mild (0.01 - 0.02mm/year) category. 

HERA Design and Construct Bulletin No. 46 is specifically written relating to corrosion of steel sections in the ground and in water.  Prior to this report being issued, the go-to standard for corrosion rates was from a Steel Construction Institute (SCI) publication which outlined a rate of 0.015mm/year, regardless of soil conditions.  Thus the HERA report set out to test this rate in relation to the following variables:

•   A natural soil pH ranging from 2.3-9.5
• Soil Resistivity ranging between 300-50,000 Ohm’s
• Soil N Values between 4-40
• the majority of all the sites included intersecting water tables with both saline and fresh water with some exhibiting groundwater flow.

The key findings are outlined below:

1. The highest corrosion rates were found in permeable soils within 2.5 metres of the surface and which were at or above the water table. Even at the upper level the corrosion rate was below the 0.015mm/year outlined above.

2. Both mean and maximum corrosion rates decrease with time after installation of the pile.

3. There is no statistical significance found between the corrosion rate and any of the following parameters: soil type and permeability, N Value of the soil, pH of soil (there was a slight increase in corrosion rate below pH of 4), soil resistivity and nature of ground water.

4. The corrosion rate was similar on all faces of the steel pile.

5. The type of steel has no influence.

These conclusions indicate that the soil conditions have very little effect on the corrosion rate of the pile, and that using a 0.015mm/year corrosion rate is not only applicable, but conservative.

As a final reference, NZS3404:2009 - the structural steel code - has design corrosion rates depending on the location (exposed, water and soil), the type of fill encountered (controlled, uncontrolled) and the level of the water table. The two cases most relevant to Piletech screw piles are buried in fill below the permanent water table and the buried in controlled fill above the permanent water table both these cases have a design corrosion rate of 0.015 mm /year.  Which is in line with the other code requirements, as outlined above.

Thus, Piletech – unless directed otherwise – typically utilise a conservative corrosion rate of 0.03 mm / year for the outside of the pile shaft and 0.015 mm / year for both sides of the pile helix.  It’s also cheaper, quicker, with no QA hassles before our site teams mobilise to site.

EXAMPLE:

Typical design values for corrosion over design life for Piletech screw piles

50 year design life (Shaft) = .03 x 50 = 1.5mm

100 year design life (Shaft) = .03 x 100 = 3mm

50 year design life (Helix) = .015 x 2 x 50 = 1.5mm

100 year design life (Helix) = .015 x 2 x 100 = 3mm 

Don't Split the Seam. Dual Spec Pipe.

By Briar Fleming, New Business Engineer

Talking about pipe specification seems pretty dull, until your screw pile splits open while it’s being installed on site.  When Piletech started out, we used to order pipe to one specification – AS1163.  However a series of incidents led us to revise that, and since 2004 we always order pipe meeting two different specifications to control the quality of the weld on the pipe.  This blog is to give more detail on what we mean when we say ‘dual spec’ pipe, and why it’s so important for screw piling.

There are two main ways of making a hollow steel tube:

·         A molten piece of steel is extruded into a tubular shape – called seamless pipe

·         A flat plate is rolled into a circular shape, and the edges are welded together (so you end up with a seam weld) – called rolled plate pipe

While seamless pipe eliminates the potential for issues associated with the seam weld, it comes with a hefty price tag.  Thus Piletech primarily use rolled plate pipe and manage the risks associated with the weld in the following way.

Initially, Piletech started off using pipe specified as AS1163 – Cold-formed structural steel hollow sections.  This specification is developed for steel used in a structural capacity – such as buildings & bridges – it is steel that is designed to transfer loads.  It’s a good specification, but not entirely appropriate for screw piling due to three key areas:

Firstly, for strength testing, a small section of the pipe is tested as per the following diagram. As you can see, it is only tested in one direction, and not tested over the welded area.

Secondly, testing and inspecting that seam weld is ‘at the manufacturer’s discretion’.  Meaning if pipe is ordered strictly as per the code, the length of weld on the pipe is potentially not inspected, examined, or tested at all. We have had a piece of pipe turn up once where a full section of the weld was missing.

Lastly for pipe ≤406mm diameter, the tolerance for manufacture is ±10% for wall thickness. This means that a pipe with 12mm wall thickness could arrive with only 10.8mm thick walls, and still meet specification, and you’ve already used all of your 0.9 structural safety factor.

It became clear to us that ordering stock standard pipe that is AS1163 specified, is actually not good enough for screw piling for the following reasons:

1.       The weld could be defective (or as mentioned, even completely missing in sections), causing failure during installation as per Image 1

2.       The wall thickness could be insufficient, causing failure as per image 2

 These two types of failure are entirely because of the high torque (turning force) the pipe experiences when it is being screwed into the ground.  Once the pile is in, the welded seam sees a lot less stress (this static state is what AS1163 is designed to satisfy).  So it’s clear that the pipe needed for screw piling is not the same as the steel pipe needed for regular structural actions.

Thus we started looking into alternative specifications to supplement AS1163 and found API 5L.

API 5L is a specification for pipe used in the hydrocarbon industry.  API stands for the American Petroleum Institute, and this specification can cover both seamless, and welded pipe suitable for use in conveying gas, water, oil and other liquids. In layman’s terms, it’s pipe used to transport flammable liquid under pressure.  Accordingly the welds are given a very high importance.

There are three key benefits of API 5L: 

1.       Every single piece of pipe is hydrostatically tested (not just a sample), and must pass without leakage through the weld seam or pipe body. 

2.       The weld on each pipe is non-destructively inspected by either electromagnetic or ultrasonic testing - giving complete confidence the welded seam is uniform and continuous.

3.       For pipe ≥ 219mm diameter, a sample of the weld and a sample of the pipe are both transversely tested as per the diagram below. 

Thus having API5L and AS1163 specified pipe (‘dual spec’), the strength of the pipe is being tested in the following ways:

·         the steel on the shaft is being tested in both directions (for 219 diameter and greater),

·         the seam weld is being tested mechanically,

·         the continuity and integrity of the weld is tested hydrostatically,

·         the weld is checked for any defects by using a non-destructive testing method.

However, after all this, API 5L also specifies the wall thickness tolerance as ± 15%! Thus, when Piletech order our dual spec pipe, we order AS1163, and API5L, and with a wall thickness tolerance of ± 5%. 

All of the above gives us the greatest confidence that the pipe that turns up to our sites will not break due to manufacturing error while we are screwing it into the ground.  It also "engineers out" the possibility of delays, additional costs, and quality issues ensuring the foundations on which your building sits will hold firm for the next 50-100+ years.

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”

BORED?? WHY NOT SCREW? Part I

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 (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?

MATERIALS: 

Blaringly obvious – concrete and some rebar will always be cheaper than the high cost of steel casings. But as we all know, the cost of a cake is more than just flour, water, sugar and eggs.

There’s more to cost of a pile than just materials…..

THE DIRT & CASINGS: 

One of the largest risks to a project is always in the work in the ground.  So you’ve got to make sure you’ve done your homework on the Geotech reports.  It may be a hard cost to swallow up front with no return on your money but money spent now will save more later.

In my earlier days I learned a rough rule of thumb: If the “N” value of the SPT test is less than 15 it will probably collapse… if the “N” value of the SPT test is above 20 it will probably hold up.  Also facto the soil types (eg. sands=uncohesive  vs clays (very cohesive), and where the water table is.

If a ground collapse is possible – then you need temporary casings. Trying to “get away with it” will condemn the piles to death as collapse of the pile bore will mix dirt with concrete around your rebar. 

“Joe-blow-contractor” can install 6-8m piles with his pendulum auger, and a small vibro attachment. Beyond this the ability of a simple excavator to remove (and not “rip”!) the casings out becomes harder. Now you need a crawler crane, with a vibro hammer – and you’re costs now start to rise significantly! 

FYI: permanently cased bored piles have more steel than a screw pile – so must be more expensive.

The crapper the soil, and the deeper they go – the more likely screw piles may be your best bet!

BIGGER PLANT = BIGGER $$$: 

A crawler crane and vibro-hammer will:

·         Cost between $10k-$30k to mobilise

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

·         Cost around $2-6k 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…..

·         Drop piling productivity. We regularly install 10-20 piles a day to 24m.  Bored piling would be lucky to install 4 piles per day.

o   Note: each day more = another $3-6k the client will have to pay for.

·         Increase risk… Contractors will now add a risk contingency sum of an extra 2-10% mark-up.

A bored piling operation typically costs more per day than screw piling, with more “one-off” costs.

THE FOOTPRINT

Crawler cranes, drilling rigs, excavators, vibro hammers, site offices, foreman’s containers, temporary casings, reinforcing cages…. Bentonite/polymer tanks?? Spoil trucks coming/going…. Concrete trucks coming/going. Tremmie pouring pile.

Now combine a main contractor starting the pile caps to reduce the overall foundations programme!

The key to quick piling is being the only contractor on site… It’s not only commercially astute, but more importantly it’s safer!

Screw piling has smaller plant, less materials/deliveries, no spoil removal and less concrete trucks.

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

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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. 

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