Tuesday, 24 May 2016

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 

Wednesday, 27 April 2016

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.

Thursday, 25 February 2016

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. 


Wednesday, 10 February 2016

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.”

Wednesday, 9 December 2015

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.  

Tuesday, 3 November 2015

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.



Tuesday, 20 October 2015

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 360o 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”.