Mythbusters - applications of screw piles

Mike Abbott - Engineering Manager Piletech

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

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

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

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

Why load test?

William Brown - New Business Manager

What is a load test? Why do we do them? Why do we sometimes recommend not doing them?

A load test is where a pile is installed on site and loaded to prove how much it compresses or stretches as load is applied. Piletech has performed hundreds of load tests over the last 16 years as a means of proving our system, particularly in the early days. In the process we have built up a large load testing database, which is a unique resource when designing piles for new projects across New Zealand. Used in conjunction with a site specific geotechnical investigation the load test database allows us to predict pile performance using real data before we ever set foot on site, producing designs that are cost effective and reliable.

Although there are costs associated with load testing, in many cases it can reduce the overall cost of the project. When considering 'to test or not to test' it pays to understand the benefits of testing, the programme implications, how this feeds into the consenting process and how it can change a pile design.

Engineering design is all about making a calculated assessment of each design element. In the absence of testing, a conservative design approach will be adopted. However, when we load test the amount of information that we have about a site increases, which in turn increases the confidence that a designer has about the soil properties and allows a more refined pile design. Generally this means that when a load test is carried out, lighter piles can be used, with less steel required.

It then becomes a question of economics. On a large project, economies of scale can be gained through refinement of the design - that is, the savings that can be made are greater than the cost of the testing. On smaller projects the opposite is true - in this case we would generally recommend that testing is not undertaken and a more conservative design approach is used. This results in heavier, more expensive piles, but a lower cost and faster construction programme overall.

Want a bit more detail? If so, read on...

All structural components of a building will experience some movement, or 'deflection' as they are loaded. Load testing consists of installing a test pile, usually before the main works, then applying weight to, or pulling on that pile and measuring how far it will compress or stretch as it is loaded.

The goals of the load test are twofold. Firstly, we want to prove that the pile can carry the load that the building will place on it, in the ground that is present on site. Secondly, we want to show the relationship between load and deflection - how much the pile will compress or stretch under a given load, and check if this is within acceptable limits.

Pile load tests need a large amount of 'reaction' - the opposite force that is required in order to push the pile to its limits. For small tests this can sometimes be provided using a kentledge system of weights, or even by using the excavator that installed the pile for dead weight. However, more often a system of 'reaction piles' is used - typically 2 - 6 piles installed in a grid around the test pile, connected with steel beams. Hydraulic jacks are used to apply pressure to the test pile in pre-determined increments, and the deflection of the pile head is measured and plotted on a graph.

Typical Load Test Curve

All of this takes time and can be a significant cost to a project, so why do it?

Structural design in New Zealand is carried out using a system called 'limit state design'. Basically this means that engineers work out how much a building weighs and how big structural components such as piles need to be to carry that weight. They then apply safety factors to scale up the assumed weight of the building and scale down the assumed strength of a given component, so that the chances of any component being overloaded are very small.

For piles, typical 'strength reduction factors' range from 0.4 to 0.7. By way of example, if calculations tell us that a pile can carry 100 tonnes, and we use a strength reduction factor of 0.4, this means that even though the pile can carry 100 tonnes before it is considered to have 'failed', we will only assume that it can carry 40 tonnes for design purposes. In other words, that pile has 60 tonnes of reserve capacity.

So here is where load testing can save you money.

On a project where we don't do a load test, we need to use a conservative factor of say 0.4 - 0.5, like in the example above. On a project where we do perform load testing, we are getting site specific information, which increases the confidence level of the design. When we load test, the factor can be increased to as much as 0.7, depending on the site and other considerations. This means that the same pile used in the example above is now assumed to carry 70 tonnes, or nearly twice what it did in the first example.

The same pile, carrying almost twice the load.

Assuming that we have been involved early enough in the process, this allows us to work with the structural engineer to optimise the design, reducing the cost of the foundation and potentially saving time on site. The large database of load testing that Piletech has compiled over the last 16 years means that we are often able to predict load test results before carrying them out, and in doing so drive this design refinement process further forward in the design programme. In this scenario the load test simply verifies design assumptions and is a happy medium between absolute refinement of the design and balancing the programme requirements of a design and construction process.

For more information on how helpful Piletech's load test database can be, watch this space for an upcoming blog. In the meantime, we'd love to have a one on one discussion about how this can benefit your next project.

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.

Look Before You Leap

William Brown - New Business Manager

In New Zealand we’re lucky to enjoy a wide variety of outdoor environments – mountains of rock forced up by earth movement, ancient forests, rocky rivers carrying stones and sand onto open plains, wetland areas, sandstone cliffs and beautiful sandy beaches. As much as we like to get out in these environments, we also like to build in them, and the ground beneath our feet can be as varied as the view from above.

If you’re building, you will probably be expected to get a geotechnical report, but how do you know that your report will be enough to design the foundation that holds up your building, and minimise the chance of nasty surprises (and hidden costs) when you start building?

Something that we often see when a customer first makes contact is that their current geotechnical investigation isn’t detailed enough to design deep foundations, and more investigation is needed. Often this is a valid approach – geotechnical investigation can be quite an iterative process and the first round of investigation often focuses on shallow foundations – why would you look deep if you don’t need to? However, often the need for additional investigation comes as an unplanned expense and time delay, which people could do without. Choosing a geotechnical engineer who is familiar with local conditions can be helpful to reduce surprises in this area, as they may have an idea of what foundation types have worked in your area in the past.

So why bother with the additional investigation?

The simple answer is so that you know what you’re dealing with before you start building. Is that ‘hard’ layer 7 metres below the surface strong enough? Is it thick enough? Does it vary in depth and thickness across the building site, or does it disappear altogether? Will the ground liquefy in an earthquake? Just as rivers meander across the ground, conditions underground can vary from one side of a building to the other. Other times the ground beneath can be quite consistent. Although a geotechnical investigation is only ever an indication of what lies beneath, the more information you have the clearer this picture becomes.

At Piletech, we’re happy to work with your geotechnical engineer to make sure the right information is gathered, saving you time and money, and helping you to identify what lies beneath so that you can plan this into your build.

Can piles founded in the intermediate gravel layer in Christchurch perform to your specification? Don’t punch above your weight!

By Mike Abbott

For those unfamiliar with Christchurch geology, Christchurch is underlain with relatively recent alluvium deposits with substantial variability of the layered strata.  Among these, there is often a dense, competent and non-liquefiable gravel layer, which is often viewed as an attractive option for piling.  Clear commercial advantages exist by founding in an intermediate gravel layer as this will be a cheaper piling option over founding piles in the Riccarton Gravel layer that are considerably deeper.  But will this layer offer the appropriate pile response required by the structure and the specification?  What considerations need addressing to ensure a shallow piling option is appropriate?  

Diagram 1: sketch of typical geology of an intermediate layer in the Christchurch region.

Firstly, the specification should identify design loads for all loading combinations as well as providing acceptable deflection criteria.  NZS1170:2002 provides information on serviceability and ultimate loading combinations.  A load case that is often overlooked is the post seismic static load case 1.2G + YQ + Su, where Su is 1.2 times the potential negative skin friction that may act as a result of settling ground following a seismic event.

Acceptable deflections will vary from structure to structure, potentially even varying within different parts of a structure.  Generally, it is the Structural Engineer who will determine the deflection criteria required to protect the superstructure.  In the absence of specific criteria, AS2159:2009 Piling Code provides a set of default deflection criteria based on pile type and size.

Once these criteria are established, it must be determined whether the intermediate gravel layer can provide the necessary strength and resistance to pile deflection.  Factors contributing to pile deflection may include:

• Elastic shortening of the pile shaft
• Structural deformation of the helix
•  Geotechnical deformation of bearing strata
•  Liquefaction induced geotechnical settlement of underlying layer

It is the punching of piles into the lower strength underlying layer (as shown in Diagram 1) that is often not considered.  It is also this factor that is most likely to determine the sufficiency of the intermediate layer to provide the required bearing and deflection performance.  Punching into a liquefiable material cannot be determined by load testing as the weaker material underlying the intermediate layer will not be in its liquefied (weaker) state during testing.  Therefore, numerical analysis and modelling is the only way to justify using piles in intermediate layers. 

We generally consider an elastic stress analysis (Boussinesq 1885) ensuring that the thickness of competent material is sufficient to ensure stress at the interface with any weaker layers is less than the weaker layers capacity.  Determination of the ground strength of liquefied sand can be estimated using equations developed by Stark and Olsen (2002) detailed in their paper ‘Liquefied strength ratio from liquefaction flow failure case histories’.

The following links may be of interest relating to this article:

•  Available on our website is a video showing the effects of load bearing piles on a dense layer overlying a weaker sub-layer HERE. 
•  Geotechnical interpretive report for the Christchurch CBD area HERE.

Installation vs Capacity: The balancing act of screw pile design

Ryan Fleming - Site Engineer

Piles are used where the ground in the near surface is poor and will not support the weight of a structure. On many sites an intermediate dense stratum must be penetrated by the piles in order to reach a more consistent founding layer. Screw piles are typically more sensitive to this than other types of piles; the screw pile designer must walk a tight rope between installation and bearing capacity, whilst providing a cost effective, low risk solution.

When designing a screw pile, we need to consider the amount of torque required to achieve the design load with a specific helix size as well as the torque needed to install the helix to the correct founding depth. As screw piles are self-tapping, when the density of the ground increases, so does the amount of torque required to install the pile. Even though the torque required to achieve the design load in the founding layer may be low, the section size of the shaft may need to be increased to provide enough torque capacity to penetrate through a dense intermediate layer.

As a screw pile designer it is critical to understand the relationship between the permanent performance requirements and the installation process. However, these are not the only inputs into defining the shaft and helix ratio. Others include:

• Composite moment capacity
• Lateral loading / spread
• Corrosion

These various aspects should all be considered to develop a robust piling solution; the probability of success is improved with good geotechnical information and specific experience – get it wrong and the piles will fail to penetrate during installation, causing significant delays to the project.

Walking the tight rope of screw pile design is a real balancing act – remember, it’s a long way down……

Good Connections

James Wood – Piletech Manager

One of the benefits of screw piling is the speed with which it is delivered on site.

The final process on site within the piling sub-contract usually involves the connection detail.  This is typically achieved by placing reinforcing bars into the concrete infill within the steel tube of the screw pile.

A typical Screw Pile Connection

However, there are many ways to form a connection, dependent on load requirements.

A few of the more common examples are:

• Reid bars with flange plate nuts
• Annuli - welded to the pile
• Cage style arrangement where pile allows
• Shear studs or dowels

Where possible, we will propose the use of straight bars - this makes it easier for following trades to place cages, saving further valuable time on site.

Typically the screw pile designer will propose a connection detail.  However as the connection detail can affect what happens above it, the structural engineer's involvement is required to ensure design requirements are met.  A collaborative approach to this element ensures Good Connections - a technically suitable and economical result.

Factors to consider when developing connection:

• Design loads
• Bar development length
• Bond between concrete and pile wall (typically this is greater than bar development)
• Minimum steel to concrete ratio
• Constructability for following trades

For more information about design and connection of screw piles, contact one of our team or visit:

 http://www.piletech.co.nz/Screwpiles.php