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

The early bird catches the worm

James Wood – Piletech Manager

So why would involving a sub-contractor early benefit my project?

The past decade in the New Zealand Construction market has seen an evolution in how Clients procure their assets: from the ‘entry price trumps all’ tendering market to the-focused-on-the-end game procurement models of design and build, Alliancing and latterly Public Private Partnerships. This progression has typically delivered greater value through early (and later) involvement of the Contractor to ensure alignment and delivery of the key project drivers.

Typically, screw piling is a design and build part of a project’s scope. The Consultant provides a performance specification as part of the tender documentation and a pile design and pricing is presented by the sub-contractor. This is typically completed in less than two weeks; little time is left for value to be added.

However, real value can be delivered through earlier involvement. The input of a specialist Engineering team refines the design through minimising scope, reducing risk and ensuring build-ability. This then flows through to input into the consenting process, procurement, removing testing from the critical path and generally ensuring that the construction phase is the encore rather than the first act.

A significant proportion of our projects, with a variety of Clients and Consultants, are secured through nomination. These Customers understand and trust that value can be delivered to their project through our early involvement. A quick survey around the office shows that these are some of our most successful projects with the best outcomes for clients, Main Contractors and Sub-contractor

.

On the other end of the scale, we are seeing projects coming to market that have been designed for competitive pricing. This is predominantly focused on the rebuild in Canterbury. In these cases there is often little thought given to rationalising overall scope of the project, or understanding of the implications of procurement, the risks associated with poor specifications and overall allocation of piling risks. In many of these projects, Clients will ultimately fail to reap the value they are seeking; the tender phase simply provides competitive tension around what has been put forward in the documentation.

We are continuously finding more areas where earlier involvement provides value to a project. Only last week one Consultant mentioned that they liked working with us because it reduced the time they invest in the piling; they could focus on the subsequent phases of the project, increasing their and the Client’s probabilities of a successful outcome.

Early involvement with a sub-contractor prior to the tender phase can appear counter-intuitive – “How do I know I am getting value for money if I can’t compare a few prices?” However, others have understood the benefits and taken the ‘leap of faith’ - becoming some of our most successful, repeat customers. They understand the overall drivers and ensure that the team is aligned, setting the project up for success.

So, on your next project are you going to be early enough to catch the worm?

We would love to hear your feedback on this or any of the topics in the screw files. Please feel free to post below and we will be sure to get back to you.

True or Screwed?

James Wood - Piletech Manager

Do you know if your helix is true?
The concept of a true helix is essential to the performance and repeatability of screw pile systems. It allows designers and constructors to predict how a given pile will perform and deliver this during installation.

So what is a ‘True Helix’ and what’s the big deal if it’s not true?  A true helix is defined as having perfect symmetry: a uniform pitch throughout the 360 degree revolution and the leading and trailing edges are parallel to each other, much like the thread on a screw. 

A true helix on the left and a 'duck-bill' helix on the right

A true helix pile has benefits in being easier to assemble and minimising the gap between helix and shaft, reducing the chance of defective workmanship: a quality pile. However, the majority of the value comes in the installation and capacity of the pile.

A true helix minimises ground disturbance and produces the lowest and most consistent torque application. The helix serves two purposes: installation and load bearing. As it is rotated, the leading/cutting edge of the true helix cuts through the soil, allowing the top surface of the helix to “pull” the pile downwards. For every revolution, the pile should penetrate the ground by the same amount as the pitch of the helix.

If the pitch is not constant the helix disturbs more ground, creating voids above and below the flight as it rotates. This requires more torque, increasing the stress placed on the pipe to penetrate to a given depth. Not a good outcome when you’re encroaching on the shaft’s torque capacity, having not reached the target founding layer.

If piles are carrying tension loads, an undisturbed soil column is even more important. A false helix will tender to ‘auger’ the soil column above and the pile’s tension capacity is significantly reduced.

Design and Pile Sign Off

The correlation between the driving torque and inferred ground strength is essential to the sign off process of screw piles. A large and accurate database of load testing information can offer significant savings and confidence by enabling a refined design, which delivers obvious economic benefits. Our database of 15 years of sustained static load testing is based on the constant of the true helix.

Conversely, feedback from false helices can vary significantly and adds a variable to the database. This either drives the design towards conservatism and higher costs or, through lack of awareness, causes inconsistent or over-estimated capacities between various sites or from pile to pile.

As with all things screw pile, there is not an industry standard that can be referred to. However, a number of useful documents exist.

So - do you know if your helix is true?

Collaborative contracting, 1+1=3

Will Brown - New Business Manager, Piletech




 What defines a successful project? On time? On budget? Was it easy?

Thinking back, what strikes me is that the most successful projects I've been involved with on all of the above measures have been the most collaborative ones. The projects where client, consultant and contractor work alongside one another from the outset, complementing one another's strengths and engaging with one another to understand and deliver the client's objectives. Rarely have I found, in either a consultancy or contracting role, that this early collaboration is wasted.

So what stifles this collaboration? Why don't we always work like this?

In my early days as a consultant I certainly felt a level of mistrust towards external parties, particularly when they called themselves contractors. Why would we look at changing the traditional model when we can write it all in a contract then get a price, and avoid discussing the 'risks'? What I missed was that where there is risk there is opportunity, and by insulating my clients from this risk I was creating hidden costs. I was then given the opportunity to work in a more collaborative model and when I look back at my 5 years as a consultant the result of that project is my proudest achievement hands down.

Fast forward to my current role where I have the opportunity to work with a large number of people from all over the industry, and guess which ones I feel most engaged in? At Piletech we strive to exceed our customers' expectations in everything we do, but you can bet that the projects where we have the opportunity to engage with our clients up front yield the best results and deliver a huge sense of satisfaction to everyone involved. People leave projects feeling energized and wanting to work with the same team again, rather than feeling like they've been for a round with Mike Tyson.

Aside from the obvious financial incentives, as a contractor, a consultant, a project owner or a developer, isn't that we all want from our work?

The Proof is in The Pudding

We were recently approached by a Client whom we had worked with back in 2007 on a building in Christchurch. Unfortunately, the super-structure was one of the limited few of around 100 structures on our piles that had suffered in the series of earthquakes. The extent of the damage was such that it was not economically viable to repair.

However, the Engineer noted that the building had been designed to over strength loads and saw that there may be value in the on-going capacity of the existing screw piles as part of the planned new building.

Having completed a desk study around the proposed new building loads in accordance with the latest version of code requirements, combined with our archived pile manufacture and installation records and on site load testing, we felt that there was a possibility of re-using the piles.

Subsequently, a pragmatic approach by our in-house engineering team, in discussion with the structural Engineer, recommended that the piles below shear walls be removed for inspection. This was a unique opportunity to view piles that have been in the ground for over six years but more significantly subjected to substantial seismic events.

The results to date are encouraging:

• No deformation of helices
• All welds NDT  tested -  no defects
• Shaft – true with no permanent deformation
• No corrosion – even in upper pile above the water table 

The future use of the remaining piles is still under investigation, with further destructive tests to the extracted piles. However, this unique opportunity has given us some validation that the many subtle aspects that we demand in the delivery of quality screw piles, results in surety to our Clients. The proof is in the pudding.

Comparing Apples with Oranges

Many Engineers are familiar with the relationship between the estimated load capacity of a driven pile and force (set) with which it has been installed. The HILEY formula is commonly used to perform this function.  The vertical displacement of the pile, for a known number of blows, with a known weight hammer, infers through this calculation ground strength and hence a pile load capacity.

In a similar fashion, the load capacity of a screw pile is determined via a known installation torque and calibration factor.  For any given material type, the load capacity of a screw pile increases through a relationship that increases relative to the rotational torque used to drive the pile into the ground. This assumes that the material is of uniform strength below the helix (in the case of a compressive load) as the installation torque will not identify softer (or harder) material below this level.  Hence the installation torque should only be used as verification that the founding strata identified in the geotechnical report has been encountered.

The torque calibration factor is best determined by on site sustained static load testing or by utilising load test results in similar geotechnical conditions. The latter option requires the use of more conservative geotechnical reduction factors.  Guidance on appropriate geotechnical factors can be found in AS2159-2009 and draft SESOC Technical Guidelines. 

For the Project Engineer there is very little guidance on appropriate screw pile torque calibration factors. Over the past 15 years we have developed a world class database of over 1000 load tests in a range of New Zealand and overseas geotechnical materials that can be called on for this purpose.

The accuracy of the inferred load capacity is therefore very much reliant on the accuracy of the torque output of the installation equipment.  We have a power head torque calibration device that measures torque utilising strain gauges directly at the output shaft.  Torque outputs were sometimes determined from relationships between hydraulic pressure and gear ratios, as provided by the manufacturer. However, through our QA procedures, we have found some of these to be flawed, with some manufacturer’s estimations being proven to be unreliable, as much as 25% different to actual measured torque.

Inaccuracies in either of the components on the left of the equation reflect on the result.  The risks to the project of inaccuracies in these parameters include:

• Piles exceeding structural capacity during installation
• Reduced pile capacity from that specified by the Engineer
• Pile settlement exceeding design values

All of this background information is critical to the performance of the piles going forward, not just in the short term settlement/gravity state, but into potential future seismic cases. It is therefore critical that the screw pile Engineer compares apples with apples - fully understanding the installation equipment being used and the pile sign off criteria, not just from pile to pile, but from across a range of power heads, piling rigs and geotechnical conditions.

A lack of awareness about this issue can leave a legacy of under-performing piles and thus compromise on-going performance. Fortunately we are here to help and are always willing to talk pile torque.