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

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?