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.

Lateral thoughts

With the Earthquakes in Christchurch, seismic design is increasingly becoming part of everyday language. One component of this is lateral load.

There are four ways of dissipating a lateral load being applied to a foundation:

1. Through passive restraint of the ground beams or base friction
2. Through passive restraint of the piles
3. Shear keys
4. Through raked piles

 We typically recommend that these are treated as mutually exclusive; differential responses will typically mean that the stiffest component will initially attract the majority of load, rather than share as a system.

Screw piles, although up to 450mm in diameter, do not have a significant lateral capacity when compared with bored piles. 

We have two ways of addressing this:

1. The bored over screw – where a shallow bored pile is installed over the screw pile.
2.  The step up pile – where the upper section of the screw pile is super-sized.

Others aspects to consider:

• Ductility factor of the pile
• Period of building oscillation
• Composite pile shaft design (concrete filled steel shaft)
• Liquefaction effects on passive restraint capacity 

When all this has been considered the system is modelled. We typically employ the tried and trusted Broms method for preliminary designs, followed by the use of L-Pile for detailed designs.  A robust design process can be achieved by using these tools in conjunction with software developed in-house.  Our software is based on Eurocode 4 to model moment capacities of steel/concrete composite tubes.

This is one of the key areas where the screw pile Engineer can add value to a project; the earlier the interaction begins with the Structural Engineer, the greater the optimisation of the foundation system and the greater the certainty of costs and performance for the Client. This is far more effectively delivered through closer relationships with the consultant and by moving away from the typical tender model of procurement to more collaborative methods of early contractor involvement (ECI) or nomination. Around 50% of our customers take advantage of this value and continue to return for their next project having witnessed the benefits.

What you don't know won't affect you?

The manufacture of steel tube in the global market is set up around petroleum and structural applications. The requirements for these industries are quite different to that of screw piling, where significant torque is applied to the tube.

Many years ago we procured tube in accordance with AS1163 (Structural Steel).  We soon revised this when it became clear that the standard did not guarantee a continuous seam to any given length of tube.

The photograph shows a section of the tube in question

The application of a torque significantly below calculated 

capacity has caused this failure.

Fortunately these issues were exposed during testing before they became permanent works.

In response to this we moved to utilising an American Petroleum Institute standard (API5L) that ensures all product is hydrostatically and radiographically tested, guaranteeing a continuous Electric Resistance Welded (ERW) seam to all lengths of tube.

However, our understanding of this continues to evolve (The API5L standard does not give suitable comfort to our team with respect to the steel’s strength.) Yield tests may be longitudinal according to the code dependent on the tube size – evidently not suitable for application of torque and in permanent works scenarios around bending moments. We now specify both transverse and longitudinal tensile strength testing on all our material.

There are a number of other areas that should be considered:

• Grade of steel – weld ability and ductility?
• Certification – what tensile strength can you use in design calculations and comply with NZ3404?
• Elongation Value
• Wall thickness tolerance

All of these specific requirements make it somewhat of a minefield to buy product from supplier’s stock or from ‘pre-loved’ applications.

Unfortunately there is no recognised standard that considers screw piling that the consultant can refer to, providing comfort that a suitable material is being used. We are continuing to evolve a specification around best practice. This is available on our website here.