Ground Screw Engineering and Performance

In order to accurately specify the application of our ground screw products, we have produced the following information document addressing the performance, load bearing capability and durability of the screw piles.

Firstly, we have the screw itself :

The screw is manufactured from hi-tensile steel grade ISO630Fe360A. The tubular section of our screws has a 3mm or 4mm wall thickness (depending on type) and are either a socket type, or have a 5mm thick bracket welded to the head. The screws are hot dipped galvanised to ISO1460 / AS/NZS 4680.

The screws are manufactured on a limited production run for SlutaGrav AB – the parent company in Sweden, in a factory with ISO9001 Quality Control Accreditation, and are stamped with the ‘StopDigging’ logo so we can be sure of consistent quality, and guard against the risk of inferior copies.

We have tested the screws, using WSP Opus, a highly regarded global infrastructure and management consultancy, at their laboratory facility in Auckland, NZ, to provide data behind the design & construction of the screw. We tested the tensile strength of the screw and more specifically the bracket designed to connect to timber posts or joists. In all tests the timber itself failed before any sign of fatigue to either the screw bracket, weld or securing bolts. These tests simulated an uplift load far greater than the screw would ever be subjected to, and therefore give us confidence in the engineering design of the screw.
We also completed a compression test on the screw – it started to fail (bend) at over 210KN and supported a constant load of over 200kn (20,000Kg). Although these tests are not relevant to the ‘in ground’ performance of the screw pile, they have proved that the screws themselves are over 5 times stronger than any load they would encounter in normal service. The full report on our testing is included as an appendix to this document. This brings us on to the second area –

In ground load bearing capability:

As logic dictates, the load bearing capability of the screw is dependant on the density of the ground it is installed in. In general the longer the screw, the deeper it is installed and the denser the ground is – our 1200mm screws therefore have a greater load bearing capacity than our 900mm screws.

We have produced a data sheet, based on multiple tests in a variety of ground conditions to show the average ‘in ground’ performance of the screws. The data sheet has results for load(compression), uplift (tension) and lateral load resistance. This testing was undertaken by SWECO, one of Europe’s leading Civil and Structural Engineering companies. The tests utilised a tension or
‘pull test’, which involved installing a screw in the ground and then recording, with a load cell, the force needed to pull the screw upwards out of the ground and measuring pile deflection. This gives us the upward or wind uplift resistance figure, and SWECO have provided a conversion multiplication factor of 1.46 to determine downward load bearing capability.

The stated figures on our data sheet have also incorporated a safety factor to allow a structural engineer to be able to confidently incorporate an appropriate amount of screws into a foundation design. This means that in practise, load ratings of the screws are higher than those stated, and much higher in very dense ground.

As a standard service on request, we provide on site testing with our apparatus to accurately specify screw performance at a given site / location. This will allow an engineer to confirm his design parameters before installation takes place or confirm a design during installation. This is an accurate and accepted way of proving performance and load bearing capacity of the screw pile system, and ensures total confidence in the product.

Durability: How long will the screws last?

Steel protection has been specified in accordance with the “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings” AS/NZS 2312 and the “Steel structures Standard” NZS 3404 including amendments and technical specifications.  We note that this is on a time to first maintenance basis.

Deciding on potential lifespan of galvanised steel’s in ground durability is a well documented and researched science. There is a wealth of information available from independent studies in North America / Canada and Australia where the use of galvanised steel ground anchors and structures are widely accepted and approved construction methods.

This method of construction offers an acceptable level of durability to exceed the current Building Code in both Australia and New Zealand. The benefits of full recycling capability of the screw piles adds to the advantages of the method for sustainable building.

Our ground screws are designed and manufactured with a hot dip galvanised coating to ISO1461 and AS/NZS 4680.
This means a minimum coating of 85microns.

An excerpt from the Specifiers Manual of the AUS / NZ Industrial Galvanisers Company:

“Corrosion Rates and Australian /NZ Standards:
The use of corrugated steel culverts has been long established in Australia. A standard has been developed and the most recent revision, AS/NZS
2041:1-2011, contains a significant amount of useful information in its Appendix E on durability issues.
There are a number of informative tables in this Standard that address corrosion issues for a variety of coatings as well as the base steel. These tables base corrosion rates on soil resistivity and pH as well as soil classifications. These tables nominate corrosion rates for galvanised coating from around 3 microns per year in well drained soils with neutral pH to over 20 microns per year in undrained acidic (pH<4) soils.

Metal loss for steel is nominated at less than 10 microns per year in well-drained soils with high resistivity and pH greater than pH5, to 300 microns per year in poorly drained soils with low resistivity (usually related to chloride concentration). Reasonable averages derived from these tables for both zinc and steel in contact with soil are for zinc, 6-10 microns per year, and for steel, 20-30 microns per year.

In designing steel products for use in soil, there is enough information available to make reasonable estimates of service life of the structure. For products like screw-in piles used for house foundations, and lighting and power poles, these considerations are important in determining their service life.

Often, the focus is simply on the performance of the protective coating. However, an understanding of the steel’s performance allows a better outcome through the provision of a corrosion allowance over and above the structural requirements of the steel.
Taking the example of a screw-in pile used as a house foundation, the steel thickness needed to support the structure is in the order of 2.5-3mm in a square or circular hollow section pier. By using a pier with a wall thickness of 4.5-5 mm, with a heavy duty hot dip galvanised coating with a minimum coating thickness of 85 microns, inside and out, the durability expectations would be as follows, in a normal domestic building site environment: Galvanised coating life : 85 micron coating/ 6 microns/year = 12 years
Steel corrosion allowance : 2 mm at 30 microns per year = 67 years
TOTAL = 79 years”

Conclusion

The use of steel in in-ground structural applications is on the increase, as the benefits of using well-engineered steel alternatives are recognised by engineers and specifiers. Given an understanding of the corrosion issues involved, it is possible to engineer an acceptable life for these buried structures.

As shown by our test procedures, it is also possible to accurately determine the ground screw’s in-ground load bearing / uplift resistance capability and so the performance of the screw pile product is easily quantifiable.

Add to this the exceptionally quick installation method and portability of installation equipment designed by StopDigging, it has the potential to revolutionise some aspects of the construction industry.
From an environmental point of view, the fact that the ground is not disturbed during installation, reduces the potential for diseases like Kauri Dieback to be spread, and minimises damage to sensitive tree roots.
The screws are easily withdrawn if needed for either relocation or recycling – which cannot be said of concrete.

Whilst not suitable for every type of design, the case for specifying ground screws as an alternative to traditional digging and concreting timber piles, is compelling.