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.