How to certify screw piles:

According to AS2159 there are only two ways to properly design & certify screw piles:

a). Design to known geotechnical conditions; and/or

b). Carry out static load testing. 

How to design screw piles to suit geotechnical conditions

Designing screw piles to suit known geotechnical conditions means using a Geotech Report that shows suitable material for founding the screw pile helix and, ideally also confirms the consistency of material at least six helix diameters below the founding depth. 

Looking longer term, the soil and groundwater corrosivity that might be identified in a Geotech' Report can guide the spec for steel thickness of the piles and forecast durability over their design lifespan.

On site, a test pile is installed at the same spot as the given Geotech' CPT, borehole and/or DCP test location. "Install torques" are recorded with particular attention given to the install torque at the designed depth. That recorded torque is then used as a reference across the job to help assure pile uniformity and flag any variations or changes in depth of the target foundation material. 

How to static load test screw piles

Static load testing screw piles is loading installed piles with the actual mass specified by the Safe Working Load plus a margin to compensate for varying conditions that might be applied over the design life of the pile. AS2159 prescribes a pile testing schedule which roughly doubles the specified SWL of a pile and sequences loading, unloading and reloading the pile over a set amount of time.

For a common 120kN SWL screw pile, a static load test may involve applying 24ton of force multiple times in a sequence over a set period of time with constant monitoring, measurement and recording.

The set up for a static load test is typically a structural steel test frame held down by a group of reaction/negative load screw piles. A high-rated hydraulic jack is held down by the test frame and used to force the screw pile into the ground. Constant records are kept with the hydraulic pressure gauge on the jack, displacement gauges on 3 axis of the test pile, a dumpy level for relative movement between the pile and frame, and ideally a calibrated load sensor. 

The set up and time involved in monitoring static load tests make them a costly exercise but the surest way to confirm actual pile performance. 

Other non-conforming methods of proving screw piles (table): 

• dynamic pile testing; and

• torque inference;

Dynamic pile testing

Dynamic pile testing involves dropping a weight onto an installed test pile so that the total force applied is in excess of the SWL. Dynamic pile testing is not a conforming method of certifying screw piles as per AS2159, with the main reason being the increased difficulty in assessing deflection of the pile helix. The full deflection/bending of the pile helix might not be tested before the force on the pile is reduced and the pile returns in the direction of its original state. 

While not being ideal for proving overall screw pile performance, dynamic pile testing may still have value in proving some degree of pile performance. If appropriately planned, dynamic pile testing can at least prove that piles are not subject to a catastrophic geotechnical failure. It may be a useful supplementary tool for testing and certification where:

• geotechnical information is known; and

• the cost of static load testing is prohibitive; or

• access inhibits the set up of a static load test or reaction piles; or

• an increase in the number of tests that can be carried out is an advantage of the speed and efficiency of dynamic testing. 

Torque inference 

Based on experience (or equally lack thereof), some piling contractors may assume ground conditions and load capacities of piles based on torque alone. This is not an acceptable way to certify piles under AS2159 and has a few problems:

• different ground materials can cause different torques; a stiff sand okay for founding a pile may have a lower torque than a highly reactive clay that isn't suitable for founding a pile;

• torque gauge readings can be easily influenced by other install factors such as straightness, hydraulic pressure, or rate of install relative to helix pitch; and

• it is more difficult to determine whether a layer of one material (possibly fill) is overlaying a much weaker strata.

In simple terms:

Torque = rotational force

Rotational force ≠ vertical load capacity

From an economical perspective, using assumed torques it is guesswork whether a pile will "pull-up" or reach the presumed torque at any certain depth, so pile design cannot be optimised for value and the overall project cost is likely to me more at the end of the day. 

Installing piles based on torque alone should only be done when you are willing to risk and repair whatever it is they're holding up. In any other case, do it right with a Geotech' Report.  

What do we need for piling platforms?

The quality of your piling platform is always the biggest factor to either: 

a). Make piling efficient and life easy across the site; or

b). Create headaches and critical safety risks throughout the process. 

Piling platforms need to be stable, made from consistent compactable material, built flat and level at or near Top of Pile height, large enough for the piling operations, and weather/groundwater resistant for continuous smooth operations.

If work continues when a piling platform is too small, soft or uneven:

• operators are tested to make unsafe decisions;

• rigs and lifting plant are at risk of tipping over;

• piling components suffer increased wear and tear, risk of failure and not working correctly; 

• pile trimming, control of spoil and water runoff are all made messier; 

• piling efficiency is significantly reduced, increasing time and cost for all parties; and 

• quality is harder to assure. 

Tips for a good piling platform: 

1. talk to your engineers and piling contractor in the planning stages;

2. consider simplifying the pile design to reduce the changes in level; and

3. take the time to prepare the subgrade and uniform compactible material near Top of Pile height... for a much better start when piling begins.