Screw pile cost blowout
Why were additional costs incurred?
You may be curious to know why Koble Constructions, an advocate for Screw piles, is writing about cost blowouts and the reasons for them. The following is an interesting case study showing that, occasionally, cost blowouts happen, and it’s essential to understand how and why it can happen.
A client of Koble Constructions provided us with structural drawings and a soil report for a small building on a large block in the outer suburbs of Melbourne. In this case, the decision was made to use screw piles instead of bored piers because of limited access along the side of an existing building and to save construction time.
The soil report showed a soil founding depth of 2 - 2.5 meters deep, but groundwater was not encountered. The structural drawing specified screw piles to be a minimum of 2.5 meters deep with a minimum load-bearing capacity of 60Kn. Koble Constructions quoted that they would supply 3.0-meter-long 80Kn piles for the installation.
On the day of the installation, Koble Constructions arrived on site with a truck loaded with an excavator, screw piling auger drive motor, a torque measuring unit and the correct amount of 3.0 80 Kn screw piles to complete the job.
The screw piles were unloaded from the truck and positioned ready for installation. The operator drove the first screw pile into the ground, but the pile didn’t make the required torque. The same thing happened for every single screw pile on the rest of the site.
The question now was how deep the piles would need to go to meet the specified torque requirements. After some deliberation between Koble Constructions' ground crew and the builder, it was agreed that a set of 1.5-meter extensions should be added to the piles to reach the required depth to make the torque for 60 kn of load per pile. The pile extensions were added to the piles, but they still didn’t make the required load-bearing torque.
A second lot of extensions had to be installed. The soil report showed foundation material at 2 to 2.5 metres deep. So, what was going wrong? Koble Constructions added an extra 1.0-metre pile extension, to an already 1.5-meter extended screw pile to see just how deep we had to go to make the required torque. The total depth needed was 5. 5 meters deep. A truckload of 1-meter screw pile extensions was ordered, plus an additional 10 1.0-meter spare screw piles, in case we had to go deeper in some locations.
In the end, five out of the 10 spare 1-metre screw pile extensions had to be used to make the required torque. Screw pile total depths varied from 5.5 metres to 6.5 metres deep. An average depth of close to double the depth stated in the soil report.
There was no option but to screw the piles down deeper. If screw piles don’t make the design torque, the supported structure has a high chance of failing. Leading to cracking and the possibility of complete building failure. An unstable foundation is a disaster for the Designer, the Engineer, the Builder and the client.
The alternative was to use bored piers instead of screw piles. For this site, bored piers would have been an enormous challenge due to the excessive depth needed to reach the foundation material. The cost for bored piers would be at least twice the cost of screw piles. The cost of deep footings with blinding would be 4-6 times more expensive than screw piling. when you include concrete pumping and spoil removal.
The key takeaways from this case.
Soil reports don’t always indicate what is beneath the ground throughout the entire site. Screw piling for foundations is an excellent solution for dynamic ground conditions. The option to use extensions to reach a foundation depth makes screw piles our preferred solution. When the unexpected happens, screw piles are the most economical solution.