It’s really impressive when screw piles are installed for a new home with compact equipment like a skidsteer or midi excavator.
We started this example custom home project with a 13,000 lbs JCB Tele-Skid which is a really slick machine. However, at nearly 40 feet deep (the depth of expected till), instead of torquing up, the piles lost torque while refusing to proceed deeper. This wouldn’t be a significant issue if we were installing to known bedrock but to lose torque into expected till is not typical.
Luckily, the CAT 320 excavator used to excavate the basement was still on site and could be fired up for an impromptu load test. Utilizing a load cell and jack installed between the pile and 50,000 lbs excavator, we were able to confirm that the pile would not satisfy design criteria. Not only did the pile settle nearly 1″ at 17,000 lbs, it settled an additional 3/4″ by the time we reached the design criteria of 30,000 lbs. The good news was that the pile did stabilize and resist additional loading beyond the design loads. The interesting news was that, upon releasing the load, the pile rebounded 3/4″.
This was soft till! Would we have to walk away?
We conduct regular lunch and learns with Winnipeg’s engineering community and we have been asked several times, “what do you do if you get to soft till?” We didn’t have a real-life example to share when asked about soft till but… we now have the answer. In soft till, you swap out the compact, slick skidsteer for a nearly 60,000 lbs CAT 321 excavator.
The additional crowd from a 60,000 lbs excavator was enough to allow for the piles to penetrate into the till and torque up instead of down.
Additional load tests proved that the piles were now well suited to satisfy the design criteria.
This case study once again proved the benefits of knowing your soils while also observing torque when installing helical screw piles. Furthermore, it is an excellent example of how you might benefit from verifying capacity with load tests.
How do screw piles perform in soft till? Quite well, when installed with equipment large enough to acquire the target torques.
If you have a project, like this one, where the soils are problematic and you require a driven or helical pile, give us a call for a fast, free and no-obligation estimate on an engineered solution – 204.793.0653.