America's Foundation with Gary Seider - Tension & Gravity Load Testing: Torque Measurement
In this edition of America’s Foundation, Gary Seider, P.E. interviews foundation experts Mike Kerker, Cary Hannon, P.E. and Kevin McNeill on the components of load testing and torque measurement. Read a summary below and watch the full video to see how each professional approaches load testing differently, and uses torque measurement and gravity loads in their foundation systems.
How Would You Determine Pile Capacity?
There are a couple different ways you can determine pile capacity.
- Treat each plate as an individual bearing element and plug the area of the plate into your bearing capacity equation. Your bearing capacity equation will be dependent on soil type. for example, clay or sand.
- Use the helical plate to advance the pile to depth. The surface area of the helix is your bearing area for a bearing pile.
How Many Plates Can You Have on a Helical Pile?
Three-helix helical piles are going to be used in most situations to penetrate dense material. Other situations you may only need one, but practically speaking three or four is the limit
More plates on a helical pile require specific spacing. We determine the spacing based on the types of soil being used. For example, in harder soils like gravel, multiple plates will be used to get the torque quicker. Most multi helical piles will have an inverted Christmas tree shape so the larger plates can drive the smaller pile into dense material.
How Do You Determine Gravity Load?
Normally you are going to be transferring the load of your structure down to a stratum that will support the load. Sometimes it’s just as important to hold something up, as it is to hold it down (if you have large tension loads). You may need just as must resistance to uplift, as you need actual capacity for compression.
How to Determine Pile Capacity:
- Torque Measurement: Torque measurement is measuring the installation energy that it takes to install the pile. Measuring the installation energy is an indicator of the piles capacity. A driven pile is much like a nail, where a helical pile is much like a screw, so the torque energy that it takes to put it into the ground will determine the amount of energy it takes to pull it out of the ground.
- Load Testing: When load testing; there is a specified method to perform any test. You are usually going to be using your load beam and continuously putting a load on the pile to test how much it can take by taking it up to a prescribed load and monitoring the safety factors. Then, just calibrate your torque to your load test.
The most accurate way to determine capacity is load testing, however torque measurement is more frequently used. In the past, measuring torque directly was problematic with the limited tools available. Most contractors would use hydraulic pressure as an indicator of torque. You can measure torque directly with a torque measuring device, or measure the differential pressure across the motor while installing the helical pile. Now, there is the CHANCE® digital torque indicator that simplifies the process of measuring torque.
Watch the full video to learn more about torque calibration and load testing.