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Joel Bryant

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How to Avoid Serious Hazards When Lifting Post-Tension Slabs

Posted by Joel Bryant on Oct 13, 2020 10:00:00 AM

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I recently provided on-site technical support on a job with Foundation Repair of Western Colorado. A post-tension residential slab had some interior settlement issues. (Post-tension slabs are constructed with embedded high-strength cables that keep them from cracking or moving.) It looked like the settlement was due to an issue with the cables, due to either a faulty installation or failure to properly stretched after the install.

When working on a slab containing post-tension support cables, you must be extremely careful. These cables are typically stressed to greater than 30,000 lbs tension. If you accidentally drill through or cut a tensioned cable, it can likely rip out of the concrete with a sudden whip-like motion. Consequences could include slab destruction, equipment damage, serious injury, or even death. A few years ago, I witnessed a geotechnical engineer accidentally cut a post-tension support cable when drilling through a slab for a soil boring sample. We were inside a building. The cable snapped with a sound like a massive gunshot. It shot out from the exterior of the foundation wall. Fortunately, no one was standing near that area at the time. Nor was there any valuable equipment nearby. However, serious damage was done to the slab.

While consulting on this job with Foundation Repair of Western Colorado, we used ground penetrating radar to locate the post-tension cables before doing any drilling. The crew then marked the “no-drill” areas and was able to safely drill through the slab for polyurethane injection. In addition to locating the post-tension cables, we used the GPR system to locate rebar within the slab and check for underground utility lines.

Ground Penetrating Radar uses radar pulses to image the subsurface. It is a non-intrusive method of surveying the sub-surface to locate underground utilities such as electrical conduit, plumbing, duct work, cables or masonry. GPR can also often identify the location of voids under the slab, providing helpful information in the injection process.

This slab lifting job went very well. The crew used AP Lift 430 to level the slab. The settlement was remedied, and the customer was very happy with the results.

Want more information on slab lifting products and procedures?

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Topics: Equipment & Accessories, All Posts, Lift Slabs

Dynamic Cone Penetrometer: An Essential Tool for Deep Lift® Jobs

Posted by Joel Bryant on Sep 17, 2020 10:00:00 AM

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As a consultant on many deep soil geotech jobs over the years, I have found myself recommending one critical piece of equipment repeatedly: The Dynamic Cone Penetrometer (aka the DCP). I highly recommend this tool as an essential part of your Deep Lift® rig set up. As Jim Spiegel explained in his blog post on this subject, this device is used to measure the strength of subgrade soils, and can be utilized to identify weak layers of soil within a soil mass. As you can see in the video below, a hydraulic pump drops a 30kg (60lb) hammer, driving the measuring rod into the ground.

To measure soil strength, the operator counts the number of blows it takes to drive the steel rod in 10-centimeter increments. Good soil requires 10+ blows to drive the rod 10 centimeters. Anything less is typically indicative of weak soil conditions.

DCP testing is a widely accepted means of in situ testing. Due to the common acceptance of DCP testing, many municipalities and agencies are willing to pay for this service.

Here is a hypothetical scenario for DCP use:

Soil strength data is needed under a concrete parking lot before a Deep Lift® job. Multiple tests would be performed, spread out evenly over the area. For thorough testing, each DCP test would be executed to a minimum depth of 15 – 20 feet or refusal (when the hammer blows no longer cause movement.) The test results can then be compared and analyzed to find where potential weak soil conditions may be present. This allows for accurate and effective application of the polymer to address the weak layers.

DCP testing can also be utilized prior to bidding any Deep Lift® job to develop effective injection plans, as well as accurately estimate material usage on projects. With that said, it’s not always possible or practical to perform pre-bid testing. When pre-bid testing is not performed, it would still prove highly beneficial to perform DCP testing prior to injection. I have personally encountered many situations where injection plans have been drastically altered after test results were analyzed (injection depth was added or subtracted, the weakest layers were identified, etc.) The Dynamic Cone Penetrometer is a portable tool which can easily be used in many locations where other testing methods are not feasible and it provides valuable insight into the soil conditions that need to be improved.

Want more information on Geotech products?

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Topics: Equipment & Accessories, All Posts, Lift Slabs, Deep Lift