Hello,
Long post, but I’m cutting out some details to shorten it and I appreciate your patience in reading through it all. My wife and I are having a house built with a post-tension foundation on concrete piers. Most piers are 12” diameter, and 15 feet below original grade. None of them sit on bedrock. Soil tests indicate mostly low-plasticity clay all the way to 20 feet below grade, with equivalent plasticity index of 17, and potential vertical rise of less than 1”.
I’ve had concerns about compaction (or lack thereof) of the built-up dirt pad, so after the beam trenches were dug, I hired a geotechnical firm to perform Dynamic Cone Penetrometer (DCP) testing of the trench bottoms to see if the bottoms would be up to the task of helping to support the beams, and not just relying on the piers. It was determined that in most areas, the trench bottoms had a layer of about 6” of not-so-compacted dirt (that layer was actually quite muddy that day due to a light rain the night before), but there is firm, well-compacted dirt beneath that – probably the original grade. The geotechnical engineer said that as long as a concrete vibrator was used, the concrete would mix with the looser dirt/mud at the trench bottoms to form a “concrete mud” capable of supporting the house. So I told the builder to be sure the concrete crew would use vibrators, and on pour day (which was the very next day), they did indeed use a vibrator. However, looking back at video footage and based on my memory from being there the whole time, I doubt the vibrator went deep enough to reach the trench bottoms, and I doubt it was held in place long enough to have the desired effect. If I assume the vibrators didn’t create the desired “concrete mud,” and the first 6” in the trench bottoms were somewhat loose (and muddy), how much support, if any, will that looser layer provide for the beams, and will it tend to settle over time, leaving only the piers to support everything? Asking the question another way - is it normal and acceptable for there to be several inches of loose-ish dirt/mud in the trench bottoms? Is it likely that the concrete would have compacted or displaced that layer to any meaningful degree as it was being poured?
Being an engineer myself (but not a soils or civil engineer), I have been attempting to calculate the pier loading, assuming there is no soil bearing support under the beams or the slab sections. The foundation engineer told me verbally that each pier is capable of supporting about 27 kips, after dividing by a safety factor of 2 (i.e., 54 kips before the safety factor). He said this also assumes the first 3 feet below grade contribute nothing to the skin friction. When I look at the soil report, it says the first 9 feet are the active zone, producing an uplift pressure of 420 psf on the pier through skin friction when wet, while the bottom 6 feet are the anchor zone, producing 600 psf of downward pressure. The report then goes on to say that the pier load capacity can be calculated using 600 psf, but it’s not clear if that is meant to be applied over the entire pier length (minus the top 3 feet), but doing it that way, and adding the end bearing capacity, is the only way that I can arrive at anything close to the 27 kips. But I’m not sure it makes sense to do that, since the active zone could seemingly lose its load-carrying capacity when the soil in that zone dries up and stops gripping the pier. In such a situation, the load would be carried only by the anchor zone (bottom 6 feet), along with the end bearing, and that would not produce anything close to 27 kips. Am I right in viewing it this way? I contacted the engineer for further clarification, and all he would say is that his firm only ignores the top 3 feet. If I use his approach, and ignore any load bearing by the dirt beneath the beams, then my calculations arrive at a safety factor of almost 2 in terms of pier loading. But if I ignore the top 9 feet and only use the bottom 6 feet for skin friction, plus end bearing, then I show very little safety factor (around 1.0). I’m looking for folks who understand this stuff to help me understand better, and hopefully to provide me with some reassurance that my foundation will be ok.
Thanks for any insight you can provide.
Long post, but I’m cutting out some details to shorten it and I appreciate your patience in reading through it all. My wife and I are having a house built with a post-tension foundation on concrete piers. Most piers are 12” diameter, and 15 feet below original grade. None of them sit on bedrock. Soil tests indicate mostly low-plasticity clay all the way to 20 feet below grade, with equivalent plasticity index of 17, and potential vertical rise of less than 1”.
I’ve had concerns about compaction (or lack thereof) of the built-up dirt pad, so after the beam trenches were dug, I hired a geotechnical firm to perform Dynamic Cone Penetrometer (DCP) testing of the trench bottoms to see if the bottoms would be up to the task of helping to support the beams, and not just relying on the piers. It was determined that in most areas, the trench bottoms had a layer of about 6” of not-so-compacted dirt (that layer was actually quite muddy that day due to a light rain the night before), but there is firm, well-compacted dirt beneath that – probably the original grade. The geotechnical engineer said that as long as a concrete vibrator was used, the concrete would mix with the looser dirt/mud at the trench bottoms to form a “concrete mud” capable of supporting the house. So I told the builder to be sure the concrete crew would use vibrators, and on pour day (which was the very next day), they did indeed use a vibrator. However, looking back at video footage and based on my memory from being there the whole time, I doubt the vibrator went deep enough to reach the trench bottoms, and I doubt it was held in place long enough to have the desired effect. If I assume the vibrators didn’t create the desired “concrete mud,” and the first 6” in the trench bottoms were somewhat loose (and muddy), how much support, if any, will that looser layer provide for the beams, and will it tend to settle over time, leaving only the piers to support everything? Asking the question another way - is it normal and acceptable for there to be several inches of loose-ish dirt/mud in the trench bottoms? Is it likely that the concrete would have compacted or displaced that layer to any meaningful degree as it was being poured?
Being an engineer myself (but not a soils or civil engineer), I have been attempting to calculate the pier loading, assuming there is no soil bearing support under the beams or the slab sections. The foundation engineer told me verbally that each pier is capable of supporting about 27 kips, after dividing by a safety factor of 2 (i.e., 54 kips before the safety factor). He said this also assumes the first 3 feet below grade contribute nothing to the skin friction. When I look at the soil report, it says the first 9 feet are the active zone, producing an uplift pressure of 420 psf on the pier through skin friction when wet, while the bottom 6 feet are the anchor zone, producing 600 psf of downward pressure. The report then goes on to say that the pier load capacity can be calculated using 600 psf, but it’s not clear if that is meant to be applied over the entire pier length (minus the top 3 feet), but doing it that way, and adding the end bearing capacity, is the only way that I can arrive at anything close to the 27 kips. But I’m not sure it makes sense to do that, since the active zone could seemingly lose its load-carrying capacity when the soil in that zone dries up and stops gripping the pier. In such a situation, the load would be carried only by the anchor zone (bottom 6 feet), along with the end bearing, and that would not produce anything close to 27 kips. Am I right in viewing it this way? I contacted the engineer for further clarification, and all he would say is that his firm only ignores the top 3 feet. If I use his approach, and ignore any load bearing by the dirt beneath the beams, then my calculations arrive at a safety factor of almost 2 in terms of pier loading. But if I ignore the top 9 feet and only use the bottom 6 feet for skin friction, plus end bearing, then I show very little safety factor (around 1.0). I’m looking for folks who understand this stuff to help me understand better, and hopefully to provide me with some reassurance that my foundation will be ok.
Thanks for any insight you can provide.
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