Foundation on Expansive Soil - PTI vs Safe
Foundation on Expansive Soil - PTI vs Safe
(OP)
I have been working on foundation design for a 4-story wood frame multi-family design on a very high plastic soil. The geotech has given the following parameters for the soil.
Bearing pressure = 2500 psf
PTI Parmeters
Center em = 8.3', ym = -2.2"
Edge em = 5.1', ym = 1.8"
ks = 125pci
Differential Settlement = 1:15
The exterior loading on the wall is fairly light (3500plf).
The contractor is a decent sized company with a lot of local projects. The geotech and the contractor claim that all the projects they work on are a 5" slab with some conventional reinforcement and "top steel".
WE have designed the slab with 2 methods. Both designs are much more robust than the contractors experience. The first method we used is the PTI method, which gave us the following design (post tension 4" slab with 18" x 31" ribs at 8' on center or a 10" equivalent slab).
The 2nd method, we modeled the building in SAFE. In the model, we removed the outer 5' of soil supports in an attempt to model the "Center lift" condition. Our model cantilevers the building 5'. As you can imagine, in order to get the model to work, the slab becomes very large. Even with the large slab, at the corners and jogs, the bearing pressure is higher than allowable. We tried flat slabs and ribbed design. All our designs were either not practical or bearing pressure was exceeded.
My questions are as follows:
1.) Can we use the PTI method even though the PTI method does not consider corner and building jogs?
2.) Are these soil parameters compatible with a shallow foundation or is a deep foundation required?
3.) Is our Safe model too conservative?
FYI, we asked the geotech to confirm his numbers and he stood by his original report.
Thanks for your feedback.
Bearing pressure = 2500 psf
PTI Parmeters
Center em = 8.3', ym = -2.2"
Edge em = 5.1', ym = 1.8"
ks = 125pci
Differential Settlement = 1:15
The exterior loading on the wall is fairly light (3500plf).
The contractor is a decent sized company with a lot of local projects. The geotech and the contractor claim that all the projects they work on are a 5" slab with some conventional reinforcement and "top steel".
WE have designed the slab with 2 methods. Both designs are much more robust than the contractors experience. The first method we used is the PTI method, which gave us the following design (post tension 4" slab with 18" x 31" ribs at 8' on center or a 10" equivalent slab).
The 2nd method, we modeled the building in SAFE. In the model, we removed the outer 5' of soil supports in an attempt to model the "Center lift" condition. Our model cantilevers the building 5'. As you can imagine, in order to get the model to work, the slab becomes very large. Even with the large slab, at the corners and jogs, the bearing pressure is higher than allowable. We tried flat slabs and ribbed design. All our designs were either not practical or bearing pressure was exceeded.
My questions are as follows:
1.) Can we use the PTI method even though the PTI method does not consider corner and building jogs?
2.) Are these soil parameters compatible with a shallow foundation or is a deep foundation required?
3.) Is our Safe model too conservative?
FYI, we asked the geotech to confirm his numbers and he stood by his original report.
Thanks for your feedback.






RE: Foundation on Expansive Soil - PTI vs Safe
RE: Foundation on Expansive Soil - PTI vs Safe
I would expect allowable stresses to be exceeded, and redistributed, locally. Would that improve your results significantly? As for the specific questions:
1) I would say so. All buildings have corners and most have jogs.
2) The soil parameters don't bother me.
3) I suspect so.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Foundation on Expansive Soil - PTI vs Safe
Here is a tutorial of the proper way to model a PT slab in adapt-SOG. The process is much more involved than simply modeling a lack of support at edges, you need to follow the PTI flowchart.
https://www.youtube.com/watch?v=ZQqdJUJO270
With your PTI beam spacing I suspect you have around 60 PI value. I wouldn't mess around with that and be influenced by the contractors.
RE: Foundation on Expansive Soil - PTI vs Safe
RE: Foundation on Expansive Soil - PTI vs Safe
PTI DC10.5-12 R6.1.3 "Geotechnical approaches should reduce ym-center to less than 2.0 in. and ym-edge to less than 1.0 in."
RE: Foundation on Expansive Soil - PTI vs Safe
26" deep beams are a starting point in my book, so 31" is certainly reasonable given the soil conditions.
Maybe you could explore reducing the interior beam widths if you do not need the bearing capacity there?
If you want to triple check your design, you can download the WRI procedure for free here:
Link
http://wri.support/TF700-R-07.pdf
RE: Foundation on Expansive Soil - PTI vs Safe
RE: Foundation on Expansive Soil - PTI vs Safe
Thanks for the feedback. We have decided to go with a 10” flat slab. Our slab meets the output from the PTI method. Our design did not satisfy our SAFE model but it seems that most engineers agree that he model was conservative.
The 10” slab did have the benefit of eliminating almost all the interior footings.
RE: Foundation on Expansive Soil - PTI vs Safe
Refer to Mitchell method and design software slog
Subset national a deep perimeter footing will solve the issue