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Coronagrey

Pt foundation over compacted select fill

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I'm working on a project where the site had to be built up with 4 feet of select fill, compacted to 95% in 8 inch lifts, as the geotechnical report recommended.  The perimeter beams are to go 30" into the select fill.  The contractor doing my pt foundation is telling me he has never done a foundation where the beams don't go at least 12" into undisturbed soil.  That would mean the beam should be 5 ft down, the 4 ft built up plus 1 foot into the ground.  I contacted my geotechnical engineer, and he said as long as you compacted it down in lifts, and had it tested, which I did, I should be fine.  But my contractor is like no, they are wrong.  What are your professional thoughts?

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What did the Geotechnical report say?   

  • Did the GE know that it was a fill site?
  • Did the GE know that PT Slabs were being used?
  • Did the GE give you the Design Parameters for the PT Slab(s)?
  • Id the whole site a fill site or just part of it?
  • Is any of the bottom of the of the beams, footings, slab in natural ground or is it all in fill. 

 

Go through the testing reports and make sure everything passed or was accepted by the testing company based on the recommendations outlined Geotechnical Report.

Read the Geotechnical Report again.  Challenge the GE on anything that is unclear.  Get a supplement report if you encounter any information that is not in the report.

Ask the Contractor what his issue is?  Is he/she/they concerned about differential settlement?

Get all the out and issues, addressed them, and close out any concerns.

 

 

 

  

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The beams do not have to be bearing on native/undisturbed soils, assuming your fill is suitable and properly compacted. My concern would be the 4 feet of fill inducing settlement. Where is the project located? We need more information including borings logs, structural loads, grading plan etc. to help.

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If the soil was compacted and tested during compaction it should be sufficient to support beams. Code usually requires virgin soil so that you have a solid base and not just going on top of loose fill that is going to compact once the load is put on the foundation. If I'm not mistaken, the compacted fill would probably have stronger bearing capacity thanot the virgin soil.

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41 minutes ago, youngmotivatedengineer said:

If I'm not mistaken, the compacted fill would probably have stronger bearing capacity thanot the virgin soil.

Depends on the existing soil conditions, the type of fill material, the density to which the fill materials were compacted to, etc. The native/virgin soils could be preconsolidated.

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@Coronagrey @Jmcc06 @youngmotivatedengineer

IBC "1809.4 Depth and Width of footings. The minimum depth of footings below the undisturbed ground surface shall be 12 inches......The minimum width of footings shall be 12 inches."

Im sure you guys have run into this, but wanted to post it. However, I have run into municipalities requiring a minimum of 6" of virgin soil for shallow foundations. Also, 

IBC "1804.6 Compacted fill material. Where shallow foundations will bear on compacted fill material, the compacted fill shall comply with the provisions of an approved geotechnical report as set forth in Section 1803. 

If the existing soil was not considered better or met select fill strength conditions, I would remove it and fill. This is an interesting case being 4 ft is being added. Keep us updated please @Coronagrey.

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In regards to the 4' depth, the compaction level will only matter where the footing will bear. If you have proper compaction at the desiredoor footing level, the rest of the soil can be place non-compacted without affecting the strength of the footing. However, this would most likely cause grading and settlement issues around the exterior of the foundation.  It could cause huge problems is that area is going to get a solid surface like concrete or asphalt pavement. 

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I would follow the geotechnical report. It’s fairly standard stuff here in FL in flood zones. 

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