conflicting foundation advice 3

[gringor]

building a 2 story waterfront home on Intracoastal in Florida. Most of the building pad is:
Stratum 1 - Loose light gray to dark brown fine sand to 2 to 4 ft below grade
Stratum 2 - dense cemented sand and shell to 9 to 12 ft below grade
Stratum 3 - refusal (presumed limestone, sandstone, or cemented coquina)
water was 3.5 ft below grade at test time (i suppose this varies with tide)

exception is a corner of the building pad that has a layer of very soft grey silt between 3 and 5.5 ft below grade. It spans 100 to 200 sq foot area of the building pad (estimated from digging test pits).

building pad will be raised a 3.5 over current grade to comply with BFE. Foundation is stem wall with overpour.

soils engineer is advising to remove silt, refill with clean compacted fill. General Contractor (GC) is saying its better to NOT remove the silt layer (because he says it can never be compacted as good as undisturbed state) and beef up foundation with extra steel and maybe some concrete/rebar poured down past the silt layer and on top of the cemented shell/sand. In his view doing anything is being over cautious because he thinks it is not enough silt to be concerned with. Architect and soil engineer dont like GC's beefed up foundation in that area idea, because of differential settling concerns. Soils engineer says clean fill replacing the silt can indeed be compacted adequately and will be just fine.
GC has got a years of "nuts and bolts" experience builing in this area (Florida) so I dont want to just discount his concerns. So Im just double checking with the experts here. Who is right? Would the above described silt layer come back to haunt me if I dont remove it? Or am I creating an unecessary problem by disturbing the soil to remove the silt layer?

Thanks in advance for any thoughts on the matter.

 

[Mad Mike]

The soil engineer is correct- you have a "very soft" silt layer in-situ, with the recommendation being to over-excavate it to spoil and replace it with an engineered granular material. This is good practice, not overly cautious at all.

The contractor's only argument appears to be that the suitable granular material will not achieve the in-situ densities of a very soft silt. That point of view is dangerously flawed so I would not even consider any of his other arguments/recommendations.

There are probably other factors at play, but I would rather run the contractor's concerns past the soil engineer than the other way around.

All the best,
Mike

 

[EireChch]

I agree with your Soil Engineer and Mad Mike. It sounds like the contractor is talking about reusing the silt, when he says it will never be compacted as good as its undisturbed state.

The contractor may have a point to leave the silt in if it was extending to 9/10ft as this is getting to the level of being uneconomical to remove. (Even then i probably would still remove it). Architect/Soils engineer also correct re the potential for differential settlement.

 

[Ron]

I practice in Florida. Your soils engineer is correct. Over-excavate and backfill. Compaction can be done properly and fully support the foundation. If your contractor can't do that....get another contractor!

 

[gringor]

you guys are the best! thanks for the responses. Will make sure excavators are mobilized today. as an added precaution I plan to get the soils engineer to inspect the excavated pit and supervise the fill and compaction.

 

[oldestguy]

In my experience some contractors with years of experience somehow get their minds stuck on certain ideas, sometimes with no merit to back them up. I have been amazed many times to find I have saved them lots of bucks by changing those long held ideas. Result: they are then great customers thereafter.

 

[cvg]

suggest your geotech be there during the excavation, especially since the "pit" will end up with 6 feet of water. Make sure the contractor has a good work plan for dewatering. And make sure the geotech is on board with the contractors choice of imported "granular material". He should approve it before it gets hauled on-site.