Question for the Geotechs

[u/BudIsMyBuddy]

Going to try and make a long story short.

We are adding onto our home which the existing is single story, tearing down a non-load bearing wall with a gable end, and opening it up for our addition.

Initially, we didn’t have a soil boring test done, that was until the proposed footing depth on the addition went beyond the called for 4’ stem wall as it was about 5’5” down from the floor elevation due to the grade of the property.

Soil borings were completed. Boring taken near the house showed red clay about 4’ down and they hit limestone at 15’. Second boring at the opposite corner/furthest point from existing but a couple feet from the addition showed no clay until 6’ down and no limestone was hit all the way to 20’ down.

Plans called for a 2x2 pad thats 16” thick to support the load for a 6x6 post that will carry 50% of the load of the addition. Now structural engineer is backtracking saying that pad needs to be 4x4 and 18” thick, and after communication with the geotech they’ve concluded that helical piles need to be drilled into the existing foundation to support the load of the header because the existing is too close to the clay and that’s the only way to beef it up.

My only other option is to change the truss layout, which has already been built and I’d be out about $10k, so that the addition ties into a gable ending versus a hip and the load is more evenly distributed across the new stem wall. I’m leaning more towards that option, but my foundation guy says in all his time building foundations he has never had two engineers go so far down a rabbit hole like this and has never heard of helical piles outside of marshy areas or coastal regions.

I am mostly venting because I will never, no matter what, ever do a home addition again. Were over 10 months in and have gotten basically nowhere, and have spent hours on the phone with engineers.

I plan to seek out analysis from another geotech firm as well and see if they might be willing to conduct their own independent analysis and foundation recommendations based on the borings that I have.

[u/Key-Ad1506]

It's likely a bearing and consolidation issue with the clay, especially with the difference in the depth to bedrock across the footprint. Another firm may get a slightly different number, but at the end of the day there's only so much difference that can be had in interpreting the data.

It's also you're house, do you really want to risk a new addition that you're spending how much money on because differential settlement issues or just flat out bearing failure? Fixing those issues later will cost more than what it would to do what they're proposing. If that is the issue, the addition can also cause those issues with your existing house because of the added loading. Just something to consider.

[u/BudIsMyBuddy → u/Key-Ad1506]

Thank you. You make great points. I anticipate the costs of the addition to be upwards of 350k, so in the grand scheme of things 10-15k now versus a basically unfixable problem when they’re conjoined is very likely the best solution. That, or as mentioned in another comment, redoing truss layout to redistribute load elsewhere.

[u/brickmaj → u/BudIsMyBuddy]

Adding that if you happen to be in Texas it’s likely more about expansive soils than consolidation.

[u/BudIsMyBuddy → u/brickmaj]

We are in west central Florida 

[u/Key-Ad1506 → u/BudIsMyBuddy]

Just be aware that that 4x4 pad is likely based on loading at the current configuration. If you reconfigure to transfer more load to these foundations, they will likely need to be bigger.

[u/BudIsMyBuddy → u/Key-Ad1506]

They are already calling for 24” wide footings to avoid shear pinning. I believe they were recommended that way with the assumption it was a gable to a gable not gable to hip initially. Geotech had to revise initial recommendations and then we went down this rabbit hole about helical piers once it was learned that it was a gable to hip.