Wet Footer Base 2

[SteveFehr]

I just had footers dug for my addition (2 story residence, appx 28x40') and, being an EE, I hired what I thought was a professional to do the job. I'm in Chesapeake, VA, 8' above sea level and the ground here is flat and always wet. The soil the footers will be on is hard clay directly beneath the footer and a softer (way stickier) clay underneath that for at least 2'. The footer trenches are also unfortunately wet and there is really no practical way to dry them, given the water table- about half the footers are soppy like that, and the others are hard enough you can leap into the trench and barely leave a mark. We've got a pipe trench cutting through 18" beneath the footers and I'm pumping that, but that only really prevents the whole shebang from flooding and doesn't help the puddling and soppiness. My contractor insisted that we could just shovel 2" of pea gravel on the wet areas, but the city inspector rejected it, saying we need engineering.

For reference, I was intending to use use 8x22" footers with a 3-block stem wall (6" block), and slab-on-grade over compacted sand as the floors, to match the existing house.

Is pea gravel the right way to base the footers? The inspector wrote that an engineer's repair or approval is required. Will an engineer approve the design as it is, or can I expect costly redesign/repairs?

 

[jgailla]

Overexcavation and replacement with 6" to a foot of 57 stone is a typical fix for a wet subgrade down here.

You don't really characterize your soft clays, but if the your existing building is OK the subgrade should be fine for a small, lightly loaded addition.

8" wide footings seem too small for a 6" wall. I would expect 12" at a minimum. I believe the footing per IBC is required to be twice the width of the wall it supports.

Someone in here won't like this, but you shouldn't need an engineer to sign off on such a small structure. Unfortunately, what the inspector wants the inspector gets.

Talk to a local geotech firm and see if you can get someone out to "approve" the subgrade prep.

 

[SteveFehr]

They're 8" thick, 22" wide, which is the max width I can have an 8" thick footer with 6" block per IRC 2003 (8" CBU can be max 24")

After I made this post, I remembered one of my old coworkers is a PE who does stuff like this on the side and is happy to give free advice- he advised I wait until it dries out He said the gravel did nothing to help and could only hurt and said that I had to shovel all the muck and gravel out and go back and fill in with packed sand so that you could walk around the entire footer without feeling any give. Which I really wouldn't need an engineer for if I did that! He said he'd come out and look at the site and give me a stamp for $100/hr if I needed, but is leaving town for 2 weeks friday afternoon so I'll have to get him tomorrow.

I'm going to call the city first thing tomorrow and see if I can get the inspector to tell me exactly what he's looking for, that's the surest way of passing inspection. By my whole attitude is that inspection is just a formality- this is my house, I want it done RIGHT regardless of whether the city's signed off on it.

Does it matter what sand I use to pack if I use sand? I've got 3 yards of fill sand and 3 yards of masonry sand on site I can use. What's 57 stone? Would that be easier to pack with wet clay underneath or would it still mush, even with 6" of stone? Unfortunately, trying to dig out the mush just seems to make more mush...

 

[Ron]

SteveFehr...overexcavation and backfilling with gravel in clayey soils is an appropriate fix for your condition. Waiting for it to "dry out" is impractical and actually might never occur.

Have a competent geotech take a look at it. You're not shopping for a stamp...you're shopping for competent, relevant advice. The geotech will visit the site, characterize the conditions, probe the subgrade and will likely tell you to overexcavate and backfill with gravel,usually a bit larger than pea gravel (jgailla mentioned No. 57 stone, which works fine in this application). No. 57 stone is typical coarse aggregate for concrete. It has a nominal 3/4-1" max. size and is graded down to about pea gravel.

Compaction will likely not be successful. You'll likely cause the subgrade to "pump" which is a condition wherein the water content of the material is too high (at or near saturation) and when load is placed on it (during compaction, for instance), the water in the pores of the soil takes the load and does not weep from the soil as it should.

You need to make sure the subgrade has adequate stability. The geotech will probably stick a probe rod in the soil to check and see how deep the unstable material is then recommend excavation to stable material before backfilling with gravel. The gravel will allow the pore pressures to dissipate when the footing is cast and some slight settlement will occur during construction.

 

[SteveFehr]

I had an engineer come over and he probed the ground and recommended digging out another 6" in some spots and 12 and 18" in others, and filling with 57 stone. Perhaps you can aid my understanding here- from digging, it's clear the underlying clay is identical under this entire foundation. If the only difference between these sections of footer now is the depth of water saturation TODAY- how can that possibly affect the strength of the footer 10 years from now when the saturation levels will be completely different? Why should I dig out a foot more in one spot just because we had standing water from a rainstorm last week that saturated that section more than another?

 

[Ron]

SteveFehr....the reason you dig out the sloppy mud is that it has such a high moisture content and little or no consolidation of the solid materials that when you put the weight of the footing on it, the result will be very localized quick settlement, likely resulting in cracking of your foundation.

The No. 57 stone bedding will provide uniformity of the loading on the underlying clay materials. Saturation doesn't hurt anything...it's the corresponding loss of stability in the unconfined (top) part that gives problems. Once the loose material is removed and you have a relatively solid subgrade, you can then place the stone in there, place your concrete, and your footing should be fully and properly supported as well as providing a uniform loading to the subgrade. At that point, it doesn't matter if the area gets saturated again.

A problem you might have with the clayey subgrade subgrade is that if the clay dries out to a very low water content it will shrink and allow your footing to settle. If it then gets re-saturated, it might swell slightly causing your footing to heave. This can happen from rainfall or from the uptake of water as trees nearby grow and remove water from the soil. This all depends on the "activity" of the clay and whether it has a great deal of shrink/swell potential. In any case, not a good idea to plant trees near your footing that will grow large or take a lot of water if you have a clayey subgrade.

Good luck.