Building on Organic Soils 2

[DocDirt]

We will be building a lightly loaded commercial warehouse (1 storey). Organic layer about 15 ft thick is present below 20 ft from proposed floor level. Building loads will not structurally affect the organics. We are concerned about settlement due to secondary effects such as volume change to decomposition. The organics are below water table and we think that they will not oxidize( aerobic) as it happens in most swamps settling. However we are concerned with the anerobic reduction of volume causing settlement.
Need Help in estimating settlements. Then we can decide on foundation options.
Any help?

 

[msucog]

first, what kind/percentage organics you talking about?
how far below the water table are these organics? you currently experiencing a drought or in a wet area?
without knowing any real details, it sounds like i'd be more concerned about the consolidation of the materials themselves even before the results of organics...but that's just me.

 

[DocDirt]

Thank you. Organics are about 8 ft below water table. Organic content is expected to be about 10%

 

[fattdad]

I would not worry about organic decay if you are confident that they are below the seasonal low water table. I would also expect that the stress attenuation from the foundation loads would be pretty nominal by the time you get to the "organic" layer so there is little to worry about with respect to consolidation. Then again. . . .

Secondary compression can be a concern on organic soils, but again, I'm not quite sure these are truely organic soils. Do the liquid limits using both the wet-prep and the oven-dry-prep methods and see if the oven-dry prep liquid limits are 25 percent lower then the wet-prep LL. If not you are not dealing with an ASTM "organic soil" and unlikely to expect "organic soil" behavoir (i.e., high coefficient of secondary compression. Additionally, if there is no additional load from the proposed construction (and no appreciable fill) this is a moot point.

Hope this helps (or at least doesn't further confuse - ha).

f-d

¡papá gordo ain’t no madre flaca!

 

[msquared48]

Have to go look at a project on Saturday where, supposedly, a single story building was built in an area that was possibly an old site where wood chips are mixed in the soil. Building is about 30 years old, and according to the client has seen settlements up to 4". The building rests on exterior concrete strip footings and about 4' high stem wall, with two interior strip footings with beam and post framing. The floor joists are hung from the wood plate. Supposedly, the building is in a flood plain area, and some of the water from the parking lot sheet flows to one side of the structure. In looking at the settlement measurements, there is no logic to the settlement pattern, and the strangest thing is that apparently there is no cracking in the foundation.

Sounds real strange to me.



Mike McCann
McCann Engineering

 

[fattdad]

In the Northwest (and in Alaska) the use of "Hog Fuel" as fill has been used. Specifically for road construction through muskeg/peat deposits. Hog fuel (wood chips processed through a hog mill) when saturated won't decay, is lightweight and is frictional (i.e., good angle of internal friction). You take a geotextile separation fabric, pile up the hog fuel and then place more typical aggregate for the subgrade. Just work it so that when settlement is complete the hog fuel subbase is submerged.

Just an aside. . . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[DocDirt]

thanks to fatdad and msucog. The site with settlement problems
that msucog referred to is quite interesting. My take on the cause of settlement: It would be the effect of moisture on wood chips. Wood is affected by fluctuating water conditions. In the site, since water drained into during rain, the wood chips are subjected to on and off water conditions. This causes the wood to rot and sometimes causes sinkhole-like features visible on surface. Such conditions have been observed over abandoned mine areas where wooden supports rot after sometime. To make a log storey short, If possible, a test pit or two may be dug near the buildings to observe the conditions. Also look for any sinkhole features on surface especially after heavy rains.

 

[DocDirt]

In my previous post, I should have stated that the site mentioned by Mike McCann and not msucog. sorry

 

[msquared48]

Thanks for the comments fattdad and DocDirt. My only question is if geotextile fabric was used 30 years ago to control settlement.

The lack of cracking in the foundation still puzzles me no end. I can't believe that the framer was that drunk when he built the building, and, for some unthinkable reason, I can't percieve concrete to be rubber. I'm missing something here. Maybe the sinkhole idea is the answer, but why no cracking in the foundation?

I'm going to look at this one real hard.

Mike McCann
McCann Engineering

 

[DocDirt]

Dear msquared48
I have seen from reports on projects constructed during 1977 that geotextiles were used then. I think that at that time, geotextiles were in the initial stages of evolution. Now they have come a long way.
Regarding cracking of footings: Without knowing the details and looking at the site, it would be difficult to even speculate the cause. Let me guess anyway.Subsurface directly under the footing may not contain enough wood chipa. So there may not be enough magnitude of settlement and/or differential settlement directly under footing to cause cracking.

 

[msucog]

doc, i'm with fd for the most part...while the organics might contribute a little, i'd say focus more on the soil itself. however, be cautious of knowing how nature will work next year or 5 years down the road and also account for the structural tolerances. if these organics are related to undocumented fill, it may be a different story--you might have hit the less organic spots and who knows what happens in between the data points. if the organics are part of flood plain sort of areas, then at least it's possible that it might be a more consistent scenario. if it is indeed a fill situation, i personally would be much more stand-offish on giving any sort of definite answer (not that i give definite answers about things i can't see anyway). even if i performed lots of borings and ran bunches of tests and swanky analysis, you just don't know what you've missed. even with residual, you never know 100% of anything...you know more or you know less...never all. (and yes, as usual, i'm wishy washy with my wording at times...if i knew everything i thought i knew, i dang sure wouldn't be making moderate pay in one of the lower paying engineering disciplines--at least i like what i do--for the most part anyway).

not to get too far off the initial question of the thread but msquared48, when you say there's no cracking in the foundation, are you able to actually see all the foundation? if the footings aren't all tied together in to one "rigid" foundation, the individual pieces might be moving. maybe there's only 2 or 3 locations that the thing is cracked and/or rotating. maybe look at the exterior cont footing as one thing and the others as others. if you find the trend in the perimeter cont. footing, that might give you more clues since the interior footings may be moving more or less from the perimeter footings.

it's always hard to figure out what's going on with houses built that far back...you never know if it was built out of whack or if something is moving/decaying/settling or all of them. heck, even today, residential construction is so shotty that coming to a resolution can be tough. thank goodness we (my firm) stays away from residential work...not that commercial is much better but at least we've got some backup from the permit requirements standpoint. i'd say the forensic work on residential work is pretty interesting since you do have to fit all the many unknown pieces together to come to a logical "solution" (maybe the word "logical" should've had the parenthesis). anyway, enough of my ramblings...good luck with both of your situations.

 

[fattdad]

For undocumented fill, I'd consider using a dilatometer to evaluate the soil modulus profile. I would also hope that you would execute a field exploration program that addresses items such as the position of the water table and the ASTM soil classification (i.e., as referenced above regarding "organic" designation), natural moisture contents and if required a 1-d consolidation test.

So far we are just having fun in the absence of data. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[msquared48]

msucog:

The crawlspaceis 4 to 5 feet deep and all the exterior stem wall is visible. The interior strip footings are fully visible too from the inside of the crawl. The client took some pretty good pictures for me to get an initial idea of the problem.

I will let you know how the inspection goes today.

Mike McCann
McCann Engineering

 

[DocDirt]

Regarding my original post, borings are going to be dome next week, at the end of which I will have more data. Then I will report
Thanks to all the responders.
DocDirt

 

[msquared48]

Well, there was a crack in the theory, several of them, and I feel much better now. No problem fixing this with many concrete piers to good subgrade.

Thanks for all the comments

Mike McCann
McCann Engineering

 

[DocDirt]

We did 7 borings. Average thickness of Peat/organics is 12 ft. Existing soil on top of peat is fill, but has been in place for the last 25 years, average thickness is about 15 ft. Fill is in decent shape. Owner wants to raise the site by 3 to 6 ft. I am currently leaning towards surcharging the site and then dynamically compacting it so that the fill will act as a bridge to counteract the secondary settlements. Consolidation testing will be started soon (Long term) to get secondary compression parameters etc in addition to getting the required data for surcharge design.
DocDirt

 

[fattdad]

Not sure about deep dynamic compaction, but surcharging to manage primary and secondary compression is a good idea. Any classification test data yet?

f-d

¡papá gordo ain’t no madre flaca!

 

[DocDirt]

Dear Fatdad
We have just started testing. As soon as classification data becomes available, I will let you know

 

[aeoliantexan]

I would favor surcharging. Charles Ladd provides guidelines for design on surcharge to reduce secondary settlement on nonorganic soils.

Might the organic soil generate methane? If so, methane barriers, collection systems and vents may be needed to keep the interior safe.

 

[DocDirt]

Dear Aeoliantexan
Thanks, we already considered that due to a last minute suggestion by a friend of mine. We tested for gas during boring by using portable gas meter and also installed gas wells and tested for gas. While we noted concentrations of upto 5% in volume by air for methane in a couple of pockets in the peat during drilling, we did not detect any gas in the fill and in the monitoring wells. Our conclusion is that while there are some isolated areas in which methane is present, there is not enough volume of methane to come up to fill or in the well in concentrations high enough to be detected.

Consol testing is in progress to determine the settlement parameters. This is long term