Organic layer beneath spread footing 1

[Cosimo]

A soil boring identified an organic layer approximately 2' thick approximately 8' below grade. Bottom of proposed spread footing is approximately 3' below grade. Structure is 2 story residential dwelling. Material above and below this layer is medium density sand. When consulting the Bousssinesq chart, foundation pressures at 8' are approximately 0.03q which seems negligible. Future significant settlement is a concern. Should I be evaluating the allowable bearing capacity of the organic layer base on 0.03q? Any input would be appreciated.

 

[BigH]

What is q? Is the organic layer fibrous or amorphous? If the allowable bearing pressure is 100 kPa, then you would have only 3 kPa at the top of the organic layer; this is quite low and I would presume that there is some "overconsolidation" built into the organic layer by the overlying sand (which I am assuming is natural and not fill). However, one does not have any control on subsequent landscaping that the owner might wish to do. My suggestion is to place a preload on the site - extend 1.5 m each side of the house footprint and say 1.5 m high. Monitor it's settlement. When it basically stops, remove the preload and then build the house. I would doubt that any landscaping would be higher than 1.5 m (or 25 kPa) of the fill placed as preload.

 

[oldestguy]

More info needed. Any boring data like N values? What is the loss on ignition. In some cases these "tests" would indicate that you need not worry about the layer.

 

[msquared48]

If time is a problem, you could place piers (or technically underpinning) to a level below the organic layer, and build a standard foundation atop that starting at the three foot level below grade. The stem wall could be designed to span between the piers.

The other option would be to over-excavate and backfill with structural fill to the 3 foot level.

Mike McCann
MMC Engineering

 

[crcivil]

Shouldn't the foundation pressure be considered at 5' as opossed to 8', given that the footing will be bearing the soil at 3' below ground grade? Regards.

 

[darthsoilsguy2]

if the layer above the organics was placed by man.....
you will find that soil conditions get worse the deeper you get until you get under that layer, either during geotech exploration or construction.

 

[GeoPaveTraffic]

If it were my house, you can bet that the footings would bear below the organic layer.

Mike Lambert

 

[cvg]

if it were my house, I would have asked the geotech to make a recommendation, not get one from a website

 

[Cosimo]

Thanks for input. Some additional information. The home is existing with a spread footing at approximately 3' depth. Home is approx. 25 yrs old and has undergone approx. 0.2' of differential settlement over time. The home is being elevated approx. 4' due to flood concerns. One option we are exploring is utilizing the existing block (CMU) foundation and adding 6 course of additional CMUs. N values above organics 10-12 Below organics N = 25-30 Organic layer itself, N reported as 2. q is estimated at 1,600 psf including additional block courses. We are leaning towards installation of helical piers around the existing foundation with helicals beneath (new) interior masonry piers as well.

 

[msquared48]

Or 2" diameter pipe pile at a spacing recommended by a Geotech. This could easily be done at the exterior walls. Interior bearing lines are a little harder, depending on the headroom at the crawl or basement. The garage slab should probably become a structural slab on pin pile too for best performance.

Lotsa money, but problem solved...

Mike McCann
MMC Engineering

 

[fattdad]

I find it hard to believe that a 25 year old house has much remaining settlement potential from a 2-in organic-rich layer that's been under load for that long.

Where's the water table?
Is this layer really buried topsoil (e.g., is the overlying material really fill)?

I doubt I'll get further involved, 'cause I built my practice avoiding residential work.

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

f-d...that's 2 feet thick, not 2 inches. I would be concerned as secondary consolidation in such a layer is likely to be much longer and much greater than primary. This is particularly an issue if the water table changes...particularly down.

 

[oldestguy]

My experience with organics is that they compress at a decreasing rate, linear on a semi-log plot. Thus, rate now is pretty low. However, the added loadings will increase it for a while. If the differential is fixed (wall patching, etc.) maybe consider living with what comes. Also, in the added block construction use "Dura Wall" reinforcing on every layer of block, staggering and lapping so as to make a continuous "beam" effect. Even consider installing simple screw jacks for future leveling, as needed.

The helical piers , if used, they can be spaced some to cut down on the number needed. Screw jacks on each also.

 

[fattdad]

oops on the 2 inches!

Carry on. . .

f-d

¡papá gordo ain’t no madre flaca!