Residential Slab Foundation Recommendation

[sunpat2]

I am planning to build my house and I have some issues about the subsurface. I have following details for my house as well as the lot:
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HOUSE PLAN DETAILS:
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+ Size about 1800 sf 1.5 story. 1800 sf on first floor to be finished. Attic space (more than 5 ft in height) of about 1400sf to be left unfinished.
+ 2 car attached garage.
+ Exterior 6" RCC walls made of ICF(Insulated Concrete Forms with total wall width of 11"). This will be extra load to be considered in desiging foundation footings.
+ Foundation designed for floating (4" 3000 PSI monolihthic slab) with 4" of gravel min under slab (with soil bearing capacity used is 2000 PSI). Slab perimeter (under exterior walls) is min 12" wide at bottom and min 12" below grade. 12"x8" wall footing for all interior loadbearing walls.

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LOT DETAILS AND SUBSURFACE EXPLORATION:
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+ Lot has a very light grade of 1(vertical):50(horizontal) from rear line to the frontline on a cul-de-sac street.
+ SUBSURFACE: Subsurface was done in 1995 by an engineering firm with a DCP (Dynamic Cone Penetrometer) test to estimate the soil strength and relative density of the subsurface soils. Originally there was a small pond at the site and it was filled in 1990 by developer with "stiff brown fine sandy silt" fill for depth ranging from 0.5' to 3.5' from the top. Below this fill Residuum contians "firm slightly clayey fine sandy silt" for depth ranging from 1' to 8' form top.
Here are the Recommendation for spread footings:
HOUSE WITH A SPREAD FOOTINGS SHOULD BE FOUNDED ON WASHED STONE THAT IS USED TO ESTABLISH THE FOUNDATION BEARING LEVEL AFTER THE FOOTINGS ARE UNDERCUT TO ADEQUATE BEARING MATERIALS, AND THE ESULTING
UNDERCUT EXCAVATION BACKFILLED WITH COMPACTED WASHED STONE. AN EVALUATION OF THE UNDERCUT EXCAVATION SHOULD BE CONDUCTED AT THE TIME OF CONSTRUCTION TO DETERMINE WHEN ADEQUATE BEARING MATERIALS ARE REACHED. USE 3000 PSF OF ALLOWABLE BEARING PRESSURE FOR FOOTINGS BEARING ON COMPACTED WASHED STONE.
Minimum wall and column footing dimensions of 18" and 24", respectively, should be maintained to reduce the possibility of localized punching type shear failure. Exterior foundations should be designed to bear at least 18" below finished grade for frost protection.
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Here is my story.

I would like to get some good tips on foundation desgin changes like depth and width of exterior/interior loadbearing wall footings, depth of washed stone fill under slab.

I will appreciate all tips and coments. Thanks to all.

Sunil

 

[Ron]

Your conditions are site specific and a Geotechnical engineer should look at it. You have one geotech opinion already, but if you have stated their recommendations verbatim, they are weak and too generalized to be of much use. There is no statement as to the expected excavation depth to achieve adequate bearing...just the statement to check it during construction. That's a bit late for overall planning, though it should be done as a matter of course during construction to VERIFY the anticipated conditions. Your fill material concerns me as does the underlying stratum.

 

[cdh61]

sunpat,

The geo's recommendations must address foundation requirements better than you stated above. Ask what depth the adequate soils are founded at, and other items such as what are the compaction requirements of stone, just leaving it to a contractor to say compacted can lead to problems during construction as to what compacted means. Overall, the geo report should detail out how to prep the surface for the footings. Word of caution, giving a bearing capacity without stating depth of adequate soils or thickness of stone can lead to problems since adequate soils or depth are not defined. Adequate soils with a 2000 psf allowable bearing pressure is ok for some structures, but not all.

regards

 

[fndn]

Sunpat;

In your post you indicate there are firm to stiff soils, however the term sandy silt is also mentioned. Usually we use the terms stiff and firm for clays and plastic silts, while dense, loose or medium dense are used for granular soils. Either case, if stiff here means of adequate bearing capacity, then what is the reason for the undercut and placement of wash stone?

You may contact another soils engineer and have them review the soils report and give you a second opinion. In a single family residences like you describe, you have light loads and you may be able to just spread footings for 2000 psf bearing pressure like CDH1 suggested.

The money you pay for the second opinion will more than offset undercut and spread footing concrete costs-which are the recommendations from the current soils report.