I have been testing a site with about 10' of mixed fill. Mostly a clay loam and loam mix. below the fill is about 2 feet of original topsoil (river flood plan) before reaching the inorganic alluvium. Total depth to the stiff alluvium is 14'. Does over-excavation sound like the best resolution for the site considering the thickness of original topsoil beneath the fill. The fill was placed about 5 years ago. the client is seeking a foundation design. Thought about piers but this amount of topsoil has me concerned.
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fill overlying topsoil
- Thread starter MWB1
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Well, my first reaction: "the client is seeking a foundation design." - what is the project?
If it is a high speed paper machine you will have different considerations than if it is for a small shed (extreme examples).
First advise of the nature of the structure, sensitivity to total and differential settlements, etc. and perhaps one can point you in one of several directions.
If it is a high speed paper machine you will have different considerations than if it is for a small shed (extreme examples).
First advise of the nature of the structure, sensitivity to total and differential settlements, etc. and perhaps one can point you in one of several directions.
- Thread starter
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aeoliantexan
Geotechnical
I never trust an undocumented fill. the topsoil is a clue that the site was not filled under accepted specifications. An uncompacted or poorly compacted clay and loam fill can compress drastically when it gets wet, no matter how long it has been in place. All it takes is a neglected downspout or poor drainage along the wall and you can have severe differential settlement.
If the cost is acceptable, I would favor overexcavating and replacing the fill with the same material placed near optimum water content and compacted to at least 95% of maximum standard Proctor density. Widen the excavation to include the slabs next to the building and the outboard column footings. Remove the topsoil too. Put a good drain around the basement wall below top-of-slab level because the compacted fill will be pretty impervious, so any water that infiltrates the backfill will become perched on the fill.
If the earthwork is too expensive, you can consider piers or piles to support the structure, but there is still a risk of the basement floor slab and exterior slabs settling some time in the future. A structural basement floor supported on the piles will surely be more expensive than the overexcavation. Resisting the unbalanced lateral loads of the walkout basement becomes awkward with a pile or pier foundation.
If the cost is acceptable, I would favor overexcavating and replacing the fill with the same material placed near optimum water content and compacted to at least 95% of maximum standard Proctor density. Widen the excavation to include the slabs next to the building and the outboard column footings. Remove the topsoil too. Put a good drain around the basement wall below top-of-slab level because the compacted fill will be pretty impervious, so any water that infiltrates the backfill will become perched on the fill.
If the earthwork is too expensive, you can consider piers or piles to support the structure, but there is still a risk of the basement floor slab and exterior slabs settling some time in the future. A structural basement floor supported on the piles will surely be more expensive than the overexcavation. Resisting the unbalanced lateral loads of the walkout basement becomes awkward with a pile or pier foundation.
The fear about topsoil is common, but why? Usually topsoil at the surface is loose due to may reasons and likes to take on water and become softer than the same non-organic soil. Other than those fears it acts similar to ordinary soil.
Many a building sits on competent topsoil and no problem.
I see you have a basement, so that, in itself unloads the ground some. In effect you have had a partial pre-load already. To use a suspenders and belt approach, initially you can over-load the site with a surcharge. Many a fill has been surcharged in my work and all work fine. Measuring the settlements is required. Many of these have settled to a halt in a short time, usually less than a month, since no squeezing of water out of soil is required.
Just make sure the pressure from your surcharge is sufficient to exceed the pressure from footings imposed on that old fill. This may take undercutting some at the footing locations and compacting in a competent fill there. Full depth of footing undercut to good soil is not needed if the imposed pressure at the bottom of the undercut does not exceed the excess pressure applied by the surcharge.
Surcharge heights usually are between 5 and 10 feet, depending on what other details are. I'd go for the higher if in doubt. You also can move it if limited as to source of surcharge soil, but only count on the area of the surcharge top, not the bottom of the pile as affecting the ground below. We term that a "rolling surcharge". Also, plan to surcharge an area extending outside the building equal to the thickness of poor stuff below your building. since that area also contributes to building support. Thus your actual area of fill for the surcharge is well beyond building limits.
Many a building sits on competent topsoil and no problem.
I see you have a basement, so that, in itself unloads the ground some. In effect you have had a partial pre-load already. To use a suspenders and belt approach, initially you can over-load the site with a surcharge. Many a fill has been surcharged in my work and all work fine. Measuring the settlements is required. Many of these have settled to a halt in a short time, usually less than a month, since no squeezing of water out of soil is required.
Just make sure the pressure from your surcharge is sufficient to exceed the pressure from footings imposed on that old fill. This may take undercutting some at the footing locations and compacting in a competent fill there. Full depth of footing undercut to good soil is not needed if the imposed pressure at the bottom of the undercut does not exceed the excess pressure applied by the surcharge.
Surcharge heights usually are between 5 and 10 feet, depending on what other details are. I'd go for the higher if in doubt. You also can move it if limited as to source of surcharge soil, but only count on the area of the surcharge top, not the bottom of the pile as affecting the ground below. We term that a "rolling surcharge". Also, plan to surcharge an area extending outside the building equal to the thickness of poor stuff below your building. since that area also contributes to building support. Thus your actual area of fill for the surcharge is well beyond building limits.
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