"Pad Ready" Definition

[pdxdeacon]

Hi,
Can someone tell me if topsoil can be used as fill to make a site "pad ready". For example, if the "pad ready" elevation is supposed to be 100, can the subgrade be 99.5, and the top 6" be loose topsoil?
Thanks, Kendra

 

[msucog]

it all depends on the owner's expectations. i personally would not word it like "topsoil as fill". i would phrase it that the pad with filled up to subgrade with structural fill and then the upper 6" was topped off with topsoil (which will be removed to reach subgrade later). for the sake of the owner, i would never use "topsoil" in my reports relating to structural fill. i would probably suggest to the owner that even grey soil be mixed to more or less disappear so that another testing firm or future buyer doesn't inadvertently classify a jar sample as topsoil (which would put a big question mark on the entire fill area).
if the pad will sit there for a long period of time and is grassed and exposed to weather, then the upper 6" will likely need remediation anyway...however, i would not place organic topsoil in a building pad since it should probably not be there.

 

[civilman72]

Simple answer - topsoil should not be used under anything structural (roads, buildings, etc.)

If your pad ready elevation is 100, you should not use topsoil to "fill" the last 0.5' Rather, you should bring subgrade to elevation 100, and then place topsoil on top of this. This topsoil will have to be removed prior to constructing the building.

 

[genomty]

Usually "top soil" refers to those soils near to the surface containing organic material. However, sometimes and I really mean very few times,at some project sites where top soils consit of harpan or caliche or any other soil without vegetation remnants top soil could even be used as Select or Structural Fill. Nevertheless, and that's really for sure if your project will support loads, even ligth load the soils supporting concrete slabs or pavement structures should be compacted.

In some cases where water migration due to capilarity is expected or soil heaving is predicted, coarse graded gravel or sand are left in loose state. I'm not totally agree with this solutions but I have seen some projects where these solutions are specified.

Ask to the geotechnical guy the reason of this specification on your project

 

[msucog]

you bring up a good point genomty. the definition of topsoil should be clearly defined. for example, in my part of the world, lots of people classify old, cultivated soils as topsoil. when the terminology is not corrected, this can cost the owner a significant amount of money if the material is hauled off or treated as topsoil. and it causes problems if someone says that topsoil (referring to the excavated cultivated soil after the true topsoil was already removed) was placed as fill.
as far as topsoil (remaining after stripping the surface organics) placed as fill in roads or some slope areas, i see no problem with it as long as it is blended with cleaner soil, well compacted, observed as stable during placement and is capped with ~5' of well compacted, clean soil. the owner must acknowledge that there is a slightly higher risk of adverse effects by utilizing the material in this manner. and they must also acknowledge that buildings will never be located in these areas. and i'm sure that some on this board would object simply because of their location and localized geology. my statement applies to my part of the world where it is not a big problem as long as it is done properly and site conditions considered. and of course, there are limitations based on slope size, inclination, owner not willing to accept the risk, possibility of buildings moving or expanding, etc.

 

[BigH]

I'm not sure that I would use topsoil as structural fill unless it is of "new" origin with a low organic content. As far as the thickness of it goes, this can and does cause many problems - in investigations we use to dig a few small pits adjacent to the boring to define the topsoil thickness (which I would suppose should be defined as the agronomist's "A" horizon). Way back when, there was a paper in Canada by a chap named Walker - they left the topsoil in place for a floor slab (if I remember correctly) as to remove it would have caused much construction problems (sensitivity to disturbance of the underlying material). They determined that the topsoil would have neglible effect on the behaviour of the slab and might be better than mucking about removing it.

 

[oldestguy]

An interesting subject. When engineers, contractors, etc. say, "Strip the topsoil". I ask, "Are you predudiced against black?" Then, I ask "why strip it", and the answers generally are without foundation. In my experience with loss on ignition at 5 percent or less, I have never seen a problem if topsoil is compact before something goes on it, including footings, slabs and roadways. That's in a period of over 50 years.

I hasten to add that topsoil has the tendency to soften more rapidly than the same non-organic soil when wetted, so in wet weather it is more of a problem in that respect. Yes, problems do occur when soft soil, including topsoil, is buried under pavements or other features. But the color and low organic content should not be the reason you strip it.

On the other hand, if we are cost conscious with our recommendations and not "c-y-a" type guys, this subject is brought to the attention of the owner with the cost differences involved, so he can decide for himself.

I first came to realize the lack of knowledge about the plus or minus factors involved when I saw contractors "get away with it" and it worked. The relative compact condition of the stuff makes all the difference (as with any soil)

More often than not, my experience has been that it is left in place (or even used as compacted fill) if the stuff is tight, with success.

 

[epongra2]

While most topsoils where I practice have a low percentage of organics, they still have a visibly porous structure and can contain large roots, burrows, etc. There is a potential, borne out by laboratory testing, for consolidation and hydroconsolidation. As such, no on using topsoil for foundation support, maybe on using it for slab support, and it is usually fine to remove and recompact the surficial soils, i.e. to turn the surficial soils into compacted fill that can then be used for foundation or slab support.

As a geologist, I do make the distinction between an actual topsoil and fill placed over the site that, with time, develops a weak soil profile at the surface.