Earthwork Calc w/ variable shrinkage

[smeltron]

Hello all,

I am currently working on calculating the total earthwork (using Civil 3D) for a proposed site and I have some questions with respect to shrinkage. Specifically, the site geotechnical report states that the upper 18 inches of existing soils has a 25% shrinkage, while soils below 18 inches have a shrinkage of essentially 0%.
I am wondering what is the best method of approach to calculate the actual quantities to see if the site is currently balanced. Somehow I have to differentiate between the drastically different layers of soil. I've tried several methods with varying results so I just wanted to get some input from others.
Thanks.

 

[Ron]

The shrinkage that the geotechnical engineer is referencing is likely a result of additional compaction. The upper 12-18 inches of material, fill or native, is typically subjected to compaction. 25% might be a bit high, but nevertheless, significant "shrinkage" will occur.

 

[655321]

Since Civil 3d gives you the total cut / fill volumes…what you need to know is the volume of cut from 0”* to -18”.

You could do this by generating cut/fill ticks, drawing polylines around the cut regions to get the area, estimating the average depth of cut (with 18” as your lower bound) multiply the area x times depth and then you have the volume of your shrinkable soils.

You may be able to generate contours from your cut / fill surface, that might make the delineation easier.

*Make sure your considering site clearing/stripping in your qty estimates.

What exactly have you done so far?

 

[oldestguy]

Not familiar with the program, but I'd strongly suspect the Geotech report as being wrong on that advice.

Sure the upper materials in some areas are loosened due to various reasons, but, traditionally all of the cut, except for rock excavation,usually has shrinkage. Numbers can run from 10 to 30 percent, depending.

I'd lump everything into one category, since when do excavators separate out the top 18 inches from the rest of the job?

Can you go to a nearby DOT and see what they typically experience in the same type of soil? If this is for a DOT, no harm in asking either.

 

[smeltron]

Thanks for the replies.

The procedure I have used with Civil 3D is to prepare two separate volume calculations: One from my existing ground to proposed ground. For the second calculation, I dropped the entire existing surface 18" and calculated the volume from that dropped surface to the proposed ground. I then subtracted the value of cut obtained from the second calculation from the value of cut from the first calculation. This gives me the total cubic yards of cut from 0" to 18" below the existing ground and I applied the shrinkage to this value. Kind of confusing, but it seems like a reasonable approach.

As far as the validity of the info provided by the geotech, I think that it is actually fairly accurate. It appears that the site was a spoil disposal site sometime in the past with fill evenly distributed across the site without any compaction. So its basically just a bunch of loose soil across the entire site. His estimate of 0-5% shrinkage below 18" is probably more like 10%, but there is not much cut below 18" anyway, so it's almost negligible.

 

[civilman72]

Sounds like the way you calculated the separate volumes was the correct way to do it with C3D, but not sure if you took the advice from 655321 above to consider the site clearing and topsoil stripping.

I would also use the shrinkage values determined by the geotech - if you don't use their values to determine your earthwork quantities, it opens you up for liability.

 

[cvg]

rather than go to the highway department and ask them for advice (which in my past experience, unless you have a buddy over there in the materials lab, you won't get much of an answer anyway) or make your own, uninformed assumptions - why not pick up the phone and call your geotech and have him explain it to you and verify that the deeper strata will in fact, not shrink at all? You may want to look at the boring logs also, high blow counts or a description such as "stiff or very stiff or very hard" for layers below 18 inches may be an indicater of the density of this soil.