Earthwork Calc w/ variable shrinkage
Earthwork Calc w/ variable shrinkage
(OP)
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.
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.





RE: Earthwork Calc w/ variable shrinkage
RE: Earthwork Calc w/ variable shrinkage
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?
RE: Earthwork Calc w/ variable shrinkage
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.
RE: Earthwork Calc w/ variable shrinkage
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.
RE: Earthwork Calc w/ variable shrinkage
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.
RE: Earthwork Calc w/ variable shrinkage