Newbie cut/fill questions...

[Mayms]

Greetings,

I am an architect trying to do basic cut/fill calcs for a residential discretionary permit. We have AutoCAD models of the topo and the architecture, so it is easy to add, intersect, and subtract volumes, then do a massprop on the result to find the volume. I have gleaned various details from various posts on this site, but thought I would bundle them toghther and post them to verify that I am on the right track. My assumptions are as follows:

1) Cut calculations should include the volume of retaining walls, footings, slabs, sand & gravel under slabs, and topsoil removed.

2) Cut calculations should be factored up for "swell" of 15% to 25% to account for expansion if earth is to be exported.

3) Fill calculations should account for "shirnkage" or compaction of the earth after grading.

4) If earth is cut then recompacted elsewhere as fill, the resulting volume will be approximately 15% LESS than before grading.

I also have a bonus question:

I am on a canyon in San Diego and anticipate that the soil will be comprised of several feet of silty sand/gravel fill with sandstone below. If we are going to plant native species on the site after grading, will we need to import "topsoil" or can we re-use the top several feet of the cut and call that topsoil?

Any other insights or comments you have would be most welcome.

Thanks!

Steve

 

[ccor]

Check out "Estimating Earthwork Quantities", by Daniel Atcheson. It provides a treatise on how to adjust the raw earthwork quantities that AutoCAD LDD generates. It also covers hand methods that it sounds like you may be using. The book is quite good and very concise.

 

[cvg]

your geotechnical engineer should answer questions 2, 3, 4 and the bonus. Relying on rules of thumb such as the percentages you quote could get you into trouble, as these vary from site to site. The geotech is there to evaluate these things for you and should provide that information.

 

[Mayms]

ccor & cvg:

Thanks for your posts. I would love to learn more about this as I am genuinely curious, but I probably should have explained that all I am looking for is guidelines for ROUGH calculations sufficent to get throgh the discretionary process. When (and if!) obtain the discretionary permit we will hire a geotech and civil for the soils report and to obtain a grading permit. These services are extremely expensive for a single-family residence here in So. California so we don't want to spend that money until we know exactly what will be required. The other thing I didn't mention was our time constraint. We need to submit tomorrow! So...did the 4 assumptions above seem GENERALLY reasonable, or are some of them out-and-out wrong?

Thanks again!

Steve

 

[cvg]

I would divide your earthwork into
1) mass grading which would include stripping of top soil (probably only 6 inches), leveling the site and creating the subgrade under your building slab.
2) structural excavation / fill which would include footings and retaining walls and imported materials like sand and gravel bedding.

If you are excavating sandstone, swell could go higher for volume of your exported material. shrinkage is probably ok at 15%

I wouldn't count on several feet of topsoil unless you are in the bottom of the canyon. Strip it from the site first and stockpile for later reuse. Amend it if necessary to improve its fertility.

 

[Mayms]

Excellent! Just what I was looking for. I'm sure you are right about the "topsoil." Infertile sand and rock is a better description. Only the heartiest scrub will grow there as it is so I am sure we will have to amend it to get even "native" species to grow.

Thanks!

Steve