Over-Excavation?

[bones68ss]

I am fairly new to complete earthwork takeoffs. I've been using INSITE software to do takeoffs for 12 to 20 acre sites. I am running into problems when I encounter jobs with big over-excavations at the pads. When comparing my numbers to others they don't seem to add up. I guess what I'm asking is how do you perform a takeoff on a site with over-excavation?

 

[msucog]

i guess i'd have a couple of questions to ask:
-with whom are you comparing numbers? contractors, engineers, owners?
-is someone accounting for shrink/swell of the material?
-how far apart are the numbers?
-how close are the survey readings?

 

[bones68ss]

Well I work for a GC and I'm comparing my numbers to the subcontractors so we can justify our earth price to the owner.
As always the soils report is vague and tells us there is "X" amount of undocumented fill and boulders on site onsite to be removed. So it's really a guess of what the loss is going to be. Obviously since I don't have a risk my numbers were more conservative than the subs. But the owner also had an outside source do a takeoff but his seems to be way off. The pad is to be over-ex'd 6 feet and the site 1 foot. The outside guy took the square footage of the pad and multiplied by 6 foot for the pad and the same with the site. You can't do that with an over-ex to figure your cut right? I don't know the process of taking off a site with an over-ex.

 

[msucog]

boy, that sounds like a mess. contractors always have a high number and owners always have a low number. for the site, if you take the total acres minus the pad area, then multiply by the depth, that should give you the in-place neat quantity. however, when you dig it out it will take up more room in the truck (because it fluffs up) but when you put it back in, it should shrink due to compaction. if it happens to be rock that you take out, the quantity should swell since you'll never get it as tight as it was originally. there's many different ways to look at the scenario...does removing well compacted fill swell more or less than residual soil when you dig it out.
to answer your question, it all depends on how the specs are written and how the contractor gets paid. i always ask for how everyone figures their numbers. that way you can see where the differences are. if they say they used a computer program, i would be suspect of them. i've seen that almost cost the owner 22,000cy of mass rock on one particular shot of many (they forgot to account for the 20' cut which had already been made -- i.e. had 20' too much rock in the figures). get everyone to agrees on shrink/swell factors, areas, depths, etc...then it should be a matter of getting the math to work out...lock everyone in a room to fight it out...that usually works.

 

[DRC1]

I try to do the take off in the same progression as the work. I calculate the quantity of topsoil, usually estimating a depth if not specified. Again estimate the amount I can stockpile vs waste. I then remove asphalt ,concrete etc. Then I adjust existing contour lines for this. Identifying the areas with suitable and not suitabal material for backfill, I calculate quantities for each to the orginal depth of cut and then to the depth of over excavation. I also keep any benching for side hill fills as a seperate number. Once I have the total cut, I look at how much material I can use from the cut for the fill and how much impoted mateial must come in.

The trick is to do it in definable steps that logically segergate the work. Remember, a) good takeoff takes practice and patiance, so depending on who the owner had do it, it could be wrong. b) this is not an exact science. if you get everyone with in 15%, thats as good as it gets

 

[TerryScan]

Bones,

If I understand you correctly, the "outside" guy did not account for proposed vs. existing grade.

"took the square footage of the pad and multiplied by 6 foot for the pad and the same with the site."

This could easily explain why it might be way off.

 

[bones68ss]

Terry you are completely right the "outside" guy didn't figure in existing grades, so that explains the bust. Msucog and DRC1 thank you for the replies, you answered my questions, I appreciate the help.