TIN surrface Volume issue

[hristev12]

Hello, fellow Civil3D users and masters. I have a bit of a issue with calculated Volume using TIN surfaces. A bit of prerequisites to take into account - we've surveyed with drone two waste cells where each month there were groundworks. For each month we've calculated the volume of the groundworks using the EG surface from the previous month and the EG surface at the current moment. The results are the following:

For me the strange thing is when calculate the sum of cut's and fill's volumes they differ significantly from cut and fill Volume of the initial surveyed surface EG (base) compared to last surveyed surface EG (comparison). BUT when I calculate the net: sum of cuts - sum of fills it is practically the same. Am I missing something obvious? My logic is that sum of all cut's and fill's should be closer to the cut and fill of initial - last surveyed EG.  I've checked areas of the surfaces from different months which are constant (like paved roads etc.) and the differences are within 1-3 cm or 0.1'. Any hints and recommendations are welcome. Thanks in advance

[nkiakas]

Hi @hristev12

I think you included the errors in the elevations from the drone in your calculations. But when you use the "net" these errors are expunge from calculation, that's why you are near to zero net in the end.

Assuming that the old surfaces are not exist, you can make a check by collecting sample elevations with total station (or GNSS) from the latest surface and compare them with the elevations from the drone.

Personally i believe that the most accurate volume calculation is the one from the first and last month, not the interim.

[chriscowgill7373]

is it possible that some of the dirt is moving in place?  not being added or subtracted, but moved to a different location, so it shows as a cut/fill, but it is a net wash in the begin vs end month?

[GKNIGHT892NE]

I think you may be thinking that the cut and fill are something different than what they are. I think the cut and fill is the change from surface to surface, the cut is what has been lowered, the fill is what is higher, but that would change from surface to surface. The net on the other hand is the volume increase, it looks like the net is always increasing, but the volume of each cell is always getting smaller because there's a bigger cut each time than there is a fill.   The net is the actual change in volume of the cell, the cut and fill numbers are the change from surface to surface so there is no justifiable way to correlate those numbers in the overall calculation, but the net is a number that can be justified because it's the change in volume, the cut and fill is the change in volume of the part of the volume that is higher or lower than the previous surface.

[ChrisRS]

I'm not sure that I fully understand your situation. This looks like a borrow pit rather than a waste dump, but in either case I would not expect to see both cut and fill. I concur with @chriscowgill7373.

I think you are seeing some movement of dirt from month to month that double counying.  One month it gets cut and the next month that area is filled as Earth is moved. Typically I think you're only going to want to pay for the difference between the original existing grade and the finish grade at the end of the project. If that is the case you're September to May figures appear to be correct.

I think that the May figures should actually be the volume figured from the reconstruction and May flights, less the September and October volumes.

My calculations put that at 672 rather than 2000 cut and -602 fill rather than 1300 fill. I am not sure that I fully understand what is going on.

I would suggest a different approach to this. For the the September volumes I would use the pre-construction EG and the September flight to figure out the volume. For the following months, I would use the original EG and that months flight to figure a volume then subtract the cumulative volumes of the previous months. In this way any corrections for just moving earth around, scooting around,  rather than true cut and fill, will be adjusted monthly rather than in a larger lump sum at the end of the project.

@chriscowgill7373  might be able to confirm that this makes sense.

Good luck.

It would be nice if you reported back what you decided.