Calculating volumes between surfaces

[jamescMX2DQ]

I'm trying to design a crane standing area in Civil 3D and want to calculate the volume of hardcore material required to create the crane stand. For clarification, the crane stand is 67.5m long x 30m wide x 0.3m deep.

I initially made an alignment for the crane stand and then modelled a corridor along this alignment using a simple lane assembly which had a depth of 0.3m. I created two more alignments, one following the left side of the crane stand and another following the right and set the elevations of these alignments to match the top of the lane corridor. I added these two alignments to the corridor as baselines and modelled them with assemblies which were composed of drain and daylight subassemblies (see attached 'Crane Stand Assembly (separate components)'). I made two surfaces from this corridor, one following the top links and one following the datum links (see attached 'Crane Stand Corridor' to see the finished corridor). 

I created a triangulation volume surface to calculate the volume between the two surfaces. I expected this would be around 600m^3 (67.5mx30mx0.3m), however the volume calculated was only 270m^3. 

I deleted my corridor and created a new one which was made up of an assembly composed of the lane, drains and daylight subassemblies all in one (see attached 'Crane Stand Assembly (with drains & daylights)'). I created the same surfaces as before and this time the volume was calculated to be just over 600^3, which is correct. The only problem with using this assembly is I cannot daylight to the existing ground surface from both short ends of the crane stand which is something I wish to do to calculate cut & fill volumes.  

Would anyone know why my initial corridor is calculating the incorrect volumes? Apologies if this is long winded or complicated but I wanted to give all the information. 

[neilyj666 · Accepted solution]

For a wind farm right?

I find that corridors are a bit too involved for this work and use gradings instead (audit and save often) to calculate the cut and fill. The hard-core is simply as you have computed it.

[jamescMX2DQ · Accepted solution]

Yeah that's right. I spend most of yesterday trying different methods but I wasn't able to find anything that worked fully correct. I'll go with the grading and manual computing, cheers

[neilyj666]

Some turbine manufacturers require the cranepad to be 0% where the main crane sits, then 1% to the edge of the pad, then 2.5% to the first assist crane pad, then 5% to meet the incoming track and this is quickest to model using feature lines to create a finished pad Plane (for all pads on the project). Use feature lines offset 1m and taking level from the pad Plane surface less 0.3m to use as the basis for the grading. The 1m offset allows for the stone thickness and room for edge protection in fill and a ditch in cut.  Ensure each feature line is in its own site to avoid to much rebuilding of the gradings.