Feature Surface - Using Ground Levels Option

Just wanted to run something by you if that’s OK
I have a site that has a large area where we have to strip off 80mm but then add 150mm of new topsoil.
The finished levels are to match the existing levels (before strip).
 
The approach I have taken:
1.                  Strip 80mm
2.                  Create Feature Surface – selecting Use Ground Levels option
3.                  Apply an offset of -0.15m
 
My question is, is my offset correct or should I have applied an offset of -0.07m (0.15m – 0.08m strip). I’m unsure whether the software works to original ground levels or ground levels after strip when using the use ground levels feature?

Thanks

Hi Randallinho,

Welcome to the forum!

i think the short answer to your question is that you would be better off using the 'spread' element. This is basically the opposite of the 'strip' element - it adds a fixed thickness of material to the area you define. So you could add a 'strip' element in one phase with 80mm and then a 'spread' element in the next phase with 150mm for your topsoil.

However, I may not be understanding you quite right. If you strip 80mm and then add 150mm of topsoil you won't be matching the existing levels - you'll be 70mm above the existing.

The levels the outline uses are the 'previous ' ground levels rather than the existing, so they would be the ground levels after stripping in your case. However, the 'existing levels' option refers only the the outline itself, and not the area within it. In other words, the level of the outline will be the previous ground levels, but it won't 'track' the ground levels within the outline area. If you used this outline option you would find the software would just use this outline and fill the area within it with a triangulation respecting the outline levels, but not following the ground levels within the outline. I'm pretty sure this isn't what you want from what you've described.

The only elements that 'track' the previous ground levels within the area you define are strips and spreads.

One tip: when you're using strips and spreads if you want to strip or spread over the whole area you've defined for your existing terrain you can just draw a big box around everything. I see a lot of people carefully drawing around the outline of the existing ground, but since the software can only calculate up to the limits of the existing terrain you can save time by just drawing a big box around everything.

Hope this helps,

Leo