Landfill Cap Grading

[sendhil.kumar]

I am familiar with grading a landfill cap using a featureline along the perimeter - when the perimeter is flat. However, grading the landfill cap from an uneven perimeter (varying elevations) is a mess in acheiving accurate 3:1 slopes. Are there any techniques that can be used to achieve this grading? I use AutoCAD Civil 3D 2013. 

Thanks for your help.

[Joe-Bouza]

What are the requirements other than 3:1? are you trying to grade to a specific elevation or achieve a particular cross slope on the top?

a picture would help.

My gut tells me you want to grade to an elevation.

why is it more dificult having a toe at one elevation?

[jbalaneski]

I've done a lot of landfill caps and cell design.  To design a cap, the best way to get your side slopes correctly there's a few options to consider.

They are as follows:

1.  Constant elevation from top of berm (i..e 5.0 m above top of berm all around)

2.  Constant elevation at top of slope (i.e. elevation 100.0 m, height from top of berm varies)

3.  Maximum top of waste elevation (i.e. 100.0 m along ridge)

For option 1, you simply step off the top of your berm (if they are 3D Polylines/Featurelines) to the required height, or use grading objects.  You can then grade the top using grading objects

For option 2, you can use grading objects to set the elevation required. Allowing the grading object to determine your crest.  Then contiue the grade up to the top (usually 5%) using grading objects.

For option 3, you can create a "temporary surface" and daylight to it using grading objects.  For the temp surface create a 'C3D Site' called Temporary Surfaces.  Draw a feature line that would represent your "ridge" of the top of the cap.  Step off that feature line on each side, at the required grade (usually 5.0%) so it's outside your berm limits.  Create the tin surface from the 3 feature lines.  Daylight the top of your berm to the temporary surface.  You get the crest of your 1V:3H slope.  Once oyu have that line, you can delete the temp surface and use the centerline ridge in your final cap surface.

[Joe-Bouza]

So why cant you use a grading object for any of those?

[sendhil.kumar]

Hi Joe, the toe cannot be at the same elevation as that is the perimeter that the existing ground allows us to make the best of the landfill boundary. Its basically a hilly region. My only criteria is to grade to the top until it reaches a small flat spot, the slope cannot be steeper than 3:1. It has a 15 ft wide bench at every 35 ft vertical height. 

[jbalaneski]

You can use grading objects...I'm just from an old school world where I still offset everything and create temporary surfaces...lol

[jbalaneski]

The perimeter you show in your picture...is that the toe at existing ground, or is that the top of your berm?

[Joe-Bouza]

I'm still not seeing why you have a problem? the grading object has the tools to get to a uniform elevation at a given slope? why does that not work?

can you post the feature line and what is the top elevation?

[jbalaneski]

The main problem is if they need to create benches in the surface that are relatively smooth if he just uses grading objects at a relative elevation of 35 ft, his benches will be all bumpy because of all the grade change in his original featureline.

[Joe-Bouza]

then my suggestion, is to use the grading to develop this level elevation, to design the benches'

That is, grade to an elevation slightly above the highest elevation on the FL creating a level perimeter, then grade in the benhes going up

[sendhil.kumar]

Attached is a DWG file with the feature line... It is the perimeter of the landfill. so that will be the toe of the cap.

[Joe-Bouza]

this is how I would approach grading this:

1. determine the lowest elevation along the FL

2. deterimine the two locations where the lowest elevation +35

3. grade to elevation (LP +35) which is a horseshoe shape aroung the site

4. add the first bench +35 on the previous gradings

5. continues this process closing in on the hp

6.add transistion fl as appropriate

[Joe-Bouza]

Gee I thought is was a pretty good solution?

[Nbondy]

The trouble with using a grading object in this scenario (or a corridor and the OFTB subassemblies for that matter), is that it does not account for the slope of the feature line in calculating your 3:1 slope.  The 3:1 slope will be perpendicular to the horizontal of the feature line, not a true 3:1 slope.  Not a huge deal if your feature line is relatively flat, but in this case with 20% grades along the feature line, you end up with slopes as steep as 2.5:1 using a grading object and specifying a 3:1 slope. 

I typically use a grading object and grade a short distance to my nearest major contour interval at slightly flatter than 3:1 to account for the feature line slope.  I’m okay if it’s not a perfect 3:1 slope as long as it’s for a very short distance.  I then extract the daylight line/contour from the grading object, turn it into a 2d polyline, and stepped offset that the rest of the way up the hill at my major contour interval.  In this case, I would use the grading editor to adjust the target elevation and slope around the toe to account for the wide range of elevations and slopes of the feature line, extracting my nearest major contour as I go.  I'd love to hear of alternative solutions to obtain a true 3:1 slope from a feature line that's not flat.  

Neil Bondy

www.cadmasters.com

[Joe-Bouza]

I'm not so sure what you mean Neil? my example has 3:1 slopes from the sloped feature line. the grading computes 3:1 at point A and again at point B and so on. all radially from the FL.

In all my experience a graded slope is measured perpendicular to the contours not perpendicular to top and toe? I could be wrong. going slightly under 3:1 would be prudent I agree.

[Nbondy]

That’s precisely the problem, Joe, the 3:1 slope should be perpendicular to the contours, but it’s not, it’s perpendicular to the feature line.  Add a one point slope percent label to your surface just south of the +35 circle (near coordinates 6500,2840).  You’ll see that the slope of the surface is 40%.

Neil Bondy

www.cadmasters.com

[Joe-Bouza]

Hmmmm.............?

[Nbondy]

Hi Joe,

I've added a few slope labels along the toe, and some dimensions between contours to help illustrate the problem.

Thanks,

Neil Bondy

www.cadmasters.com

[Joe-Bouza]

Hi Neil

I see it, but it seems to be mostly where the geomtry gets squirly, which should be considered.

Thanks

[Nbondy]

No, it doesn't have to do with squirrely geometry.  Draw a straight feature line at a grade of 10%, create a grading from it at 3:1, and build a surface from that.  The result is a 34.8% slope, not a 33.3% slope.  I've attached a simple example.  It can cause serious headaches if folks aren't aware of it.

Neil Bondy

www.cadmasters.com