Subassembly Composer - Ditch Depth in Fill Condition

[Anonymous]

I am trying to create a custom subassembly using SAC 2012 that will place a 1' deep ditch at the bottom of the slope in a fill condition.  The trouble I am having is how to hold a consant 1' depth on the backslope and vary the foreslope since the daylight offset and elevation is variable depending on the slope of the existing ground.  I need the attachment point to be the edge of the shoulder so it seems like a loop that varies the foreslope length would be required but loops are not allowed.  I will also have the option to target a profile but I also need the ability to hold a constant ditch depth.

Has anyone tried to model something similar or have any idea on how to model this condition.  Any help would be appreciated.

Jim Blackwood

[KMercier_C3D]

I have an idea but a sketch would be helpful as well as what input parameters you would expect the Civil 3D end user to provide. 

Input Parameters:

Ditch Depth (1')

Backslope (?:1)

Ditch Bottom Width ?

Target Parameters:

Daylight Offset

[fcernst]

 "hold a consant 1' depth on the backslope and vary the foreslope"

No, you do not need a loop. You want to vary the foreslope to achieve your 1 foot depth relative to the backslope daylight point. 

You will have to supply either the desired ditch CL Offset or the desired Backslope Offset. Both of these could also be targeted Alignments, Feature Lines, or Polylines.

This is a well defined system  ( # independent eqn's = #unknowns) if you supply one of the above, so no loop required.

Or, if you decide you want to Hold the forelsope and let SAC push around the Ditch CL or the Backslope Offset to satisfy the 1' depth requirement, you could do that also.

[Anonymous]

I have attached a drawing of the situation I am trying to model.

The input parameters will be:

 Foreslope slope (X:1)

Backslope slope (X:1)

Ditch depth (ft)

Ditch width (ft)

Target Parameters:

Daylight Surface

Constants:

Foreslope Slope

Backslope Slope

Backslope width

Variables:

Ditch bottom offset

Daylight point offset

What I need is foreslope and backslope slopes to remain constant and have the length of the foreslope vary so that length of the backslope results in a ditch that matches the ditch depth input parameter exactly.  Setting target offsets for the ditch bottom and daylight point will not work in this case because the ditch bottom offset will vary depending on the height of the fill and the slope of the existing ground.

My current way of doing this is to draw linework into the corridor section editor at each station to manually find out the required ditch elevation at each station and then creating a ditch profile for the entire corridor.  I then use the LinkSlopetoElevation and Link SlopetoSurface subassembly to draw in the ditch.  This process is time consuming on projects that are several miles long so I figured that creating a subassembly to do the work for me would save a lot of time.

[doni49]

Just to be clear, you want to make sure the bottom of ditch elev is 1 foot below where it eventually daylights?  Or 1 foot below the EG (dimension X in the attached screen shot)?

[KMercier_C3D]

I think you are going to need to have some sort of loop or sequence of test scenarios since you want the depth measured in comparison with the unknown daylight elevation. 

Based on your sketch, what I would do is set the following Input Parameters: 

Foreslope slope (X:1)

Backslope slope (X:1)

Ditch depth (ft)

Ditch width (ft)

Start Test Ditch Offset (ft) (ex. 5')

Test Ditch Increment (ft) (ex. 1')

Then use auxiliary points and auxiliary links to create a ditch using the starting ditch offset with the foreslope and backslope. If it works (AL2.ylength>=1) then you create a point and link, if it doesn't work then you test the next offset using the incremental value. I would set it up to do somewhere between 5-10 iterations. 

Now when you are using this subassembly in Civil 3D you can either be happy with your ditch jumping at 1' increments or you can finesse it once you know a certain range is all putting the ditch at 5' with 1' increments then set the range to start at 4' (Ditch offset - increment) and set an increment of 0.1 (increment/iterations), using the Section Editor->Parameter Editor Override. This will help smooth the offset of your ditch. 

(Edited to spellcheck)

[doni49]

Assuming that it's exactly as you've sketched it, look at what you know:  the foreslope (4:1) and backslope (6:1) form two sides of a triangle and since the slopes don't vary, you know the angle between them doesn't vary either.

So now you know the angle between an imaginary line (red line labeled "Aux Slope" in the following sketch) -- or at least you can figure out the angle ONCE. 

Given a line at this Aux slope from the attachment point, you can target the EG.  That will give you a first test point (if the ditch won't fit at that point then see if continuing on this slope will eventually find the EG again (maybe this imaginary line will cross the EG multiple times).

[KMercier_C3D]

@doni49:

But you don't know the slope of the aux line just by knowing two slopes (you would need a 3rd piece of information like the xlength of both legs of the triangle). If the ditch is offset by 5' or by 10' then these are going to be two different aux slopes per your sketch. So as you said making an aux slope would just be a first test scenario, you still need to do some sort of iteration in order to obtain an answer. 

[doni49]

But you don't know the slope of the aux line just by knowing two slopes (you would need a 3rd piece of information like the xlength of both legs of the triangle).

Sure you do.  The Aux slope will never change.  The LENGTH of the aux slope line will change.  If you go from the attachment point at this aux slope and target the EG, you will eventually hit the EG (if you're in fill as the OP stated). 

I haven't actually used them myself but I have seen conditional subassemblies.  Can't you go from this EG/AuxSlope point and work backwards to see if the ditch will fit and if it doesn't fit, continue on with using the same aux slope and target the EG again?

This is just basic geometry, if you know the slopes of the back and fore slope then you know the angle between them.  That angle can't change and as such the angle between Aux Slope and foreslope can not change and neither can the angle between Aux Slope and backslope.

[KMercier_C3D]

Please see my attached 15 second sketch. As you stated the slopes of the foreslope and backslope remain unchanged. The length of the backslope will remain constant since that is defining the 1' depth however the length of the foreslope (and therefore the "aux slope") will vary as shown. 

 

[peterfunkautodesk · Accepted solution]

What if you went up first for the backslope (using slope and Delta Y) and then do a Aux point to the surface. Then from that daylight point, go down for the ditch bottom and then back up to the hinge point.

In the sketch AL1 is 6:1, AL2 is what every slope you want for the foreslope, AP2 is the daylight, P2 is the ditch bottom where L2 is at 6:1, and L1 is at the foreslope. P2 is 1' below P3.

Regards,

Peter Funk

Autodesk, Inc.

[fcernst]

Very clever!  Thank you for showing...

[Anonymous]

Brilliant!

Never thought about pulling about my geometry book and looking at similar triangles!

[premj]

FYI

Many similar tips & tricks are covered in my SAC training book and for details feel free to visit:

http://www.solidcad.ca/training/civil/autocad-civil-3d-subassembly-composer/

Regards,

PJ

[Anonymous]

So how wuold I set this up to be a condtional situation where in cut I want a ditch to be 18" deep from the edge of shoulder and in fill to have a ditch at the toe as described.

[peterfunkautodesk]

You would check to see if the P1 point is above or below the target surface and then branch the SA for cut/fill. In the cut branch, you would create the ditch down 18" at your slope, and they daylight up to the surface. In the Fill, you would use the method described above.

Cheers,

Peter Funk
Autodesk, Inc.

[Anonymous]

Ok.  So I am completely new to SA composer how would this look in the composer?

[Anonymous]

OK.  I think I have the subassebly created in SA composer, but the preview is only displaying the false condition (cut).  Is there a way to display the true condition (fill) so I can check the input?

[doni49]

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@Anonymous wrote:

OK.  I think I have the subassebly created in SA composer, but the preview is only displaying the false condition (cut).  Is there a way to display the true condition (fill) so I can check the input?

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On the Target parameters tab, set the surface's Preview Value to a negative number. 

EDIT:  After posting this, I thought maybe it would be helpful to explain that a little further.  The "surface" that the preview displays is flat and is shown as though it's at this "Preview" elevation.  If you have an end condition that is dependent upon being a specific distance above or below the surface, then change this preview value such that you're able to see what happens at the various elevation changes.

[Anonymous]

Thanks, but I must not have something correct because it still only displays the false condition (cut).