Calculating the maximum slope for grading on the Site Grading Vignette is an important concept to understand because creating too steep of a slope in your solution can easily lead to a failed score.
For this ARE vignette, you will be required to re-grade an existing site to satisfy a number of conditions and requirements. You will be presented with an existing site plan showing the contours of the site. In most cases, you will need to modify and adjust the existing grade lines in order to properly meet water drainage requirements set by the program.
Along with the water drainage requirements set forth in the SPD vignette program, you will also be asked to keep any new grading under a certain slope. To be honest, this is not that hard of a requirement to meet, but it’s commonly overlooked and forgotten about by many candidates. Although the maximum slope requirement is clearly stated in the site grading program, many people forget about it because they’re so concerned with satisfying all of the water drainage requirements.
Being aware of all requirements on the Site Grading Vignette before your actual test day is so important because you’ll save yourself valuable time if you what most of the requirements will be ahead of time. Although the maximum slope for the site grading will vary per exam, the requirement as a whole will typically be in place so understanding how to calculate it will be essential.
Typically in the given program, the maximum slope for grades will be stated as a percentage. For example, it may say something like this in the program “Grades Shall Not Exceed 20%”.
As you’re reading through the program, definitely note this requirement because one of the first things that you do before drawing will be to calculate the maximum slope for the grading.
After you check the program requirements on the SPD vignette, you’ll want to check the site plan and especially note the spacing of the existing contour lines. These existing contours may be spaces at either 1′, 2′, or even 5′ increments and this spacing is critical when trying to calculate the maximum slope.
Calculating Maximum Slope
To be clear, it’s important to remember that in this vignette you will most likely be required to re-grade a portion of an existing site plan. Any modifications to the existing contour lines will need to meet these slope requirements given in the program.
Once you have the maximum slope percentage from the given program and have noted the contour spacing, you now have enough information to calculate the maximum slope for any new contour lines that you create.
There are many different ways to calculate the maximum slope so I’ll go over the most common way right here. It’s up to you which method you use but you will need to do this calculation to figure out the MINIMUM horizontal spacing that you’ll need between contour lines.
Method 1 – Dividing The Percentage
This first method is a fairly quick calculation where you simply take the existing contour spacing and divide by the maximum slope percentage.
Let’s say that 20% is the maximum slope given to you in the vignette program and the existing contour spacing is 1′.
1′ / .20 (meaning 20%) = 5′
What you’ve done here is basically divided 1′ by the 20% to come up with a maximum allowable slope of 1:5.
This means that you’ll need to space out your new contour lines by at least 5′. If they’re less than 5′ apart anywhere on your grading plan, you will violated the slope requirement because your new grades will be too steep.
Let’s say the existing contour lines were spaced at 2′ intervals instead of one. Here’s how that calculation would work:
2′ / .20 = 10′
So now you will need to make sure you have at least 10′ between all contour lines or else you’ll be too steep.
Using the same contour intervals as the previous example, let’s say that the maximum slope given in the program was 25% instead of 20%.
2′ / .25 = 8′
Again, 8′ will be the MINIMUM spacing needed between contour lines to make sure that you don’t exceed the MAXIMUM slope requirements.
As you can see, there really aren’t extensive calculations required to calculate the maximum slope for the Site Grading Vignette, but you’ll need to have a solid understanding of what you’re actually trying to figure out. As you’ve seen above, you’ll be given the requirements and spacing and it’s your job to figure out the minimum horizontal distance between contour lines so that you don’t exceed any maximum slope requirements.
If you would like to read more about the Site Grading Vignette, see here!
As I mentioned before, this vignette is meant to be a quick test of your ability to understand simple issues related to water drainage and site topography. There really aren’t too many tricks in this vignette so it’s important that you read and follow all directions. Since the Site Planning Vignette will also be part of the SPD exam, you’ll need to manage your time so you can finish both vignettes within the allotted time.