What separates a bad gravel driveway from a GREAT gravel driveway? This series of articles discusses the Snohomish, Skagit and Whatcom County private road, encroachment, culvert and gravel driveway requirements.
In our last article, Fire Turnouts and Fire Turnarounds, we reviewed how gravel driveway curves, turnouts and turnarounds are designed. In this article, we are going to focus on road encroachments, drainage ditches, and culvert installations.
A driveway's encroachment is essential to gain entrance to a property and there are design considerations that need be applied to any new road encroachment installation such as turning radius, speed limit of the connection road, residential density of your area, for example, urban or rural, existing drainways, etc.
Let's study some drawings.
In the drawing above, you'll notice the encroachment cross section supplied by Whatcom County. It calls for a 12 inch minimum diameter culvert. The necessary culvert size depends on some site conditions and will need to be reviewed by the county engineer to make sure the proposed diameter makes sense for your site. The top of the culvert requires at least 12 inches of gravel cover plus a top driveway surface. Where the culvert protrudes on either side of the driveway, we will need to go 12 inches beyond the gravel toe slope which is a 1:1 maximum. Then, 6 inch minus gravels needs to be installed over that surface of that toe to protect it from erosion.
The culvert should also be installed with a slope of 1% towards the downhill side.
Things get interesting when we consider the roadway speed limit and the ADT (Average Daily Traffic) information where our gravel driveway encroachment connection is made. The following drawing offers us some guidance:
There's a lot to unpack here, but what we see is that we have a choice of radii depending upon the posted speed limit of the road we are encroaching. At 35 MPH or less, we can have a right and left turning radius of 10 feet. We find our opposing radius origins by measuring 10' from the edge of the new driveway and 10' from the edge of the traveled roadway or the fog line. The fog line is good to use since it cannot be easily disputed. Where our two measurements intersect is where we layout our arcs. This gives us our encroachment width. If the posted speed limit is greater than 35 MPH, we need to use 25'. It should be noted that both 10' and 25' are minimum radii.
If it was my encroachment, I would choose the minimum radius allowed and here's why: The length of your required culvert pipe is NOT calculated based on the width of your driveway (12' in the case above), but on the width of the encroachment. Those turning arcs add a lot of length and culvert pipe is not inexpensive. Remember, we need to extend the culvert 1 additional foot beyond the toes of the shoulder materials supporting your driveway.
Also take a look at the notes. On Whatcom County gravel driveways, we can connect to the roadway with gravel ONLY IF our ADT (Average Daily Traffic) volume is less than or equal to 1000 ADT. Many of the roads in Whatcom County fall under this however, the county has their fair share of busy roads as well. If the road we are encroaching is greater than 1000 ADT, we'll want to consider laying asphalt for that first 20 feet of driveway. That's really not a bad idea. It keeps rocks and mud off of the roadway and helps keep potholes from forming. Laying asphalt for the first 20' is a worthwhile upgrade. Here's what that looks like in Whatcom County:
Before ending this article, I wanted to share one more configuration that Whatcom County allows that I really like, especially on busier roadways:Whatcom County Road Standards Skagit County Road Standards Whatcom County Roadway Guidelines Whatcom County Residential Driveway Examples Snohomish County Road Standards & Examples
I hope that this series of gravel driveway articles gives you some ideas for your upcoming road encroachment project!
Once your encroachment is built and your culvert has been installed, it is important to remember that it needs to be maintained and inspected for failure or damage. I'm planning on writing another article soon that will discuss the when, why and how of culvert maintenance. Check back for that soon!
If you are looking for a gravel driveway contractor to help you with your project and you are located in Skagit county, Snohomish county, or Whatcom county, please give us a call or send us an email! WE LOVE THIS STUFF!!!
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