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.
When we think about the shape of a driveway, gravel or otherwise, perhaps we envision its turns. It's true that turns are very important. Too tight a turn may mean that EMS vehicles (ambulances, firetrucks, bullet proof SWAT transports, etc.) can't reach you or a loved one efficiently in the event of an emergency - or worse - not at all. We'll discuss what makes a turn adequate later.
First, lets discuss shape as it pertains to a gravel driveway's cross section. Lets imagine a gravel driveway sliced somewhere along its path. According to Skagit County, a good gravel driveway cross section would look something like this:
If we focus on the shape of the driveway as drawn, we notice that the driveway...
1) has a crown. In the image above, we see a high centerline with a 2% down slope in both directions from center. This is key to controlling the path of storm water. We want storm water to flow off of the driveway and do it taking the shortest path possible.
2) down slopes relatively sharply AWAY from its edges. We see the notation of 2:1 there. These ratios vary from county to county and situation to situation. Sometimes you may find a cross section calling out the ratio to be a maximum of 1.5:1. This ratio represents the rise to run ratio - in this case - 1 foot of vertical drop for every 2 feet of horizontal run down to your drainage ditch. This ratio gives your driveway height above the high water line. It also gives the driveway edges structure so that they don't crumble and cause your driveway to lose its legal width which we see as 12 feet in the drawing above. That is a minimum driveway width for Skagit County gravel driveways, and as it happens, it looks like that is also the case for Whatcom and Snohomish counties. It's a good thing to verify since the dimensions do change over time. I was bitten by this once while trying to get a site plan approved in Whatcom County. The Whatcom County planning department sent me old specs and the fire marshal rightfully had a fit when I explained to him where I had gotten them. He told me that the specs I received were 15 years out of date - YIKES! Make sure you have the latest information! If you hire me to build a new driveway for you or help with your site plan, I'll definitely make sure we are referring to the latest specs.
3) has layers. Starting from the bottom and working up, we have an assumed layer called the grade. It is the soil layer holding up the gravel layers. You'll notice it also has a shape to it. It is crowned and it is compacted. Optimally, it should be undisturbed, non-loosened soil. It needs to be crowned since the gravel isn't impervious and we want to avoid mud rising its way to the top and destroying our work. The next layer of gravel is called our Base Course. Base Course has a mix of larger stones. It is topped by crushed surfacing stone which is a mix of smaller stones and fines and compacts really hard. You'll notice that the drawing above is not opinionated about the thicknesses of these layers. But I assure you that these layers should be placed in specified thicknesses. Depending on what you are doing, there are substantial thicknesses called out by the county specs. For example 8 inch base course AFTER compaction and 2 inch crushed surfacing, again AFTER compaction. Thicker layers are a good thing and help us get the roadway height and edge slopes we need.
If you are getting ready to repair your gravel driveway or install a new one and you would like more information, please take a look at some of the following references. Please keep in mind that the building/planning departments aren't always good at keeping the most up to date information on their websites so you will want to call them and ensure you have the latest requirements. Also keep in mind, that this article is about adequately shaped driveways. If you have an older driveway and you simply wish to breath some new life into it or you find you are having drainage issues, with a little bit of grading, resurfacing work, and bringing in new gravel, you too can get the shape to a good place. Or - give us a call! We're happy to help!
In our next article, we'll discuss Curves, Turnouts and Turnarounds!Part II: Driveway Fire Turnouts and Turnarounds
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