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Aim for the Potholes, Dad!
 I live about half a mile up a gravel road shared by five other families. “Who fills the potholes” is a recurring thorn in our collective derrieres.
My kids think it’s a riot, me swerving extravagantly to miss them (the potholes, not my neighbor’s derrieres).
“Dad, why are you swerving to miss the potholes?” they shout. “It’s a lot funner if you can hit ‘em all! Whee!”
“Don’t you kids know anything about physics?” I bark back at my 9- and 11-year-old sons. “It’s the hitting them that causes continued scour brought on by micro-supervelocities at the water-particle interface, for crying out loud! And you want me to hit every one? What do you think I am, a, a… stupidhead?”
“No dad, not a stupidhead, just a fun vacuum. Hey, here comes a great big one, dad. Let’s see how big a splash you can make! Whee!”
Then my wife, Cindy, chimes in. “Tim, if you wrench that steering wheel one more time, my head’s gonna come flying right off my neck! Just bite the bullet and drive straight. It won’t kill you to roll through a few potholes. For Pete’s sake, you’d think they were land mines or something.”
“Tut, tut, my ignorant family,” I reply smugly. “You could all use a lesson in Pavement Death 101. Allow me to expound...”
Water.
That’s correct, water. If there was no water, there’d be no potholes or failed pavement. Okay, so there wouldn’t be cars or humans either, but at least no one would have to suffer undercarriage pulverization by those darned, pesky potholes.
Well, yes, there’s a little more to it than that. Proper subgrade preparation and compaction, as I’ve harped on many times before, is also critical. But, still, water is every pavement’s worst enemy. And I’m not just talking about gravel roads — this includes asphalt pavements and concrete too.
Water is a solvent. It tends to loosen small granules, then suspend them in solution until a tire comes along and splatters them across the countryside. Freeze-thaw cycles, for those of you in northern climates, accelerates this process. Granted, concrete and asphalt granules are more difficult to loosen, but given enough water and tire agitation, in time they too will succumb.
Have you ever noticed that flat roads have a lot more potholes than sloped roads? This is because flat roads don’t shed water very well. And if there happens to be a soft spot in the subgrade causing a depression, water now has a place to congregate.
“Say, Frank,” one water molecule said to the other. “There’s a new depression over by oil-pan alley. Everyone’s congregating!”
“Cool, Joe — thanks for the tip. Don’t mind if I trickle on over. Let’s party!”
How much slope is needed to ensure ample drainage? The more, the better. Of course, once you get beyond about 14%, vehicles can have trouble staying earthbound, and fire trucks may not be able to make it to the fire.
For asphalt and gravel, 2% slope is a bare minimum. Any less and you’re almost certain to get water congregation places, aka bellies, sooner or later.
Concrete is much more forgiving because water doesn’t dissolve it nearly as readily. In low traffic areas, 0.5% slope can work. With very low slopes, however, be careful of sheet ice formation.
To summarize, the following list is what you should do if you have a flat, gravel road like mine — in order of best to worst. Naturally, this is also the order of most expensive to cheapest:
- Pave it with concrete 6 inches thick over a structurally sound subgrade. I would also use 10-gauge or thicker welded wire fabric at mid-height of slab. Subgrade can usually be compacted native soils, assuming they’re at least one full grade better than oatmeal. Other than sheet ice worries, five families won’t create enough traffic to cause problems, even if the road settles a little.
- Pave it with asphalt 2 inches thick over a structurally sound subgrade. Where I live, we construct subgrade with 6 to 10 inches of thick imported sand-gravel mix plus 2 inches thick crushed rock — all very well compacted. Road fabric placed on top of native ground is common too, especially if soils are soft. Make sure your contractor builds in at least 2% side slope or crown.
- Regrade your existing gravel, adding new as necessary, with 3%-4% side slope or crown. I prefer 1-1/4” minus, crushed gravel — it tends to hold up better than smaller diameter mixes.
- Bicker with your neighbors as to who’s on “Filling the Potholes” duty.
- Do nothing and bicker with your family about the wisdom of swerving to miss as many potholes as your turning radius and reflexes will allow.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to buildersengineer@constructioncalc.com. Tim reads every one.
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The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.
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