Weighing the Options
‘Back to gravel’ doesn’t have to be the only choice for a failing road … even if funds are scarce.
By Mike Anderson
Ken Skorseth isn’t down on asphalt; he’s up on options for cash-strapped agencies facing expiring or deteriorating secondary roadways.
And, for a guy who’s been at the “Alternatives to Paving” table for at least a decade, he’s suddenly seen the spaces around that table fill up and overflow with hungry transportation engineers and planners.
“Number one, what’s driving this is economics. The rapidly escalating prices of highway construction and maintenance and the essentially flat budgets in local transportation is the first reason this is happening,” says Skorseth, program manager of the South Dakota Local Transportation Assistance Program. “But the second reason, and it’s huge for us in the Central Great Plains region, is the dramatic increase in the size of trucks and ag equipment, and that goes right along with the graphically increased yield of our crops, our grains. And then a third factor is that so many of the local roads are at the end of their design life.
“It’s all hitting us at once.”
Notice Skorseth uses “Alternatives to Paving” – which is also a title in his presentations – as compared to the industry movement’s more common refrain of “Returning to Gravel.” The latter connotation, he has come to understand, has painted him in some circles as an anti-asphalt guru of sorts. Returning the lowest-volume roads to stabilized gravel is an option – and a sound economic one for agencies, he says, as long as the road retains a low volume – but it is not the only option Skorseth offers. Some choices do, in fact, include the use of asphalt:
An asphalt surface treatment – not pavement but rather prime/chip – placed over a deep aggregate base. This is referred to as “blotter” in South Dakota.
Put to work widely in Australia, New Zealand and South Africa, a tactic that has caught Skorseth’s eye involves “building almost all of our structural strength to carry loads in a prepared subgrade and in base layers, and then simply constructing a very thin asphalt treatment at the surface – really nothing more than weatherproofing. It’s thin and it’s fragile, but will work if indeed you build all the structural strength to carry the loads underneath it.” This technique has promise here, he says, but admittedly “is a hard sell in North America, because our concept of a deep base layer falls short of what is needed to do these for a long life cycle.”
Using a 20-year lifecycle cost, a study conducted for the South Dakota DOT summarized that, for roads with up to a 170 ADT (average daily traffic) rating, a stabilized gravel surface is suitable. That threshold, says Skorseth, falls to 150 ADT if user costs are factored into the math. “If it goes higher than that, it’s not a good economic decision to go to gravel only, because the loss of gravel from high volumes of traffic and the frequency of maintenance needed will exceed the cost of pavement, if you calculate a lifecycle,” he says. “Of course, trying to find the upfront money to rehabilitate or reconstruct the asphalt surface is the big hurdle. That’s a lot of money at once.” With a gravel road, the initial cost may not be as high, but the maintenance is.
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