By: Todd Thomas, Water Resources Forester, Texas A&M Forest Service
|Mmmmm....dips. But not quite.|
Q: While looking through the blue book, I noticed that you have specifications for both broad-based dips and rolling dips. I was always under the impression that these were basically the same thing. Can you explain the difference between the two, and when you might use one instead of the other?
A: Excellent question, and one that we get quite a bit! Upon first glance, rolling dips and broad-based dips do not appear to be much different at all. They both accomplish the same goal: to provide cross drainage on in-sloped roads. The differences between the two are quite subtle, but knowing these differences can save you time and money down the road when it comes to road construction and maintenance.
Both rolling dips and broad-based dips are reverse slopes in the road surface that outslopes for natural cross-drainage. Rolling dips are designed to be used on haul roads and heavily used skid trails. Broad-based dips differ in that they are designed for use mostly on heavily used haul roads. Rolling dips can be used on roads with up to a 15% grade, while broad-based dips should be used on roads that do not exceed a 12% grade. The spacing on broad-based dips should change with every 2% change in gradient, while the spacing with rolling dips changes with every 5% change in gradient.
With broad-based dips, the reverse grade should always be 3% and approximately 20 feet in length. Rolling dips are slightly more flexible in their size and gradient. Guidelines for rolling dips call for the reverse grade to be between 3% and 8% with the length ranging from 10 to 15 feet. The slight differences between the two allow for higher vehicle speeds on broad-based dips than on rolling dips.
Since broad-based dips are used on high traffic roads, some other considerations may be necessary. On some soils, the dip and reverse grade section may require bedding with crushed stone to avoid rutting the road surface. Also, energy absorbers such as rip rap, and in some cases, a level area should be installed at the outfall of the dip in order to slow down runoff and keep erosion minimized.
With both types of dips, neither the dip nor the hump should have a sharp, angular break, but instead should be rounded to allow smooth flow of traffic. Properly constructed dips require minimal maintenance and continue to function years after abandonment, saving your road and saving you money.
*This article was published in the August 2013 edition of the Texas Logger