By: Greg Bassler
Winter is here and it is the most important road maintenance period or time of year. Technically, the critical time for establishing drainage is during the fall/winter transition (fall breakup). Provisions or structures for drainage must be installed and maintained in the road prism prior to winter freeze up to reduce erosion and potential sediment delivery. Once the road prism is frozen, road maintenance becomes much more difficult and often ineffective.
One of the most effective structures used to provide surface water drainage on both aggregate and native-surfaced roads is the rolling dip. Dips on out-sloped roads channel water running in the surface ruts off the road surface. Properly located and constructed rolling dips provide an excellent method of collecting water and dispersing it to the forest floor. Unfortunately, use has been limited in some areas of the Inland Northwest due to opposition and criticism by log haulers. In most cases, this is justified since most rolling dips were improperly constructed.
A properly constructed dip is one that is approximately 35 to 50 feet long and 12 to 30 inches deep. If dips are too short in their total length, the truck or trailers will bottom out. The road grade into and out of the dip should be level and not out-sloped. The level grade prevents the truck frame from flexing which can damage the truck’s frame and undercarriage. The most common problem observed with construction of dips is they are typically too short in length and too shallow. Dips are effective on road grades up to 8%. On grades steeper than 8%, the water will run through the dip and a water bar must be used.
Natural dips should be utilized where possible during initial road construction by “rolling” the grade. Moving the road grade slightly downhill and then bringing it back uphill is how this technique is applied. Dips should be located on straight road segments to be most effective. Dips on curves are typically not as effective and often make turning difficult for trucks. Depending on soil types, rolling dips and the fill slope located below the dip is commonly rocked to prevent erosion. Slash and sediment traps can also be used to filter sediment and disperse energy.
Whatever the drainage structures used, they must be in place and maintained prior to freeze up. Hauling during fall wet weather conditions often causes rutting which can lead to subgrade damage. Log hauling during this time may have to be suspended until freezing temperatures solidify the road. Drainage must be re-established by grading or backblading with a dozer. Suspending hauling operations early, before excessive rutting and possible subgrade damage, generally allows earlier resumption of hauling operations when freeze up occurs.
Once the road surface is frozen, maintenance is generally limited to snow plowing and building frost in the road. Snow should be windrowed to the outside edge (fill) sides of the road. Snow should not be pushed or windrowed into the inside ditch or culvert inlets. Holes should be cut or pushed in the snow berms to allow water to drain off the road during rain events and thawing conditions. Rolling dip outlets must also be kept open and functional. When spring breakup comes, drainage structures built into the road are the most effective in capturing water from melting snow and rain. Whenever possible, removal of the entire snow berm is advised. Use of these structures and techniques will help insure proper drainage and reduce erosion.