Topic 8.4: Haul Roads and Log Landings

Except in riverside logging operations, roads for hauling logs from the forest are needed, even where yarding is carried out with helicopters.  For ground-based yarding operations in which the financially viable skidding distances are 250 m (with bulldozers) or 500 m with rubber-tired skidders, about 5 and 3 m of haul road, respectively, is needed for each hectare harvested.  Where helicopters yard logs up to 5 km, only 1 m of road is required per hectare harvested.  Given that logging roads often cost $1000-$50,000 per km to construct (depending on road quality, terrain, local availability of surfacing material, and other factors), and given that many of the direct and indirect environmental impacts of logging can be attributed to roads, how and where roads are constructed is of tremendous importance.  It is not our intention here to try to summarize the extensive topic of road engineering, but there are certain features of logging roads of which all foresters should be aware.

High on the list of criteria for evaluating logging roads from environmental, operational, and economic points of view are length, steepness of grade, trafficability, and maintenance requirements.  Many logging road related problems are the result of poor layout (i.e., locating roads inefficiently).  Lack of accurate topographic maps is one reason why road networks end up being overly dense and otherwise poorly designed, but this problem can be overcome by engineers (or their proxies) spending more time in the forest ground-truthing their maps.  By combining ground-truthing with stock-mapping or other pre-felling operations, the costs can be reduced.  Given the huge cost of constructing roads that can only be used under perfect weather conditions, roads that require substantial maintenance, and roads that do not permit passage of fully-loaded logging trucks, the cost of knowing the terrain in the area to be roaded before roads are planned are comparatively miniscule.

Water is a major problem on roads because road surfaces are often severely compacted and thus all the water that falls on them must flow off over the surface.  Dispersing water from logging roads is a major engineering challenge that poor road siting exacerbates.  Forest managers that “inherit” roads from their less careful predecessors know this problem only too well. Before considering some of the ways roads can be sited and constructed so as to facilitate drainage, minimize erosion, and increase the proportion of the time they can be utilized, we need to briefly consider some fundamentals of hydrology.

Perhaps the most straightforward way of thinking about dealing with water on logging roads is to consider the factors that increase erosion.  Basically, the rate at which soil is displaced by flowing water is governed by the volume of water, its speed and turbulence, and soil surface properties. (Note: these properties and some others are often summarized in what is called the “universal soil loss” equation.)  The quantities of water flowing is obviously related to rainfall (amount and rate of fall), but is greatly influenced by the size of the “catchment” above any point being considered on the road.  For example, the catchment above a point at the bottom of a hillside road that lacks side drains is larger than near the top of the hill.  If water flows onto the road from adjacent areas, the catchment is enlarged and the volume of water flowing is correspondingly increased.  The speed at which the water flows, which greatly influences its capacity to dislodge and move sediments, depends on both slope and tortuosity of its path.  Erosive power of water increases exponentially with its rate of flow which is reduced if the water has to flow around rocks, logs, or other obstructions along its path.  The key to reducing road maintenance costs, increasing road longevity, and minimizing sediment loading of streams is to direct water off roads before it picks up volume and speed.

Nearly every country in the world has official standards for road construction that specify, among other things, the required inter-drain intervals as a function of slope.  The Food and Agricultural Organization of the United Nations (FAO) has also published a very useful forest management guide entitled “The FAO Model Code of Forest Harvesting Practices” (Dykstra and Heinrich 1996).  Here only a few of the recommended practices pertaining to unpaved logging roads will be mentioned.

Logging roads with low maintenance costs, that result in little sediment reaching streams, and that are usable under all but the most adverse weather conditions have the following characteristics (among others):

• Domed in profile so that water flows to the road margins.
• Frequent drains directing water from the roadside into the adjacent forest where it slows down, releases its suspended sediments, and infiltrates the uncompacted soil.
• Slopes <8 degrees (16%).
• Compacted subgrade covered with sufficient gravel to support loaded logging trucks (e.g., 15-25 Mg) without deformation. The amount of gravel necessary can be reduced by covering the ground surface with permeable geotextile fabrics before surfacing materials are applied.
• Minimal length on transported subgrade materials, such as cut and fill slopes.

Building roads with these characteristics may require substantial initial expenses, but these can be rapidly offset by reduced maintenance and hauling costs.  Such roads are easier to build with a combination of bulldozers and excavators, the latter being particularly versatile in constructing side drains.  Road longevity can be further increased and maintenance costs reduced by using logging trucks equiped with “central tire inflation systems” that allow drivers to reduce and increase tire pressure depending on road conditions.