Single trailer chip truck loading post-harvest forest residuals
Single trailer chip truck loading post-harvest forest residuals

Using post-harvest forest residuals, or slash, to make biojet fuel and co-products is an economic challenge. A significant cost in the supply chain is transportation associated with hauling the processed slash material to a conversion facility. To supply slash material to a conversion facility or processing depot, trucks need to travel on forest roads and often travel long distances.

NARA researchers Rene Zamora-Cristales and John Sessions have evaluated multiple approaches used to lower the transportation costs of moving slash. These approaches include improved methods to load the processed slash into a trailer so that bulk density is increased to providing logistic models used to select the most cost effective routes and equipment to haul slash out of the harvest site.

Recently, they published a peer-reviewed paper, partially funded through NARA by the USDA-NIFA, evaluating the use of single and double trailers, double trailers being one truck pulling two trailers, in steep terrain. Their goal was to examine conditions where using a double trailer configuration might be more economical than a single trailer configuration.

Are double trailers cost effective for transporting forest biomass on steep terrain

Experiment design

To evaluate these two options, the researchers developed a simulation model based on data from trucking observations at timber harvest sites in southwest Oregon. Using the simulation model, they compared the cost of using a truck pulling two single 32-foot trailers (28-feet in California due to State regulations) to a similar truck pulling a single 32 or 45-foot trailer. All truck configurations complied with state regulations. The single-lane roads used in the simulation were gravel with gradients ranging from 5% to 20% and could accommodate a single or double trailer configuration.

The travel speed of each trailer configuration was evaluated under a variety of road conditions and loads. They also evaluated the time and cost involved to connect, disconnect and load (including grinder costs) the trailers and the amount of fuel consumed. In general, double trailer configurations traveled slower than single configurations and required more time and fuel, hence cost.

Experimental results

Getting a double trailer configuration to a slash grinding location, loading it, and hauling the load to a conversion facility took 34% more time, and had a 24% higher hourly cost, than using a single trailer configuration. The time and cost difference would favor single trailer configurations, however, double-trailer configurations can carry more biomass than single trailers. As the double trailer configuration travels further, the economics favor the larger load. It turns out that if the distance from the grinding site to the conversion facility is greater than 35 miles, than a double trailer is more economical than a single 32-foot trailer. When using a single 42-foot or 45-foot trailer, then the break-even point is 56 and 70 miles respectfully.

If the double trailer requires a trailer hook-up distant from the grinding/loading site, then the cost advantage sways to a single configuration. For example, if the distance to the conversion facility is 70 miles and the trailer hookup occurs two miles from the loading site, then a 42-foot single trailer is more cost efficient.

Of course, the layout of a harvest site can vary. The authors considered a scenario featuring a centralized landing yard with a large turn around where untreated slash material is hauled from the surrounding area using bin trucks and ground at the landing. In this case, the benefits for a double trailer were marginal. Although the hook-up times were eliminated, the cost of the bin trucks eliminated the cost benefit of no hook-ups.


The authors summarize that double trailer configurations are only cost effective for long distance hauls when hook-up distance to the loading site are minimal. They point out that improvements in trailer steering and packing methods may offer better results to decrease transport costs.