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Randy B. Foltz1

1 USDA Forest Service, Washington, DC. USA.


The sediment production from a good-quality aggregate surfaced road section and a marginal quality aggregate surfaced road section was compared. Rainfall and runoff were measured for three winter seasons with logging truck traffic and one winter season without traffic. Sediment production from the marginal quality aggregate varied from 4 to 17 times as much as from the good-quality aggregate. Sections with logging truck traffic produced 2 to 25 times as much sediment compared to sections with no traffic conditions. While these ratios were specific to the test site, the processes would apply at other locations and produce a similar range of values.


The United States Forest Service road network consists of 369 000 miles - approximately 75 percent are unsurfaced, 20 percent are aggregate surfaced, and 5 percent are paved (USDA 1988). In wet climates, such as western Oregon and Washington, most roads are surfaced with 200 to 400 mm of aggregate (surface and base courses) to provide structural support during wet weather. Typical constructed profiles of logging roads in these areas consist of a 25 mm minus dense graded surfacing aggregate over a base aggregate. Even with these relatively large thicknesses of aggregate materials, sedimentation can be a problem.

In drier climates, such as the Intermountain West, placement of aggregate on the running surface is an accepted method of reducing the amount of sediment produced from unsurfaced forest roads. In these situations, the aggregate thickness is less than in the wet climate situation.

Burroughs and King (1989) developed an equation relating ground cover (for example, aggregate thickness) to reduction in sediment production. Their equation resulted in a 20:1 reduction in sediment for a 100 percent ground cover of aggregate. In a study (Burroughs et al. 1984) on the Nez Perce National Forest, Idaho, on "border-zone batholith" material of gneiss and schist using a 102 mm lift of 38 mm minus high quality, gneissic, crushed rock, simulated rainfall gave a 4.8:1 reduction in sediment compared to an unsurfaced road with the same parent material. Swift (1984) demonstrated the importance of the thickness of the aggregate layer. A 51 mm lift of 38 mm crushed rock resulted in no sediment reduction. A 152 mm lift of the same size resulted in a 12.5:1 reduction in sediment, and a 200 mm lift with a D50 of 76 mm resulted in a 33:1 reduction in sediment. Each of these studies was with high-quality aggregate and NO traffic.

High-quality road surfacing aggregates are not always readily available in many localities, and marginal quality aggregates are often used to reduce road-building costs. In some instances, marginal quality aggregates perform adequately from a structural point of view, but may generate excessive sediment. The effects of aggregate quality on sediment production have not been adequately measured.

The objective of this study was to compare a good-quality aggregate to a marginal aggregate both with and without traffic. The marginal quality aggregate needed to be "marginal, but usable."

Traffic was representative of an actual timber sale. A comparison between the two aggregate qualities with and without traffic was made. If the physical process were identified, extrapolation to other sites would be possible.


The United States Forest Service conducted a study of how aggregate quality affects sediment production with and without logging truck traffic. This study, conducted during the winter months for 4 years beginning with the winter of 1992, was part of a larger study that included the effect of tyre pressure on sediment production (Foltz 1995) and a study of the development of ruts with traffic (Truebe et al. 1995). Only the aggregate quality study is reported in this paper.

Test site

A 2.25 km long by 4.27 m wide crowned section of forest road on the Lowell Ranger District of the Willamette National Forest, Oregon, was selected for the test. This area of western Oregon has wet winters with little snow cover at the elevation of 470 m. Average precipitation from November to April is 1100 mm. The road was chosen to meet the requirements of length, constant grade, and the ability to control non-test traffic. Two 61 m long sections that had a similar grade of 12 percent and aspect were selected. One of the test sites was surfaced with a marginal quality aggregate; the other site was surfaced with a good-quality aggregate. Both aggregates contained igneous extrusive materials, but the marginal quality aggregate contained zones of weathering, which produced marginal quality aggregate. The major difference between the aggregates was sand equivalent (56 for good and 22 for marginal) and durability of fines (63 for good and 31 for marginal) (Foltz and Truebe 1995). It should be noted that, while the marginal quality aggregate failed to meet the specifications for the Willamette National Forest, other National Forest personnel have told the author that it would be similar to aggregate generally acceptable in their area. For this study, 102 mm of the specified quality of aggregate was compacted as surfacing on the existing aggregate. The existing aggregate varied from 300 to 400 mm of a 76 mm minus material on a clayey silt subgrade.

Runoff and sediment measurements

Two runoff measurement collectors were located on each test section. One collected a portion of the runoff flowing laterally from both sides of the road crown; the second collected runoff flowing longitudinally down the road. Continuous flow measurements were made. Sediment trapped in a 0.15 m3 settling box was collected approximately every three weeks for mass and size analysis in the laboratory.

Truck traffic

To simulate the effects of road use from a timber harvest, loaded and unloaded trucks were driven on the test road. Prior to each season's measurement period, the sections were graded to remove wheel ruts and to provide a consistent initial road condition. During the winter of 1992, two trucks were used. One western-US style logging truck with a 22 450 kg load of logs was driven downhill. A dump truck with the same axle spacing as an unloaded logging truck carrying its trailer was driven up the hill. Vehicle speeds rarely exceeded 50 km/h. All trucks had tyres inflated to 620 kPa.

Driving was done on eight consecutive days of each 2-week period, including during rainfall events. No driving was permitted on frozen road conditions. This paper uses the term "load" to mean the combination of one loaded logging truck passing a road section and one unloaded logging truck passing in the opposite direction. The equivalent timber haul assumed the 22 450 kg of logs per truck represented 17 m3.

During the winters of 1993 and 1994, a second loaded logging truck was added. It also had a 22 450 kg load of logs and was driven downhill. The dump truck made twice as many passes as the logging trucks to maintain the desired ratio of one loaded logging truck for each unloaded truck.

To measure the no-traffic sediment production from the road sections, measurements of rainfall runoff were taken during the winter of 1995. No logging truck traffic was permitted on either section. Only occasional light pickup traffic, less than four passes per week, was permitted.

Each year of the traffic test ran from mid-January to 1 April. The no-traffic test period was begun in mid-November and ended 1 March.

Results and analysis

Table 1 summarizes the precipitation, truck traffic, and equivalent timber haul for each year. Seasonal precipitation ranged from 147 to 782 mm during the study. In 1993, the precipitation of 521 mm included 249 mm of snow. During each of the other years, the snowfall was insignificant. The precipitation maximum daily and maximum 5-minute intensities were remarkably consistent for the four test years.

The measured precipitations varied from 41 percent of the average seasonal precipitation in 1992 to 109 percent in 1993 with only the 1993 test period exceeding the average seasonal precipitation. Based on National Oceanic and Atmospheric Administration Atlas 2 for Oregon (Miller et al. 1973) a 2-year, 24-hour storm depth was 89 mm. Only during the no-traffic year was this value achieved.

In 1992, the typical number of log loads per day was 15, which increased to 30 when the second truck was added in 1993 and 1994. These rates fall into the category of "heavy traffic" (Reid and Dunne 1984).

Water yield

Table 2 presents the water yields for the four years. The good-quality road section consistently had lower runoff than the marginal quality section, indicating a higher infiltration rate. The higher infiltration rate was likely the result of fewer fines in the good-quality aggregate. In 1993, the runoff ratios were lower than the 1992 rates, even though the precipitation was greater. Much of this was because 48 percent of the 1993 precipitation was snow, which slowly infiltrated into the road surface as it melted.

In the no-traffic year (1995), the runoff-rainfall ratio for both aggregate qualities was higher than the year (1993) with nearly equal precipitation. This ratio was also higher than the average runoff-rainfall ratio for the three years of traffic. The runoff-rainfall ratio increased with cumulative traffic for both aggregate qualities. If the aggregate had been able to recover from the effects of traffic in one season, one would expect the runoff-rainfall ratio to approach the first year of the test. Since this did not occur, the data suggest that more than one season without traffic would be required to return to the initial runoff-rainfall ratios.

Sediment production

Table 3 displays the sediment production from the road surfaces of each aggregate quality. Virtually all of the eroded material, 96 percent to 99 percent, was less than 6 mm. Silts and clays typically comprised 65 percent by weight of the sediment in the runoff.

An analysis of variance of the sediment production in g per mm of rainfall was performed using the General Linear Model of SAS. Treatments were traffic level and aggregate quality. Aggregate quality was statistically significant (p=0.058) and traffic was also statistically significant (p = 0.031).

The ratio of the marginal quality to good-quality sediment production represents the sediment penalty from the use of the lower quality aggregate. This ratio varied between 3.7 and 17.3. The sediment ratio demonstrates the value of using the good-quality aggregate. In the lowest precipitation year, for every unit of sediment eroded from the good-quality aggregate, nearly four units were eroded from the marginal quality aggregate. The difference between sediment production was more pronounced in the highest precipitation year where for each unit of sediment from the good-quality aggregate, the marginal quality aggregate produced 17.3 units. Two factors, rut geometry and aggregate resistance to wear, contributed to the differences.

Rut geometry consisted of an initial cross-slope flattening, leading to a deepening of the wheel path to form a rut. As the cross-slope of the road was flattened, the distance required for water to flow from one side of the road to the other was increased. Consider a 4.3 m wide crowned road on a 10 percent grade with a 4 percent cross-slope (Figure 1). The distance that water must travel from the road crown to the road edge is 5.73 m. If because of traffic the cross-slope is flattened from the initial 4 percent to 2 percent in the wheel paths, the flow path is increased to 7.58 m, an increase of 32 percent. This increase occurs without visible rut formation. Generally, the wheel track has reduced infiltration compared to the non-tracked portion and, therefore, produces more surface runoff. The combination of increased flow path and increased runoff results in greater erosion, even though no visible ruts are present.

When a rut does form, the runoff is prevented from flowing across the road and is confined to the rut, and the concentrated flow causes additional erosion. Concentrated flow in the rut continues until the flow overtops the rut or a cross drain is encountered. In severe cases the rut may channel water for long distances and bypass relief culverts or drains.

Both cross-slope flattening and wheel rutting were observed on the marginal aggregate road section. The good-quality aggregate exhibited only slight cross-slope flattening. Figure 2 shows representative road cross-sections taken before and after 10 500 m3 of timber haul. In Figure 2(a) the cross-slope on the good aggregate section flattened from 6.5 percent to 5 percent. Flow path calculations for these conditions were 4.5 m initially and 4.9 m for the final condition, an increase of 9 percent in flow path length. In contrast, figure 2(b) shows the marginal aggregate section began with a 5 percent cross-slope and ended with 130 mm deep ruts. They began as cross-slope flattening, became ruts in soft portions of the road, and finally extended the entire 61 m length of the test section. This flow path geometry caused flow paths on the marginal quality sections to exceed those on the good-quality sections by a factor of 12 times (61 m/4.9 m). These wheel track geometry considerations were a major contributing factor to the increased sediment production from the marginal quality aggregate.

A second factor contributing to the increased sediment production was the inability of the marginal quality aggregate to withstand crushing by the truck traffic. This crushing maintained a ready supply of fine particles available for transport by the runoff. Alternatively, the good-quality aggregate was more resistant to crushing resulting in less sediment available for transport. The crushing and increased fines content in the marginal quality aggregate also reduced infiltration, resulting in the higher runoff:rainfall ratios as shown in Table 2.

The initial particle size distribution for both test segments was taken before traffic in 1992. Another particle size distribution was taken again at the end of the 1993 test after two seasons of traffic. This permitted a mass balance for the particle sizes less than 0.075 mm to be performed. The mass balance was solved for the amount of fines generated due to crushing. The traffic on the good-quality section generated 66 percent fewer fines than the marginal quality test section, 682 kg compared to 1986 kg. These values were further evidence that the marginal quality aggregate generated more fines than the good-quality aggregate.

Another penalty due to marginal quality aggregate is increased road maintenance. As shown in Figure 2, the marginal quality test sections had deep ruts that would have had to be removed by road maintenance. During each year of the test, this section would have been bladed under normal operational conditions from 2 to 4 times. The good-quality test section did not require this maintenance.

Effect of traffic on sediment production

To account for the different precipitation depths in each of the years and to isolate only the effect of truck traffic, the ratio sediment yield in grams to sediment per mm of rainfall was calculated. This ratio better represents the sediment increase due to logging truck traffic. Although it does not remove any effect due to different rainfall intensities, Table 1 shows that the 5-minute intensities did not vary greatly.

Figure 3 presents the sediment ratio from a road with logging truck traffic to the same road without logging truck traffic for both a good-quality aggregate and a marginal quality aggregate. For both aggregate qualities, the sediment ratio increased with increased timber haul traffic. At the highest haul measured, 20 600 m3, the sediment comparison between traffic and no-traffic on the good-quality aggregate was 18:1. The similar ratio for the marginal quality aggregate was 25:1. The lowest measured haul volume caused an increase of approximately two times, regardless of the aggregate quality, indicating the importance of aggregate quality as haul volumes increase.

The ratios in Figure 3 can be compared to similar ratios taken from Reid and Dunne (1984) on a study located in the Clearwater River basin on the Olympic Peninsula of Washington. For a heavy use aggregate road compared to a light use aggregate road, Reid and Dunne implied a sediment ratio of 132:1. The values measured at Lowell were one-fifth of those for the marginal quality aggregate, and nearly an order of magnitude less for the good-quality aggregate. One obvious difference was that Reid and Dunne included the entire year, but this Oregon study included only the winter portion. However, during the winter, soils were nearly saturated and produced more runoff and erosion than similar storms would produce during the dry summer season. Bilby et al. (1989) in a study in southwestern Washington reported sediment production values of an order of magnitude less than Reid and Dunne. Aggregate quality differences among the three studies may be responsible for the reported differences.

Conclusions and recommendations

The 4-year test of the effect of aggregate quality on sediment production demonstrated that aggregate quality made a statistical difference in the amount of sediment produced. During a simulated timber sale, a section of road with marginal quality aggregate produced 3.7 to 17.3 times as much sediment as a similar section with good-quality aggregate. These sediment ratios combined with the sediment productions of 10 300 to 54 800 kg/ha were significant.

The lowest sediment ratio, 3.7, was under conditions of 147 mm of rainfall and 4 500 m3 timber haul. The highest sediment ratio, 17.3, was under conditions of 521 mm rainfall and 10 500 m3 timber haul. This implies that the quality of the aggregate increased in importance as the traffic and rainfall increased. Stated differently, for roads that will have large amounts of traffic, the attention to aggregate quality should increase.

The sediment ratios for the different precipitation amounts demonstrated the importance of seasonal road closures during high precipitation periods. The wettest year resulted in a sediment ratio of 17.3 compared to the driest year ratio of 3.7.

Lower quality aggregate resulted in the need for more frequent road maintenance to remove wheel ruts. With limited maintenance budgets, this penalty could help to offset the additional cost of the higher quality aggregate.

Without traffic, the quality of the aggregate still had a measurable effect on sediment production. On the good-quality aggregate, for every unit of sediment produced with traffic only 0.056 units of sediment were produced without traffic. The corresponding value on the marginal quality aggregate was 0.050 units of sediment without traffic.

This study found that heavily used road sections by logging truck produced sediment at 2 to 25 times as much as lightly used road sections. These values are 1/5 to 1/10 of that reported by one study (Reed and Dunne 1984) and equivalent to another study (Bilby et al. 1989) in similar climates.

While the economics and availability of aggregate depend on site-specific conditions, the use of high-quality aggregate reduces the amount of sediment produced from unpaved forest roads. The mechanisms that caused the increase in sediment production were (1) the increase in flow path, (2) the inability to resist crushing, and (3) surface rutting and concentrated flow. Since these mechanisms were not site specific to either Oregon or the aggregate used, the penalty due to the marginal quality aggregate was not site specific, only the magnitude of those mechanisms. Careful attention to aggregate quality and vehicular traffic should be exercised when selecting aggregate for unpaved roads.


The author would like to acknowledge the assistance of the United States Department of Agriculture, Forest Service, San Dimas Technology and Development Center for their financial and technical support and for the loan of a logging truck. The assistance of the Willamette National Forest, notably Mark Truebe and Gary Evans from the Supervisors Office; John Capri and Larry Tennis of the Lowell Ranger District; Dave Katagiri and Steve Bigby of the Materials Laboratory, and the Rigdon Ranger District for the loan of a dump truck were greatly appreciated. The work of Ben Kopyscianski and the other individuals who collected samples and maintained equipment in the cold and rain of four Oregon winters was commendable.


Bilby, R.E., Sullivan, K. & Duncan, S.H. 1989. The generation and fate of road-surface sediment in forested watersheds in southeastern Washington. Forest Science 35(2): 453-468.

Burroughs, E.R., Jr., Watts, F. J., King, J.G., Haber, D.F., Hansen, D. & Flerchinger, G. 1984. Relative effectiveness of rocked roads and ditches in reducing surface Erosion. In: Proceedings of the 21st annual engineering geology and soils engineering symposium. University of Idaho, Moscow, ID.

Burroughs, E.R., Jr. & King, J.G. 1989. Reduction of soil erosion on forest roads. G.T.R. INT-264, Ogden, UT. US Department of Agriculture, Forest Service, Intermountain Research Station.

Foltz, R.B. 1995. Sediment reduction from the use of lowered tyre pressures. SAE 1994 Transactions. Journal of Commercial Vehicle, Section 2, Vol. 103, pp. 376-381. Society of Automotive Engineers. Warrendale, PA.

Foltz, R.B. & Truebe, M.A. 1995. Effect of aggregate quality on sediment production from a forest Road. In: Sixth International Conference on Low-Volume Roads, Vol. 1 pp/49-57. Transportation Research Board. National Research Council. Washington, DC.

Miller, J.F., Frederick, R.H. & Tracey, R.J. 1973. NOAA ATLAS 2, Western United States, Volume X-Oregon. National Oceanic and Atmospheric Administration. Silver Springs, MD.

Reid, L.M. & Dunne, T. 1984. Sediment production from forest road surfaces. Water Resources Research 20(11): 1753-1761.

Swift, L.W., Jr. 1984. Soil losses from roadbeds and cut and fill slopes in the Southern Appalachian Mountains. Southern Journal of Applied Forestry 8(4): 209-215.

Truebe, M.A., Evans, G.A. & Bolander, P. 1995. Lowell test road: Helping improve road surface design. In: Sixth International Conference on Low-Volume Roads. Vol. 2, pp. 98-107. Transportation Research Board, National Research Council. Washington, DC.

USDA. 1988. Roads in the national forests. FS 414. United States Department of Agriculture, Forest Service. Washington, DC.

Table 1: Precipitation and Traffic Summary for the Lowell, Oregon Test.


Maximum Precipitation




Seasonal average


5-min. intensity


Timber haul





























TABLE 2: Water Yields from Road Sections with Two Aggregate Qualities.

Marginal Quality

Good Quality



Rainfall Depth

Runoff Depth

Runoff Rainfall Ratio

Runoff Depth

Runoff Rainfall Ratio





























TABLE 3: Sediment Production from Road Sections with Two Aggregate Qualities.


Rock Quality

Mass Sediment

Sediment Production

Average Concentration

























































1 - With traffic
2 - With out traffic

Figure 1. Representative cross-section of road showing runoff flow paths from the road crown to the road edge. Side (a) represents no cross slope flattening. Side (b) represents cross slope flattening in the wheel tracks due to heavy traffic.

Figure 2. Road cross-section surfaced with (a) good-quality aggregate and (9b) marginal quality aggregate. Before traffic was after grading and before logging truck traffic. After traffic was after 10 500 m3 haul (616 loads) during 1993.

Figure 3. Ratio of road section operated with traffic to section operated without traffic based on sediment production in g/mm of rainfall

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