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Randy B. Foltz1 and William J. Elliot2

1 US Department of Agriculture, Forest Service, Washington, DC. USA.

2 Research Engineer and Project Leader, US Department of Agriculture, US Forest Service, 1221 South Main St., Moscow, ID. 83843 USA. This paper was written and prepared by U.S. Government employees on official time and, therefore, is in the public domain and not subject to copyright.


The sediment production from highway tyre pressures, constant reduced tyre pressures, and from central tyre inflation tyre pressures on loaded logging trucks operating on a marginal quality aggregate surfaced road were compared. Rainfall and runoff were measured for three winter seasons with logging truck traffic. Sediment production from the constant reduced tyre pressure road sections averaged 45 percent less than from the highway tyre pressure sections. An average saving of 80 percent was measured from the tyre pressure sections of the central tyre inflation system. These results were used to calibrate a physical process based erosion model (WEPP). Once calibrated, the model was used to estimate the sediment reduction expected at two locations in the United States, one in Brazil, and one in Romania. Since the process that are responsible for the sediment reduction are not site specific and were modelled by WEPP, we feel confident that lowering tyre pressures in logging trucks on unpaved roads can reduce the sediment loss from these road surfaces. The WEPP model can be helpful in estimating the sediment reduction.


Unpaved forest roads are used to transport forest products. When these roads are allowed to become rutted, sediment production can increase from two to four times compared to freshly-graded roads (Foltz and Burroughs 1990; Foltz and Burroughs 1991). By controlling the tyre pressure in heavy truck tyres, S the impact of sedimentation can be reduced. Tyres with reduced pressure have been shown to develop shallower, less well-defined ruts, which have less concentrated flow and shorter flow paths. These factors result in less sediment eroded from the road. Two methods are available that reduce tyre pressures in heavy trucks, Central Tyre Inflation Systems (CTIS) and Constant Reduced Pressure (CRP).

Central Tyre Inflation Systems are a technology that has been used by the US Army and others since before the second world war to improve vehicle mobility. The technology allows a vehicle driver to reduce tyre pressure while in motion. The reduced tyre pressure, when used with radial ply tyres, results in a longer "footprint". This longer footprint reduces the vehicle pressures applied to the ground. The tyre pressure is chosen depending upon vehicle weight, speed, tyre type, and road surface, and can be varied while the vehicle is in motion. Modifications to the vehicle are required to install a CTIS. The system has been used on loaded and unloaded logging trucks, dump trucks, fire engines, transport vehicles, concrete trucks, buses, water trucks, and other heavy vehicles.

With the Constant Reduced Pressure, the operator manually reduces the tyre pressure in each of the vehicle tyres and retains these reduced tyre pressures during the entire hauling operation. No vehicle modifications are required. The minimum tyre pressure depends on gross weight, maximum speed, and tyre size and type. The tyre pressure cannot be changed without stopping the vehicle. With CRP, tyre pressures and, therefore, ground contact pressures are not as low as with CTIS.

Beginning in January 1992, a three-year study was begun by the US Forest Service in western Oregon to investigate the relationship of tyre pressure, aggregate quality and sediment (Foltz 1996), and the development of ruts with traffic (Truebe et al. 1995). This paper will discuss the tyre pressure portion of the study.


Test site

A crowned section of forest road, 2.25 km long by 4.27 m wide, on the Lowell District of the Willamette National Forest, Oregon in the northwestern United States was selected for the test. This area has wet winters with little snow cover at the 470 m elevation of the site. The long-term average precipitation for January to April is 480 mm. This road was chosen to meet the requirements of length, constant grade, and the ability to control non-test traffic. Three 61 m long sections with a similar grade of 12 percent were selected. Each section was surfaced with 100 mm of 76 mm diameter aggregate on top of the existing 300-400 mm thickness of 76 mm minus subgrade. The aggregate placed on the road sections was considered a marginal quality aggregate by the Willamette National Forest because it contained an excessive quantity of fines and the durability of the fines was low (Foltz and Truebe 1995). This aggregate was considered to be typical of lower quality aggregate often used by forests in lieu of higher quality, more expensive aggregate.

Two runoff measurement collectors were located on each test section. One collected a portion of the runoff flowing laterally off both sides of the road crown, while a 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 to determine mass and particle size analysis in the laboratory.

Tyre pressures

Each of the three test sections was dedicated to a single tyre pressure system. One test section received only CTIS pressures. One test section received only CRP pressures, and the third test section received only highway pressures. Pressure changes were made between test sections. CTIS pressures were chosen based on the particular load and speed of the trucks. The CRP pressures were chosen based on the load and the criteria to run on paved roads at 88 km/h indefinitely. The highway pressures were representative of typical truck operations. Table 1 presents the tyre pressures used for this test.

Truck traffic

To simulate a timber harvest, loaded and unloaded CTIS equipped logging trucks were driven on the test loop. Prior to each season's truck traffic, the sections were graded to remove wheel ruts and provide a consistent starting condition. This paper will use the term "load" to mean the combination of one loaded logging truck passing a section and one unloaded logging truck passing in the opposite direction. The 22 450 kg of logs per truck represented 17.4 m3.

Rainfall simulation

At the conclusion of the 1993 test period, rainfall simulation using a CSU-type simulator was performed. A single storm with an intensity of 50 mm/hr and a duration of 30 minutes was applied to each road test section. The purpose of the simulation was to determine mathematical model parameters under more controlled conditions than the natural rainfall events.

Modelling methods

We selected the Water Erosion Prediction Project (WEPP) model to predict erosion for the Oregon and other sites. WEPP is a physically based soil erosion model (Laflen et al. 1991). It continuously models climate, soil water, and plant growth, on a daily time step. For each storm, the model predicts runoff, sediment detachment, sediment deposition, and sediment yield from the bottom of the hillslope. The Hillslope Version predicts the distribution of erosion along a hillslope profile, as well as the sediment yield, and eroded sediment size distribution in the runoff at the bottom of the hillslope.

The results of the 1993 and 1994 field studies were both carefully studied, particularly the data from 1993. The runoff amounts to the gutters along the side of the plots and to the cross drain at the bottom of the plots were evaluated iteratively to determine a combination of hydraulic conductivity and length of road contributing to the respective collector. Estimates of hydraulic conductivity and soil erodibility were available from the rainfall simulation carried out at the end of 1993 test period, and from previous rainfall simulation studies (Elliot et al. 1995). Site observations, videos and photographs made during the rainfall simulation were also consulted to gain insight into the flow path behaviour.

Once initial estimates of the conductivity and path length for a treatment were estimated, the erodibility values were determined iteratively for each plot with results from both natural and simulated rainfall. The values were also compared to observations on previous road studies.

From the results of this analysis, we developed typical input files for the WEPP model describing the flow path lengths, climate, and erodibility. We ran the model for 30 years of climate generated by the WEPP climate generator for a climate near the Oregon test site, for a range of water flow path lengths - 7, 15, and > 30 m - and road grades - 3, 5 and 12 percent - to estimate the benefits of reduced tyre pressures for different topographies. For the 7 m flow path length, the effect of soil properties on erosion was found. For flow path lengths greater than 7 m but less than 30 m, the 7 m results for the CTI treatment were increases proportionately, whereas the flow path lengths were increased to 15 and 30 m for the highway pressure treatment. For results greater than 30 m, the 30 m results can be proportionately increased to any length of road. The length used may be a function of local topography, or of spacing between road cross drains. We then expanded the study to consider another climate in the southeastern United States, as well as climates typical of Brazil and Romania.

Results and discussion

The precipitation, truck traffic, and equivalent timber haul for each year are summarized in Table 2. Precipitation ranged from 41 percent to 109 percent of seasonal average. In 1993, the precipitation of 521 mm included 249 mm of snow. During each of the other years, the snowfall was insignificant. Maximum daily precipitation and maximum 5-minute intensities were remarkably consistent.

Sediment production

Table 3 shows the sediment production from each treatment. These values represent the amount of sediment eroded from the road surfaces. Virtually all of the eroded material, 96 percent to 99 percent, was less than 6 mm diameter. Silts and clays typically comprised 65 percent by weight of the sediment in the runoff.

The sediment production and sediment yield in Table 3 were specific for the Oregon test site. As a first approximation to generalizing these results, the percent reduction of sediment from the CTIS test section compared to the highway pressure test section and the percent reduction of sediment from the CRP test section were determined (Table 4). These values represent the sediment benefit achieved from the use of a particular reduced tyre pressure system.

The observed value for the percent reduction when combining the three test periods for the CTIS section was 80 percent. The values ranged from 87 percent to 70 percent on a seasonal basis. The greatest improvement, 87 percent, occurred in the year with the greatest rainfall, and the least reduction in the driest year.

The percent reduction, when the three test periods were combined for the CRP pressure section, was 45 percent, and ranged from 15 percent to 59 percent. The least improvement, 15 percent, occurred in the season with the least rainfall.

Rut depths

Figure 1 presents a typical pre-traffic and post-traffic cross-section for the 1994 test period. Truck traffic and precipitation on each test section were identical. Reducing tyre pressures showed a marked difference in the development of rutting. Using a peak-to-peak measure of the depth of a rut, the highway tyre pressure section resulted in a 133 mm deep rut compared to a 8 mm deep rut on the CTIS section and a 32 mm deep rut on the CRP section. We believe this degree of rutting was one of the major factors responsible for the differences in sediment production observed on each test section. The development of ruts 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 path length for water to flow from the road crown to the road edge was increased. Generally, the wheel track has reduced infiltration compared to the non-tracked portion and, therefore, more surface runoff. This combination of increased flow path length and increased runoff results in greater erosion, even though no ruts are visible.

When a rut did form, the runoff was prevented from flowing across the road, was confined to the rut, and the concentrated flow caused additional erosion. Concentrated flow in the rut continued until the flow overtopped the rut or a cross drain was encountered. We have observed other sites where the rut channelled water for long distances and bypassed relief culverts or drains.

Both cross-slope flattening and severe wheel rut formation were observed on the highway tyre pressure road section. For the 1994 test season, the CTIS cross-slope change could not be detected. It was initially graded too flat - 2 percent cross-slope - resulting in a long flow path of 13 m both before and after traffic. A better choice of cross-slope would have been at least 4 percent. The CRP section flattened from an initial 5.6 percent to 2.2 percent for a flow path increase from 5.1 m to 10.7 m, over twice the initial path length. The highway test section flow path increased from the initial 4.2 m to the entire length of the section, 61 m, because of the severe rutting.

A second factor contributing to the increased sediment production on the highway pressure treatment was the degree of aggregate crushing. This crushing maintained a ready supply of fine particles available for transport by the runoff. Alternatively the lower tyre pressures did less crushing of the marginal aggregate and did not provide as large a sediment supply.

The initial particle size distribution for each test segment 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 CTIS section generated 46 percent fewer fines than the highway test section, 1 076 kg compared to 1 986 kg. The corresponding value for the CRP section was 24 percent, 1 505 kg compared to 1 986 kg. These values were further evidence that the lower tyre pressures had less impact on the aggregate.

Rut development

Figure 2 shows how the rut depths increased with truck traffic during the 1993 test period. The CTIS section showed little change of depth with traffic. The CRP section deepened only slightly with traffic. Both of these are in contrast to the highway pressure treatment, which developed a deep rut of 89 mm. Half of the increase in rut depth on the highway section occurred after only 15 percent of the total truck traffic.

Figures 1 and 2 reflect the differences in road maintenance requirements among the three tyre pressure treatments. The highway tyre pressure section would have required maintenance two to four times to remove the deep ruts. Both of the two lowered tyre pressure sections would not have required any road maintenance.

WEPP modelling

The WEPP model provided an improved estimate of the effects of lowered tyre pressures on sediment production. The infiltration and erosion parameters determined from the Oregon study are presented in Table 5. Our analysis showed that the differences in erosion on the two treatments were more influenced by flow path length and conductivity than by soil erodibility properties. For the CTIS section, we estimated flow path lengths of 7 m compared to a 30 m flow path on the highway section. The modelling effort showed us that we needed different flow path lengths for the tyre pressure treatments and our field measurements confirmed that we did indeed have those flow path differences.

The three-year average soil losses observed on the site were 177 kg and 893 kg for the CTI and highway treatments, respectively, compared to the 30-year average annual WEPP-predicted losses of 225 kg and 1 181 kg. The WEPP averages were well within the range of observed losses, especially considering that the measured losses were for only the winter months (Table 3).

Road length and steepness modelling

The results of the computer simulation comparing the effects of road length and steepness are presented in Table 6. The field plot length was 61 m long, and the observed reduction in soil loss averaged 80 percent. In the simulation results, the predicted reduction was 81 percent, showing the ability of the WEPP model to compare different topographic scenarios. The greatest sediment benefit of the lowered tyre pressures is to be found at the steeper road grades. In these conditions, other benefits of lowered tyre pressures, such as ability to pull steep, wet grades unassisted, are also more profound (Keller 1992).

One of the major findings of the WEPP calibration aspect of this study was the major role that flow path geometry plays in road erosion. This role has led us to focus one aspect of our research on better understanding flow path relationships and flow path properties. Such an understanding of flow geometries is of great benefit in addressing such issues as insloping versus outsloping roads, in evaluating appropriate spacing of road cross drains, and in better understanding the relationship between road physical properties and the impact of the road on the surrounding environment.

Effect of climate on sediment reduction

A second set of runs was carried out to compare the benefits of CTI pressures for climates from different areas of the world, including the Oregon climate, a climate from the southeastern United States, a climate from Brazil, and a climate from Sinaia, Romania. The topography was assumed to be similar to the research plots, so Table 7 allows the comparison of different climates on the effectiveness of CTI tyre pressures to reduce erosion.

The differences in Table 7 show the effect of climate only. It is likely that the soils chosen for Brazil were more permeable, which would result in less benefit from CTI. A greater percent of the runoff in the Romanian climate was from snowmelt than from any of the other climates, which is generally at a lower rate than from rainfall. This may account for the generally higher predicted runoff amounts but lower sediment reduction benefits. Table 7 demonstrates that CTIS is beneficial in all climates for sediment reduction.

Management opportunities

The results of the three-year study had several management opportunities for forest road management. One was that tyre pressures in logging trucks had an impact on sediment production. Simply reducing the tyre pressure from 620 kPa to 480 kPa resulted in a 45 percent reduction in sediment loss from the road surface. This level of sediment reduction could be achieved without modifications to existing vehicles. The benefit of CRP is that no modifications are needed to the truck.

The use of Central Tyre Inflation System vehicles reduced sediment production by 80 percent. Although there is a cost of equipping vehicles with CTIS, the ability to reduce sediment production could mean the difference between operating in sensitive areas and not operating.

The WEPP model demonstrated the ability to model the effects of lowered tyre pressures on different road grades and different climates. The use of this model, or the Variable Tyre Pressure (VTP) 1.00 programme (USDA 1996), would allow tailored estimates of the sediment reduction attributable to lowered tyre pressures.

A second benefit of lowered tyre pressures was less frequent road maintenance. This reduced maintenance frequency would be an additional cost advantage for both lower tyre pressure systems.

Summary and conclusions

A three-year test of the effect of tyre pressure on sediment production demonstrated that reducing the tyre pressure in logging trucks results in less sediment from the road surface. Depending upon the amount of tyre pressure reduction and the road grade, up to 80 percent reduction in sediment could be expected. Both the field results and the physical process based model indicated that steeper slopes and longer flow paths would experience the largest sediment reduction.

The mechanism by which the reduction in sediment occurs is the prevention of surface rutting and concentrated flow. Since ruts and concentrated flow are not site specific, the existence of the benefits of lowered tyre pressures is not site specific. Only the magnitude of the benefits is site specific. The WEPP model was able to predict the observed sediment reductions and we feel confident that it can be used to closely estimate the magnitude at other locations world-wide.


The authors would like to acknowledge the financial and technical support and the loan of a logging truck from the USDA Forest Service San Dimas Technology and Development Center. The assistance of the Willamette National Forest notably Larry Tennis of the Lowell Ranger District and Mark Truebe and Gary Evans of the supervisor's office was appreciated. A special acknowledgement to Ben Kopyscianski of the Intermountain Research Station who led the individuals who collected samples and maintained equipment in the cold and rain of three Oregon winters.


Elliot, W.J., Foltz, R.B. & Luce, C.H. 1995. Validation of Water Erosion Prediction Project (WEPP) model for low-volume Forest Roads. Proceedings of the Sixth International Conference on Low-Volume Roads, Vol. 1, pp. 178-186. Transportation Research Board, National Research Council, Washington, DC.

Foltz, R B. 1996. Traffic and no-traffic on an aggregate surfaced road: sediment production differences. Presented at the IUFRO Seminar on Environmentally Sound Forest Roads and Wood Transport, Sinaia, Romania.

Foltz, R.B. & Burroughs, E.R., Jr. 1990. Sediment production from forest roads with wheel ruts. Proceedings from Watershed Planning and Analysis in Action. American Society of Civil Engineers, Durango, CO.

Foltz, R.B. & Burroughs, E.R., Jr. 1991. A test of normal tyre pressure and reduced tyre pressure on forest roads: Sedimentation effects. Proceedings from Forestry and Environment...Engineering Solutions, Forest Engineering Group, American Society of Agricultural Engineers. New Orleans, LA.

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

Keller, R.R. 1992. The results of operational testing of central tyre inflation systems proves the benefits of low tyre pressure in logging operations. Proceedings Eighth Pacific Northwest Skyline Symposium, pp. 196-205.

Laflen, J.M., Lane, L.J. & Foster, G.R. 1991. WEPP - A new generation of erosion prediction technology. Journal of Soil and Water Conservation 46(1): 34-38.

Truebe, M.A., Evans, G.L. & 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. 1996. Variable Tyre Pressure (VTP) 1.00 computer software. U.S. Forest Service, San Dimas Technology and Development Center, San Dimas, CA.

Table 1. Tyre pressure treatments used for the Lowell, Oregon, test

Tyre pressure system

Unloaded Truck

Loaded Truck

Steering axle

Other axles

Steering axle

Other Axles

Central Tyre Inflation System





Constant Reduced Pressure





Highway Tyre Pressure





Table 2. Precipitation and traffic summary for the Lowell, Oregon, test



Maximum Precipitation



Seasonal Average


5-min. Intensity


Timber Haul






















Table 3. Sediment production from 61 m long road sections - Lowell, Oregon

Tyre Pressure

Mass Sediment (kg)





Central Tyre Inflation System





Constant Reduced Pressure





Highway Tyre Pressure





Table 4. Sediment reduction due to the use of lowered tyre pressures - Lowell, Oregon

Tyre Pressure

Sediment Reduction (%)





Central Tyre Inflation System





Constant Reduced Pressure





Table 5. WEPP erodibility values for two tyre pressures on a road with marginal quality aggregate.



Flow Path Length


Rill Critical Shear




(kg m/s4)







3 000 000

3.5x 10-4






3 000 000

3.5x 10-4


Table 6. Percent reduction in sediment by employing CTIS tyre pressures rather than highway tyre pressures


Flow Path Length



> 30













Table 7. Rainfall, runoff, and percent reduction in predicted sediment loss for a road segment with a length of 61 m and a uniform gradient of 12%


Average Rainfall


Sediment Reduction




Corvallis, OR, USA





Collowee, N.C., USA





Campinas, Brazil





Sinaia, Romania





Figure 1. Road cross-sections from sections with (a) CTIS tyre pressures, (9b) CRP tyre pressures for 1994 test season. Before traffic was after grading and before logging truck haul. After traffic was after 20 600 m3 (1205 loads)

Figure 2. Rut depth development with logging truck traffic for 1993 test season. Timber haul was 10 500 m3 from 616 loads

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