United States 

Department of 

Agriculture Soil Density and Moisture Content on Two Unused Forest I 

Forest Service 

Northeastern Forest 

Experiment Station Roads During First 30 Months 

Research Paper NE-629 After Construction 

J. D. Helvey 

J. N. Kochenderfer 
Abstract 

Many foresters and loggers believe that trafficability, or resistance to rutting, 

improves with time after logging roads are built. However, no one has attempted to 

quantify the changes or identify the processes involved. In this study, dry density 

and moisture content of the surface foot of soil were measured at 315 locations on 

two roads in the central Appalachians over a 30-month period. Density increased 

slightly during the measurement period at most locations. ~lmost all of the density 

changes occurred during the first few months after construction. Moisture content 

decreased during the first few months after construction, then fluctuated with 

precipitation amounts. 

Authors 

J. D. HELVEY is a research hydrologist, USDA Forest Service, Timber and 

Watershed Laboratory, Parsons, WV 26287. 

J. N. KOCHENDERFER is a research forester, USDA Forest Service, Timber and 

Watershed Laboratory, Parsons, WV 26287. 

Manuscript received for publication 11 July 1989 

Northeastern Forest Experiment Station 

100 Matsonford Road, Radnor, PA 19087 

January 1990 
Forest access roads are required for most forest 

management programs because forest products cannot be 

removed economically without roads. If all-season access is 

needed, gravel is used to increase traction and stabilize 

road surfaces. Since road surfacing with gravel is a major 

cost item ($10,000 per mile in the central Appalachians), a 

practice that reduces gravel requirements will provide 

considerable savings. The most obvious way to reduce 

gravel requirements is to allow traffic only when the road is 

dry. Also, we have observed that roads constructed and not 

used for several weeks are more resistant to rutting by 

logging trucks than newly constructed roads. This increased 

trafficability was confirmed by loggers and other users of 

low-standard forest access roads. 

Trafficability is the capacity of a soil to withstand traffic of 

wheeled and tracked vehicles. Silversides and Koroleff 

(1949) point out that trafficability depends on soil with 

sufficient bearing capacity to support vehicles and enough 

traction capacity to provide the necessary forward thrust. 

The bearing capacity of the soil is a function of the shearing 

strength and is attributed to friction and cohesion between 

soil particles. Shear strength is increased when soil density 

is increased because of increased friction between soil 

particles. Cohesion of soil is a function of moisture content 

and it also may increase as fine soil particles fill voids 

between larger particles. 

We hypothesized that the soil in a road unused for a few 

eeeks after construction is drier, or denser, or both, than 

when it was first built. To test this hypothesis, we built two 

roads in 1982 near the Fernow Experimental Forest and 

allowed no vehicular traffic for the next 2% years. Soil 

density and soil moisture measurements were made 

periodically during this period. The purpose of this paper is 

to report the results of those measurements. 

Study Area 

The study was located in the central Appalachians where 

annual precipitation averages 58 inches, evenly distributed 

throughout the year. The two roads-Canoe and Haddixwere built in contrasting geologic formations and soil types. 

The Canoe road is in the Calvin channery silt loam type of 

the Hampshire formation, whereas the Haddix road is in the 

Berks channery silt loam type of the Chemung formation. 

Soil depth on both areas is 5 feet or less. The percentage of 

coarse fragments' is highly variable; cove areas have 

deeper soils and lower coarse fragment content. Under 

similar moisture regimes, the potential for rutting is 

inversely correlated with the coarse fragment content of the 

soil. 

Study Methods 

The minimum standard roads used in this study were 

described in detail by Kochenderfer and others (1984). The 

roads were constructed from a flagged centerline. There 

was no formal road design or construction staking. All rightof-way clearing was done with a bulldozer in conjunction 

with road building. Standing trees were pushed or pulled 

over and pushed away from the roadbed with the bulldozer. 

Cut banks were left vertical if bank height was less than 5 

feet, otherwise they were rough sloped with a bulldozer. 

Water from all streams and live seeps is conveyed under 

the road through culverts. Finally, broad-based dips spaced 

at intervals of about 150 feet were used to control surface 

runoff from precipitation. Figure 1 shows a section of a road 

immediately after construction. 

Figure 1 .-A minumum-standard road section with a broadbased dip. 

'Coarse fragments are mineral or rock particles ranging from 2 mm to 25 cm in cross section. 
Figure 2.-A minimum-standard road section with a high percentage of coarse 

fragments. Note the density sampling sites in the upper half of the photograph. 

The numbers refer to the measurement points on a transect. 

Each road was divided into segments at least 200 feet long. 

Within each segment, nine transects were established 

across the road. Each transect (Fig. 2) contained five 

measurement points: (1) the inside portion next to the cut 

bank, (2) the inside track, (3) between the tracks, (4) the 

outside track, and (5) the fill. Only road sections with low 

rock content were selected for study because these are the 

sections most subject to rutting. The Haddix road had four 

sections and the Canoe road had three. 

A Troxler Model 341 1-B Nuclear Surface Moisture Density 

Gauge2 was used to measure dry density and moisture 

content at the 4-, 8-, and 12-inch depths. The procedure at 

each measurement point was as follows: (1) a metal rod 

0.75 inch in diameter was pounded into the roadbed and 

*The use of trade, firm, or corporation names in this 

publication is for the information and convenience of the 

reader. Such use does not constitute an official 

endorsement or approval by the US. Department of 

Agriculture of any product or service to the exclusion of 

then extracted, (2) the soil surface was leveled to avoid air 

spaces at the soil instrument interface, (3) a I-minute 

reading was made at each depth as defined earlier, and dry 

density and moisture content were recorded, and (4) after 

the measurements were made, a numbered Cinch square 

piece of plywood was placed over the hole. Repeat 

measurements at each subsequent date were made at the 

same locations. The plywood cover prevented water and 

transported soil from getting into the hole. 

A complete set of measurements on the two roads (945 

readings) required about 5 days to complete. We used 

weather forecasts to avoid periods of unsettled weather so 

that measurements were not interrupted by rain storms. The 

first set of measurements was started on August 26, 1982. 

others that may be suitable. 
Subsequent sets began on November 8, 1982, May 9, 1983, 

August 1, 1983, October 31, 1983, May 16, 1984, July 30, 

1984, and March 6, 1985. To test for compaction by loaded 

log trucks, additional measurements were made at one road 

section on April 14, 1989. In addition to a few weekly trips 

by pickup trucks between 1985 and April 1989, about 

500,000 feet of logs were trucked over the roads between 

1986 and 1987. 

Data Analysis 

Dry bulk density and moisture content were each averaged 

by road, by sampling positions across the roads, and by 

sampling depth. Average values at each depth and 

measurement position were tabulated and tested by 

regression for time trends. A regression model of the form 

,. 

Y = a + bLn(x) (1) 

was fitted to the data. In this model, 9 is predicted dry 

density or moisture content, a is the intercept, Ln is the 

natural logarithm, x is the number of days after road 

construction, and b is the slope. The logarithmic model was 

chosen because we expected density changes to be most 

rapid during the first few weeks after road construction and 

more gradual later. 

The 1989 measurements were averaged by road position. 

The effects of traffic on dry density were estimated by 

Table 1 .-Average dry density and moisture content to a 

12-inch depth by sampling positions on the two logging 

roads 

comparing density at each sampling depth between the 

tracks with density of the same depths in the track 

positions. Presumably, density between the tracks would 

not be affected by truck traffic. 

Results and Discussion 

Average dry density and moisture content to a 12-inch 

depth for the two roads were: 

Road Dry density Moisture content 

(Pounds/ft3) (Percent by volume) 

Haddix 102.56 19.16 

Canoe 1 12.90 13.26 

The roadbed of Canoe road averaged about 10 percent 

denser and 30 percent drier than the roadbed of Haddix 

road. Average density and moisture content across the road 

were as expected (Table 1). Lowest density was on the fill 

slopes, and density increased uniformly toward the cut 

bank. Although the values for the two roads were different 

from each other, relative values across the road were the 

same. Thus, for both roads, density at the cut banks 

averaged about 12 pounds per cubic foot greater than at the 

fills. Moisture content increased slightly from the cut bank to 

the fill. Table 2 shows that dry density increased slightly and 

moisture content decreased slightly with depth in both 

roads. 

Table 2.-Average dry density and moisture content by 

sampling depths on the two logging roads 

Dry density Moisture content Dry density Moisture content 

Sampling Haddix Canoe Haddix Canoe 

position 

Pounds per ft3 Percent by volume 

Cut bank 107.5 119.2 18.8 12.4 

Inside track 104.7 117.6 18.4 11.9 

Between tracks 102.8 1 1 1.4 19.2 13.4 

Outside track 102.0 108.8 19.1 14.6 

Fill 95.8 107.6 20.3 13.9 

Sampling Haddix Canoe Haddix Canoe 

depths 

(inches) 

Pounds per ft3 Percent by volume 

100.7 110.6 19.4 13.4 

102.9 113.6 19.2 13.2 

104.0 114.5 18.9 13.1 
The regression analysis indicated a slight increase in soil 

density over time at most sampling locations and depths 

(Table 3). An exception was the cut bank position of the 

Canoe road where density decreased over time. This may 

have been caused by soil swelling after the overburden was 

removed. 

Figure 3 is a plot of Canoe road density over time by road 

position at the Cinch and 8-inch depths. The 12-inch depth 

followed the same trend as the &inch depth. As stated 

earlier, the sampling sites nearest the cut bank became less 

dense over time, possibly because the removal of the 

overburden allowed these soils to expand slightly. Density 

of the other sites increased slightly, but the amounts were 

small. Most of the change occurred during the first winter 

after road construction. Subsequent changes were very 

small and probably related to weather. Density changes on 

the Haddix road were inconsistent as shown by the R2 

values (Table 3). Only the track sampling positions showed 

consistent density changes over time. The lack of a 

significant density change at the fill position was surprising, 

and we have no reasonable explanation for it. 

Moisture content of both roads decreased sharply soon after 

construction, but it increased at the end of the study in 

response to rainfall. Since all sampling sites across the 

roads showed similar trends, only the average values are 

presented (Fig. 4). 

Logging traffic had only minor effects on road density at the 

section measured in 1989. Density at the 4- and &i'nch 

depths averaged only 2 percent greater than that between 

the tracks. The 12-inch depth was unaffected by traffic. 

Table 3.-Regression coefficients for predicting dry soil 

density over time, after road construction 

Road Depth Haddix Canoe 

position road road 

Cut bank 

Inside 

track 

Between 

track 

Outside 

track 

Fill 

Inches 

4 

8 

12 

4 

8 

12 

4 

8 

12 

4 

8 

12 

4 

8 

12 
8-INCH DEPTH 

130 I 

Cut bank 0-0 Insidetack A-A 

Between tracks +-+ Outside Fill tracks A-A 

0-0 

w 4-INCH DEPTH 

0 120 

Figure 3.-Average road density of the Canoe road over time at the 4- and 8-inch 

deposits. 

A 

'A 

12 

0 

I 

w A-A Canoe Road 

Figure 4.-Average moisture contents of the two roads over time. 
Conclusion Literature Cited 

The results of this study show a small, but consistent 

pattern of increasing soil density over time at most sampling 

locations. Some possible causes for the density changes 

are as follows. When soil is exposed in a roadbed, it 

becomes subject to the bombardment of raindrops. Hewlett 

and Nutter (1969) stated that 1 inch of rain, falling at 

terminal velocity, generates 50.5 foot-pounds of energy on 

each square foot of surface area. In the study area, where 

annual rainfall averages 58 inches, the average annual 

energy would be more than 2,900 foot-pounds per square 

foot. This energy compacts the surface layer of soil and 

overland flow carries the fine soil particles away. The result 

is a slight increase in soil density over time, as shown by 

our measurements. Also, average moisture content of the 

soil decreases slightly over time because infiltration is 

reduced. 

Although the combination of increased density and 

decreased moisture content of the roadbed would logically 

improve trafficability, we did not attempt to quantify the 

degree of improvement. Several tests, including shear 

strength, resistance to compression and penetration, 

bearing capacity, and so on, have been developed for rating 

soils. However, to our knowledge, these tests have not been 

correlated specifically with trafficability. Such correlations, 

along with measurements of bulk density and moisture 

content, would be very useful and should be developed as 

suggested by Silversides and Koroleff (1 949). 

Hewlett, John D.; Nutter, Wade L. 1969. An outline of 

forest hydrology. Athens, GA: University of Georgia 

Press. 137 p. 

Kochenderfer, J. N.; Wendel, G. W.; Smith, H. Clay. 1984. 

Cost of and soil loss on "minimum-standard" forest 

truck roads constructed in the Central Appalachians. 

USDA For. Serv. Res, Pap. NE-544. 8 p. 

Silversides, C. R.; Koroleff, A. 1949. Construction and 

maintenance of forest truck roads. Woodlands 

Research Index No. 60 (B-8b), Montreal, Canada: The 

Pulp and Paper Research Institute of Canada. 137 p. 
Helvey, J. D.; Kochenderfer, J. N. 1990. Soil density and moisture 

content on two unused forest roads during first 30 months after 

construction. Res. Pap. NE-629. Radnor, PA: US. Department of 

Agriculture, Forest Service, Northeastern Forest Experiment Station. 6 p. 

Reports results of soil density and soil moisture measurements on two 

roads in the central Appalachians over a 30-month period. Density 

increased slightly during the measurement period at most locations. Almost 

all of the density changes occurred during the first few months after 

construction. Moisture content decreased during the first few months after 

construction, then fluctuated with precipitation amounts. 

ODC 116.5:383.4:114.11 

Keywords: Forest roads, soil density, soil moisture, central Appalachians 

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