Effect of Skid Trail Compaction on a Volcanic Soil in Central Oregon1

ROBERT F. AixBROOK2

ABSTRACTA skid trail in the Qchoco National Forest, Oregon, was examined

to assess the degree of compaction that had occurred as a result oftraffic during log harvesting. The soil, classified as a Lithic Cryor-thent is a sandy loam derived from rhyolitic and tuffaceous collu-vium. Measurements of bulk density and shear strength were madeat five positions across the trail and two locations in undisturbedsoil. Cores were taken to measure bulk density, moisture content,and moisture retention. Compaction resulted in increases in bulkdensity of up to 25%, in shear vane strength up to 157%, and incone index up to 220% along with a decrease in pore space >10 Mmof 69%. Bulk density, vane shear, and penetrometer measurementswere found to be highly correlated.

Additional Index Words: bulk density, soil strength, forest soils,entisols, porosity, andepts, harvesting.

1 Contribution from the Earth Science Dep., Univ. of Waikato,Private Bag, Hamilton, New Zealand. Received 21 Nov. 1985.

- Senior Lecturer, Earth Sciences, Univ. of Waikato, Hamilton,New Zealand.

Allbrook, R. F. 1986. Effect of skid trail compaction on a volcanicsoil in central Oregon. Soil Soc. Sci. Am. J. 50:1344-1346.

THE EXTRACTION OF TIMBER from fofCStS in thePacific Northwest of the USA sometimes in-

volves the use of ground-based skidders. These ma-chines, with their loads of 3 to 7 Mg, exert high groundpressures. Mathematical models to determine staticground pressures were developed by Lysne and Bur-ditt (1983). These pressures compact the soil, increas-ing the bulk density by 10 to 17% (Froehlich andMcNabb, 1983; McKyes et al, 1979). Dickerson (1976)found that an increase in bulk density due to com-paction resulted in a decrease in pore space. Byrd andCassel (1980) showed that compaction increased soilstrength as measured by a cone penetrometer.

The effect on tree root growth resulting from skid

ALLBROOK: SKID TRAIL COMPACTION ON VOLCANIC SOIL 1345

Table 1. Bulk density as measured by nuclear densimeter.

Position

Table 2. Bulk density as measured by using undisturbed cores.

Position

Depth

cm5

10152030

It

0.08 (0.06)t0.96 (0.04)1.06 (0.06)1.05 (0.06)1.11 (0.04)

2T

————— Mg1.01 (0.04)1.14 (0.04)1.15 (0.02)1.16 (0.02)1.11 (0.04)

3t

0.80 (0.07)0.93 (0.03)0.96 (0.04)1.04 (0.05)1.06 (0.09)

Undisturbed

0.82 (0.02)0.99 (0.01)0.92 (0.01)0.93 (0.04)0.93 (0.03)

11 = edge of trail, 2 = rut, 3 = center of trail.t Standard error in parenthesis.

trail compaction was reported by Froehlich andMcNabb (1983) and the effect on forest productivityby Wert and Thomas (1981).

This paper reports the results from work carried outon a volcanic soil in the Ochoco National Forest,Crook County, Oregon.

MATERIALS AND METHODSThe site chosen for the study has a 10° slope with a north-

erly aspect. The vegetation consists of a mixed conifer forestdominated by ponderosa pine (Pinus ponderosa Dough). Theclimate is characterized by abundant sunshine, high evap-oration, and wide daily, monthly, and yearly temperatureextremes. Annual precipitation varies from 400 to 2000 mmdepending on altitude (Paulson, 1977).

The parent material of the soil is a colluvial mixture ofrhyolitic and tuffaceous sediment. The area was mapped asan Entic Cryandept (Paulson, 1977); however, no ash wasfound at the site, which would suggest Lithic Cryorthent tobe more suitable (Soil Survey Staff, 1975; Leamy, 1984).

Three transects across a 4-m-wide skid trail were exam-ined. Three positions were characterized, (i) on the edge ofthe trail that was only slightly compacted; (ii) where the rutswere deepest; and (iii) in the center of the trail that hadmainly been affected by having logs dragged over it. Undis-turbed sites were sampled approximately 5 m from the trail.

Bulk density was measured in the field, using a doubleprobe, nuclear densimeter (Campbell Pacific Nuclear stra-togauge),3 at 5.1, 10.0, 15.2, and 30.5 cm from the surface.A vane shear device was used to measure soil strength atthe same depths as for bulk density. A recording penetro-meter fitted with a 13.2-mm-diam cone was also used tomeasure soil strength. Using an impact sampler, relativelyundisturbed samples 60 by 50 mm in diameter were takenclose to where the densimeter measurements were made andsealed in plastic bags to maintain them at the field moisturelevel.

In the laboratory the undisturbed samples were used tomeasure bulk density and moisture content, and this valuewas used to convert to dry bulk density both the core andnuclear densimeter values. The core samples were also usedto estimate the volume of pores < 10 /urn from the moistureretained when the soil was equilibrated on a pressure plateat 0.03 MPa. Depths were measured from the soil surfaceand not from a fixed datum line. Thus, depths in Position2 are about 10 cm deeper as compared with the undisturbedsoil surface and positions 1 and 3.

RESULTS AND DISCUSSIONValues presented are the means of the three tran-

sects. Thus, six sets of data were obtained from the

Depth

cm0-68-14

16-22Sample no.

It

1.051.261.26

2

2t

1.111.311.21

2

3t

0.921.091.07

1

Undisturbed

0.960.990.98

2

11 = edge of trail, 2 = rut, 3 = center of trail.

Table 3. Cone index as measured by penetrometer.

Position

Depth

510152030

It

850 (200)J1680 (322)2230 (310)2450 (339)1990 (371)

2t 3t

O.J. C*

1750 (294) 660 (202)2630 (282) 1000 (150)2580 (237) 930 (219)2130 (367) 1130 (231)1580 (237) 1400 (583)

Undisturbed

720 (122)980 (400)820 (143)760 (131)860 (167)

' Mention of trade names does not constitute endorsement byWaikato Univ. to the exclusion of other products that may be suit-able.

11 = edge of trail, 2 = rut, 3 = center of trail.t Standard error in parentheses.

undisturbed soil and positions 1 and 2 and three fromposition 3.

Table 1 shows the results for dry bulk density usingthe nuclear densimeter. The lower bulk density at 5cm was due to organic matter and possible additionof Mazama ash. Position 3 does not differ significantlyfrom the undisturbed site while Position 2 shows thehighest densities. Percentage increase in bulk densityof Position 2 over the undisturbed is about 22%.

Table 2 shows the results of bulk density deter-mined on the core samples. As would be expected thesefigures correlate well with those in Table 1, as shownin Eq.[l]ph (densimeter) - 0.97 (pb cores) — 0.05

R2 = 0.94 [1]Percentage increase in bulk density of the com-

pacted soil, Position 2, over the undisturbed soil usingcore data is 23%.

Table 3 gives the results for the penetrometer ex-pressed as cone index values. There was a significantincrease in strength, over the undisturbed site, foundin all positions. Percentage increases found over theundisturbed site were 157% in Position 2, 125% inPosition 1, and 25% in Position 3. Since strength var-ies with moisture content it should be noted that themoisture content was farily constant at 38.6% w/w(SE, 1.12).

Table 4 gives the results for shear strength estimatesusing the vane shear. In positions 2 and 1, a maximumshear strength was found at about 15 cm below thesurface. There was a significant increase in strength,over the undisturbed site, found in all positions. Per-centage increases found over the undisturbed site were125% in Position 2, 115% in Position 1, and 43% inPosition 3. The pattern parallels the values for coneindex. Correlation between the two sets of data werehighly significant as shown in Eq.[2]Vane shear (kPa) = 9.4 + 0.031 (Cone index, kPa)

R2 = 0.90 [2]

1346 SOIL SCI. SOC. AM. J., VOL. 50, 1986

Table 4. Shear strength as measured by vane shear.

Position

Table 5. Pore size distribution.

Depth

510152030

IT

36 (7.27)t61 (12.13)86 (12.58)87 (11.34)74 (10.90)

2t

43 (6.85)85 (6.84)90 (5.64)77 (13.53)63 (8.67)

3T&.r a.

30 (6.44)52 (11.07)40 (5.96)52 (15.75)58 (18.00)

Undisturbed

27 (3.43)39 (6.83)35 (7.38)25 (5.36)33 (7.81)

11 = edge of trail, 2 = rut, 3 = center of trail.t Standard error in parenthesis.

There was found to be a good correlation betweenvane shear strength and penetrometer resistance andbulk density as determined by densimeter Eq.[3], [4].

0.76 + 0.0043 (vane shear, kPa) R2 = 0.71 [3]Pbph = 0.80 + 0.00014 (Cone index, kPa)

R2 = 0.67 [4]Since the range of bulk densities is much smaller thanthe range for soil strength measurements the latter aremore sensitive for estimating compaction.

Table 5 shows the results for the pore size distri-bution. Total percent porosity was calculated from thebulk density, Table 2, and the particle density, 2.52Mg m~3. Volume of pores <10 nm from moistureretention and volumes of pores >10 /on were foundby difference. The most compacted soils, Position 2,show a 16% increase in <10-Atm pore space over theundisturbed site and a 47% decrease >10-jum porespace. There is a 15% loss in total percent porosity.

Equations [5] and [6] show there is a strong corre-lation between pore space > 10 nm (P) and soil strengthmeasured by either vane shear or penetrometer.

Vane shear (kPa) = 114 - 2.7 PR2 = 0.76 [5]Cone index (kPa) = 3483 - 91.6 PR2 = 0.81 [6]

The increase in strength is therefore associated withthe reduction in >10-jum pore space. The reductionfound here is less than that reported by Dickerson(1976) for a decrease in >50-jum pores, showing thatthe larger the pores the more readily they are dimin-ished by compaction.

CONCLUSIONCompaction of the soil by ground-based skidders has been

found to occur in soils having a low initial bulk density.Depth of ruts is a good indication of compaction. Increasesin bulk density of up to 25%, of vane shear strength up to157%, and of penetrometer resistance up to 220% were re-corded. These values were paralleled by a decrease in ma-croporosity.

Position

It

2T

3t

Undisturbed

Depth

cm0-68-14

16-22Mean0-68-14

16-22Mean0-68-14

16-22Mean0-68-14

16-22Mean

<10 iaa

37353636363834363732293330293431

>10 ion—— % v/v ——

22151417

2010181627252927

32322730

Total

59505053564852526457586062616161

11 = edge of trail, 2 = rut, 3 = center of trail.

Maximum effects of compaction are found about 15 to 20cm below the surface as was reported by Soane (1973).

ACKNOWLEDGMENTI would like to thank Professor B.P. Warkentin for making

available the facilities in the Soils Dep., Oregon State Univ.during my study leave. I also acknowledge the help fromDr. H.A. Froehlich and Mr. D.W.R. Miles of the Dep. ofForest Engineering, Oregon State Univ. both in the planningof the work and in carrying out field determinations.