Off-road Motorcycle Effects on a Desert Soil

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Abstract
The effects of controlled motorcycle traffic on a Mojave Desert soil in California were studied in order to describe soil compaction quantitatively. Four experimental trails, representing 1, 10, 100, and 200, passes each with an off-road motorcycle were established in a loamy sand at 6.2% by weight moisture content. Penetration resistance, density, infiltration rate, and response to rainfall, were measured for the undisturbed soil and for the experimental trails immediately after the conclusion of the impact, and soil cores were measured in the laboratory to determine pore-size distributions. The soil density was remeasured again one year after the impact, to determine the amount of recovery.Immediately after motorcycle use, the average density of the soil in the 0 to 60 mm depth increased from an average of 1.53 metric tons/cubic metre in the undisturbed soil to 1.77 t/m3in the 200 pass trail, and the average density in all the trails could be expressed empirically as a logarithmic function of the number of motorcycle passes. The soil penetration resistance profile for the 200-passes-trail showed that the maximum compaction occurred between 30 and 60 mm depth, and that changes in penetration resistance could be measured to a depth of 0.21 to 0.25 m. The infiltration rate after 2 hours decreased from an average of 98 mm/hr in the undisturbed soil to 29 mm/hr in the 200 pass trail.The undisturbed soil infiltrated artificial rainfall up to an intensity of 46 mm/hr before surface ponding occurred in the 100- and 200-passes-trails at intensities greater than 25 mm/hr. Decreases in the soil-pore volume were greatest in pores with effective radii greater than 150 μm, with incremental decreases occurring in pores with radii between 150 and 4.5 μm; the total volumes of smaller pores were similar in the compacted and undisturbed soils. One year after the impact, the density of the soil at a depth of 0–30 mm had decreased significantly in the 10-, 100-, and 200-passes-trails but the densities at 30–60 mm depth remained unchanged.