Soil--The Stuff Under Foot, Hoof, and Wheel
Most backcountry trails are constructed on native soils. A review of some basic concepts of soil and its engineering characteristics will help explain why trails degrade, and why soils and physical site conditions are important components in trail management.
Soil is unconsolidated material on the Earth's surface. It is composed of mineral and organic particles, the voids surrounding the particles, and the water and air within the voids. The composition of the particles and their relationship to the voids strongly affect the physical characteristics of soil. That composition, called soil structure, describes the character of soil aggregates. These aggregates of individual soil grains form unique shapes depending on the soil's origin and the surrounding environmental conditions. The shapes of aggregates include granules, plates, prisms, columns, and blocks. The voids between the aggregates form passageways for air and gas exchange, as well as for water movement within the soil body.
The character of soil varies from place to place across the landscape. A soil's mineral and organic content, structure, moisture content, depth to bedrock, ability to support vegetation, and other characteristics vary, depending on the soil's origin and the environment in which the soil is located. As a result, individual soil types cover the Earth's surface like a mosaic. Five soil-forming factors control the character of a soil at any given location (U.S. Department of Agriculture 1993):
Parent Material is the material in which a soil develops. Examples of mineral-based parent materials include alluvial deposits, weathered bedrock, glacial remnants, wind deposits, and marine sediments. Organic parent materials include leaf litter and decomposing wetland vegetation. The parent material influences the texture of the soil--the relative amounts of sand, silt, clay, and organic material that make up the finer components of the soil and the percentage of boulders, cobbles, and gravel that make up the larger components.
Topography is where the soil is located on the landscape. It includes the elements of slope, aspect, elevation, and landscape position of the soil. Slope strongly influences the risk of erosion, aspect (relative sun exposure) influences daily temperature variations, and elevation influences climatic environment. Landscape position describes the soil's location on the landscape--such as: ridgeline, alluvial terrace, highly dissected upland, foot slope, floodplain, and high terrace.
Time is the period parent materials have been subject to weathering and soil-forming processes. In general, the older the soil, the greater the chemical and physical modification of the original parent material and the more developed a soil's internal structure. Through time, soils develop distinct layers. Soil scientists generally recognize four layers: 'O' for surface organic layers, 'A' for the organic-rich surface mineral layers, 'B' for the weather-altered subsoil, and 'C' for the unaffected parent material.
Climate indicates the effect of local weather on soil development. Climate influences chemical weathering, soil temperature, and soil moisture levels. Within a localized area, topography moderates climate to some degree.
Organisms are the plants, animals, and humans that affect soil development. This includes effects from vegetation growth, leaf litter accumulation, soil microorganisms, bur-rowing animals, and human agriculture, recreation, and construction. Organisms can dramatically affect a soil's development. Vegetation enriches soils by contributing organic material and aiding in the development of internal soil structure. Unfortunately, many human activities have a disruptive effect on soil development.
Starting with raw parent material, topography, time, climate, and organisms work together to weather, mix, and transport soil. Soil is continuously evolving and modifying its capacity to support plant, animal, and human use over time.
Soil's Characteristics as a Structural Component for Trails
Unlike bedrock, asphalt or concrete, soil is an unconsolidated material composed of loosely bonded particles and the voids surrounding them. The lack of solid bonds between particles means that soils are susceptible to impacts from trail use in a number of ways. These include crushing, lateral displacement, and erosion. A soil's suitability as a structural component for trails is controlled by two factors, its bearing strength (its ability to support a load without being deformed) and its cohesion (the ability to resist displacement). Those suitabilities are primarily controlled by two related factors: the relative size of soil particles (soil texture) and the relative water content of the soil voids (soil moisture level).
Soil texture is the relative amount of organic matter, gravel, sand, silt, and clay in a soil. In general, soil texture can be broken into two major classes:
Finely textured soils--those with high percentages of organic matter, silt, and clay
Coarsely textured soils--those with high percentages of sand and gravel
In general, the coarsely textured soils have good bearing capacity. This is because of their large particle size, good drainage characteristics, and low shrink-swell potential. Conversely, finely textured soils generally have poor bearing capacity because of their small particle size, poor drainage characteristics, and a tendency to shrink or swell under different moisture conditions. Both classes of soils have moderate to poor cohesion, depending on other factors such as vegetation cover and roots that help hold individual soil particles in place.
Soil moisture level measures the relative amount of water in soil pores. A soil's texture controls the percentage of pores within a soil. Surprisingly, finely textured soils have more pore space than coarsely textured soils. Finely textured soils can have up to 60 percent void space, while coarsely textured soils typically have around 40 percent.
Soil moisture can range from bone dry to totally saturated. Because water acts as a lubricant between soil particles, the relative amount of water within a soil can dramatically affect its structural stability. While coarsely textured soils tend to have good bearing strength across a wide range of moisture conditions, finely textured soils have reduced bearing capacity as moisture levels increase. At saturation, when all soil voids are filled with water, finely textured soils typically have little bearing capacity. Finely textured soils store and retain water over long periods so their bearing capacity can be low for prolonged periods.
Besides soil texture and soil moisture, other environmental and site factors contribute to a soil's structural capability and suitability for trails. These include:
- Soil temperature
- Depth to bedrock
- Type of surface cover
- Root mass
- Landscape position
These factors largely control how well a soil will support surface traffic. These characteristics also provide insights on how soil should be managed and on the options that might be employed to increase its suitability for use. Table 1 provides some general guidelines on broad categories of trail suitability based on these factors. The table segregates site characteristics into three classes of suitability for each soil factor: poorly suited (highly sensitive), limited suitability (moderately sensitive), and generally suitable (slightly sensitive).
The information in table 1 can help trail managers identify where they may have problems with existing or planned trail routes. For example, sites with all 'generally suitable' ratings shouldn't pose any inordinate management or environmental concerns; those with 'limited suitability' ratings may require some special attention; and those with 'poorly suited' ratings may require significant attention and a high level of management. Poorly suited sites should be avoided during new trail construction. Existing trails with "poorly suited" ratings should be assessed for environmental impacts and evaluated for relocation.
Table 1--General guidelines on trail site suitability and sensitivity to impact.
|Soil factor||SUITABILITY/SENSITIVITY CLASS|
|Soil texture||All organic soils; soils with an organic surface layer thicker than 4 inches||Silt greater than 70 percent or clay greater than 40 percent in the soil surface layer; sand component is greater than 80 percent in the surface layer||Soils with a high percentage of gravel or rock in the surface layer|
|Soil temperature||Ice-rich permafrost is within 40 inches of the surface; soils at or near freezing||Low ice permafrost within 40 inches of the surface||Deeply frozen soils (winter activities)|
|Soil moisture||Poorly or very poorly drained soils; the water table is within 12 inches of the surface; water is ponded at the surface; soils are at or near saturation||Somewhat poorly drained soils; the water table is between 12 and 24 inches of the surface||Well- and moderately well-drained soils; the water table is deeper than 24 inches below the surface|
|Type of surface cover||All wetland vegetation communities; permafrost-influenced vegetation communities; alpine tundra communities|
|Root mass||Fine, thin, poorly developed root mass||Root mass that is 2 to 6 inches thick, primarily fine roots||Root mass is more than 6 inches thick with a high percentage of woody roots|
|Soil depth||--||Less than 2 feet to bedrock||More than 2 feet to bedrock|
|Slope||Slopes steeper than 40 percent if the slope length is longer than 50 feet; slopes 20 to 40 percent if the slope length is longer than 100 feet||Slopes between 6 and 20 percent (with appropriate water control)||Slopes less than 6 percent (with appropriate water control)|
|Landscape position||North-facing aspects in some climatic conditions||Ridgelines (if shallow soils); foot and toe slopes (if wet or there are seep zones); floodplains (seasonal flooding); slopes (depending on percent of slope, see above)||South-facing aspects; gravel bars, terraces, and alluvial benches; outwash plains; alluvial fans (depending on slope)|
How Soils are Degraded
Trail use damages soils when the type and level of use exceed the soil's capacity to resist impact. A soil's capacity to resist impact varies depending on textural class, moisture level, and other environmental and site characteristics, but the processes by which soils are impacted are generally the same. Trail use damages soils directly by mechanical impact from surface traffic and indirectly by hydraulic modifications, soil transport, and deposition.
Direct mechanical impact has several components: abrasion, compaction, shearing, and displacement.
Abrasion strips surface vegetation and roots.
Compaction reduces soil voids and causes surface subsidence.
Shearing is the destructive transfer of force through the soil.
Displacement results in the mechanical movement of soil particles.
Indirect impacts include hydraulic modifications, such as the disruption of surface water flow, reductions in infiltration and percolation, surface ponding, and the loss of water-holding capacity. Other indirect impacts include those associated with erosion--both the loss of soil particles by wind or water erosion and deposition of transported particles. An associated impact is the hydraulic pumping that occurs when a destructive flow of water is forced through a saturated soil.
Both direct and indirect impacts degrade trail segments. The impacts generally occur in the following progression:
Abrasive loss of protecting surface vegetation and root mass (direct impact)
Compaction and surface subsidence (direct impact)
Hydraulic disruption (indirect impact)
Breakdown of soil structure from shearing and pumping (direct impact)
Soil particle erosion and deposition (indirect impact)
While most of the stages in this progression are familiar concepts, the shearing and pumping components may not be as familiar to some readers.
Shearing describes a transfer of force through a soil. When an applied force exceeds the capacity of the soil body to absorb it, a portion of the soil body can be displaced along a shear plane--that place where soil particle cohesion is weakest.
The most common example is when the passage of a wheeled vehicle forms ruts. The downward force of the wheel shears--or displaces--the soil beneath it, forcing the soil to bulge upward beside the wheel. This process is illustrated in figure 2. The shearing action destroys soil structure by crushing soil peds (natural soil aggregates) and collapsing voids. Shearing is most likely to occur on finely textured soils under moist to saturated conditions. It is uncommon in coarse soils.
Figure 2--Diagram of shearing action.
Pumping action occurs when soils are saturated with water. Saturated soils are most common in wetlands, but may occur on other sites during spring thaw, periods of high rainfall, or where water is ponded. Pumping occurs when the downward pressure of a passing force--such as a vehicle wheel--forces water through soil voids and passages. When the pressure is released, water rushes back into the vacuum. This process is illustrated in figure 3. The force of this rapid water flow erodes internal soil structure and clogs soil voids with displaced sediment. Pumping occurs within all soils, but is most damaging to finely textured soils because of their fragile internal structure.
Figure 3--Diagram of pumping action.
Shearing and pumping actions reduce soils to a structureless or "massive" condition. This condition is characterized by the loss of distinguishable soil structure and a reduction in pore space voids, and interped passages (the space between peds). An example of soil in a massive state is a dried mud clod or an adobe brick. In a massive state, soils have significantly reduced infiltration rates, percolation, water storage capacity, and gas exchange. This reduces a soil's ability to support vegetation growth, leads to surface ponding of water, and increases the soil's sensitivity to additional impacts.
Surface Erosion, Surface Failure, and Trail Braiding
Trail use has a predictable path of surface impact. The degree of impact is modified only by the natural resilience of the soil and the intensity of trail use. In an ideal situation, a natural balance is maintained between soil resilience and use, and trail use occurs without significant degradation. However, on sites with wet, unstable, and sensitive soils, that equilibrium is easily upset. Even low levels of trail use can have significant environmental consequences.
Typically, trail degradation follows one of two pathways: surface erosion or surface failure. Surface erosion occurs when wind or water displaces exposed trail surfaces. This usually occurs on steep terrain or on sandy soils that are susceptible to wind erosion. Surface failure occurs when trail surfaces degrade into muddy tracks with deep muck holes. This usually occurs on flat areas with organic or finely textured soils. Either pathway can lead to significant environmental impacts that are extremely difficult to stabilize or reverse. Without stabilization, a destructive cycle of degradation can begin that expands the impact to adjacent surfaces. That cycle begins with the widening of trail surfaces as users avoid degraded surfaces and expands to the development of multiple parallel trails.
The two degradation pathways are diagrammed in table 2.
Table 2--Trail degradation pathways.
|Both pathways of impact begin when:|
|Surface vegetation and roots are stripped by surface traffic.|
|Exposed soil is compacted and the trail surface subsides relative to the adjacent surface.|
|Compaction and subsidence destroy soil structure and disrupt internal drainage patterns.|
|Impact follows one of two paths:|
|Surface erosion||Surface failure|
|Exposed surface is eroded by wind scour or||Water collects on the trail surface.|
|Water drains onto the trail surface.||Water pools in low areas.|
|Water is channeled along the trail surface.||Pooled water saturates the soil.|
|Water erodes the trail surface.||Shearing and pumping damage the soil structure.|
|Deep erosion ruts form.||Muddy sections and deep muck holes form.|
|The impact continues:|
|Trail segments develop rutted or muddy sections.|
|Trails widen as users avoid degraded sections.|
|Trail users abandon degraded sections.|
|New routes are pioneered on adjacent soils.|
|Vegetation and roots are stripped by traffic along the new route.|
|Soil is compacted and the soil structure is destroyed, leading to surface erosion and/or surface failure.|
|And so the cycle repeats itself.|
The first consequences of pioneering a trail across a virgin landscape are the stripping of surface vegetation, the abrasion of roots, and the compaction of surface soil layers. These impacts destroy soil structure, reduce water infiltration, and break bonds between soil particles. Soil particles are more vulnerable to displacement and loss from wind or water erosion. Soil compaction also leads to surface subsidence--the lowering of the trail relative to the adjacent ground surface. Trails become entrenched. This lower surface intercepts and drains water from adjacent surfaces and channels that flow along the trail. This dramatically increases the risk of water erosion on sloped areas and the pooling of water in low-lying sections. As trail surfaces degrade due to rutting or the formation of muck holes, users widen the trail and seek new routes, usually on adjacent soils where environmental conditions are identical to the original impact site. As this new route degrades, it is abandoned. A third route is pioneered, and then a fourth--until finally the area is scarred with a number of routes in various stages of use and abandonment. This condition is called trail braiding. Trail braiding significantly expands the environmental impacts of trail use. Trail braiding occurs because trail use levels repeatedly exceed the carrying capacity of soils to support that use. Figures 4a and 4b illustrate the process.
Figure 4a--Ponded water in ruts and muck
holes prompt riders to pioneer new routes in
adjacent undisturbed areas.
Figure 4b--Results are an adjacent degraded
alignment and the development of a braided trail.
In braided trail sections, abandoned trail segments may slowly recover from impact through natural revegetation. However, the impact has usually dramatically altered the site's thermal, soil, and hydrologic characteristics. These changes affect the composition and structure of vegetation that can grow on the disturbed site. For example, a site that supported shrubs and grass before disturbance may only support sedges or other water-tolerant plants after disturbance. Abandoned routes may also recover enough to support subsequent trail use, but they are generally more sensitive to impact than virgin sites.
The impacts associated with braiding are a major concern for land managers because they dramatically increase the area of impacts associated with trail use (figure 5). Studies conducted in one area of Alaska documented that the average OHV trail had an impact area 34.6 feet wide (Connery 1984)--that's four times the width necessary for a single OHV track. Using that average width (34.6 feet), each mile of trail affects 4.2 acres. A single-track trail (8 feet wide) of the same length would affect just 0.97 acre per mile. Braided trail sections more than 200 feet wide are not uncommon within Alaska. For resource managers, the increase in area affected by braiding is significant in terms of resource destruction, habitat loss, and esthetics.
Figure 5--A braided trail in Alaska. More
than a dozen separate routes have been
pioneered in this section crossing a wetland.
Note the wet trail conditions and numerous
potholes. At this site, trail impacts affect an
area more than 250 feet wide. Braiding significantly
extends the area of impact by modifying
vegetation cover, surface hydrology, and soil
In Alaska, the cycle of degradation is well studied and documented (Connery 1984; Connery, Meyers, and Beck 1985; Ahlstrand and Racine 1990, 1993; and Happe, Shea, and Loya 1998). Responding to the impact has been more difficult. The problem is also compounded by rapidly expanding OHV use and increased OHV trail mileage. One study conducted by the Bureau of Land Management documented a 76-percent increase in miles of trail from the early 1970s to the late 1990s (Muenster 2001). Significant increases have also been observed in many other areas of Alaska. These increases in trail mileage and their associated environmental impacts on soil, vegetation, habitat, and water resource values have given resource managers a legitimate reason to be concerned about the impacts associated with degraded OHV trails.