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This report sorts through the various choices for the most "economical and sustainable" types of trail surfacing options along the proposed Rio Grande Trail corridor from Belen to Sunland Park, New Mexico.
See The art of building crushed stone trails by Tony Boone
Download full 20-page report (pdf 852 kb)
Prepared by Anasazi Trails, Inc.
The following section on trail surfacing is an excerpt from the study.
Considerations When Choosing Trail Surfacing Options
There are many factors to consider when deciding which trail surface is most suitable. Aside from funding source and initial capital cost considerations, the following items must be addressed before reaching a decision on trail surfacing:
Crushed rock trail whose curving, undulating shapes of nature
are appropriate to the setting
What are the existing soil and environmental conditions in the area?
• characteristics of the native soils
• soil constraints and possible solutions
• hydrologic patterns of the area
• topographic relief
What are the aesthetical considerations?
• curving, undulating shapes in nature
• dealing with the feel of straight, dusty roads (levee/canal roads)
• diversity of views during trail experience
What is the overall management strategy and what will be the long-term operating costs?
• effects of user groups on different surfaces, depths and widths
• maintenance and long-term durability
• law enforcement and park/trail patrol
• vandalism issues (graffiti on hard surfaces, deformation, arson)
What is the anticipated trail use?
• volume of use
• types of use
• seasons of use
What is the availability of surfacing materials?
• cost/benefit analysis for surfacing types
• distance of material source to project site
• methods and equipment required for proper installation
• embodied energy requirements from processing and transportation of materials
Natural surface trail for horses and mountain bicycles
Even when a trail surface has a longer life expectancy (e.g. 25+ years for concrete), agencies responsible for long-term management must understand what level of routine maintenance will be required to manage hazards and associated risks and protect their trail investment. Maintenance tasks include sweeping, corridor trimming, hazard removal, mowing, tread work, weed control, and routine safety inspections.
In the case of severe weather events like flash flooding which can cause massive soil erosion, or even the normal freeze/thaw cycle that some areas experience, most agencies would have significant challenges managing these occurrences without additional funding and staff.
Surfacing decisions must factor a variety of aspects including economic, social, and environmental conditions specific to each surface type and balance these with the desired trail characteristics and the site-specific conditions. Visitor desires must be weighed against potential negative impacts to the natural, cultural, and historical resources and consider the economic limitations of the managing agency.
New Mexico is fortunate to have with large deposits of alluvial sediments that are readily accessible along the Rio Grande and its tributaries. There are over a dozen sand and gravel companies within the southern 270-mile stretch from Belen to Sunland Park. Usually they are located within a few miles of the river and rarely further than 30 miles from any potential project site up and down the RGT corridor.
Other consideration when selecting trail surfacing options should include the desired modes of recreational use, topography, and hydrology; single-use verses shared-use; type of trail experience and level of resource protection desired; trail funding/budget; available grants; existing soils and their limitations; and the ability of the agency to manage visitor use.
After assessing these considerations, trail managers may find many surfacing options are immediately excluded from further consideration. For instance, if the capital available for the project is low, more expensive materials such as asphalt, concrete and boardwalks may not be economically feasible. If there is limited budget or human resources available for long-term maintenance, a grant for a hard surface trail may be appropriate in areas of high use and poor soil conditions.
As discussed, there are many factors to consider when deciding whether to surface a trail or not. If evaluation of the factors point toward surfacing a trail, there are various types of surfaces and surfacing products available.
Concrete greenway trail in Santa Fe
Trail Surfacing Options
Concrete is a mixture of sand, gravel, and mortar and provides the hardest, fastest, and smoothest surface for recreational activities. It typically has the longest lifespan of any trail material assuming it was designed and installed correctly. When properly constructed and maintained on a regular basis, concrete can last 25 years or more. The high cost of concrete trails is often the most limiting factor since it is one of the most expensive surfaces to install. Concrete may be used for constructing trails with grades exceeding 15% without significant erosion concerns.
Concrete trails are generally more appropriate for urban settings and areas where there is a high volume of trail use. These trails are appropriate for the widest variety of trail uses, ages and abilities. When placed in an intimate, natural setting, the wider, bright white concrete often detracts from the aesthetic beauty. However, the lighter color of concrete reflects more heat and is cooler than asphalt. Concrete can be tinted to blend in with native soil color but ultimately increases the cost of the material.
Concrete is also produced in the form of blocks, bricks and pavers. When placed on a well-drained base course, pavers can provide space for vegetation to grow through, permeability of surface water, and provide excellent support for trail users. Numerous types of permeable concrete treatments are available and help address concerns associated with non-permeable trail and parking surfaces. These are very costly and may be appropriate for heavily landscaped areas or crosswalks, especially since they rarely conform to ADA accessibility guidelines.
Concrete prices continue to soar due to escalating fuel prices and are rarely guaranteed for more than a few weeks from suppliers. This makes the grant fund budgeting process more challenging than in the past.
Asphalt is composed of a small aggregate held together by either a hot-mix or cold-mix bituminous (coal-based) compound. The resulting trail surface is hard and smooth, also very suitable for a wide range of recreational activities. Asphalt may be used on steeper alignments without significant erosion concerns.
Trail managers should pay close attention to the sub-base preparation that will underlay asphalt trails as the surface is typically thinner than concrete and more susceptible to cracking by frost heaving and tree roots. Proper drainage is also required since asphalt and concrete channel the surface water, often resulting in erosion concerns.
Numerous types of permeable asphalt are now available and help address some of the concerns associated with non-permeable trail and parking surfaces. Recycled materials are also becoming common in asphalt paving, including Recycled Asphalt Product s (RAP), which reuses old recycled asphalt or glassphalt which uses recycled crushed glass as an aggregate substitute.
When installed properly on suitable sub-grade, asphalt products typically have a life span less than half that of a concrete trail, averaging approximately 10 years. As with any surfaced trail, proper installation is imperative. Poorly installed asphalt or concrete trails may not last three
years before problems begin to arise. Like concrete, asphalt prices are directly influenced by the by rising fuel costs and price quotes from suppliers are rarely guaranteed for more than a few weeks.
Alternative Surfacing Products
As new products evolve and innovative technologies emerge, trail professionals will continue to experiment with a variety of alternative surface types in search of building the perfect trail.
Whether organic or natural by-products from agricultural or industrial processes, alternative surfacing products may be considered viable because a firm, stable and slip-resistant surface can result with proper installation. As with concrete and asphalt, these products require specific installation procedures and ideal site conditions to perform optimally. Many require soil testing and specific soil textures to ensure performance/longevity.
Innovative trail surfaces derived from industrial processes, use recycled materials or by-products to bind native soils with or without imported aggregates. Bottom ash and fly ash by-products from coal-fired power generation have been used in trail construction. Other unique examples include the use of crushed ceramics, crushed oyster or pecan shells, wood chips or shredded wood as a alternative trail surfaces. These surfacing alternatives have had mixed results and may only be considered feasible, economical and appropriate if the source of the material is close to the project site and suitable to the various aspects discussed.
A growing number of products are available as stabilizers for trail surfaces. Many of the most common include:
Soiltac – This is an eco-safe, biodegradable, liquid copolymer used to stabilize and solidify soil or aggregate and is also used for erosion control and dust suppression.
Poly Pavement – This product is a liquid soil solidifier that converts native soils into a durable wear surface.
Natural Pave – Natural Pave XL resin pavement binder emulsion is mixed with aggregate materials to produce compacted pavement surfaces that retain the natural coloration and texture of the constituent aggregate material. Resin pavement mixtures contain no petroleum ingredients
and are appropriate for use in sensitive natural environments, including access to beach, estuary and riparian areas.
EMC Squared – EMC Squared is highly effective in improving the stability behavior of a broad spectrum of aggregate and soil materials for service applications in a wide variety of climatic conditions. The product technology is both user-friendly and environmentally affable.
StaLok/Stabilizer – Made from ground seed hulls of the plantego plant native to Arizona. Stabilizer is a nontoxic, non-staining organic soil stabilizer. StaLock is a polymer enhanced version of the 20-year-old product.
Soil-Sement – Soil-Sement is an environmentally safe, advanced powerful polymer emulsion that produces highly effective dust control, erosion control and soil stabilization.
Roadbond EN-1 – This product contains a strong oxidizer, a powerful solvent and a natural dispersant. The interaction of these components activates the naturally occurring mineral cements in the soil and bonds the soil grains together.
Mountain Grout – Mountain Grout is a soil stabilizer. Sprayed onto or mixed into the sand, Mountain Grout binds with the sand to form a hardened surface within hours.
Dura Road PX-300 – This is a liquid copolymer soil stabilization product which produces an abrasion and water resistant surface made of natural soil.
Lignosite – This is a byproduct of the calcium bisulfite pulping process.
RoadOyl – This product is a resin-modified emulsion that provides treatment for bare earth or unpaved surfaces. Formulated from tree resin, this state-of-the-art emulsion is unique in its high bonding strength and is appropriate for use even in close proximity to wetland areas and other areas of environmental sensitivity.
Klingstone – Klingstone 400 is a moderate viscosity, single component, moisture curing liquid (polymer) designed to stabilize soils for foot traffic and light vehicular traffic.
Permazyme 11X – This product produces all weather roads, increases compaction up to 15% with no extra effort, it is environmentally safe.
Earthzyme – This product is a totally natural bio-degradable product. It improves a soil’s physical and chemical properties, which result in significantly less mechanical effort to achieve greater densities. For use in soils less than 20% clay, binds only with clay particles, not silts, sands or gravels.
All of these products come in powder and/or liquid form and are applied topically or are mixed in with the soils or imported aggregate material. More information on each of these products can be found using the website links provided in the Appendix.
• Fly ash is a byproduct obtained from the stacks of coal-burning power plants.
• Bottom ash is the coarse, granular, incombustible by-product collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both.
Natural Plant/Animal Byproducts
• Crushed Pecan Shells
• Shredded Wood
• Engineered Wood Chips
• Crushed Oyster Shells
Natural surface interpretive trail
Trails constructed in native soils are most often the least costly to build, especially in areas with undulating topography, loamy soils, and good drainage. However, these ideal conditions are not always the norm along the RGT corridor. Soil types vary from those with higher clay content to almost pure sand; sandy soils being most typical. Soil survey maps can generally aid in determining the soil types in your area can be obtained from the USDA’s Natural Resource Conservation Service (NRCS). The NRCS website link is provided in the Appendix. High clay content soils typically cause trails to be slick and muddy when wet. Water often puddles and when horse hoof pock marks or a linear bike tire rut dry out, they are very difficult to smooth out. Clay based soils take significantly longer to dry out since their extra fine particles have a high rate of cohesion and refuse to give up water easily.
Bentonite, a naturally occurring clay, has also been used as a soil additive in other areas of the United States for successfully amending soils with high sand content to achieve a firmer surface. Like other additives, it requires costly preparation, transportation, and installation unless the material is readily available on or near the construction site. Soils with high sand content are usually course and drain very well, but rarely do they provide a firm, stable and slip resistant surface. Deep loose sands are not uncommon along the Rio Grande and are typically the least preferred surface for most users; some equestrians and ATVs being the exceptions. Hikers and mountain bikers typically do not seek out deep loose soils because of the significant difficulty required to efficiently travel over them.
Crushed aggregate is simply crushed rocks and boulders. The raw materials originate from glacial till and alluvial sediments from the eroding Rocky Mountains to the west and north. This random mixture of crushed rocks and aggregate becomes valuable and useful once it is separated into distinct sizes and materials of sand, gravel and rock using screens of various dimensions. In many places along the Rio Grande flood plain these alluvial deposits are several miles deep providing an almost infinite source of material for surfacing trails with crusher fines.
Crusher fines trail
The rock dust screenings or fines are usually the smallest particles that fall out of the crushing bin. The larger rocks keep moving further and are separated depending on operator’s goals and range in size from 1/4 inch and larger. Typically, those materials with the suffix of "minus" contain the smaller particles rendered from the crushing process and are referred to as “crusher fines”. Other aggregates are simply crushed stone with minimal to no fines (i.e. gravel), separated into different sizes for different uses and range from 1/4 inch to 1-1/2 inch and up.
Surfacing sections of unsuitable soils with 4-6 inches of compacted crusher fines over a landscape fabric and/or a compacted base course material can eliminate many of the problems inherent with unsuitable native soils.
It must be noted that even with a firm, stable surface when dry, trails constructed of crusher fines often result in rutting and pocking by all users when travelled upon when slightly wet or when saturated conditions from snowmelt or extended periods of rain exist. Road base material consisting of 3/4 inch diameter particle sizes or larger are often more resistant to deformation by user traffic than smaller crusher fines.
Over 20 samples of crusher fines and other crushed aggregates from the RGT corridor study area were tested for suitability using the dish test methodology as explained in “Natural Surface Trails by Design”. Numerous products performed excellent, while others were not suitable for trail use. Materials change in color from brown to mauve and red to grey along the river corridor. The parent stone and boulder materials crushed into fines and other products have a significantly high content of inherent soil binders. This allows for harder compaction, which enhances durability to all modes of user traffic. A simple, soil penetrometer is used to compare the durability and hardness of the sample materials.
It should be noted it is not recommended to surface equestrian or ATV trails. These users most often prefer native soils and less stringent construction standards. Some mountain bikers also fall into this category of preferring native soil or natural surface trails assuming it is not deep sand or wet muddy clay. Loamy soils, like sandy-clay-loams that contain rock or cobble are often more durable under hooves or forces associated with accelerating wheels.
Trails on soils with fine and homogeneous textures are more erodible and often have greater potential for tread incision. Loam and sandy-loam soils, because of their even mixture of silt, clay and sand, provide the fewest limitations for trails.
Since crusher fines trails usually cost in excess of $10.00 per linear foot, it is important to construct them correctly. Success relies on obtaining the right materials and installing them professionally. Mistakes often result in poor compaction, soft surface conditions, non-accessibility and, inevitably, costly repairs. Do not allow crusher fines to sit on the trail for a season before they are wetted and compacted. Typically, the fines will sink to the bottom and the larger particles will float to the top creating loose, unstable tread conditions.
Gravel and crusher fines differ from one another in that gravel is screened to remove the fines which contain the natural binders/cements. Gravels remain loose because of dead air or pore space within the matrix which allow them to drain well and resist compaction.
Crusher fines contain inherent soil cements and binders, which promote their compaction. Fines that contain too many rounded particles, like some decomposed granites, are more difficult to interlock and often yield a loose and unconsolidated surface. Angular particles like andesite, dolomite, and certain types of granite can easily be wetted and compacted to meet the ADA accessibility guidelines.
A good indication of the strength of a rock binder is the hardness of the parent material. The harder the source rock, the stronger the binders will be. Crushed fines contain the original rock cements and binders within the rock dust. These binders combined with water and subsequently compacted with a vibratory roller or plate compactor should produce a solid surface that resists significant deformation from horse hooves, hiking boots and mountain bike tires. On a well compacted crusher fines trail it is not uncommon to find baby strollers and road bikers.
See The art of building crushed stone trails by Tony Boone
Download full 20-page report (pdf 852 kb)