By Eric West, P.E.
Design and construction guidelines for asphalt trails.
What's the best trail surface? This question has been debated for decades, and the answer is still the same: it depends on intended use, the setting, the budget, and available materials. Asphalt has been the material of choice for trails of many kinds throughout America. This article provides the technical data, based on wide experience, for building the most durable and cost-effective asphalt trails.
The selection of surface material for trails and paths is primarily based on anticipated type and intensity of trail use, as well as terrain, climate, design life, maintenance, cost, and availability. Soft surface materials are low cost, but require substantial maintenance and are not suitable for many of the recreational activities today's trails and paths are used for. Hard surface materials, specifically concrete and asphalt, provide years of service with low maintenance.
The following are the key elements to consider in building quality asphalt trails:
Bike paths and trails should be constructed to match the existing topography as closely as possible, however, longitudinal slopes should not exceed five percent and a cross slope of two percent is desirable to provide adequate drainage away from the pavement surface. Proper drainage is one of the most important factors affecting pavement performance. Proper drainage entails efficient removal of excess water from the trail. Surface water runoff should be handled using swales, ditches and sheet flow. Catch basins, drain inlets, culverts and underground piping may also be necessary. These structures should be located off of the pavement structure.
Not all hot mix asphalt is the same, and the type used for a highway may not be the appropriate mix for a trail or bike path. Specific mixes are designed for specific applications.
The hot mix asphalt specified for your project should provide adequate strength and durability. The overall objective for the design of asphalt paving mixtures is to determine a cost-effective blend of aggregates and asphalt that yields a mix having:
Specifications for trail and bike path hot mix asphalt should address the specific application and use, and allow the use of locally available aggregate, where its quality is adequate for the project. The gradation specification should be consistent with local specifications. It is recommended that a SX 1/2" nominal maximum size gradation, or meeting the state Department of Transportation criteria, be specified.
Bike paths and trails are not subjected to heavy loading. Many of these paths are also constructed in terrain difficult for large construction equipment to access. Thus the hot mix asphalt should have a reasonably high asphalt cement content. This "rich" mix will provide excellent durability and allow for ease of placement and compaction. In addition, high asphalt binder content mixes reduce segregation potential and improve the surface texture of the mix for this type of application.
The first step in analyzing pavement thickness is determining the loading the pavement will be subjected to. Pavements need to be designed to support wheel loads from vehicles that will have access to them. These may include emergency, patrol, snow removal, maintenance and other motor vehicles.
The next step is to determine the load carrying characteristics of the native soil. A soils investigation should be performed to determine the sub-grade strength, load support capabilities, ground water conditions, and swell potential of the native soils. The soil investigation should be performed with test hole locations at appropriate intervals to account for the varying soil conditions that may be encountered.
Pavement thickness is dependent on the loading that will be applied to the pavement, the asphalt mix design and the ability of the underlying soil to support the loads. Full depth asphalt pavement is the overwhelming choice to distribute loads to the sub-grade. However, depending on the existing soil's ability to support the loads, an aggregate base course and/or geo-textile may be used to improve the stability and/or load carrying capability of the native soil.
The geo-technical engineer performing the soils investigation should recommend design thicknesses for the pavement based on the anticipated loading conditions and the results of strength testing performed on the native soils. As soil conditions vary across the site, recommended design thickness may change. The standards for determining the supporting capabilities of the native soil vary depending on location.
The most common test performed is the R-value, American Association of State Highway and Transportation Officials (AASHTO) designation T -190 and T-99, American Society for Testing and Materials (ASTM) designation D 2844. This test provides a relative soil strength to be applied to nomographs, or design equations, which include environmental and loading criteria for determination of a required structural number for the pavement. The required structural number must be achieved by an adequate thickness of pavement. Each pavement layer is assigned a strength coefficient based on the type of material used. A dense graded hot mix asphalt is assigned a coefficient of between 0.34 and 0.44, based on research done by AASHTO, and the properties of the mix.
In an area with reasonably good soil (R-value 20), occasional maintenance vehicle use, and good drainage, a required structural number of approximately 1.6 is determined from design nomographs. To determine the necessary thickness of hot mix asphalt, divide this structural number by the strength coefficient of the material. For a typical hot mix asphalt, we will assume a strength coefficient of 0.40. The calculation of 1.6/0.4 provides a recommended pavement section of 4 inches of hot mix asphalt.
The above example is typical of the method used by geo-technical engineers to provide recommended pavement sections. This example is based on numerous assumptions and should not be used for actual construction. Your geo-technical engineer or landscape architect will provide site specific information for your project.
In general, it is recommended a minimum 3" of hot mix asphalt be used for bike paths and trails where loading from vehicles will be negligible. As soil conditions deteriorate and loading increases, the pavement thickness should be increased.
Prior to construction, vegetation should be cleared and stumps and roots removed along the trail for a minimum of five feet outside the edge of the proposed pavement. This will allow construction equipment access and help prevent roots and growth from eventually encroaching on the path. If adequate access width cannot be provided, the contractor will be forced to use less efficient equipment with increased costs.
The asphalt should be placed on compacted sub-grade that extends a minimum of two feet beyond the edge of pavement. The edge of pavement should be feathered with native soil to avoid any sharp drops from the trail edge. The sub-grade should be prepared by removing topsoil and unstable soil, shaping to grade, scarifying the surface to a minimum depth of six inches, moisture conditioning, and compacting. The sub-grade should be compacted to a minimum of 95% of standard Proctor density, AASHTO T 99, and the moisture should be maintained within 3% of optimum. If aggregate base course is used in the pavement section it should be compacted to a minimum of 95% of modified Proctor density, AASHTO T 180, ASTM D 1557.
Depending on the soil conditions, compaction and moisture criteria may vary. Consult your landscape architect or geo-technical engineer for site-specific information. After compaction a soil sterilant and/or root inhibitor should be applied. Application should be carefully controlled to the pavement area only. Typical shaping, grading and compaction crews consist of a motor grader or blade, landscape tractor with back box for grading, and a rubber tire roller for compaction. Additional compaction equipment and access to water may be required.
Prior to placement of the asphalt pavement it is recommended the sub-grade be proof rolled to highlight areas of uncompacted or unstable soil. This may be accomplished using a loaded single axle or tandem dump truck or a loaded rubber tire loader. Soft or unstable areas should be recompacted or removed and replaced with stable soil. It is also important that all utility installations, including sprinkler systems, be complete prior to paving.
Placement of the hot mix asphalt should be accomplished with a self-propelled paver, where possible. Where pavers cannot be used, a spreader box attached to a dump truck may be used. Minimum paver width is generally eight feet. For widths less than eight feet cutoff shoes may be placed in the screed to reduce the width of paving. The screed controls mat thickness and crown. Vibratory screeds are typical and provide a small amount of compaction prior to rolling. In general, the uncompacted mat should be 1/4" thicker than the final desired thickness to allow for densification during rolling operations.
The hot mix asphalt should be delivered to the paver at a temperature adequate to allow proper compaction. This depends upon the type of asphalt cement used, but generally ranges between 235 to 300 degrees Fahrenheit. The contractor's ability to achieve compaction is dependent on the mix temperature, pavement thickness, subgrade temperature, ambient temperature and wind velocity.
Compaction should be accomplished immediately after placement by the paver. Steel wheel vibratory rollers are generally used for initial breakdown rolling behind the paver, followed by a steel wheel finish roller. Depending on the compactibility of the mix, a pneumatic tired roller may also be used. Minimize their tendency to pick up fine aggregate from the surface with proper tire temperature or the use of a release agent. It is recommended the hot mix asphalt be compacted to 92-96 percemt of the Theoretical Maximum Specific Gravity, AASHTO designation T 209, ASTM designation D 2041.
Joint construction should be carefully done to ensure a uniform mat. Longitudinal joints, which occur where mats are laid side to side, should be constructed with a vertical face or a step taper. The step taper should have a 1.5" vertical face at the surface, tapered at a 3:1 slope from this point to the subgrade. Prior to placing the adjoining mat the joint should be tack-coated. Asphalt placed against a longitudinal joint should overlap the existing asphalt by 1". Compacting longitudinal joints should be accomplished by rolling from the hot side of the asphalt. The steel wheel roller is placed with the majority of the drum on the hot, newly placed asphalt, with approximately 6" of the drum extending over the cold asphalt.
Transverse joints occur at any point the paver ends work and then resumes at a subsequent time. The end of the paving mat should be cut off vertically to allow the full lift thickness to be placed against it. Lumber is used as a bulkhead, paving over the lumber and leaving a taper that is removed along with the bulkhead prior to resumption of paving. Another method is to form a papered transverse joint where heavy wrapping paper is placed along the entire face of the vertical edge of the pavement. The paper extends approximately three to four feet onto the subgrade. The paver resumes paving over the paper to form a taper. Prior to resumption of paving, the paper and material on top of it is removed forming a vertical edge.
When paving resumes the vertical edge is tack-coated, heated and the paver backed over the existing asphalt with the screed resting on the previously placed mat. The shims should have a height equal to the expected compacted thickness, i.e. 1/4" per inch of material. Mix is delivered to the paver and the paver starts forward slowly. Excess mix left by the paver is bumped back to the joint location and/or removed. The joint is then rolled transversely from the cold side beginning with the roller approximately six inches on the newly placed mat and continuing across in six to twelve inch increments. Timbers should be placed along the outside edges of the mat to support the roller and minimize distortion of the outside edges.
Composite sections, consisting of asphalt pavement overlying aggregate base course, have an advantage in the ease of grading the base course to the proper level for placement of the asphalt pavement. If base course is used it should be a minus 1-1/2" and minus 3/4" aggregate size. The strength coefficient of base course ranges from 0.12 to 0.14, depending on the R-value of the material. Based on this strength coefficient, 3" of base course are equivalent to the strength of 1" of asphalt pavement. However, when using aggregate base course, the asphalt pavement thickness should be maintained at 3" and should never be less than 2". The minimum thickness of the aggregate base course should be 6" for an asphalt trail, or thicker for poorer quality sub-grade material.
The subgrade should be stripped of vegetation, shaped to grade, and compacted at the proper moisture content prior to placement of the pavement structure. In general, compact the sub-grade to a minimum of 95% of the maximum density as determined by AASHTO T 99, Standard Proctor and maintain moisture content to within 3% of optimum. Again, your geo-technical engineer or landscape architect should provide guidelines for proper compaction of the existing soil.