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Trail Design Charette analyzes accessibility for pathways

California State Parks sponsors problem solving and trail construction project at Prairie Creek Redwoods State Park.

By Don Beers, California State Parks

California MapIn October of 1998 Whole Access and the North Coast Redwoods District of the California Department of Parks and Recreation co-sponsored a Multi-Disciplinary Trail Design Charette in Eureka, California. The five day synergistic problem solving session focused on developing a process for evaluating and designing recreational trails to determine the greatest amount of accessibility that can be obtained without adversely affecting the natural and cultural resources of the land.

The charette team members represented a wide variety of disciplines and expertise including disability/accessibility design, trail design and construction, assistive technology, archeology, geology, landscape architecture, park planning, physical engineering, ecology, inclusive recreation, surfaces and safety engineering and soil stabilization technology. A film crew led by Jay Moss and Bruce Schmiechen and still photographer Jack Hopkins documented the entire charette exercise and the follow up trail design and construction work.

The charette process began with a field review of two trails (Prairie Creek and Foothill) both located at Prairie Creek Redwoods State Park. These trails served as the outdoor laboratory for the charette design process. Both trails were totally inaccessible and had numerous trail maintenance and resource degradation issues. Prairie Creek trail closely followed a stream and meandered through a broad and heavily forested stream valley. Its linear grades ranged from zero to thirty-six percent with most of the trail grade being under ten percent. Steep linear grades, step carriages and bridges with step approaches represented obstacles to accessibility. Roots protruding into the trail bed and wet saturated tread surfaces also created obstacles and barriers.

Foothill trail was located on a steep hill slope and traversed through an old growth redwood forest winding around the giant trees both standing and fallen. The trail undulated around these ancient trees and the linear grades ranged from zero to fifty-nine percent with most of the trail being under twelve percent. Steep linear grades, severe cross slopes, narrow tread widths, precipitous drop-offs, step carriages and bridges with step approaches represented significant obstacles to accessibility. Roots protruding into the trail bed and seasonally wet and saturated tread surfaces were also obstacles and barriers.

Once the field review was completed and a thorough image of both trails and their issues was conveyed to the charette participants each member gave a brief presentation on their respective disciplines and how it related to trails, accessibility and resource protection. This cross training exercise enabled the participants to obtain a fundamental understanding of each other's disciplines and their views regarding improving accessibility and protecting resources.

Once the cross training was completed, the charette participants identified the commonalties and conflicts (conditions /design elements) between sustainable trail design, resource protection and accessibility.

Commonalties Between Accessibility, Resource Protection & Sustainable Trail Design

  1. Curvilinear alignment produces a good sustainable trail and more moderate linear grades
  2. Moderate linear grades have less erosion, greater sustainability and higher access potential
  3. Firm, stable and uniform tread surfaces promote good sheet drainage, reduce tread deformation, soil saturation and improve access.
  4. Tread surfaces with a good coefficient of friction provide a safer and more pleasurable hiking experience and improve accessibility

Accessibility Conflicts With Resource Protection & Trail Sustainability

  1. Lower linear grades require longer trail alignments
  2. Wider tread widths increase the size of the trail's footprint
  3. Lower cross slopes and edge protection may reduce sheet flow and increase erosion and trail maintenance
  4. Lower linear grades may require more engineered structures and increase construction and maintenance cost
  5. Wider tread widths and hardened tread surfaces may change the character of the land or the user's experience
  6. Lower linear grades, wider tread widths, engineered structures and hardened tread surfaces may increase the trail's impact on the resources

Resource Protection and Trail Sustainability Conflicts With Accessibility

  1. Steeper linear grades are difficult to traverse up or down depending on the assistive device used
  2. Steeper linear grades, when combined with higher cross slopes and change of direction, cause most wheeled assistive devices to track off the trail
  3. Trail tread with a low coefficient of friction does not provide the traction necessary for many assistive devices
  4. A steep drop-off along the trail edge may creates a safety issue
  5. Narrow trail widths (less than 36") do not accommodate most wheeled assistive devices
  6. Obstacles in the trail tread such as roots and rocks may create barriers
  7. Vertical obstructions such as tree limbs and over hanging down trees may create barriers and hazardous conditions
  8. Traditional trail structures such as steps, water bars and open culverts represent barriers

Once the charette team completed identifying the potential conflicts between accessibility and resource protection and sustainable trail design they used their collective knowledge and experience to identify potential solutions and or mitigations to those conflicts.

Solutions and/or Mitigations to Maximize Resource Protection and Sustainable Trails

  1. Linear trail grades are kept moderate between 5 and 8%, with short runs up to 10%
  2. Layout of trail maximizes side hill construction to provide a fuller native trail bench for better durability, drainage and sustainability
  3. Trails located on flat terrain are turnpiked to provide drainage and protect tree roots
  4. Trail structures such as retaining walls, bridges and drain lenses are kept to a minimum and are used to protect resources and maintain good linear grades
  5. Trail cross slope is kept at 5% but may be up to 8 percent depending on the linear grade, tread width and type of tread surface
  6. Edge protection is provided only when conditions warrant it. Native vegetation and natural features such as rocks and logs can serve as edge protection
  7. Edge protection is installed in a manor to facilitate sheet flow
  8. Trail tread widths are designed for 36 inches minimum and are widened only to mitigate conditions affecting accessibility
  9. Tread surfaces need only to be firm, stable and have a good coefficient of friction
  10. Mobility assistive devices that have a greater capability of traversing recreation trails need to be developed, manufactured and made available

Solutions and/or Mitigation to Maximize Accessibility

  1. Steeper linear grades are mitigated by increasing the tread width and improving the coefficient of friction of the tread surface
  2. Combinations of steeper linear grades, higher cross slopes and changes of directions are mitigated by widening the tread width, improving the tread surface and or providing edge protection
  3. Locations with a steep drop-off are mitigated by widening the tread width and or installing edge protection
  4. Trail tread surfaces are well shaped, compacted and constructed on fuller benches to provide better firmness and stability
  5. Native soils are augmented with crushed shale rock when additional strength and firmness is required
  6. Tread surfaces are constructed to be uniform but have enough roughness and texture to provide a good coefficient of friction
  7. Trail structures that create barriers such as steps, water bars and open culverts are eliminated
  8. Trail tread widths are designed to be a minimum of 36 inches
  9. Tree roots protruding into the trail tread are covered with native soils or a combination of crushed shale rock and native soil
  10. Rocks protruding into the trail tread are removed or covered with native soils or crushed shale rock
  11. Vertical obstructions are either removed or mitigated by installing features that warn visually impaired trail users

Upon the completion of the design charette, a core trail design group comprised of Phyllis Cangemi, Barry Atwood, Jay Moss, Karl Knapp and Don Beers re-evaluated Prairie Creek and Foothill trails armed with the design solutions and mitigations developed by the charette team. This group also relied upon their many years of experience in accessibility and trail design to develop the prescriptions necessary to improve the access of these trails without compromising park resources.

In performing the redesign of these two trails the core group followed a process that could be duplicated by others regardless of the specific trail conditions or setting that may be encountered..

The landform was thoroughly researched and evaluated to identify potential trail routing needs, resource concerns, boundary issues, land use capabilities and political constraints.

The trails were evaluated to ascertain if their initial layout and construction work was fundamentally sound. Trail conditions reflecting resource damage and sustainability problems usually are a manifestation of poor trail design or construction. These problems are usually attributed to not thoroughly understanding the land base and its capabilities and limitations. Also not identifying and laying out between major and minor control points (locations where the trail has to be or stay away from), not following curvilinear alignment principles and constructing trails that disrupt the surface and shallow ground water hydrology. These conditions or deficiencies needed to be corrected as a standard practice in trail reconstruction. In doing so, resource protection and sustainability issues were corrected and the trail's accessibility potential was significantly enhanced.

Once major and minor control points were established the linear grade between those control points was identified and compared with the sustainable grade of the landform and the linear grade required too comply with accessibility standards.

If the linear grades between the established control points were within accessibility standards then the trail was realigned between those control points if there were no resource or esthetic issues. New trail alignments incorporated curvilinear layout and maximized side hill construction. All abandoned trail segments were fully rehabilitated by recontouring the trial bench and re-vegetating the trailway with plants salvaged from the new alignments.

If the linear grades between the control points were in excess of the accessibility standards then minor control points were evaluated to determine if they could be modified to reduce the linear grade between them. Example: constructing a retaining wall under a large tree (minor control) to allow the trail to pass under it and reduce the linear grade or lengthening a bridge at a stream crossing site (minor control) to elevate the approaching trail grades to reduce the linear grade. If the minor controls could not be modified without causing resource damage or sustainability issues the land base was evaluated to determine if additional linear run could be achieved through the use of properly placed topographic turns, climbing turns or switchbacks (turns).

If minor control point modifications or turns could not be achieved to reduce the linear grades then those segments of trail could not comply with the accessibility standards for linear grade. However, those segments still were reconstructed to comply with those standards that they could meet i.e. tread width, cross slope, tread firmness and stability, overhead clearance, etc. Those trail segments that could be modified to comply with accessibility standards were so as to make the trail as accessible as possible.

Trail re-design and reconstruction efforts focused on the simplest solution then graduated to the more complex as the situation dictated. Segments where the trail alignment met all the resource, sustainability and accessibility requirements were left untouched.

Tread width and surface: Trail segments with minor trail bench and tread surface deficiencies were reconstructed to obtain a uniform linear grade and outslope. Locations where the tread surface failed to meet firmness and stability requirements were augmented with a crushed shale aggregate, that when blended with native soils produced a firm and stable surface that closely matched the color and texture of the native soils. This material also provided a good coefficient of friction during wet conditions and when the trail was littered with organic material. It also allowed water to percolate and evaporate through it minimizing resource impacts.

Obstacles in the tread surface: Rocks or small roots were removed from the tread surface to provide a uniform and smooth trail tread. Larger roots were covered or capped with crushed shale rock and native soils to bridge over these obstacles (turnpiked)

Vertical clearance in the travelway: Limbs protruding into the travelway or trees spanning across the travelway were removed if they did not comply with accessibility standards and their removal did adversely impact the resource or the user experience. Slight trail re-alignments were made if adjusting the trail route a few feet could provide proper clearance. Logs and vegetation were used along the side of the trail to guide the visually impaired user to the clear opening. When significant down trees (specimens) were encountered the trail was rerouted around them but came close enough to provide the user with an up close tactile experience.

Combinations of steeper linear grades and cross slope: Where the linear grade was steeper (8 to 10%) and the cross slope was at 5% the trail was widened to between 48 and 60 inches and or the tread surface was improved by adding crushed shale rock.

Combinations of steeper linear grades, cross slope and change of direction: Where these conditions existed the trail tread was widened to between 48 and 60 inches, the tread surface was improved by adding crushed shale rock and edge protection was installed along the radius of the turn.

Edge protection: Edge protection was provided at locations where there were (1) a steep hazardous drop off along the outer edge of the trail, (2) steep linear grades, cross slopes and the trail had a precipitous drop off, and (3) a combination of steep linear grades, cross slopes and a change of direction. Edge protection consisted of native vegetation (if it was strong enough to restrain the user), logs and large tree limbs securely fastened to the ground, large rocks firmly attached to the ground, and combinations of hand rails, diagonal rails and or bull rails on bridge and puncheon structures. All edge protection used was installed to facilitate overland sheet flow and blend in with the natural surroundings. Edge protection was only installed at the locations where it was warranted.

Drainage crossings: Small ephemeral drainages were traversed by constructing armored rip rap crossings (cobblestone), drainage lenses or installing culverts. Low volume perennial drainages were crossed using puncheon structures (boardwalks). Bridges were constructed to cross the larger perennial streams. All of these structures were designed to have a finished elevation of trail grade and the puncheon and bridges had edge protection.

In performing the construction work every attempt was made to match the building materials to the natural environment and the local architecture. If synthetic materials were used (cellular confinement, geotextile fabric, etc.) they were hidden and covered by native soils and vegetation. Every attempt was made to make the finished trail seamless with the natural environment and provide a quality outdoor experience for all trail users.

The charette design exercise proved to be a very educational experience. It brought people together with very diverse backgrounds and experiences and helped them learn from each other and expand their own knowledge and understanding of trail and accessibility design. It helped close the gap between trail/resource specialist and accessibility specialist and advocates. It helped identify the commonalties between good trail design and accessible design. It also showed that the conflicts between those two disciplines could be solved or mitigated with simple design and construction solutions that already exist and are commonly used by trail designers and builders. Finally, it defined a process for re-designing existing trails or designing new trails to maximize accessibility, protect resources and provide sustainable trails. This process can be applied universally to all trails regardless of their location or conditions.

December 2003

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