invisible structures, inc.

Garden of the Gods

Located in Colorado Springs, Colo., Garden of the Gods Park draws close to two million visitors per year. The site features miles of trails for both pedestrian hiking and horse riding use. The equestrian trails have been difficult to maintain due to the soft, and highly erodible, soils and high volume of daily traffic. Parks and Recreation department contacted Invisible Structures, Inc. to investigate the feasibility of using Gravelpave2 on the worst portions of trails to alleviate these problems.

Garden of the Gods Park is located in Colorado Springs, Colorado and features glorious views of Pikes Peak and dramatic geologic formations of uplifted red sandstone shoulders and spires. Located only minutes from downtown, the park draws more than 1.7 million visitors per year.

It retains a near "wilderness" character, while providing miles of trails for both pedestrian hiking and horse riding use. City Park and Recreation Department staff have used the site’s undulating terrain and natural vegetation of pine and scrub oak, to create a one-way vehicular circulation system that is separate from pedestrian and equestrian trails and paths. Parking areas are kept relatively small and screened from view, with close access to picnic areas and long-range views. The park is closed after dark and avoids the need for large area lighting fixtures.

The equestrian trails have provided maintenance staff monumental pains over the years, due to soft, and highly erodible, soils and high volumes of private and commercial horse trips per day (140+). Commercial guides offer excellent park environment and history tours from the top of a saddle, and provide a memorable experience for the common city-dweller visitor.

Horses tend to walk nose to tail and create a narrow pathway. Steel-shoed horses are extremely tough on trail surfaces – especially unimproved surfaces like those found in the park. During dry weather park soils are loose and "fluffy", but when wet, deep ruts form from traffic and surface runoff moving quickly down the hilly terrain.

Some of these ruts repeatedly reached depths of more than three feet during rains in 1999. Soils moved downhill by water were removed from roots of native trees and shrubs (causing loss of this vegetation), and deposited in normally dry streambeds. When later rains occurred, stream flow capacity was reduced, causing water levels to rise and remove soils from root zones of riparian plants along the stream bank, with flooding downstream.

Previous efforts to intercept and divert concentrated runoff, such as the use of wood diversion bars, energy dissipation, and re-grading, were largely unsuccessful and required substantial maintenance efforts, usually by a track modified skid steer and manual labor. Several locations were redressed almost daily.

Early in the year 2000, the Parks and Recreation department contacted Invisible Structures, Inc. to investigate the feasibility of using Gravelpave2 (also promoted as Surefoot4 for the equine market) on the worst portions of trails to alleviate these problems.

Key concerns of the City staff were:

• Low visual impact provided by product color and surface.

• Secure and stable footing for horses.

• Suitability of use by hikers and bicyclists.

• Reduction of erosion, maintenance effort and costs.

The site contained severe rutting caused by horse traffic and water.

The native soils are red, so we made the structure a Terra Cotta red color to match the soils. The ring and grid structure is molded with a polyester filter fabric attached to the bottom side. The fabric serves to separate the base course fill from the top wearing course, and prevent undercutting by erosion. Anchor nails and washers are used to tightly secure the mat to the base course and prevent any edges from lifting.

The park design and maintenance staff wanted to experiment with the use of native soils to fill the mats. After filling ruts, regrading, placing and hand tamping a porous base course, and installing the product, the mat was filled with native soils. The initial objectives of lower installation cost and surface color were met. Large scale rutting had been stopped. However, following several rain events, evaluation of the first plot determined that small areas of surface erosion still existed where water removed fill between rings on the mat. Maintenance efforts were reduced! Additional improvements were still necessary for the desired maintenance level.

We made two suggestions for a second test location: Substitute Slopetame2 with crossbars between the rings, instead of Gravelpave2 which has no crossbars; and substitute the native soil fill material with a gravel of 3/16" minus size, with no fines, tumbled in a cement mixer with 7% color tinted cement. Color had to be added for camouflage because all of the locally available stone was light in color.

In building surfaces for wheelchair access, Invisible Structures found that adding 7% to 10% cement to 3/16" gravel, then misting with water, covering with a tarp for 48 hours, locks the wearing surface together in the rings on the mat. After a hurricane in Savannah, Georgia, the installation withstood more than four feet of floodwater, with no erosion.

The Parks Department was willing to try this similar installation method. The only difference was that they decided to add a small amount of native soil above the rings to completely hide the Terra Cotta colored rings, which normally are still slightly visible. The effect is now more like rough surfaced native rock with a bit of sand on the surface. The preference for completely hiding the trail so that no manmade influence is present is the dominant philosophy of most trail managers and government entities. Certainly, this purist goal is a difficult challenge for any manufacturer.

To fix the problem, Invisible Structures made the Gravelpave2 structure a Terra Cotta red color to match the soils. The ring and grid structure is molded with a polyester filter fabric attached to the bottom side. The fabric serves to separate the base course fill from the top wearing course, and prevent undercutting by erosion.

Another equine test area was a visitor trail along a concrete pathway. People drive their personal horses to Garden of the Gods, then use any or all of the trails, and not just the ones dedicated to group tours. Several stretches of different installation methods are being evaluated.

A three-year study of the wheelchair-user trail design at the Rock Mountain Easter Seals Camp in Empire, Colorado, shows that hand-tamping soils, gravel or crushed asphalt into our mats is working well. Adding switchbacks to wheelchair trails will prevent any significant erosion from expanding on steep slopes, and the switchbacks add safety when bordered

by rocks.

Testing at Garden of the Gods will continue into 2002 so that a full season of horse traffic can be evaluated. To date the results of 2001 are astounding - the maintenance cost savings are dramatic, the test areas are holding up to high use, and more attention can be given to other areas of this spectacular natural wonder.

Bill Bohnhoff is a registered Landscape Architect, founder of Invisible Structures, Inc., and inventor of ISI’s current product line.