The number one reason we don’t we use permeable walkways and trails more throughout our communities - potential for water damage.
Letting Water Slip Through the Cracks
A large part of why we have no water in southern California is we don’t capture and store it when we do have it. Paving the land with impervious surfaces (roadways, sidewalks, etc.) is the primary reason we don’t have the ability to store rain water efficiently. Part of the answer is to pave more areas with more permeable surfaces. However, there are challenges in doing this and we need to understand those challenges and address them through quality design.
Decomposed granite (DG) trails are a great solution. The product of the weathering of granitic parent rock material, DG, is a granular, compatible soil type that is commonly used for paths and other paved surfaces. Decomposed granite is much more permeable than conventional paving materials and can usually be installed at a much lower cost. Its permeable nature allows water percolate down through the granite, reducing runoff problems and allowing moisture to reach underground roots from nearby trees and shrubs. But, if not designed and installed correctly, the positive qualities can quickly be overshadowed by negatives.
Don’t Run Off…Stay for Awhile
DG trails are a great addition to any rural community for many reasons including:
- Soft paving for aerobic jogging or walking
- Great surface for equestrian
- Reduced heat island effect (reflective heat)
- Blending better into a natural setting
In many studies throughout the country, walkers, bikers, and equestrians ask communities for natural surfaces to enjoy. Begin the trail with the disabled in mind and continue that access throughout. Discover if there are any interesting places or other trails to connect to. Figure out if this trail will be for transportation (kids getting to school), recreation, or both.
The intended benefit of the DG path throughout the community is to give residents an amenity within their community that can be utilized by all. The added benefit is that this amenity allows the community to give back to their own surrounding environment.
When Water Trips You Up
DG trails sounds like a win-win right? So, why isn’t DG more commonly used?
Water damage or erosion on trails is the number one reason. Water damage can create unsightly erosion and unsafe deep trenches exposing tripping hazards. This is typically caused by poor initial trail design, which can rarely be overcome, even by regular maintenance. Another cause is inadequate or inappropriate maintenance, which can both increase trail problems and wastes valuable crew time.
Trail construction and maintenance is an inexact science with many variables. Much depends on the location of the trail, the soil, the climate, and the types of uses. However, there are certain general guidelines which, if adhered to, will prevent most trail deterioration and minimize maintenance costs. Here are some design rules to follow when using DG for your community’s trail system.
Rules of a Crack-Free Road
- Get the high water flows from heavy storm events into vegetated areas where the water has more time to percolate.
- Pay attention to the gradient. Highly erosive, sandy soil requires a flatter surface, so even a 5% gradient could be too steep. Granitic soils are more forgiving and therefore can hand up to a 15% gradient, but keep in mind trails less than 10% are far more comfortable to hike and ride.
- DG requires stabilization, which is a very specific treatment during installation including scarifying the DG material, applying top coats (understanding when and how many to apply based on how the DG responds) and knowing when to compact the DG. Make sure to work with someone who has experience with this material before proceeding.
- Install and maintain water bars to divert water off a trail at controlled points along the trail. They can be incorporated in the original construction of a trail, or they can be installed later as a maintenance measure. Done well, a series of water bars can effectively eliminate erosion and stabilize a trail for years. Done poorly, water bars can accentuate trail erosion and become dangerous tripping hazards.
Ultimately, the most influential component of trail maintenance is the original trail design, alignment and installation. A well-designed trail will be easier to maintain, will deteriorate more slowly and will be more pleasant to use.
On the other hand, a poorly-designed trail is difficult to maintain, deteriorates quickly and, once you lose it, there’s not much that can be done to restore it. In addition, a poorly designed trail will always be less pleasant to hike or ride. Shown in the accompanying photos is a comparison of a well-designed trail versus one which has design flaws that allowed erosion.
Just the same, if a well-designed trail isn’t maintained properly, your trail will still suffer from many of the common problems that occur with DG.
Keep a look out for my next blog post on DG, which will focus on my recommendations for properly maintaining your DG trails. In the meantime, please feel free to share your experience using and working with DG and any design tips you believe should be added to the mix.
Also, next Wed, Feb 22 at 2PM ET/11AM PT, I will be discussing permeable surfaces and DG live on Twitter. Follow and join the conversation by tuning into #landscapechat.