Author: Libby Hanson

All too often, rainy weather generates more water than a landscape can handle.  If this water is not managed properly, it leads to several issues. These can be minor annoyances such as puddles, or more serious issues such as:

• Reduced groundwater recharge
• High rates of runoff and erosion
• An increased likelihood of natural disasters (e.g. flooding or mudslides)

These issues are compounded by the use of non-porous paving materials, such as concrete and asphalt.  To compensate for the lack of permeability, additional landscaping element⁠s⁠—such as bioswales⁠—are sometimes incorporated into a landscape design.

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What’s a Bioswale?

A bioswale is a landscaping feature used to manage stormwater runoff by slowing, collecting, filtering, and infiltrating stormwater.  They are often seen alongside large parking lots, business parks, and industrial areas. Similar to a storm gutter in function, a bioswale normally involves the use of vegetation to reduce the speed of the runoff for maximum effectiveness.

At first glance, a bioswale seems like an ideal way to manage stormwater.  However, it’s simply a Band-Aid that has its own host of issues.

As an additional element added to a design, bioswales drive up a landscape’s installation costs.  Bioswales also require regular maintenance. If a bioswale is not properly taken care of, there’s a risk that water will not properly drain; a lack of drainage can allow pollutants water to pool on the surface long enough to allow pesky insects to breed.  A bioswale must be inspected on a regular basis to ensure adequate vegetation is maintained, there are no blockages, and that any sediment that has built up is adequately removed.

The Bottom Line

There’s a better solution for stormwater management: permeable pavement.  Permeable pavement allows rainwater to percolate directly into the ground – reducing runoff and all of its associated issues and eliminating the need for a bioswale or similar landscaping elements.  Permeable pavement is also accompanied by environmental and financial benefits.

Environmental benefits include:

• Reduces stormwater runoff rate and volume
• Reduces water pollution by trapping pollutants in the pavement
• Replenishes groundwater, which can, in turn, be used by plants surrounding the pavement

Financial benefits include:

• Reduces the need for conventional drainage features, such as curbs and gutters
• Lower installation costs, since underground piping and storm drains are not needed
• Reduced water-usage costs, since surrounding plants can benefit from the replenished groundwater

There are many different permeable paving options available, and choosing the right one may require a little more research than choosing a traditional paving option, but it’s well worth it.  We explore the different options more in-depth here.

GraniteCrete – Your Stormwater Management Solution

GraniteCrete is a specialized permeable paving product; use of GraniteCrete eliminates the need for additional landscaping elements like a bioswale.  Mixed with decomposed granite, it has a solid and durable surface with the near-strength of concrete. It contains no polymers, oils, or resins; so, no toxic materials will leach into the ground as water passes through.  GraniteCrete will stand up to both foot traffic and light vehicular traffic, making it appropriate for patios, driveways, sidewalks – and more!

For over two decades, landscape designer Christian Douglas has explored his passion for creating “beautiful and productive outdoor spaces.”  Three key criteria inform his designs: Sustainability; that gardens be edible; and that they have a high end, stylized design sensibility.

His Californian Modern Edible design perfectly embodies these traits. This masterpiece can be found at a private residence in Mill Valley.

At 5,000 square feet, the limited lot size presented a challenge.  Douglas maximized the space by creating multiple living areas that allow several distinctive experiences as visitors move throughout the landscape.  He accomplished this by using raised vegetable planters, strategically placed anchor points such as a water feature, and carefully chosen plants and fruit trees.

To increase the sustainability of the landscape, Douglas incorporated a rainwater catchment system into the front yard, which made using a permeable paving material a crucial aspect of the landscape design.

Decomposed Granite Failed
He originally used decomposed granite as the substrate.  The decomposed granite served its purpose … for about two years. By that time, it had deteriorated and tracked away so extensively that it needed to be replaced.  He attempted a workaround: he tried a polymer hardener in an effort to make the decomposed granite more stable and to keep it in place.  Unfortunately, this failed; not only did it look “awful,” but the decomposed granite would still track away into the house.  Plus, polymer hardeners can reduce permeability and run the risk of allowing unnatural substances to leach into the groundwater.

GraniteCrete Was The Right Replacement
Douglas decided it was necessary to remove the material altogether and start over with a better paving material.  That’s when Howard Lasker of SBI Building Materials and Landscape Supplies recommended he use GraniteCrete for its permeability, natural look, and durability.

To complete the installation, Douglas brought in Dave Washer and his team at Art Gardens.  Washer has extensive experience installing GraniteCrete and is a recommended installer of the product.

Recycling the Old Decomposed Granite – a bonus!
Since GraniteCrete is mixed with decomposed granite, Washer first made sure the old decomposed granite fit GraniteCrete specifications.  Once confirming it did, Washer was able to satisfy Douglas’ goal of sustainability by recycling the old material and using it for the pedestrian pathways in the garden.*

These new pathways support the original goal of utilizing a permeable paving material that augments the landscape’s rainwater system. Additionally, because the material is not “loose,” it won’t track away and these beautiful pathways will be a lasting and attractive feature of this garden for years to come!

“GraniteCrete is more expensive [than plain decomposed granite], but after using decomposed granite for years we found that while decomposed granite is cheaper up front, the lifetime cost is higher [when you take into account] repairs, redoing it, et cetera.”

-Christian Douglas, landscape designer

See our story about the Marin Headlands for another example of reusing on-site material with GraniteCrete.

The Right Decomposed Granite for a Quality Job

What is Decomposed Granite?

Decomposed granite (DG) is a material formed by quarrying and pulverizing solid granite.  It is like gravel, but finer and—when compacted—more stable.

DG can also vary in hue, or color, depending on where it is quarried from.  The color of DG from within the same quarry can have differences if it comes from different “veins” in the same quarry.

Types of DG Used in Hardscaping

“Hardscape” is any non-living aspect of a landscape design, and can include decomposed granite.  The two main types of decomposed granite used in hardscaping are “unstabilized DG” and “stabilized DG.”

Unstabilized DG is simply loose granite with no binders or additives.  While it has excellent drainage and the initial cost is low, loose decomposed granite can be costly to maintain.  It easily washes away due to rainfall, or gets tracked away from the installation site (and into homes, which may damage hardwood flooring).

Stabilized DG is comprised of a stabilizer combined with the granite aggregates.  This provides the decomposed granite with varying levels of permanence, depending on the stabilizer used.  Unfortunately, not all stabilizers are permeable and some may allow environmentally-harmful materials to leach into the groundwater.  Common stabilizers are explored in more detail here.

One such stabilizer that can be used is GraniteCrete Admixture.  This is an ideal stabilizer because it requires little to no maintenance, does not fail over time, and has excellent erosion control.  It is also permeable, allowing water to easily pass through; since it does not contain any oils, resins, polymers, or enzymes, it will not cause water pollution.

To ensure quality installations of GraniteCrete, it’s important that the GraniteCrete admixture is combined with decomposed granite that meets the specifications in our Specification Guide.

Let’s Get Technical

Decomposed granite always has a sieve analysis associated with it.   The sieve analysis is the percentage of sizes of the aggregate particles.  The DG used for landscaping is often composed of ⅜” minus (or smaller) particles.  The ⅜” aggregate is sometimes referred to as “bone” and makes for easier, and more-beautiful installations.

The sieve analysis of DG that creates the best GraniteCrete installations is below.

As one can see, these ranges vary without affecting the integrity of the final installation. It is critical that the decomposed granite used contains at least 2% to 5% of true ⅜” sized aggregate.  

It is also critical that the non-expansive clay fines—sieve 200 and sieve 400—do not exceed 18%.  If the clay fines exceed 18%, this can lead to several issues:

• Since it is more or less clay dust, the clay fines will make the DG too “creamy” once it’s combined with water.
• During compaction, the fines will rise to the top.  This will cause an undesirable “slick” look, and discoloration of the finish.
• The mixture will be difficult to work.  As a result, surface imperfections will be tricky to remove and creating an acceptable finish will be difficult.
• Reduced final permeability.  Larger stone creates voids in the final installation; these voids allow water to pass through to the ground below.

The importance of using DG with a low clay content and a healthy percentage of true ⅜” stone cannot be emphasized enough.

Decomposed granite that falls within the ranges noted above are found throughout California.  Ask the supplier to provide a sieve analysis of the decomposed granite they provide.

We’re Here to Help

Please take the time to find a decomposed granite that meets our specifications.  If you have any questions about a sieve analysis, don’t hesitate to reach out to us.  We are happy to review it to ensure an acceptable DG is used.

There are some paving materials that technically can be installed under conditions that are less than ideal.  However, doing so is risky and increases the likelihood that the paving material will fail shortly after installation, causing issues such as large cracks, uneven surfaces, and holes in the pavement.  The need for avoidable repairs can drive up both product and labor costs in the long-run … and cause unnecessary headaches!

Fortunately, there are a few paving solutions out there that are not as prone to such issues.  Our favorite of these solutions is GraniteCrete.  Factors that might totally prevent the installation of other paving materials either don’t affect—or actually help—the installation of GraniteCrete.

Low temperatures can affect the integrity of some paving materials, but not GraniteCrete!  It can be installed at temperatures as low as 33° Fahrenheit – just one degree above freezing.

Moisture in the air can also negatively impact the installation of some paving materials.  While GraniteCrete cannot be installed during rainfall, light fog and mist are actually beneficial and aid its installation.  According to Geoff Smith, who developed GraniteCrete, even a drizzle won’t necessarily impede proper installation, provided adjustments are made to the moisture level added during the install process.