A forensic approach found the best decomposed granite solution for Kenyon College.
By Neil Budzinski and Matthew Girard
Decomposed granite pavement (DG) is a textured and responsive paving material used on paths and plazas. Yet the quiet appearance of DG masks material and construction complexities that shape the outcome of the built work and belie what may appear to be a simple installation. In 2010, Michael Van Valkenburgh Associates (MVVA), where we are senior associates, was hired to prepare a master plan for Kenyon College in Gambier, Ohio. Over several years of our working with Kenyon to renovate its historic Middle Path, the challenges of this material were revealed and met through a program of design-phase mock-ups, manufacturer’s product development, and innovations in installation methods. We have learned several lessons regarding the product and the methods that change the way we specify and oversee the installation of this seemingly simple material.
Kenyon’s landscape is organized around Middle Path, a 3,600-foot-long walk made from a local river stone. The material of Middle Path, cherished for its color, texture, looseness, and sound, was a large part of its charm, but also a significant contributor to its challenges. In fair weather, the existing path had an active surface that shifted and crunched pleasingly underfoot. In foul weather, Middle Path became impassable, the river stone washing away to reveal potholes and soft, muddy spots that harbored puddles or ice depending on the season. The college recognized that the surface needed to be improved and made accessible in all types of weather and to all visitors, but few in the community could stomach the replacement of the historic material with a rigid pavement, such as exposed aggregate concrete. In the 2012 master plan for Kenyon College, MVVA recommended the renovation of Middle Path with DG pavement to address accessibility issues while preserving its feel and appearance.
Replacing Kenyon’s Middle Path posed a problem that stabilized DG seems invented to solve. Unlike asphalt or concrete pavement, DG will not degrade over time. It does not fade, crumble, chip, crack, or stain. The dense surface releases a thin layer of aggregate that is tactile and audible underfoot and, with regular maintenance, the surface is consistent and endlessly repairable. Replacing the river stone of Middle Path with DG provided continuity of texture and character while meeting the need for a firm surface. It was also possible to nearly replicate the hue of the existing stone, given the wide color range of native granites available.
Whereas the river stone had natural rounded edges and was clean-graded (i.e., without fines), DG is crushed, creating an angular stone with sharp edges that lock together when compacted. It includes a high percentage of fines to fill voids. DG can be installed as is, but a key component of the Kenyon project was the integration of a plant-based binding agent that reacts upon exposure to water to bond the aggregate together. Adding the “glue” makes this a stabilized DG. It remains somewhat permeable and regularly releases loose aggregate on the surface, but stays firm and dense.
To demonstrate these qualities to the campus community, a design-phase mock-up was undertaken during the summer of 2012. Two panels, each eight feet by 20 feet, were built to test the selection of a custom granite aggregate color blend. A third panel composed of regionally available crushed limestone was included to offer a lower-cost option. The work was performed by a single contractor over a two-day period. Thorough hydration throughout the DG profile is a key step in activating the stabilizer within the DG mix, and an early sign of trouble was the difficulty the contractor had in hydrating the mock-up panels with water. The specification called for the DG to be wetted down after placement of every two-inch lift (before compaction), but the DG had become hydrophobic. Water would barely penetrate the surface before running off. As a result, copious amounts of water were used and the panels took several weeks to firm up. They finally hardened off during the dry fall weather, but the freeze/thaw cycles of a mild winter made the paths soft and muddy, and the mock-ups were deemed to have failed, as their performance did not surpass that of the existing path. Intended in part to assuage the campus community about the forthcoming change to Middle Path, the failed mock-up fueled skepticism about the new material, and for some, the need for change in general.
We began the forensics, and the following factors were evaluated as possible contributors to the failure:
Insufficient compaction of the subgrade or the base course: Density tests were performed and revealed compaction greater than 95 percent, an acceptable number. This was not a contributing factor.
Insufficient compaction of the DG: A water-filled sod roller was used. This was approved by the manufacturer’s representative, but it was likely too light and was considered a contributing factor.
Insufficient activation of the stabilizer: The in situ DG resisted absorption of the water, prompting aggressive watering that may have caused the stabilizer to be washed out of the upper portion of the profile. This was considered a contributing factor.
Insufficient stabilizer in the mix: A sample of leftover DG mix was lab tested by the stabilizer manufacturer and determined to meet the specification.
This was not a contributing factor.
Insufficient drainage: The soil adjacent to the paths was compacted and did not drain well. This likely contributed to the failure.
Generally poor installation technique: The stabilizer manufacturer claimed the installing contractor “just didn’t get it; some do, some don’t.” This claim may have provided us some solace, but was not actionable.
During our investigation, we examined past DG installations in MVVA projects and contacted landscape contractors who performed this work on other jobs with good and bad results. We were unable to pinpoint a consistent approach that would guarantee success, and therefore we had no obvious way forward. The inconsistent performance was corroborated by the aggregate manufacturer for the Kenyon mock-ups, Kafka Granite in Wisconsin. Kafka staff informed us that they knew of multiple failures, and that the failures were not easily explainable, but they suspected that the problem lay with the stabilizer additive. To assist MVVA in developing a solution for the college, Kafka accelerated its testing of an alternate stabilizer manufactured by the Envirobond company called Organic-Lock. Having the manufacturer’s participation in testing and developing the stabilized DG was critical to moving the process forward.
In the spring of 2013, Kafka Granite conducted its own trials at its facility, making several test panels that varied the stabilizer type, quantity, and the DG aggregate size. At the end of a two-week observation period, some panels were deemed successful. On the strength of this preliminary work, in the summer of 2013 two new test panels were installed at Kenyon College using the new product. Each panel was installed by a different contractor. The installation procedures of the two contractors were generally the same, but the increased participation of multiple parties fostered innovations. The use of formwork, for example, was a direct outcome of this process. The two panels held firm through the difficult Ohio winter, and on the success of this demonstration, the project proceeded.
During the design-phase mock-ups and the subsequent installation of Phase I (10,500 square feet), Phase II (22,000 square feet), and Phase III (11,000 square feet), we learned the following lessons:
The right stabilizer is critical: We now specify Envirobond Organic-Lock as the stabilizer for all our DG installations. It is a Canadian company, but the mixing of the stabilizer into the aggregate is typically done at the aggregate supplier, which addresses concerns about buying American products and shipping the stabilizer long distances. Kafka Granite of Wisconsin and Read Custom Soils near Boston provide stabilized DG with Organic-Lock.
Prehydration needs to be done correctly: At Kenyon, the stabilized DG was prehydrated to 14 percent moisture content before it was spread, which is comparable to ambient conditions in much of the country (always check local conditions). The mix will appear quite dry. The hydration was performed at Kenyon using a skid steer loader to turn over the material while it was being misted with water. The alternate “dry-soak” method of spreading the DG in two-inch lifts and then misting it with water in situ may not hydrate the aggregate thoroughly, or may excessively hydrate the top surface. Agitation of the media is required for a good blend, and this is hard to do for DG wetted in situ. Contractors might think it is easier to spread the material dry and then hydrate, as it seems like fewer steps, but the Organic-Lock stabilizer repels water even more than the initial product.
Get the compaction right: A one-ton roller (a Wacker Neuson RD 27) seems to hit the sweet spot for compaction of the DG. Heavier than that and the DG is pushed aside by the roller. A two-ton roller was periodically used at Kenyon, but had to be operated very slowly or “waves” were produced in the surface.
The use of formwork aids installation: At Kenyon, the contractor used two-by-six wood formwork to define the limit of the path. This gave a very clean edge and was useful for grade control as the DG was leveled to the top of the formwork. The formwork should be set roughly one-half inch above the desired grade to allow for the compaction of the material. The formwork should be removed as soon as possible, because having the formwork higher than the compacted DG will prevent water from draining off the surface.
Well-designed drainage is a must: Good movement of water off and away from the DG is essential. A cross slope of 1.5 to 2 percent should be used, even when running slopes exceed these numbers. Low spots where water gathers adjacent to the DG should be avoided to allow water to shed beyond the path. Pay attention to micrograding at lawn edges so they are lower than the DG, because a thick lawn set high could trap water at the DG edge.
When possible, have the manufacturer’s representative on site: Unless a contractor is very familiar with this installation, the DG manufacturer’s representative should be on site early in the installation. At Kenyon the representative played a significant role addressing questions of moisture content before compaction and repair techniques.
Work directly with the manufacturer on the mix ratio: Let the stabilizer manufacturer specify the mix ratio for stabilizer to aggregate. They will need a sample of the aggregate to do this. For Envirobond, expect to see 30 to 35 pounds of stabilizer per ton of aggregate. The key is that every stabilizer product is different; work directly with the manufacturer to find the best mix for a particular locale and condition.
Grain size affects bonding: Kafka Granite performed Kenyon’s mock-ups at its facility using 3/8-inch minus stone to get a coarser, grainier look, but based on these trials they observed that a quarter-inch maximum aggregate size bonded more firmly.
Minimize vehicles: Plows and trucks were driven over the mock-ups without incident during frozen months. A truck could do some slight damage when the DG is saturated owing to wet conditions, as we’ve noted in the final installation.
With Phase I and Phase II of Middle Path completed, we have had more than a year to observe the DG installations. The DG has held up well and the college is pleased with the results, but there are several postoccupancy comments to consider before specifying DG.
Even with shallow running slopes, erosion from heavy rain can occur anywhere water gets channeled. This can occur at path edges if the adjacent grade prevents water from sheeting away. Kenyon uses a clay court broom/drag brush to regularly smooth these areas and redistribute DG. In very wet weather the path can be soft, but not muddy, underfoot; it will firm up again as it dries. This has periodically been an issue at Kenyon. In the winter of 2015–2016 they experienced a rapid, partial thaw coupled with heavy rains, which caused the top of the DG profile to become saturated while the lower portion remained frozen. This created soft conditions on the path for a period of time. The situation remedied itself when the path dried out and then refroze. When the path stayed frozen, there were no problems. This issue reinforces the importance of proper slope for drainage, though there will likely still be times when a perfect storm of significant precipitation coupled with impediments to drainage (such as snowbanks or frozen ground) will make mushy paths unavoidable.
The Kenyon grounds crew finds winter maintenance challenging. Half-pipe PVC baffles or lifts are attached to plow blades to prevent gouging of the DG during snow removal, but the complaint is that this special blade leaves behind a thin layer of snow, which can lead to icy conditions, especially if students have already walked on and packed the snow. Spreading sand and gravel is not advisable because it is visually dissimilar to the DG surface. To address this, Kenyon keeps a stockpile of the original DG mix to use as winter grit, which has been effective but requires greater coordination and cost. Salting the DG is not recommended because the salt thaws the upper layers of DG while the lower remains frozen, resulting in soft conditions. Also, though it was not an issue with the Kenyon project, DG near building entrances can track inside the building and damage floor finishes.
Climate and microclimate can also affect the performance of the material. The DG installed under new trees (with small canopies) was exposed to full sunlight and firmed up more quickly than DG installed beneath the shade of the existing sugar maples. The biggest difference is that the DG under the sugar maples feels less granular and has a whitish cast to it. In general, the solar heat of a full-sun environment will result in DG that is easier to establish and maintain.
Like all materials, DG has its challenges. It should never be represented as carefree in either installation or maintenance. Winter maintenance in temperate climates, for example, will always be a consideration. But its advantages are many, including the absence of distracting joints or color shifts and the rich experiential dimension in the sound of it underfoot. It is a unique material with a beguiling mix of casualness and sophistication that encourages a slightly slower pace, and it provides a special vantage from which people can appreciate their surroundings.
Neil Budzinski and Matthew Girard are senior associates at Michael Van Valkenburgh Associates, Inc.