Testing a Permeable Paving System in a Truck-Washing Area
A fire station in Oceanside, CA, was selected as the
site for installation and testing of a new permeable paving system,
Soil Retention Products Inc.’s Drivable Grass. A permanent deflection
and endurance study was performed on the system.
The linear grooves and holes between the concrete muffins allow for root penetration and moisture drainage.
fire station’s adjacent truck washing area has been recently paved with
the Drivable Grass system. This location offered the ability to control
and measure varying loads, drive passes, and the moisture percolation
capability of the plantable concrete paving mat. During the course of
the study, the mat was actually subjected to a higher load frequency and
larger loads than would be expected over the “typical use” service life
of the mat.
This study used both qualitative and quantitative
measures of the paving system’s performance. Manometer readings and
photographic documentation were used to evaluate how the concrete mat
performed with the repeated loading of various fire trucks. Test results
showed negligible permanent deflection and deformation of the product.
Grass is a permeable, flexible, and plantable concrete pavement system
that is manufactured from poly-reinforced concrete. The pavement mat
gains tensile strength by incorporating polymer grids, spaced at 2
inches on center, that connect the concrete muffins (raised concrete
cubes in the mat that supports most of the load). The grid maintains its
tensile strength from the polymer reinforcing while also allowing large
amounts of flexure facilitated by the linear grooves and holes between
the muffins. These cracks and holes also allow for root penetration and
moisture drainage. Typical areas of installation include emergency and
service vehicle access driveways, overflow and recreational vehicle
parking, patios, golf cart paths, residential driveways, roundabouts,
vehicle wash areas, V-ditches, and other non-driving areas such as
This has been the first study to research and collect
quantitative data on the long-term performance of the product as it is
related to permanent deflection. The study took place at an active fire
station where the pavement system was subjected to loads from fire
Table 1 shows the properties of the mat.
Station No. 5 on North River Road in Oceanside is located adjacent to
the San Luis Rey River and walking trail. In the spring of 2005, the San
Diego Regional Water Quality Control Board made the Oceanside Fire
Department aware that the practice of washing fire trucks on the
station’s asphalt pavement was resulting in runoff draining directly
into the riverbed. To temporarily mitigate the problem and contain the
runoff, the fire station installed a gravel bed with a polyurethane
geomembrane liner. The method proved to be ineffective, and other
alternatives were considered by the fire department. Soil Retention’s
Drivable Grass was chosen based on its ability to handle large vehicle
loads and limit runoff by allowing water to infiltrate the pavement
Construction of the fire truck wash area was completed
at the end of September 2005. Drivable Grass concrete paving mat was
installed underlain by a structural section consisting of 8 inches of
Class II aggregate base course placed over a compacted subgrade. The
subgrade was scarified to a depth of 6 inches, moisture conditioned, and
compacted to at least 90% relative compaction. A separator fabric of
MirafiHP-570 was placed between the subgrade and the aggregate base. The
aggregate base was moisture conditioned and compacted to 95% relative
compaction (Figure 1).
1.0- to 1.5-inch topper consisting of sand or sand and organics was
overlaid on the aggregate base. The topper with organics was placed to
facilitate grass growth through the mat. The grooves and holes in the
concrete mat allow roots to penetrate through the mat and reach the
topper soil with organics. The topper consisting of sand only (i.e.,
with no organics) was used as a leveling course where grass was not
The native subgrade consisted of two different soil types:
Dark olive clayey sand with gravel (SC) with a maximum dry density of
131.0 pounds per cubic foot (pcf), optimum moisture content of 9.5%, and
an R-value of 77.
2. Dark brown clayey sand (SC) with a
maximum dry density of 122.0 pcf, optimum moisture content of 12.0%, and
an R-value of 71.
The unit weights and water contents were
different for the two soils due to gravel content, but their R-values
were fairly similar.
Pavement section design was based on the
lowest R-value of 71. The design was completed using the California
Department of Transportation (Caltrans) design method assuming a Traffic
Index of 5.5. Additionally, extra base material was required by the
City of Oceanside. In the design method, the Drivable Grass was
disregarded in computing the structural section.
Fig. 2: Infill Areas
different types of infills were used on top of the concrete mat: sandy
soil with grass seed, sod, and pea gravel with varying amounts of cement
binder. These different infills allowed for direct comparison both
quantitatively and qualitatively between each section. It should be
noted that the topper with organics was only used below infill areas 1
and 2 (Figure 2).
Following the placement of the sod and seed,
the grass areas of the Drivable Grass installation (i.e., Infill Areas 1
and 2) were cordoned off and access was not allowed, because the grass
needed time to become established.
fire trucks were allowed to be driven and washed over the mat
installation with pea gravel only. In January 2006, after the grass
sections became established, they were opened to normal use, and
conclusions were drawn from the acquired data. Figure 13 shows an
example of the mat’s response to a load being applied by a fire truck.
Table 2 shows the specifications for each fire truck that drove over the
area, and Table 3 shows the number of fire truck passes by date. A
visual inspection of the pavement surface was completed at each site
visit. At no time was any cracking of the concrete muffins, or any signs
of permanent deflection, noticed.
Fig. 3: Deflection Measurement Lines
measure any permanent deflection of the concrete mats, manometer
surveys were performed. A manometer can measure very small differences
in elevation by recording relative water level changes. Measurements
were taken on every other muffin cube on marked lines and charted by row
number. The first reading was then set as a baseline and relative
movement measurement was taken from that baseline. Figure 3 shows the
layout of the mats and the location of each of the measurement lines.
first baseline reading was taken on September 28, 2005, before any
driving use was allowed. Measurements were also taken on November 9,
2005; November 30, 2005; December 15, 2005; and February 10, 2006.
1 and 2, as shown in Figure 3, are the grass-planted sections of the
concrete-mat-paved area and therefore were not subjected to as many
passes as the other mat-paved areas. The error of the manometer survey
is ±0.2 inch. Most of the movement detected falls into the error range
(0.2 inch) with potential permanent deflections of less than 0.2 inch
occurring where the wheels passed directly over the mat. Drivable Grass
with the grass infill areas performed very well considering the organics
present underneath the mat.
The concrete muffins never cracked; nor was there any sign of permanent deflection.
For Rows 3 and 4, as shown in Figure 3, no
discernable pattern of movement could be detected, and all movement was
within the error of the survey. As shown in Figure 3, these two rows run
perpendicular to the driving direction of the fire trucks. Row 5 runs
parallel to the driving direction, and a pattern of deflection can be
seen with a maximum deflection of 0.4 inch ±0.2 inch.
has been able to quantitatively and qualitatively describe the
performance of Drivable Grass with repeat high vehicle loading. During
the study, Drivable Grass was subjected to a higher load frequency
and/or higher loads than what would be expected over the service life of
the product for typical uses. Drivable Grass performed very well in
regard to both deflection and structural performance. No cracks in the
concrete muffins were found, even in the areas where very minor
permanent deflections occurred. Comparison of the different infills has
shown that there is no connection found between the type of infill used
and permanent deflection or performance.