Permeable interlocking concrete pavement (PICP) is gaining increased use in residential and commercial projects thanks to the need by U.S. and Canadian municipal governments to curb stormwater runoff.
Many PICP designs take advantage of the soil infiltration to reduce stormwater runoff even in clay soils. Unlike interlocking concrete pavement (ICP), PICP is generally built on native, undisturbed, non-compacted soils in order to promote infiltration. Compaction greatly reduces a soil’s infiltration capacity. If the soil subgrade is compacted, the amount of water infiltration decreases 50 to 90 percent with the higher percentage applying to clay soils. Therefore, the Interlocking Concrete Pavement Institute (ICPI) recommends not compacting soils under PICP in many designs beyond grading and trimming for drainage.
Figure 1. ICPI recently launched a PICP certificate course for contractors. Besides meeting training needs within the industry, the ICPI is encouraging PICP course certificate holders as a requirement in federal, provincial, state and municipal construction specifications, as well as private sector projects.
Figure 2. Elmhurst College in Elmhurst, Illinois used very thick bases to accomplish underground detention/infiltration for landscape irrigation in certain parts of a large PICP parking lot. The thick bases provided additional structural support for heavy trucks.
Figure 3. Equipment spreading aggregate and rides on the aggregate to keep it clean.
Figure 4. Vacuum equipment draws out dirt and soils stone from a PICP. The openings are refilled with stones. Such equipment would likely be required in removing dirt and sediment pressed into PICP openings during construction.
Figure 5. Truck washing equipment might be a cost-effective solution to keeping mud and sediment from PICP surfaces during construction.
Figure 6. Typical cross section of a residential driveway using dense-grade base berms for anchoring edge restraints. Drainage at the lowest end (typically at the street) can be handled with a pipe running into the adjacent lawn with a pop-up drain at the surface.
Figure 7. Dense-graded aggregate base material forms berms along the sides of this PICP residential driveway. Open graded aggregate base is placed in the center to store and infiltrate water.
Figure 8. Both bases—dense-graded and open-graded—are compacted to the same elevation.
Figure 9. A plastic edge restraint is nailed into the compacted, dense-graded base material. Permeable pavers are installed per manufacturers and ICPI recommendations.
Figure 10. PICP that meets asphalt should be separated with a concrete curb or beam.
Figure 11. PICP and poured-in-place concrete meet. A best practice is to separate the bases between these two pavement systems with an impermeable liner.
All photos courtesy of the Interlocking Concrete Pavement Institute
Excavation and grading equipment passing over the soil subgrade surface results in some compaction, and increased density results in decreased soil infiltration. Compaction incidental to construction equipment passing over soil is generally not to the same depth from deliberate soil compaction with equipment typical to parking lot or road construction. Nonetheless, such incidental compaction helps defeat the infiltration benefits of soil in PICP. Designers need to consider this condition and provide reduced soil infiltration in their base and drainage designs. Better still, designers and contractors can specify ways to preserve or restore soil infiltration during construction and some ways to accomplish this are presented here.
If a contractor inadvertently compacts the soil subgrade with construction equipment, the soil can be raked with the teeth of a bucket to loosen the compacted soil layer. An interesting research project by engineer and University of Tennessee Professor John S. Tyner examined the potential for restoring infiltration rates in clay Tennessee soil prior to installing pervious concrete pavement. The researchers’ summary noted the following:
Several types of treatments were applied to the clay soil prior to placement of the stone aggregate base and pervious concrete in an attempt to increase the exfiltration rate, including:
(1) Control – no treatment;
(2) Trenched – soil trenched and backfilled with stone aggregate;
(3) Ripped – soil ripped with a subsoiler; and
(4) Boreholes – placement of shallow boreholes backfilled with sand.
The average soil infiltration rates from lowest to highest were:
(1) Control – 0.013 in./hr (0.8 cm/day)
(2) Boreholes – 0.0625 in./hr (4.6 cm/day)
(3) Ripped – 0.375 in./hr (10.0 cm/day)
(4) Trenched – 0.4375 in./hr (25.8 cm/day)
While the aggregate-filled trenches drained the fastest due to their 9-foot-plus depth, the ripped soil cut to more than 17 inches deep drained reasonably well using a soil-ripping device on a bulldozer. Clean course sand was spread across the surface to fill in cracks in the soil, which further facilitates infiltration. While the depth of the ripped soil in this research project is substantial, the effects of ripping demonstrate some infiltration improvement over the control, which was native, uncompacted soil. Its infiltration rate could have been lower still if compacted by construction equipment.
When there is a period of time between excavation and aggregate placement, sometimes excavating all but the last 6 inches of soil can help preserve soil infiltration. The open excavation can collect water and sediment, and this area can be traversed with construction equipment. When the aggregate is ready to install, the final 6 inches of soil can be excavated, and the base/subbase aggregate installed.
Another approach is using equipment that can excavate from the sides of the opening without entering it and compacting the soil subgrade. Power shovels with a long reach can accomplish this. The horizontal and vertical reach of these shovels is limited, so their effectiveness is governed by the dimensions of the excavated area.
For PICP subjected to regular truck traffic, deliberate compaction of low-California Bearing Ratio soils (CBR less than 4 percent, typical slow-draining clays) may be necessary to attain sufficient structural support and minimize rutting. These soils should be compacted to at least 95 percent of standard laboratory Proctor density. Subdrains in the open-graded base will likely be required to remove water, since compaction will greatly reduce the soil’s permeability. As previously noted, the design engineer determines the compaction required for weak soils. Most low-CBR soils have low infiltration rates, so compaction to achieve additional structural support may not greatly impact infiltration.
Another approach is to use thicker bases or place deep bases (more than 2-feet thick) that function as detention facilities while providing additional base thickness to support concentrated loads from trucks. Figure 2 illustrates this at a parking lot in Elmhurst College near Chicago. A portion of the parking lot provides underground detention to provide water for landscape irrigation.
Keeping aggregates clean
One of the big challenges in PICP construction is keeping the aggregates clean during delivery, storage and placement. If dirty aggregate for the subbase or base is installed in sufficient quantities, the dirt can reduce subgrade soil infiltration. Dirty aggregates (and the fines on them) may also impair their ability to lock together during compaction, and remain as such during service. A best practice is clean tires and tracks on equipment that pushes the aggregate in front so the equipment rides on aggregate during distribution and spreading. The equipment does not ride over the bare soil subgrade (see Figure 3).
If aggregates are stored on the job site, they should be placed on an impervious pavement, on a finished portion of PICP, or over geotextile if stored on grass or bare soil. The best situation is having aggregates delivered and dumped into an excavated area for a residential driveway, commercial parking lot or road. A prepared excavation means one graded to specified elevations with drain pipe and geotextile in place as specified by the designer. Although geotextile may or may not be specified to cover the top of the soil within the excavation, it is a best practice to cover the sides. This helps prevent soil eroding into the base and subbase materials over time.
Project planning before and timing during construction are important to keeping aggregates clean. Most PICP is built just before or during the construction of buildings or site features. It is rarely installed last. Therefore, PICP can be incorrectly designated as an access road and be subject to dirt deposited from truck tires or from minor soil erosion with no specifications (or funds) to clean the surface. Some options that should be included in the projects specifications and discussed at the pre-construction meeting, are as follows:
Option 1: Establish a temporary road for site access that does not allow vehicular traffic to contaminate the PICP materials and surface. Post signs and inform all trades to use the temporary road rather than the PICP. Cost: High.
Option 2: Construct the PICP aggregate subbase and base. Protect the surface of the open-graded base with geotextile and place an approximately 2 inch-thick layer of the same base aggregate over the geotextile. Thicken this layer to match elevations at adjacent impervious pavement surfaces where vehicular traffic enters/exits the PICP area. This should provide a transition to and from existing driveway, parking lot and road surfaces. When other construction around the PICP is completed, remove geotextile and soiled aggregate and install the remainder of the PICP system (i.e. bedding, pavers and jointing materials). Any settlement due to construction traffic can be repaired prior to placing the bedding, paver and jointing materials. Cost: Low.
Option 3: Allow construction vehicles to use the finished PICP system (i.e. paver joints and/or openings filled with aggregate). Prior to opening, cover the surface with a geotextile and a 2-inch-thick open-graded aggregate layer. This “sacrificial” layer is the riding surface during construction. Transition areas to other pavements will require additional aggregates for vehicles to ride from impervious pavements to PICP. Remove the soiled aggregate and geotextile when construction is complete. Cost: Low.
Option 4: Allow construction vehicles to use the finished PICP. At the conclusion of construction activities, vacuum out the dirt from the joints. The vacuuming process should also remove some of the stones, at least the portion that jammed with sediment. Refill the cleaned out joints with clean stones. The vacuuming operation requires use of powerful full or true vacuum equipment. Regenerative air vacuums or sweepers will not be effective in removing dirt jammed in the joints. The operator of the vacuum equipment will need to adjust the vacuum force such that the dirt is drawn out with some stones, but not so much that all the stones are withdrawn from the paver joints and/openings. Figure 4 illustrates this type of equipment cleaning PICP openings and joints. Cost: Low to medium.
Another option is providing a truck washing system on the job site. These are portable systems (in various sizes) that wash mud from truck tires and undercarriages and capture the sediment for later removal from the site. These systems require a water source and are very effective in reducing sediment deposits on PICP from construction equipment. Figure 5 illustrates this equipment in action.
Like interlocking concrete pavements, edge restraints for PICP are essential. Commercial vehicular areas should use cast-in-place concrete, precast concrete or cut stone curbs. Commercial and residential pedestrian applications and residential driveways can use these types as well. A less expensive alternative can be installing a compacted base at the PICP perimeter with plastic or metal edging spiked on it with typical 3/8-inch diameter by 10-inch-long nails. The center area of the driveway or pedestrian application utilizes open-graded base materials. Figure 6 illustrates a typical cross section. While Figures 7, 8 and 9 illustrate the construction process.
When concrete curbs are poured, they are often positioned over a layer of open-graded subbase material such as ASTM No. 2 stone. The interior of the curb formwork will need a layer of geotextile placed at the bottom to prevent loss of wet concrete poured over the No. 2 stone. Without the fabric to stop the concrete, it will easily migrate into the openings among the stones. When using precast curbs or granite curbs, a cradle-type footer of concrete haunches to support the curbs should be considered.
When PICP pavement abuts a compacted, dense-graded aggregate (DGA) base (typically surfaced with asphalt), a vertically placed impermeable barrier is recommended to segregate it from the open-graded aggregate supporting the PICP. An at-grade concrete beam is sufficient to divide asphalt from the PICP surface (see Figure 10). The beam is minimum 6 inches wide, resting on the No. 2 subbase or extending to the bottom of the No. 57 material. The DGA should have geotextile under it separating it from the soil subgrade.
The PICP surfaces that abut poured-in-place concrete slabs without a curb (as shown in Figure 11) will need a vertical liner to segregate the DGA aggregates and soils from the PICP base materials. Whether next to asphalt or concrete pavements, the PICP subgrade should slope away from the DGA area at a minimum of 1 percent. This helps move water away from the DGA under the impermeable pavements.
PICP construction requires special attention to details not normal to constructing interlocking concrete pavements. Avoiding soil compaction, keeping aggregates and the PICP surface clean, plus using edge restraints appropriate to the application require cost and execution planning when preparing a bid. This article only covered a part of many PICP construction aspects. The ICPI PICP Installer Technician Course is an opportunity to fully dive into project planning, execution and maintenance (see sidebar for more information).
Article provided by the Interlocking Concrete Pavement Institute (www.icpi.org)
Tyner, J.S., Wright, W.C. and Dobbs, P.A., “Increasing Exfiltration from Pervious Concrete and Temperature Monitoring,” Journal of Environmental Management, Vol. 90 (2009), pages 2636-2641.
PICP Installer Technical Certificate Course Student Manual, First Edition, Interlocking Concrete Pavement Institute, Herndon, VA, 2009.
PICP Installer Technician Certificate course
To increase the assurance of quality installations, the Interlocking Concrete Pavement Institute (ICPI) recently initiated a PICP Installer Technician Certificate course aimed at educating contractors on the specifics of PICP construction (see Figure 1). This one-and-a-half day course covers a broad range of topics that help ensure contractors understand that PICP requires a higher level of construction skill and attention to details. The course covers residential and commercial construction including:
Comparison of PICP to ICP
Job planning and documentation
Job layout, flow and estimating materials including average productivities for various job functions
Soil and site characteristics
Subbase and base materials
Bedding and jointing materials
Selection and installation of permeable interlocking concrete pavers
The courses are sponsored by ICPI members across North America. Contractors who take the course and pass the exam receive a certificate which is being promoted by ICPI in PICP guide specifications. A recent example is requiring that the contractor have a certificate as part of guide construction specifications submitted to Caltrans. ICPI is planning to submit such requirements in PICP guide specifications to other provincial, state and local agencies.