West Hills Innovative Stormwater Demonstration
Final Report, Chapter 5: Implementing Recommended Practices, Porous Pavement

What is it? A stormwater facility that you can walk or drive on.

How does it work? When rain falls on it, the rain passes through the surface, into the base rock below, and finally into native, uncompacted soils. For a pavement to be porous, the porosity of the entire section must be carefully protected or created.

Fig 5-7 The highly trampled landscape of project 59 at the school is not just unappealing, it's probably generating as much stormwater runoff as if it were conventionally paved.
Fig 5-8 Permeable pavers could be carefully installed around existing infrastructure and would reduce runoff from this area by 90% annually.

How does this reduce runoff and protect water quality? For the area where they are installed, they reduce runoff by at least 90% annually, even in heavy clay soils.

Where can it be done?

  • Directly next to other infrastructure: Porous pavements can be located anywhere impervious pavements might be used as long as the soils are not so compacted that they don't infiltrate (see design criteria below). Because the facilities manage rainfall and not runoff, basement flooding or pavement undercutting have not been found to be an issue.
  • Not on steep slopes: Mulch and gravel paths and permeable pavers should not be placed on a slope steeper than 10%; instead, boardwalks or decks should be used.
  • Not below impervious surfaces or unstable slopes: Mulch and gravel paths and permeable pavers should not be placed where impermeable surfaces will drain onto path, as this will transport sediment that may clog the surface. Also, any landscaping upslope from walkway must be stabilized with vegetation or stepped walls to prevent erosion of dirt onto walkway. Boardwalks and decks are not subject to these limitations.

What’s the maintenance? Maintenance varies with the surface type and is much lower with proper design (see below). Maintenance activities consist of ensuring that the surface stays porous. Inspections for bare dirt uphill from the pavement should be performed once a year and landscapes stabilized with temporary erosion control and appropriate native plants.

Maintenance of the porous gravel project is only needed if a large portion of the surface is clogged, which, on a pedestrian path that is properly designed and annually inspected and stabilized, will likely not happen for decades. When it does happen, scrape the first inch of rock off the surface and pour water on the surface below to ensure that the pavement drains. If not, scrape off another inch and re-test until it does drain. Rock that has been scraped off can be washed off in an area where adequate sediment control practices (i.e. compost berm at the bottom of a grassy area) have been installed to capture the sediment. Washed rock should then be re-placed on the path. Close the trail until the clean rock has been installed to insure that foot traffic does not clog the exposed subsurface of the path.

Maintenance of permeable pavers should again be relatively low with adequate initial design and ongoing inspections and stabilization. When it does become clogged, the crushed rock between the pavers should be commercially vacuumed out and replaced with clean rock.

Site Investigation Criteria

Site suitability tool. Site suitability can be investigated using the porous pavement siting wizard on the Oregon State University's Stormwater Solutions website.

Infiltration testing. A geotechnical engineer should perform infiltration testing and should also inspect the downhill site to ensure that the site is suitable for infiltrating 90% of our average annual rainfall without causing slides.

Existing pavement areas. All the projects proposed will replace existing impervious pavement areas. This means that the soil below the pavement has already been compacted by heavy machinery. Excavation beyond the depth needed for hydrologic controls and structural stability may be needed to reach a soil strata that can adequately infiltrate. This cannot be predicted, only tested with infiltration testing as recommended above.

Design Criteria

Who can implement these? The fact sheet discusses porous walkways including boardwalks & decks, mulch paths, gravel paths, and permeable pavers (both manufactured and homemade), which can be implemented independently by the HOA as long as infiltration testing confirms adequate drainage. A porous roadway project should be designed by a licensed professional such as a civil engineer or landscape architect and each project should include a licensed geotechnical engineer as well.

Pavement section recommendation. During the design of a porous pavement, especially in a roadway, engage a geotechnical engineer to provide a pavement cross section, which includes the pavement and rock below it (aka base rock) for the site's soils in a wet, uncompacted condition. Traffic loading must be accounted for, both during and after construction. For instance, if heavy equipment will be used to construct the porous walkway, then a thicker pavement section will likely be needed (see construction section below for more information).

Infiltration rate. Guidance on infiltration testing is provided in the fact sheet or your geotechnical engineer will use a method they prefer. To infiltrate the 10-year storm in Portland, soils should drain at 0.10 inch/hour, which is a very low infiltration rate, but which works because we're managing only that rainfall that falls on the pavement surface and not concentrated runoff from other locations.

Aesthestics & cost considerations of surface types. To minimize cost, porous gravel has been proposed to replace the existing gravel pathways at the HOA. For roadway projects, while other surfaces such as pervious concrete and porous asphalt are available, permeable pavers will create a pleasing aesthetic and will be easier to unclog, should that become a problem in the future. For the small projects proposed, the cost of these three different surfaces is unlikely to be very different.

Additional general information on design criteria for roadways can be found in a peer reviewed fact sheet on the Oregon State University's Stormwater Solutions website.

Construction Criteria

Protect trees & native soil infiltration rate. Construction activity and traffic should be limited to the path or roadway areas and not be allowed under the dripline of trees outside of the path or roadway areas; however, construction equipment may not drive over exposed native soils at the bottom of the proposed porous pavement section or the infiltration rate will decrease. For the porous walkway project, where trees will surround the path, construction should occur in stages, excavating the portion of the path that can be reached with the equpiment used and installing porous pavement on that section. Cover the pavement with plywood or similar rigid material to protect it from tracked-on sediment from the construction equipment. Now the porous pavement section that has been installed can be driven on and the next section can be installed.

Where trees will receive a porous pavement surface under their canopy, such as the project featured above, an air spade, which uses compressed air, should be used to excavate around all roots.

If the native soils at the botttom of the porous pavement must be left exposed for any period of time, especially during the rainy season, cover them temporarily with a geotextile fabric. Clayey soils are easily clogged by the action of rain drops hitting it and resorting the particles, which creates an impermeable sheen. If this is allowed to happen, it will negate the value of the porous pavement, essentially creating an impervious surface at the bottom of the pavement. If soil gets clogged during construction, experience has shown that the contractor will have to wait until the soil dries out, then rake it. (If it's the middle of the winter, this could set the project back months.)

Track, floatation tire, or small equipment. Porous pavements should be installed with equipment designed to spread the weight of their own load out over a large area or equipment that's small and not very heavy to begin with.

Clogging narrative. The contractor should be asked to write a brief narrative of how he or she will prevent clogging from occuring throughout the installation of the entire cross section of the pavement.

Washing the rock. Most crushed aggregate is delivered "clean" to a standard known as a 2% wash loss; however, the sediment that's left on the rock may still be enough to eventually clog the geotextile fabric. For this reason, all rock should be washed on-site. This may be done by hosing the rock off while still in the delivery truck or after stockpiling. Scoop from the top and place rock. Hose off as needed as the pile diminishes since fines will migrate to lower levels of the pile.

Schematic Details
Porous Gravel Schematic Detail

This detail is proposed to replace the existing gravel paths:


Permeable Paver Schematic Detail
This detail may be used for roads or walkways:

Specifications & Other Definitions

Crushed aggregate. Rock quarried with a crusher is angular, not rounded. Angular rock is required to ensure structural stability and all rock used in pavements should be crushed, not rounded.

Open-graded (aka poorly graded) rock. Rock diameters are all similar in size (i.e. 1.5-1 inch rock, AASHTO No. 57, AASHTO No. 8), which creates voids between the rock where water is stored until it can infiltrate. This is the only kind of rock gradation that should be used in porous pavements. "Drain rock" may be open- (poorly graded) or well-graded rock, so take care to make sure you’re ordering open-graded rock.

The specification for gravel walkways is provided above in the detail itself as 1/4"x10 landscape rock. For roadways, additional specifications are needed as followings:

Adapted from materials courtesy of Tom Cahill of Cahill Associates:
A. Base Course
1. All aggregates beneath the pavement shall meet the following:
a. Maximum Wash Loss of 0.5%
b. Minimum Durability Index of 35
c. Maximum Abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions
2. Unless otherwise approved by the Engineer, coarse aggregate for the aggregate base course shall be uniformly graded with the following gradation (AASHTO No. 57)
U.S. Standard
Sieve Size
Percent
Passing
1 ½” (37.5 mm)
100
1” (25 mm)
95-100
½” (12.5 mm)
25-60
4 (4.75 mm)
0-10
8 (2.36 mm)
0-5
3. Unless otherwise approved by the Engineer, the bedding course shall be uniformly graded with the following gradation (AASHTO No. 8)
U.S. Standard
Sieve Size
Percent
Passing
½” (12.5 mm)
100
3/8" (9.5mm)
85-100
4 (4.75 mm)
10-30
8 (2.36 mm)
0-10
16 (1.18 mm)
0-5

Non-woven (free draining) geotextile fabric. Non woven geotextile (drainage filter fabric) shall conform to the following:
1. Minimum flow rate of 95 gal/min/ft2 ASTM D-4491-85
2. Grab tensile strength min 115 lb ASTM D-4632-86
3. Burst strength min 150 psi ASTM D-3786-80a
4. Puncture resistance min 45 lb ASTM D-4833-88
5. Apparent opening size 60-90 U.S. Standard Sieve

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