Permeable pavement: Plan review
- Do the plans include an appropriate construction sequence that is specific to the construction of the permeable pavement?
- To avoid soil disturbance and compaction during construction, will the proposed infiltration area be clearly marked before any site work begins?
- Do the plans include an appropriate cross-sectional detail for the permeable pavement?
- Verify the drainage area directed to any proposed porous pavement. If DCIA runoff from other areas on-site, in addition to the direct (1:1) rainfall onto the porous pavement, is directed onto porous pavement, the porous pavement cannot be considered disconnected. It must instead be considered, and modeled as, a porous pavement structural SMP. The porous surface must be considered, and modeled as, DCIA. Porous pavement structural SMPs require infiltration testing.
- For porous pavement structural SMPs, if infiltration is feasible, verify that the porous pavement design meets all Design Guidance Checklist design standards noted in Appendix F.10, Subsurface Infiltration.
- For porous pavement structural SMPs, if infiltration is infeasible, verify that the porous pavement design meets all Design Guidance Checklist design standards noted in Appendix F.14, Subsurface Detention.
For porous pavement DIC systems:
- Verify that the porous pavement DIC is installed on-site such that it does not create any areas of concentrated infiltration or discharge.
- Verify that the surface slope in any direction across porous pavement does not exceed 5%.
- Verify that the choker course depth is a minimum of two inches.
- If an underdrain is proposed, verify that the first 1.5 inches of runoff are stored below the lowest invert of the underdrain.
- Verify that an appropriate porous pavement curve number value is used when performing Flood Control calculations.
- Verify that the stone storage bed depth is a minimum of eight inches, except when located beneath walkways or play surfaces, for which a depth of four inches is allowable.
- Verify that stone is separated from soil media by a geotextile or a pea gravel filter to prevent sand, silt, and sediment from entering the system.
- Verify that the stone storage system has a level bottom. Terraced systems may be used to maintain a level infiltration interface with native soil while accommodating significant grade changes.
- Verify that pretreatment is provided for all runoff entering the porous pavement, including pretreatment of runoff from all inlets. At a minimum, this can be achieved through the use of sumps and traps for inlets, sump boxes with traps downstream of trench drains, and filter strips for overland flow.
- Verify that, when SMPs are used in series, the storage areas for all SMPs provide cumulative static storage for the WQv.
- Verify that any impervious liner, if necessary, is not interrupted by structures within the basin footprint. The plans must indicate that the impervious liner is to be continuous and extend completely up the sides of any structures that are located within the lined basin footprint to the ground surface. If additional liner material must be added to extend up the structures, the additional liner sections are to be joined to the rest of the liner with an impervious seam per the manufacturers’ recommendations.
- Verify that underdrains, if proposed for porous pavement DIC systems, meet the following requirements:
- Underdrains must be surrounded by a sand or stone layer to filter sediment and facilitate drainage.
- The minimum allowable thickness of a sand or stone filter layer is six inches both above and beneath the underdrain.
- To prevent clogging, underdrain pipes must be surrounded by a geotextile fabric if a sand layer is used.
- Verify that an adequate number of appropriately placed cleanouts, manholes, access panels and other access features are provided to allow unobstructed and safe access to the porous pavement structural SMP for routine maintenance and inspection of inflow, outflow, underdrains, and storage systems.
- Verify that an observation well is provided for a storage system that includes stone storage and that it meets the following requirements:
- The observation well must be placed at the invert of the stone bed.
- An observation well must be located near the center of the stone bed system to monitor the level and duration of water stored within the SMP (drainage time).
Adequate inspection and maintenance access to the observation well must be provided.
- A manhole may be used in lieu of an observation well if the invert of the manhole is installed at or below the bottom of the SMP and the manhole is configured in such a way that stormwater can flow freely between the SMP and the manhole at the SMP’s invert.
- Verify that access features are provided for all underground storage systems that are not stone storage beds.
- Verify that a sufficient number of access points in the system are provided to efficiently inspect and maintain the infiltration area.
For cast-in-place vault systems, verify that access features consist of manholes or grated access panels or doors. Grated access panels are preferred to maintain airflow.
For grid storage or other manufactured systems, verify that the manufacturer’s recommendations are followed.
- Verify that ladder access is proposed for vaults greater than four feet in height.
- Verify that header pipes, at minimum 36-inch diameter, connected to manholes at each corner of the subsurface system are provided. Alternatively, smaller header pipes may be used if cleanouts are provided on every second manifold pipe/header pipe junction, on alternating sides of the SMP.
- Verify that porous bituminous asphalt, if proposed, is specified on the plans as meeting the following specifications:
- Bituminous surface must be laid with a bituminous mix of 5.75% to 6% by weight dry aggregate.
- In accordance with American Society of Testing and Materials (ASTM) D6390, drain down of the binder must be no greater than 0.3%.
- Aggregate material in the asphalt must be clean, open-graded, and a minimum of 75% fractured with at least one fractured face by mechanical means of each individual particle larger than ¼-inch, and it must have the following gradations:
Porous Asphalt Binder Course Aggregate Gradation U.S. Standard Sieve Size Percent Passing By Weight 1” 100% 3/4” 90-100% 1/2” 80-100% 3/8” 50-80%
- 4 10-20%
- 8 5-10%
- 40 3-8%
- 200 0-3 %
Porous Asphalt Wearing Course Aggregate Gradation U.S. Standard Sieve Size Percent Passing By Weight 5/8” 100% 1/2” 95-100% 3/8” 70-95%
- 4 20-40%
- 8 10-20%
- 40 0-8%
- 200 0-3%
Neat asphalt binder modified with an elastomeric polymer to produce a binder meeting the requirements of PG 76-22 as specified in American Association of State Highway and Transportation Officials (AASHTO) MP-1. The elastomer polymer must be styrene-butadiene-styrene, or approved equal, applied at a rate of 3% by weight of the total binder. Hydrated lime should be added at a dosage rate of 1% by weight of the total dry aggregate to mixes containing granite. Hydrated lime must meet the requirements of ASTM C 977. The additive must be able to prevent the separation of the asphalt binder from the aggregate and achieve a required tensile strength ratio of at least 80% on the asphalt mix when tested in accordance with AASHTO T 283. The asphaltic mix must be tested for its resistance to stripping by water in accordance with ASTM D-1664. If the estimated coating area is not above 95%, anti-stripping agents must be added to the asphalt. The asphaltic mix must be tested for its resistance to stripping by water in accordance with ASTM D 3625. If the estimated coating area is not above 95%, anti-stripping agents must be added to the asphalt. Verify that porous concrete, if proposed, is specified on the plans as meeting the following specifications: [Section 4.2.4, 3] Porous concrete must use Portland Cement Type I or II conforming to ASTM C 150 or Portland Cement Type IP or IS conforming to ASTM C 595. Aggregate must be No. 8 coarse aggregate (3/8-inch to No. 16) per ASTM C 33 or No. 89 coarse aggregate (3/8-inch to No. 50) per ASTM D 448. An aggregate/cement ratio range of 4:1 to 4.5:1 and a water/cement ratio range of 0.34 to 0.40 should produce porous pavement of satisfactory properties in regard to permeability, load carrying capacity, and durability characteristics.
Verify that permeable paver and grid systems, if proposed, are specified on the plans as meeting the following specifications:
- Permeable paver and grid systems must conform to manufacturer specifications.
- The systems must have a minimum flow through rate of five inches per hour and a void percentage of no less than 10%.
- Gravel used in interlocking concrete pavers or plastic grid systems must be well-graded and washed to ensure permeability.
- Verify that stone designed for stormwater storage, if proposed, is specified on the plans as being uniformly graded, crushed, clean-washed stone and that it is noted that PWD defines “clean-washed” as having less than 0.5% wash loss, by mass, when tested per the AASHTO T-11 wash loss test. AASHTO No. 3 and No. 57 stone can meet this specification.
- Verify that all aggregates used within a porous pavement system meets the following requirements:
Maximum wash loss: 0.5% per AASHTO T-11 Minimum durability index: 35 per ASTM D3744 Maximum abrasion: 10% for 100 revolutions and 50% for 500 revolutions per ASTM C131 Verify that all choker course aggregate meets the specifications of AASHTO No. 57 and meets the gradation listed in Table 4.2-3 of the Manual. Verify that sand, if proposed, is specified on the plans to be AASHTO M-6 or ASTM C-33 sand and to have a grain size of 0.02 inches to 0.04 inches. Verify that storage chambers for porous pavement structural SMPs, if proposed, are specified on the plans as meeting the following specifications: Pipe used within a subsurface infiltration SMP must be continuously perforated and have a smooth interior with a minimum inner diameter of four inches. High-density polyethylene (HDPE) pipe, if proposed, must meet the specifications of AASHTO M252, Type S or AASHTO M294, Type S. Any pipe materials outside the SMP are to meet City Plumbing Code Standards. Verify that geotextile, if proposed, is specified on the plans to consist of polypropylene fibers and to meet the following specifications (AASHTO Class 1 or Class 2 geotextile is recommended): [Section 4.2.4, 9] Grab Tensile Strength (ASTM-D4632): ≥ 120 lbs Mullen Burst Strength (ASTM-D3786): ≥ 225 psi Flow Rate (ASTM-D4491): ≥ 95 gal/min/ft2 UV Resistance after 500 hrs (ASTM-D4355): ≥ 70% Heat-set or heat-calendared fabrics are not permitted Verify that underdrains, if proposed, are made of continuously perforated HDPE plastic piping with a smooth interior and a minimum inner diameter of four inches. HDPE pipe must be specified on the plans to meet the specifications of AASHTO M252, Type S or AASHTO M294, Type S. Verify that observation wells are specified on the plans as consisting of perforated plastic pipe with a minimum inner diameter of six inches. Verify that cleanouts are made of rigid material with a smooth interior having a minimum inner diameter of four inches.