Permeable Interlocking Concrete Pavement

This page serves as an introduction to the design considerations for PICP when it is part of an MSD-approved stormwater management plan.

• Designer Notes
• Plan Notes
• Manufacturer’s Prequalification
• Material Certification
• Contractor Prequalification
• As-Built Certification

MSD has developed details illustrating typical sections for PICP.

PICP Section Detail
PICP Sign

1. The designer should consider the following when designing a Permeable Interlocking Concrete Pavement system:
2. Permeable Interlocking Concrete Pavement may be used to provide water quality volume (WQv) provided it is designed and constructed in accordance with Permeable Pavement Performance Criteria or is used in conjunction with a properly designed infiltration basin/trench, sand filter, or other approved BMP per the 2000 Maryland Stormwater Design Manual. The supplemental BMP shall be located downstream from the permeable interlocking concrete system, not underneath it.
3. The design engineer shall reference the following resources published by the Interlocking Concrete Pavement Institute; Permeable Interlocking Concrete Pavements Manual, and the PICP Guide to Construction Specifications. The Permeable Design Pro software is recommended to aid in the design; all are available from the www.icpi.org website. Also reference the MSD Provisional Use Level approval letters for PICP to obtain additional design guidelines, available at www.stlmsd.com.
4. Consideration should be given to the type of traffic loads placed upon the PICP. An engineer experienced with pavement design shall provide input on this aspect early in the design phase. The minimum pavement section subject to vehicular traffic shall be (from top to bottom) 3 1/8 inch thick paving units, 2-inch thick bedding stone (typically ASTM No. 8 or 9 stone), 4-inch thick layer of base stone (ASTM No. 57 or similar size), and a layer of subbase stone (ASTM No. 2 or similar size), all underlain by MSD Type 4 filter fabric (see note 11) where needed. The thickness of the subbase will vary depending on storage and anticipated traffic loads as described in ICPI guidance literature. However, a minimum 12-inch thick subbase will be required for all applications.
5. PICP will not be allowed on sites whose land use or activity generates higher concentrations of hydrocarbons, trace metals or toxicants than are found in typical stormwater runoff. Examples of such “hotspots” are listed in table 2.6 in the 2000 Maryland Stormwater Design Manual.
6. A perforated PVC underdrain system will be required. The horizontal and vertical locations shall be determined by the design engineer, depicted on the drawings, and be in general accordance with the MSD Permeable Interlocking Concrete Pavement detail.
7. To provide an infiltration bed, the primary underdrain system may be perched or configured as internal water storage within the subbase rock. The perched height is dependent on the infiltration characteristics of the underlying subgrade and the desired storage. The underdrain system shall not be perched if the underlying soils are plastic or plastic soils that have been stabilized with lime. Design the open graded storage bed to completely drain within 12 to 72 hours.
8. When PICP is used in conjunction with stormwater runoff volume reduction practices, MSD recommends the suitability and infiltration rates of the underlying soils shall be confirmed by geotechnical evaluation prior to final design. Soil boring logs and a soil survey shall accompany all test results. Designers shall also consider the depth of the porous pavement system in determining the applicability of the information. If infiltration testing is not conducted prior to design, then the underlying soils shall be assumed to have an infiltration rate of 0.05 in/hr. (“default” subgrade saturated hydraulic conductivity). If use of a greater design rate is desired for the purpose of estimating volume reduction capabilities, at least one infiltration test shall be provided for each onsite soil type where porous pavement is proposed. In no case shall the frequency of testing be less than 1 per 5000 S.F of PICP. All infiltration test reports shall be submitted for MSD review prior to final project approval.
9. Provide an overflow system or additional storage to prevent water in the open graded storage bed from rising into the bedding course or PICP for the 2-yr, 24-hr storm event. Any emergency overflow pipes placed within the subbase should discharge only when the storage volume is exceeded. The horizontal and vertical location of all underdrains and overflow systems shall be determined by the design engineer and shown on the plans. Emergency overflow pipes and underdrains systems shall discharge to a storm water structure or daylight to a stabilized outfall.
10. Observation wells should be provided in low areas within the permeable pavement system, and shall extend to the bottom of the storage bed. The wells shall consist of a six inch schedule 40 PVC pipe with cast iron frame and cover, as shown on the MSD PICP details. The number of wells required on an installation will vary on a site specific basis, and will be determined by the design engineer.
11. A minimum of 2 feet vertical clearance should be provided between the bottom of the storage bed and the seasonally high water table.
12. Placing PICP over areas of recent fill or compacted fill is not optimal for infiltration.
13. The volume in the voids of the open graded base and storage bed shall be considered the total water storage capacity. A void ratio not greater than 40 percent will be used to calculate storage volume. No storage volume will be allowed in the paving units or bedding stone layer. The emergency overflow apparatus, if any, should discharge only when the storage volume is exceeded.
14. Prepared subgrade soil should be separated from the porous subbase. The prepared subgrade soil shall be dry and, where needed, covered with MSD Type 4 filter fabric (such as Mirafi 140N or approved equal) on the vertical sides of the typical section. Separation of the prepared subgrade along the bottom of the section may be provided by MSD Type 4 filter fabric or a well graded filter layer at the discretion of the design engineer.
15. Subgrade should normally be compacted to a minimum density of 90% to 95% of the theoretical density per AASHTO T 180. However, the designer should be aware that increasing the subgrade density decreases its infiltration capacity. Since soil conditions, grading requirements, and end user demands are site specific, this requirement shall be considered from a case by case standpoint. A geotechnical engineer shall provide input in balancing the infiltration and load bearing demands that the PICP must satisfy.
16. A permanent sign shall be posted warning that care should be taken during snow plowing; and prohibit the following: resurfacing, the use of sand abrasives for winter tire traction, and the use of power washers.
17. The property owner must execute a Maintenance Agreement with MSD, and the entire PICP area shall be platted in a Reserve Area for Stormwater Management.
18. The design of the finished grade of the pavement surface shall take into consideration the Americans with Disabilities Act (ADA) and local code requirements.
19. Subgrade should be flat, and where topography requires, terracing of the subgrade is allowed. The volume of water stored behind each terrace will be considered part of the infiltration bed.

The following notes, specifications, and details for Permeable Interlocking Concrete Pavements shall be included on the plans proposed for MSD approval:

1. The contractor shall obtain the concrete paver manufacturer’s certification that the paving units supplied to construct the PICP have been approved by MSD and meet the requirements in ASTM C936. This certification shall be provided to the MSD Division Inspector. The certification shall include the manufacturer’s name, and state that the PICP supplied meets the ASTM C936 specifications, (testing should be current within previous 12 months) and that the paving materials meet all requirements as evaluated under the manufacturer’s quality control program.
2. Prior to obtaining a construction permit from MSD to construct the permeable interlocking concrete pavement (PICP) for a given project, the engineer providing the as-built certification shall verify that the installing contractor has:
   • Past history demonstrating applicable experience.
   • The PICP installation contractor must have a current Level 1 certificate from the Interlocking Concrete Pavement Institute’s concrete paver installer program.
3. The contractor shall prevent and divert sediment from entering the subbase and pavement surface until the tributary areas are deemed stable by the assigned MSD inspector.
4. Vehicular traffic shall be prohibited on the PICP until the site is stable to prevent mud from being deposited by vehicles.
5. No product or material substitutions are permitted unless previously approved by the MSD plan review engineer or by the MSD field inspector assigned to the project. All substitutions shall be presented to MSD through the engineer responsible for the design of the PICP system.
6. Stone should be clean, washed, 90 percent fractured faces with a Los Angeles Abrasion Index of less than 40 and conform to the grading requirements in ASTM D448.
7. Do not clean the paver surface with high-pressure hoses or abrasives. When cleaning is necessary, combination cleaning machines that combine a wet spray and vacuum process have been found to be effective.
8. A permanent sign shall be posted warning that care should be taken during snow plowing; and prohibit the following: resurfacing, the use of sand abrasives for winter tire traction, and the use of power washers.
9. At completion of the project, prior to final dedication, an as-built certification, signed and sealed by a Missouri Professional Engineer, shall be provided.

The PICP manufacturer shall be responsible for establishing and maintaining a quality control program to assure compliance with requirements in ASTM C936. Prior to use on projects requiring MSD approval, the manufacture shall submit five (5) copies of a completed pervious paving application as well as documentation describing the quality control program. The completed application and other documentation shall be submitted to:

MSD BMP Committee
Metropolitan St. Louis Sewer District
2350 Market Street
St. Louis, Missouri 63103-2555

The contractor shall obtain the concrete paver manufacturer’s certification that the paving units supplied to construct the PICP have been approved by MSD and meet the requirements in ASTM C936. This certification shall be provided to the MSD Division Inspector. The certification shall include the manufacturer’s name, and state that the PICP supplied meets the ASTM C936 specifications, (testing should be current within previous 12 months) and that the paving materials meet all requirements as evaluated under the manufacturer’s quality control program.

Prior to obtaining a construction permit from MSD to construct the permeable interlocking concrete pavement (PICP) for a given project, the engineer providing as-built certification shall verify that the installing contractor has:

1. Past history demonstrating applicable experience.
2. The PICP installation contractor must have a current Level 1 certificate from the Interlocking Concrete Pavement Institute’s concrete paver installer program.

At completion of the project, prior to final dedication, an as-built certification, signed and sealed by a Missouri Professional Engineer, shall be provided certifying:

1. The PICP system was built in accordance with the details, dimensions, and materials as approved by MSD for this project.
2. The PICP system was installed by a qualified contractor, and has satisfied all applicable quality control and performance tests.
3. The PICP system installation was witnessed by the certifying engineer or a representative under his direct supervision.