HomeMy Public PortalAbout4747401
October 2012
POROUS ASPHALT PAVEMENT
This document serves as an introduction to the design considerations for Porous Asphalt
Pavement when it is part of a stormwater management plan.
DESIGNER NOTES:
The designer should consider the following when designing a porous asphalt pavement system:
1. Porous asphalt pavement may be used to provide water quality volume (WQv),
provided it is designed and constructed in accordance with the March 16, 2012 Stand
Alone Water Quality BMP policy change letter (Insert Link Here) 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 porous asphalt system, not
underneath it.
2. Designer should reference National Asphalt Pavement Association (NAPA)
Information Series 131 for additional guidelines.
3. Consideration should be given to the type of traffic loads that the pervious pavement
will be required to endure. An engineer with expertise in pavement design should
provide input on this aspect early in the design phase.
4. Porous Asphalt Pavement 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.
5. 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.
6. A minimum of 2 feet vertical clearance should be provided between the bottom of the
storage bed and the seasonally high water table.
7. Placing Porous Asphalt Pavement over areas of recent fill or compacted fill is not
optimal for infiltration.
8. Provide an overflow system or additional storage to prevent water in the open graded
storage bed from rising into the Porous Asphalt Pavement for the 2-yr, 24-hr storm
event. Any emergency overflow pipes placed within the subbase should discharge
2
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.
9. When Porous Asphalt Pavement 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 Asphalt 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 on site soil type where Porous Asphalt is
proposed. In no case shall the frequency of testing be less than 1 per 5000 S.F. of
Porous Asphalt. All infiltration test reports shall be submitted to MSD for review
prior to final project approval.
10. Observation wells should be provided in low areas within the Porous Asphalt
Pavement system, and shall extend to the bottom of the aggregate 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 Porous Asphalt Pavement detail. 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 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.
12. The property owner must execute a Maintenance Agreement with MSD, and the
entire Porous Asphalt Pavement area shall be platted in a Reserve Area for
Stormwater Management.
13. 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 Porous Asphalt Pavement detail.
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.
3
15. 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.
16. The volume in the voids of the 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 porous asphalt. The emergency
overflow apparatus, if any, should discharge only when the storage volume is
exceeded.
17. 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 Porous Asphalt Pavement must satisfy.
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.
PLAN NOTES:
The following notes shall be included on plans, details, and specifications for porous asphalt
pavement designs proposed for MSD plan approval:
1. The contractor shall verify that the porous asphalt producer that will supply the Porous
Asphalt for this project has been prequalified by MSD. The name of the producer and
their facility location shall be provided to the MSD Division Inspector prior to
construction.
2. If porous pavement area is used for temporary sediment basin during construction, the
bed shall be excavated at least one foot above the final elevation of the bed. After the
sediment is removed, the bed shall be excavated to final grade before the installation of
the porous pavement system.
3. Construct porous pavement late in the project schedule so that all of the dirty work such
as grading and landscaping is completed first. Porous pavement and the stone bed shall
not be installed until all areas tributary to it are established.
4
4. Porous pavements must be protected from sediment during and after the paving process.
At no time shall sediment or other material capable of clogging the surface be allowed to
contact the pavement.
5. Any grade adjusting requiring fill shall be done using an open-graded material, such as
the stone subbase.
6. MSD Type 4 filter fabric shall be used on the sides of the stone bed and between the
subbase and the storage bed to prevent sediment entry. The filter fabric shall not be
installed between layers of aggregate.
7. Stone for storage bed shall be 2.5” clean crushed stone with minimum 12” thick storage
bed.
8. Place aggregate for the stone recharge bed with care (not to damage the filter fabric).
Aggregate should be dumped at the edge of the bed and placed in layers of 8 to 12 inches
using track equipment. Compact each lift with a single pass of a static steel wheel roller.
Vibrator plate compactor may be used for areas that cannot be compacted with the steel
wheel roller.
9. A thin choker course layer evenly placed over the storage bed is optional. The gradation
of the choker course should be selected based on the gradation of the storage bed. If
AASHTO No. 3 is used for the storage bed, then AASHTO No. 57 is acceptable for the
choker course.
10. Porous asphalt shall be transported in covered, clean dump beds that have been sprayed
with a non-petroleum release agent or soap solution to prevent the mixture from adhering
to the dump beds. Mineral filler, fine aggregate, slag dust, etc. shall not be used to dust
truck beds.
11. Haul distances shall be limited such that porous asphalt shall be placed within 90 minutes
of being loaded.
12. The porous asphalt is placed in 2-inch to 4-inch thick lifts using track pavers and
normally compacted with only a few (1-4) passes of a 10-ton static roller.
13. Traffic shall be restricted for the first 48 hours or until the placed material has been
allowed to cool below 100° F. Use of water to cool the pavement is not permitted.
14. Porous asphalt shall not be placed when the ambient air temperature of the pavement site
in the shade away from artificial heat is below 60° F or when the actual ground
temperature is below 50° F. The contractor shall not pave on days when rain is
forecasted.
15. The full permeability of the pavement surface shall be tested by application of clean
water at the rate of 5 gpm over the surface, using a hose or other distribution device. All
5
applied water shall infiltrate directly without large puddle formation or surface runoff and
shall be observed by the certifying engineer or his representative.
16. Do not clean the Porous Asphalt pavement surface with high pressure hoses or abrasives.
When cleaning is necessary, combination cleaning machines that combine a wet spray
and vacuum process has been found to be effective.
17. 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.
18. At completion of the project, prior to final dedication, an as-built certification, signed and
sealed by a Missouri Professional Engineer shall be provided.
MAINTENANCE NOTES:
1. Maintenance personnel: Do not seal-coat the porous pavement.
2. Do not use sand or ash for control of snow and ice.
3. Inspect the porous pavements several times in the first few months after construction, and
at least annually thereafter.
4. Inspection should be conducted after rain storms to check for surface ponding that might
indicate possible clogging.
5. Semi-annual vacuum sweeping is recommended to prevent clogging.
6. Non-porous patching shall not exceed 10 percent of the paved area.
7. Do not store soil or other material capable of clogging the surface on or adjacent to
porous pavement.
POROUS ASPHALT PRODUCER’S PREQUALIFICATION:
The Porous Asphalt manufacturer shall be responsible for establishing and maintaining a quality
control program. 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
MATERIAL CERTIFICATION AND QUALITY CONTROL:
The contractor shall obtain the Porous Asphalt manufacturer’s certification that the Porous
Asphalt has been approved by MSD. This certification shall be provided to the MSD Division
Inspector. The certification shall include the manufacturer’s name, and state that the Porous
6
Asphalt has been approved by MSD and that the paving materials meet all requirements as
evaluated under the manufacturer’s quality control program.
CONTRACTOR PREQUALIFICATION:
Prior to obtaining a construction permit from MSD to construct the Porous
Asphalt for a given project, the engineer providing as-built certification shall verify that the
installing contractor has past history demonstrating experience and/or training in installing
Porous Asphalt.
AS-BUILT CERTIFICATION
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 Porous Asphalt system was built as designed.
2. The Porous Asphalt system was installed by a qualified contractor.
3. The Porous Asphalt system installation was witnessed by the certifying engineer
or a representative under his direct supervision.
7
8
9
10
PERVIOUS PAVEMENT AS-BUILT CERTIFICATION
DATE:
To: Metropolitan St. Louis Sewer District
Department of Engineering
Attn: Keith Milson
From: (Consultant Engineer’s Name)
Title:
Consulting Engineer Company Name & Address
Re: Engineering/Construction Certification for the following project:
MSD Project Name:
MSD P#:
Project Address:
For the project’s pervious pavement practice, this correspondence shall serve as a certification of
the following:
1. The Pervious Pavement was built in accordance with the details, dimensions, and
materials as approved by MSD for this project.
2. The Pervious Pavement was installed by a qualified contractor, and has satisfied
all applicable quality control and performance tests.
3. The Pervious Pavement installation was witnessed periodically by the certifying
engineer or a representative under their direct supervision.
Printed Name:
Discipline:
License No:
(P.E. seal with signature and date)