Integrating Structural and Hydrologic Design - Belgard

INTEGRATING STRUCTURAL AND HYDROLOGIC DESIGN
CONSIDERATIONS IN PERMEABLE PAVEMENT
David R. Smith, Technical Director
Interlocking Concrete Pavement Institute
13921 Park Center Road, Suite 270
Herndon, Virginia 20171 USA
Tel: 703-657-6900 Fax: 703-657-6901
dsmith@icpi.org
ABSTRACT
Pervious concrete, porous asphalt and permeable interlocking concrete pavements combine
stormwater infiltration, detention and a riding surface for vehicles into one location. These three
sustainable pavements rely on an open-graded crushed stone base for water storage, infiltration and
vehicular support. Much research has been conducted on the hydrologic and water quality aspects
provided by these bases. State and municipal best management practice (BMP) and low impact
development (LID) manuals have incorporated design guidelines developed from university
research, industry guidelines and experience by various agencies. However, stormwater agencies
generally possess a paucity of information on the structural aspects of pavements for sustainable
urban drainage. This information is essential to providing durable designs that can withstand
repeated vehicular traffic.
This paper reviews hydrological and structural solutions for permeable interlocking concrete
pavement for use by design professionals and stormwater agencies. Using a software program, the
hydrological analysis within it determines if the volume of water from user-selected rainfall events
can be stored and released by the pavement base. User defined parameters determine how much
water infiltrates the soil subgrade and/or is carried away by subdrains or flows off the pavement
surface. Structural capacity for repeated vehicular loads is determined using the American
Association of State Highway and Transportation Officials (AASHTO) 1993 structural design
equations. This paper explains commonly used structural and hydrological design methodologies
with examples. The non-proprietary software program illustrates the range of input design
considerations as well as outputs for integrated stormwater drainage and pavement design. These
considerations apply to all types of sustainable pavements.
Keywords: sustainable paving, permeable pavements, permeable pavement structural design,
permeable interlocking concrete pavement, stormwater infiltration
INTRODUCTION
The three primary sustainable pavements are porous asphalt, pervious concrete and permeable
interlocking concrete pavement (PICP). Typical cross sections are illustrated in Figures 1 – 3.
Figure 1. Typical porous asphalt pavement
Figure 2. Typical pervious concrete pavement
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Their hydrological design generally relies on the
following variables:
• design storm or storms, typically issued by the
local stormwater agency
• long-term soil infiltration rate, estimated from soil
samples or field measured
• base/subbase reservoir thickness and storage
capacity
Figure 3. Typical permeable interlocking concrete pavement (PICP)
Hydrological design is patterned after infiltration trench design that accounts for rainfall entering
directly into the pavement surface plus runoff from adjacent contributing pervious and impervious
surfaces. Permeable/porous/pervious pavements may be designed to contain water for a few days for
the purposes of nutrient reduction. In such cases, detention pond design principles can be applied to
inflow, storage and outflow calculations. Excess water that cannot be contained by the base is
allowed to exit via swales or bioretention areas adjacent to the pavement, through pipes in the base,
and/or into catch basins. Figures 4, 5 and 6 illustrate some overflow options.
Figure 4. Overflow to bioswale at Elmhurst
College parking lot, Elmhurst, Illinois
Figure 5. Catch basin overflow at Wal-mart
parking lot Rehobeth Beach, Delaware
For all permeable/porous/pervious pavements, the
base/subbase thickness is determined for hydrological
and structural (traffic loading) needs, and the
thicker section is selected. Hydrological design for
permeable/porous/pervious pavement is described in
books and BMP manuals, but structural (base
thickness) design is not due to a lack of information
on the structural performance of open-graded bases
used in them. This paper reviews structural design
approaches for these pavements published by their
representative industries and it presents some tools
to integrate hydrological design.
Figure 6. Outflow via perforated drain pipe is set to drain into a utility structure. The pipe is elevated
above a low infiltration soil subgrade to detain water in the open-graded stone base, promote nutrient
reduction and eventual infiltration.
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Inflow-Outflow Analysis
Performed at 5 min. time intervals
Pavement
•Slope
• Infiltration
Amount of Rainfall
•Storm pattern
•Storm year
Adjacent Areas
•Size
• Drainage properties
Inflow Analyses
Drainage Characteristics
• Results of Structural Analysis
• Porosity of pavement layers
• Permeability of pavement layers
• Subgrade infiltration
• Drainage pipes
• Layer thickness
•Slopes, etc.
Outflow Analysis
• Amount of surface runoff
• Depth of water on the pavement surface
• Volume of water entering the pavement surface
• Depth of water in pavement granular layers
• Volume of water entering subgrade
• Outflow through drains, etc.
Consider changing
• Drainage characteristics
Yes
Is the
drainage
adequate
No
Consider changing
• Thickness of pavement granular layers
To structural analysis
Figure 8. Hydrological analysis flow chart for permeable pavement
PICP Structural Analysis
To assess the structural capacity of PICP, the program uses the AASHTO 1993 structural design
equation to develop base thicknesses for supporting vehicular traffic. This design method relies on
inputs such as traffic information, soils and pavement material information, reliability and
serviceability levels. As previously noted, the AASHTO empirical design method calculates a
structural number (SN) which is the sum of layer coefficients, a dimensionless characterization of the
stiffness of each pavement layer. Designed for use in the United States and Canada, the user can
instantly move from SI or U.S. customary units even while inputting data.
To determine the thickness of the required pavement layers, layer coefficients (default values or
those assigned by the user) determine if the open-graded base types and thicknesses meet the design
structural number or design SN. While the user can change default values, the program assumes that
layer coefficients for open-graded bases are lower than those associated with dense-graded bases
used under conventional impervious pavements.
The PICP paving layer thickness is consistently specified at 3 1/8 in. (80 mm) for the pavers plus 2 in.
(50 mm) for the bedding layer. While a conservative default value of 0.3 per inch layer coefficient is
assumed for the pavers and bedding layer, the user can nominate a paver-specific layer coefficient
should it be available. Since the paving layer thickness is constant and its stiffness is characterized
by this layer coefficient, only the open-graded base (usually the ASTM No. 57 stone held constant at
4 in. or 100 mm thickness) and the subbase thickness (ASTM No. 2 stone layer) requires designing.
The subbase thickness is rounded up to the nearest inch (25 mm) to ensure a reasonable and
conservative value for constructability. The software program applies to PICP subject to axle loads
up to 24,250 lbs (11,000 kg) or a maximum vehicle load of 50,000 lb (22,680 kg) trafficked up to 1
million 80 kN (18-kip) equivalent single axle loads (ESALs). Users are cautioned when the design
load exceeds 600,000 ESALs. Figure 9 illustrates similar truck loads on a PICP project.
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INTEGRATING STRUCTURAL AND HYDROLOGICAL DESIGN IN PICP
The designer can conduct sophisticated sensitivity analysis on structural and hydrological input
variables using a software program recently released by the Interlocking Concrete Pavement Institute
(ICPI 2008). The flow chart shown in Figure 7 illustrates the integration of hydrological and
structural design using this software. Like many software programs, this one enables the user to test
the sensitivity of input variables with limited data and therefore design conservatively. Besides
traffic loads and soil infiltration rates, other difficult-to-predict variables that can be modeled can
include the long term surface infiltration rate, possible lower soil subgrade strengths when near
saturation, antecedent water conditions in the base reservoir, as well as the long-term soil subgrade
infiltration rate.
Structural Analysis
Hydrological
Analysis
Pavement Structure:
Type and thickness of
pavement layers
Drainage Design:
Drainage features
and characteristics
Yes
Is the
drainage
adequate
No
Consider changing
• Drainage characteristics
Consider changing
• Thickness of pavement
granular layers
Figure 7. An overview of structural and hydrological analysis for permeable pavement
PICP Hydrological Analysis
Figure 8 summarizes the hydrological modelling process. The hydrological analysis determines if the
volume of water from user-selected rainfall events can be stored, infiltrated and released by the
pavement structure. All water is modelled as a water balance using small time steps to characterize
water inflow from precipitation into the PICP surface and runoff contributed from adjacent areas.
The program holds a significant rainfall library for many cities in the U.S. and Canada consisting of
5, 10, 25, 50 and 100 year return periods for 24 hour events.
Besides characterizing contributing runoff from adjacent areas, the user can elect to include
perforated drain pipes in the base to accommodate outflow in low infiltration soils. These can be
modeled at the bottom of the subbase or raised within it to create some detention. The program also
calculates the curve number and runoff coefficient for user selected rainfall events for the site.
Outflow is also estimated by calculating infiltrated water flowing directly into the PICP and from
contributing areas, as well as drainage from the base into the soil or to drainage pipes during each
time step. The combined process continually estimates the water level in the base and the amount
draining from the PICP, during and after the storm. Output includes hydrographs for the rainfall,
inflow from contributing areas, infiltration and outflow through drain pipes if required. Output
variables can be set for water storage and harvesting if needed.
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Traffic
Load
Design
Axle
Load, lbs
(kN)
Repetitions
Pervious Concrete
Thickness, in.
(mm)
Low 4,000 (18) Unlimited 4 (100) Unlimited
Moderate 12,000 (53)

The number of ESALs is determined by the weight of each of the axles and dividing them by a
‘standard’ ESAL of 18,000 lbs (80 kN). A five axle tractor-trailer truck provides an example: two
rear axles on the trailer each exert 18,000 lbs or 80 kN; two on the back of the truck at 15,800 lbs or
70 kN; and one in the front (steering) at 11,000 lbs or 50 kN. AASHTO uses the following
relationships called load equivalency factors or LEFs for each axle to estimate ESALs. LEF and
ESALs for this truck are as follows:
Trailer: (80/80) 4 = 1 (x 2 axles) = 2 ESALs
Truck rear: (70/80) 4 = 0.6 (x 2 axles) = 1.2 ESALs
Truck front: (50/80) 4 = 0.15 ESALs
When added together, all LEFs = 3.35 ESALs. For every pass across a pavement, this truck exerts
3.35 18,000 lb (80 kN) ESALs. To put automobile axle loads into perspective, the weight of one
passenger car placed into the formula yields about 0.0002 ESALs. Therefore, pavement design is
primarily considers trucks because they exert the highest loads and most damage. In contrast,
thousands of cars are required to apply the same loading and damage as one passage of a truck.
POROUS ASPHALT STRUCTURAL DESIGN
Porous pavement thickness design is addressed in industry literature. Table 1 indicates thicknesses
for porous asphalt design (NAPA 2008). This is excerpted from the National Asphalt Pavement
Association porous asphalt manual which does not quantify maximum traffic loading.
Minimum Compacted
Thickness, in. (mm)
Traffic Loading
Parking – Little or no trucks 2.5 (65)
Residential street – Some trucks 4.0 (100)
Heavy Trucks 6 (150)
Table 1. Minimum compacted porous asphalt thicknesses
Regarding AASHTO layer coefficients per inch (25 mm) of pavement layer thickness for porous
asphalt, NAPA (2008) recommends the following in Table 2.
Material
Layer Coefficient (per in.)
Porous Asphalt 0.40 – 0.42
Asphalt-treated permeable base 0.30 – 0.35
Porous aggregate (open-graded) base 0.10 – 0.14
Table 2. AASHTO layer coefficients for porous asphalt, treated base and open-graded base
PERVIOUS CONCRETE STRUCTURAL DESIGN
The National Concrete Ready Mix Association manual, Hydrologic Design of Pervious Concrete
(2007 Leming) provides a methodology that utilizes the Natural Resource Conservation Service
(NRCS) Curve Number method. The manual reviews software for hydrological calculations. While
the software program does not include structural design, some recommendations are provided as
charts in the appendix. Depending on axle loads, pavement thickness charts recommend 4, 6 and 8 in.
(100, 150 and 200 mm) thick pervious concrete on soils with infiltration rates no lower than 0.1
in./hr. (0.3 cm/hr).
There is no design guidance on the thickness of open-graded base required for traffic load support.
The user is directed to consulting a design professional (e.g. civil engineer) for determining pervious
concrete and base thicknesses on soils less than 0.1 in./hr (0.3 cm/hr) infiltration rate. Table 3 below
replicates the pavement thickness information by Leming. The right hand column is added by the
author to demonstrate the ESAL ranges supported by the various pervious concrete pavement
thicknesses. The design charts note that a design professional should be consulted for lifetime
designs greater than 720,000 ESALs.
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BASIS FOR STRUCTURAL DESIGN
For structural design of impervious (conventional) roads and base, many local, state and provincial
agencies design methods published by the American Association of State Highway and
Transportation Officials (AASHTO). While the AASHTO methodology is familiar to some civil
engineers, stormwater agency personnel who do not deal with pavement design are encouraged to
become more familiar with them and reference them in permeable pavement design
recommendations for local, state or provincial BMP and LID manuals, and regulatory documents.
Highway engineers are increasingly using AASHTO 2002 Mechanistic-Empirical Pavement Design
Guide which relies on mechanistic design and modeling, i.e. analysis of loads and resultant stresses
and strains on materials and the soil subgrade. The AASHTO 2002 mechanistic design model was
developed and calibrated by state, provincial and federal highway agencies across a wide range of
highway loads, load testing, soil types and climatic conditions. This model has not been calibrated
for permeable pavements (subject to significantly less traffic loads) constructed with open-graded,
crushed stone bases.
Many local agencies use the empirically-based AASHTO 1993 Guide for Design of Pavement
Structures whose underlying concepts emerged from test pavements in the 1950s repeatedly
trafficked by trucks that established relationships among materials types, loads and serviceability.
The AASHTO equation in the 1993 Guide calculates a Structural Number or SN given traffic loads,
soil type, climate and moisture conditions. The designer then finds the appropriate combination of
pavement surfacing and base materials to meet or exceed the Structural Number. This empirical
design approach appears to be applicable to permeable pavements with consideration given to input
variables.
Due to their load distribution means, porous asphalt and PICP are considered flexible pavement
systems and the flexible pavement design concepts in AASHTO can be applied to them. A key input
for flexible pavement design is the layer coefficient which characterizes each pavement layer with a
number. The higher the coefficient, the stiffer the material and the coefficient is expressed per inch
or per millimeter of pavement layer thickness. The thickness of each material is multiplied by the
layer coefficients and all coefficients are added to equal or exceed the Structural Number.
For example, given site specific inputs on subgrade soil strength, climate, moisture and traffic loads
into the AASHTO equation yields a required structural number of 2.5. The designer then identifies
the combination of pavement layer materials whose layer coefficients total at least 2.5. In this
illustration, a compacted, dense-graded crushed stone base typically has a layer coefficient of 0.14.
Asphalt typically has a layer coefficient of 0.44. If 3 in. (75 mm) of asphalt are selected, then the SN
for this layer is 0.44 x 3 = 1.32. The base layer thickness required would then be 2.5 – 1.32 =
1.18/0.14 = 8.4 or about 9 in. (225 mm) which satisfies the required Structural Number.
CHARACTERIZING TRAFFIC LOADS
When a vehicle passes over a pavement, it damages it. The cumulative effects of many passes
eventually causes ruts or cracks making the pavement unserviceable and needing rehabilitation.
Vehicles passing over a pavement exert a wide range of loads. Compared to cars, trucks and busses
do the most damage to pavements because their wheel loads and tire pressures are much heavier and
higher than cars. One pass of a fully loaded truck will do more damage to pavement than several
thousand cars passing over it. The AASHTO Guide characterizes traffic loads as the number of
18,000 lbs or 80 kN equivalent single axle loads or ESALs. The 18,000 lb load emerged from the
AASHTO road tests conducted in the 1950s and has remained as a convenient means to quantify
loads.
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CONCLUSION
Stormwater agencies provide permeable
pavement design guidelines through BMP
and LID manuals and other publications.
Most of the guidelines focus on design for
managing stormwater and its pollutants.
There is little information provided on
structural design or it is imprecisely
quantified. Better structural design
information is needed to raise designer
confidence. Compared to impervious
pavement structural design, permeable
pavement structural design is in its
formative stages. Industry guidelines for
each permeable pavement system should
be followed with advice from a qualified
design professional.
Figure 9. PICP demonstrates an ability to support
loaded trucks at Elmhurst College, Elmhurst, Illinois.
The structural behavior of open-graded bases requires more research and testing to determine
relationships between load and deformation (e.g., surface rutting and/or cracking). There is limited
laboratory and field (accelerated traffic load) testing information on open-graded bases as reliable
inputs for structural design. The AASHTO 1993 methodology appears to be a suitable empiricallybased
design approach until more design data is developed from the structural response and
serviceability of permeable/porous/pervious pavements under a range of climates, soils and traffic.
The author wishes to thank D. J. Swan with Applied Research Associates, Inc. for the flow charts in
this paper.
REFERENCES
AASHTO 1993. Guide for Design of Pavement Structures, American Association of State Highway
and Transportation Officials, Washington, DC.
AASHTO 2002. Mechanistic-Empirical Pavement Design Guide, American Association of State
Highway and Transportation Officials, Washington, DC.
ICPI 2008. Applied Research Associates, Inc., Permeable Design Pro Software, Interlocking
Concrete Pavement Institute, Herndon, Virginia.
Leming 2007. Leming, M. L., Malcom, H. R., and Tennis, P. D., Hydrologic Design of Pervious
Concrete, EB303, Portland Cement Association, Skokie, Illinois, and National Ready Mixed
Concrete Association, Silver Spring, Maryland, 72 pages.
NAPA 2008. Porous Asphalt Pavements for Stormwater Management, Information Series 131,
National Asphalt Pavement Association, Lanham, Maryland, 24 pages.
Smith 2006. Smith, D. R., Permeable Interlocking Concrete Pavements – Design Specification
Construction Maintenance, Interlocking Concrete Pavement Institute, Herndon, Virginia, 48 pages.
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