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Permeable Design Pro saves time by:

  • Characterizing run-on volumes from adjacent area and roofs into PICP with Curve Numbers
  • Referencing an extensive library of 24-hour rainfall depths for U.S. and Canadian cities up to 100 year events
  • Analyzing infiltration and outflow from the library of rainfall depths, or user-supplied rainfall depths
  • Using program default values for soil subgrade strengths and infiltration rates, or input user-supplied data
  • Designing for no infiltration for detention or water harvesting, or for partial or full infiltration into the soil subgrade
  • Analyzing base/subbase thickness requirements for vehicular applications up to 1 million ESALs (Caltrans TI = 9)
  • Modeling infiltration and outflow from 1 to 7 days inclusive of the design rainfall event
Permeable interlocking concrete pavement cross-section, partial infiltration
  • Conducting outflow sensitivity analysis using various underdrain layout, slopes and elevations on low infiltration soils
  • Calculating storage volumes and outflow rates, Curve Numbers and coefficients of runoff
  • Accounting for residual water depths stored in the base/subbase from antecedent rain events in low infiltration soils
  • Saving/printing input/output hydrographs, summary reports and CAD drawing (dwg) of PICP cross section designs
Software screen capture showing Calculating the Subgrade Layer > Gradation
Software screen capture showing Calculating the Structural Design > Traffic
Software screen capture showing Calculating the Granular Layer > Base Gradation

(Click to enlarge images)

How Permeable Design Pro works:

Hydrologic designs are is generated from a non-proprietary program by the U.S. Federal Highway Administration called Drainage Requirements in Pavements or DRIP. The program uses a water balance approach that allocates all rainfall directly onto or contributed as run-on from adjacent surfaces. The program allocates these volumes to infiltration and outflow based on default values and/or user prescribed input values. The model uses time steps to generate inflow and outflow hydrographs. Time step intervals for modeling rainfall and flows can be user-designated in 01, 0.2, 0.3, 0.4, 0.5 and 1 hour increments. 24-hour rainfall events can be modeled for 2, 5, 10, 25, 50 and 100 year return periods using synthetic NRCS Type 1, 1A, 2, and 3 storm patterns for many U.S. and Canadian cities, or with user designated events. Underdrains are modeled as pipes at the perimeter of rectangular or square PICP areas, or along an alley or street. Outflow pipe and slope can be varied to balance outflow with infiltration for selected rain events especially in low-infiltration soils. The program generates a chart illustrating the required base/subbase thickness to store and infiltrate water for desired storms.

Structural design relies on the flexible pavement design methodology in the 1993 Guide for Design of Pavement Structures published by the American Association of State Highway and Transportation Officials (AASHTO). The user characterizes the soil subgrade strength and expected traffic determined by lifetime 18,000 lb (80 kN) equivalent single axle loads. The program calculates the required Structural Number and provides conservative layer coefficients for the concrete pavers and bedding stone, base and subbase aggregates. The program calculates the required subbase thickness to satisfy the Structural Number. After running the program, the designer is provided with a base/subbase thicknesses for water storage and for supporting traffic. The designer selects the thicker subbase for the design solution. A major strength of the program is enabling the designer to conduct sensitivity analysis on hydrologic and structural inputs especially when field data for some inputs is incomplete or absent.

Analysis Process

The design of porous pavements can be somewhat complicated due to the interaction of the hydrologic properties and the structural properties of the pavement structure.

Schematic of the Analysis Process

Structural Analysis

The key component of the structural design of a pavement is to ensure that the pavement surface reduces the stresses and strain at subsequent layers to prevent any significant plastic deformations.

Schematic of the Structural Analysis

Hydrological Analysis

A hydrological analysis has been incorporated into the design procedure to assess if the rainfall can be stored and released by the pavement structure provided.

Schematic of the Hydrological Analysis

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Software screen capture depicting the water inflow distribution for a PICP pavement area