The Permeable Future
Under development for three years, the American Society of Civil Engineers Transportation and Development Institute will likely release a national standard guide on design, construction and maintenance of permeable interlocking concrete pavement (PICP) later this year. The 100+ page draft was written by a committee of consultants, government agency personnel, stormwater advocacy groups, contractors and industry suppliers. The group is chaired by David K. Hein, P. Eng. with Applied Research Associates, Inc. Final stages include balloting by his committee, comments from the public and resolution of persuasive public comments.
Titled Design, Construction and Maintenance of Permeable Interlocking Concrete Pavement, the document is divided into five chapters. The first chapter provides a table to assist users in determining if a particular site is suited for PICP. Then the material components of PICP are covered including system infiltration options; full, partial and none.
Structural and hydrologic analyses are provided. Structural analysis includes subbase thickness tables developed from mechanistic modeling and validation using full-scale accelerated load testing at the University of California Pavement Research Center (UCPRC) in Davis. These introduce a new design approach where the designer must estimate the number of days annually water will be in the subbase. This is conservatively estimated by determining the infiltration rate of the underlying soil subgrade and then, using daily rainfall data, determining the average number of days per year with rainfall greater than the 24-hour infiltration rate of the subgrade. This approach addresses the reality that some PICP projects have water standing in the base and soil, i.e., a saturated condition, for a number of days. This weakened state of the soil subgrade is factored into tables used to determine the subbase thickness.
The other influence on subbase thickness is the required water storage. The amount of storage and resulting base thickness are determined by the infiltration rate of the soil, given requirements from the local government on how much water should be managed. The standard provides equations for estimating subbase thicknesses that infiltrate all of the water into the subgrade, and equations for infiltrating some of the water into low infiltration soils while draining the remainder into pipes. The guide encourages using computerized hydrologic models including ICPI’s Permeable Design Pro software. Model selection is left to the designer on using a single rain-event or continuous event model that simulates rainfall, infiltration and runoff for as long as a year. Whatever computational model is selected, it balances inputs from rainfall against outputs expressed as infiltration and outflows from the pavement subbase.
Construction guidelines describe the essential construction steps. The PICP installer must have a PICP Specialist, an ICPI-trained person, in charge of the construction and present on the job site. The standard includes a construction inspection checklist for use by contractors and inspectors. This aligns with ICPI’s recently released continuing education presentation on PICP inspection. The checklist also closely aligns with the one in the 2015 ASCE book, Permeable Pavements.
To address the number-one question by project owners and stormwater agencies, the standard covers maintenance surface cleaning methods and surface repairs. Specifically, the standard describes routine preventive maintenance to keep surface infiltration flowing and more concentrated remedial maintenance techniques should the surface become clogged and render very low infiltration. Like construction, the section on inspection deserves a checklist covering most aspects. A foundational inspection item is checking surface infiltration by observing ponding during or just after a rainstorm. Clogged areas should be checked using ASTM C1781 Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems.
After an extensive References section, the Appendixes include a design example for structural design that leads the user through the UCPRC subbase thickness design charts. Additional hydrologic design examples quantify water volume and flow through full, partial and no-infiltration configurations. Appendixes include a guide construction specification with direction on using a lightweight deflectometer for deflection testing the compacted subbase and base. (See Engineer’s View in this issue for more information on this device.)
For comparison purposes, the Appendix also has subbase thickness design charts calculated from the flexible pavement design method (for non-permeable pavements) in the AASHTO 1993 Guide for Design of Pavement Structures. These yield conservatively thick subbase thicknesses. These are similar to those found in the UCPRC tables under the highest number days per year water stands in the subbase.
The ultimate purpose of this emerging ASCE standard is for provincial, state and local governments to reference it in their green infrastructure and low impact development manuals, as well as in stormwater management and road agency design guidelines. While the standard’s propagation and acceptance may take years, the expectation is less time will be required than an entirely new technology. PICP has been in use in parking lots, alleys and streets for over 15 years, representing well over 150 million sf (14 million m²).
In 2010, ASCE released ASCE 58-10 Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways. This design guide is for roadways experiencing up to 10 million lifetime 18,000 lb (80 kN) single axle loads or ESALs. The emerging ASCE standard guide for PICP will complement 58-10, covering designs up to 1 million ESALs. Additional research may see an increase in lifetime ESALs with hybrid systems that include other pervious materials. Finally, as the paving slab and plank market grows, a need may arise to develop an ASCE design, construction and maintenance standard for these segmental concrete paving products and systems. Looking ahead, the vision is using such standards to further institutionalize all of these paving systems among designers, owners and government agencies.