Advanced Pavement Technology

Stormwater Management for Urban Environments

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Courtesy of Advanced Pavement Technology


Elmhurst College, a hundred-thirty-five year old private learning institution, is proceeding with a campus redevelopment project that will include a one hundred seventy bed residence hall. In addition, the college continues to be a role model for sustainable site design and an educational resource for both students and the community.

The need for additional housing, due to increased enrollment demanded creative ideas as supplied by long time planning partner, Wight and Company. The Wight Company is a full service firm with a strong resume featuring LEED™ (Leadership in Energy and Environmental Design) professional’s committed to sustainable design principles. As demonstrated by past projects, they have created a plan that will incorporate cutting edge water management techniques that will enhance land usage and decentralize rainwater impact. The stormwater from building roofs and other impervious surfaces will be captured by post structural BMPs, (Best Management Practices) treated, stored and made available for groundwater recharge.

An integral part of this ecological train will be a permeable pavement system designed for a hundred year storm event that will serve as an underground retention facility as well as a parking lot surface. This system will be part of a sustainable site design that will also utilize bio-swales, rain gardens and numerous runnels to move and control overflow rainwater above ground. In addition, Wight has created a plan that will also contribute to better stormwater management, naturally. The existing oak trees, as part of this campus, a registered arboretum, will continue to prosper by implementing areas of native plants in lieu of turf grass and the natural wildlife habitat will also prosper in this urban environment. These methods of sustainable site design as applied to a more natural water management style will produce less stormwater runoff and loading of an already over worked city stormwater system.

The keystone for this site plan is a two acre area, with over a two hundred car parking lot that will serve as a detention/retention facility replacing the need for a surface water retention facility there being no land available for such a structure. Underground storage will be provided beneath the permeable pavement surface in the void areas of the aggregates. In addition, this system of aggregate layers will act as an infiltration trench and will collect and treat first-flush pollutants and improve water quality. Because this system will provide more time on site via detention and retention, groundwater recharge will be promoted and also a better microbial action will be established as this system will mimic Mother Nature regarding natural surface infiltration and time on site, while creating a peak time controlled release format. Control structures are integrated in the bio-swales and will also provide access for water samples by the students.

Design for this site considered pre-development rates and post-construction rates as well as adjoining topography run-off to develop a storage volume of 3.3 acre feet for retention. The use of CA-1 aggregates as shown in the cross-section, Figure 1 (Bio-Aquifer Storm System) will provide storage in air voids between each aggregate as determined by a quarry void ratio test that will be approximately 41 percent-43 percent. Figure 2 (Vulcan Void Ratio Report). A conservative credit was determined to be 40 percent that will yield an excavated area beneath each phase approximately 20,000 square feet, seven feet deep and filled with CA-1 clean crushed aggregates. These areas will be fed with rainwater and stormwater run-off that has been captured thru openings in the concrete paver surface that has been filled with open-graded aggregates. The aggregate structures will now serve as a holding area or retention pond that will accommodate a hundred year storm. The bottom of this excavation will be of an in situ clay matrix that will have low permeability as well as the sides. There will be a layer of geo-textile placed at the sub-grade and CA-1 interface, but it normally is not required, however, it was a decision made by the design team, therefore it was used. Typically, CA-1 aggregates will bridge clay soils more efficiently and provide a stronger sub-base than CA-7 aggregates. The stresses on the soils at this point do not exist and the primary function of this area is permanent storage for water with some infiltration occurring and the filter fabric should be considered a detriment to this process and therefore only becomes a place for potential clogging over time. When the detention is analyzed this fabric may be viewed as a separator and will help reduce migration of clay soils entering the aggregate under wet soil conditions. However, again, we have found that the CA-1 materials bridge poor soils, especially in a saturated condition better that the use of smaller aggregates or DGA aggregates.

The detention area, which includes a sub-base, base, setting bed and wearing course, has been designed to capture and treat first-flush pollutants, promote a flow path to the retention area and also support vehicular and pedestrian traffic. This area will support approximately .75 acre feet of storage or five inches of rain. In addition, the structural loading of this cross-section as depicted by Figure 3 Permeable paver typical section will, with each layer co-efficient as assigned, yield a structural number of 3.75 equal to a fourteen inch full depth asphalt roadway. This design is typical for this system and not driven by loading or vehicular applications, but when phase II was constructed, this area did accept over one thousand fully loaded semi-trucks with over twenty thousand pound axle loading without distortion or damage to the permeable pavement system. The aggregates specified for these layers are also IDOT specific and graded so that the CA-7 layer acts as a choking layer and precluded the need for geo-textile fabric as a means of segregation.

Another facet of this design involves the use of bio-swales Figure 4 ( Bio-Swale) to provide an area for capture and treatment of first flush pollutants should there be runoff from the permeable pavement surface. Grades are established that will promote flow to inlet points that have been created in the chair back of the curb. In these bio-swale areas manholes and inspection tubes are placed for future access to monitor water level and take water samples to determine the water quality and effectiveness of this system. Plant material will be chosen that will best perform in this environment as their root system will also breakdown the pollutants that may get swept into the bio-swale during an intense storm and resultant runoff. The underground conveyance for this site is essentially a gravity feed system with a vertical outlet structure that has control of the release of treated stormwater to the city’s over-burdened stormwater system. The water flow also has been treated at this point and will be used to protect and promote groundwater recharge and as a resource for Salt Creek.

After the design/development phase and during the construction drawing phase, the LEED issues were evaluated by the design team and many ideas were discussed-some relevant and others not feasible. One of these issues regarding the retention area was the use of recycled concrete. An IDOT certified crusher was contacted regarding supplying material for this project during phase II and could provide material that met the required BASS™ sieve analysis requirements, but did not have the ability to wash the materials or control the amount of one hundred and two hundred particles that would prove detrimental over time. In addition, it was pointed out by the engineering of Wight, that the possible introduction of small lime and cement dust particulates would eventually create an impervious crust at the bottom of the retention structure and preclude any infiltration over time. The cost savings when washing was applied made the recycled materials cost prohibitive and therefore standard crushed limestone aggregates were used.

However, some other areas that would contribute to a LEED (Reference #1) rating for Elmhurst College, which will be the regions first college or university housing unit to achieve this status, may be achieved from sustainable site design practices. Improvement of stormwater run-off SS6.1 and water quality SS6.2 are two areas that this system will have an impact for a more natural stormwater management methodology. Past studies regarding infiltration support rates as high as eight to nine inches per hour (Reference #2) and also studies being conducted at University of North Carolina State, (Reference #3) support the design for this project. In addition, Muth (Reference #4) has determined curve numbers associated with the rational method for stormwater run-off as low as 0.15. These actions are due primarily to the use of open graded aggregates that possess flow rates in excess of one thousand two hundred gallons per hour assuring engineers that the rapid entry of water into the system will not be impeded, even when heavy clogging occurs. The mature infiltration design life at twenty years will still be three inches per hour and more than capable of handling an average two year twenty four hour storm event of one to one and a half inches per hour rate. Water quality improvement that will meet the requirements for NPDES (National Pollutant Discharge Elimination) has also been tested and documented as shown by charts from the University of Connecticut Figure #5&6 (Objective to decrease runoff) (Objective to Decrease Water Pollution) and the University of North Carolina State Water treatment coupled with a TSS (Totally Suspended Soils) removal rate greater than ninety percent, assures this permeable pavement system will provide this site with a long-term and low maintenance solution for stormwater management. Maintenance procedures for permeable pavement systems are similar as for impervious pavements, yet will develop cost savings that conventional pavements have not demonstrated. Typical post-structural BMP maintenance requirements by EPA (Environmental Protection Agency) will include an implementation and schedule for street sweeping to gather and remove debris and surface pollutants from the pavement surface. This schedule of once to twice per year would be the same for permeable pavement systems. The maintenance costs were estimated for Morton Arboretum by Gold Figure #7A&B (Life Cycle Cost Recovery) for a heavy duty and light duty asphalt impervious pavement as compared to the proposed permeable pavement system for the same area. Including capitalization, the break even point was at twenty two years and in fifty years, the asphalt pavement maintenance costs would be nearly ten times the cost of maintaining the permeable pavement. Theses types of maintenance recovery costs have been determined by the facilities director of the Arboretum to be too conservative, as they are about half as planned. We also see evidence in winter maintenance conditions as demonstrated at Kane County Government Center, where pavers over a DGA base with a sand setting bed (in an impervious handicap parking area) will require salting after a twenty four hour freeze thaw occurrence where permeable pavers in the same area do not. This will lead to large savings as we see areas of permeable pavement freezing at lower temperatures and requiring less snow plowing and also immediate absorption of snowmelt in the openings without reforming on the surface. Therefore, less salt is required on parking and walking areas which results in large dollar savings.

Aesthetics for the campus was a design consideration that proved more important to the owner than utilizing SS7.1 LEED credits for Heat Island Effect, non roof thermal pollution reduction based on a solar reflectance testing of a combined albedo and reflectivity of greater than 0.29. While this credit is achievable with concrete pavers, the shape, color and pattern were selected to maintain the ambiance of the college buildings and existing environment. Thermal pollution reduction will be achieved using trees and other shading techniques, in addition, bio-swales have been employed as well. Mock ups for color were provided in a herringbone pattern and were larger than normal to convey shading that would be developed when using mechanical equipment for installation.

There are other non credit LEED items that this system will supply to help provide additional sustainable site practices e.g. local materials supplied within five hundred miles, local labor, recycled post consumer content in slag and fly ash used in manufacturing of concrete pavers, and innovative ideas and applications. Construction of a permeable pavement system using trained contractors experienced with open graded aggregate placement and compaction as well as the logistics and equipment required to staff a mechanically installed permeable pavement project are essential to the successful performance of this system. Sequence of construction including a method statement from the manufacturer and paver contractor are prerequisites prior to the start of a project. Another item to be employed is a preconstruction meeting with all parties present for the design team and owner and contractors. This project derived good information and understanding by all parties from this type of meeting that yielded a project that was completed on time and within budget even under severe winter conditions as we shall see in the following photos.

During the completion of phase I, the college has since received a grant from the DuPage County Stormwater District for partially funding of this innovative strategy for managing and monitoring stormwater. In summary, it was best said by Dr. Bryant Cureton, President of Elmhurst College, “what better place than a learning institution to promote our mission by developing our campus as a model of sustainable design that will be a hands on educational resource for our students and the community.”

1. USGBS (United States Green Building Council) LEED (Leadership in Environmental Design) rating system version 2.1 and 2.2 10/2005
2. US Environmental Protection Agency-New England Regional Office, Jordon Cove Watershed National Monitoring Project 10/2004
3. Study on the Surface Infiltration Rate of Permeable Pavements by North Carolina State University, Dr. William Hunt 5/2004
4. Drainage with Interlocking Pavers Professor W. Muth, Karlsruhe University 1994

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