During 2005, Charlotte Storm Water Services (CSWS) had the unique opportunity of partnering with a local developer to construct a pervious concrete parking lot at the new Wilmore Walk residential development near South End. Pervious concrete remains a relatively new Stormwater Control Measure (SCM) concept in the Charlotte area and this project, designed by Estes Design of Charlotte, served as the City's first pilot study to test the effectiveness of pervious concrete as a SCM in piedmont soils. Prior to installing the concrete, the parking lot site was excavated to a depth of approximately 2 feet. The bottom of the excavation was scarified to allow infiltration and pre-project infiltration tests were conducted. The excavation was covered with permeable filter fabric and filled with double-washed #57 stone. PCI Systems - The Pervious Company of Woodstock, Ga. installed the 6" layer of pervious concrete provided locally by Thomas Concrete. As part of the design, three monitoring wells were installed in the parking lot to monitor water levels within the stone sub-base. In 2007, CSWS contracted with the University of North Carolina at Charlotte to conduct research on the performance of the pervious concrete by monitoring rainfall and the water level response in the stone sub base under the concrete. The monitoring information was then used to calculate the sub soil infiltration rate and determine the effectiveness of the pervious concrete as a stormwater control measure in Charlotte. Dr. Craig Allan and Megan Gray of UNCC conducted the study and performed the research on the site.
The following points give general facts on the SCM pilot project and results of the study.
• Pervious Concrete parking lot constructed in 2005 at cost of $78,124
• Parking lot surface area is approximately 6,500 square feet
• Pervious concrete thickness is six (6) inches with a #57 double washed stone sub base averaging 18 inches in thickness
• Rainfall amounts and sub base water level response was continuously monitored by UNCC during the period March 2007 through May 2009
• Thirty one (31) storm events that produced a water level response in the stone sub base were monitored during the study period.
• The rainfall amounts for these storm events ranged from 0.03 inches to 5.70 inches
• The porosity or void space ratio of the stone sub base is approximately 38.5%
• The average infiltration rate of the sub soil under the pervious concrete is 0.036 inches per hour or 0.86 inches per day
• From the study, we can conclude that a six (6) inch stone sub base would store the 1-inch water quality storm volume, which would then take approximately 28 hours or 1.2 days to infiltrate into the subsoil
• From the study, we can conclude that a ten (10) inch stone sub base would store the 1-year, 24 hour storm volume for Charlotte, which would then take approximately 65 hours or 2.7 days to infiltrate into the subsoil
• From the study, we can conclude that a twelve (12) inch stone sub base would store the 2-year, 24 hour storm volume for Charlotte, which would then take approximately 78 hours or 3.3 days to infiltrate into the subsoil
• From the overall project, we can realize the importance of scarifying or tilling the sub soil excavated surface prior to installing the washed stone sub base to improve sub soil infiltration capabilities in clay type soils