The design engineering for this project was done by Cahill Associates, West Chester PA.
The pervious concrete site is located in a courtyard between two dormitories, Sheehan and Sullivan Hall. This area is known as the "Quad". The Quad renovation project was initiated to create an aesthetically pleasing courtyard that functioned as a Stormwater Infiltration Best Management Practice (BMP). Figures 1 & 2 show the Quad and the surrounding area as it was prior to the renovations.
The pervious concrete site is designed to infiltrate small volume storms (1-2"). From these smaller events, there is essentially no runoff from the site. In this region, infiltration of the two inch storm event accounts for approximately 95% of the total annual precipitation. This BMP provides groundwater recharge and helps maintain baseflows of nearby first order streams.
The pervious concrete is outlined with decorative pavers. The pavers divide the pervious concrete into three separate sections as seen in Figure 3. Below these three sections are individual storage beds. Since the site lies on a significant slope it was necessary to create earthen dams that isolate each storage area. At the top of each dam there is an overflow pipe which connects the storage area with the next downstream. The final storage bed has an overflow that connects to the existing storm sewer. The beds are approximately 4 feet deep and are filled with a #4 stone, producing about 40% void space within the beds. A geotextile liner was laid down to separate the storage beds from the undisturbed soil below. The liner's primary function is separation. The idea is to avoid any upward migration of the in-situ soil, which could possibly reduce the capacity of the beds over time. (See Figure 7 & 8).
The Infiltration/Storage Area table below breaks down the three infiltration beds into their respective runoff volume capacities. The beds illustrated below in Figure 4. The three beds can hold approximately 9,700 ft3 (725,000 gallons) of stormwater runoff can be stored for infiltration. This capacity of the beds coupled with well draining characteristics (k = 1.67 x 10-4 cm/s) of the underlying soils presents a very promising situation where infiltration of the smaller storms can be easily accomplished. A cross section and longitudinal profile of the infiltration beds are illustrated in Figure 5 and 6 respectively.
The contributing drainage area for the site is about 65,000 square feet and is approximately 40% impervious. The roof drains of both adjacent dorms are tied into the storage beds. A rough outline of the watershed is illustrated below. During a storm event, any rain that falls within the outlined area will eventually make its way into the infiltration beds. The Runoff Volume table below shows the expected runoff volumes for a 2-year storm broken down into the major components that make up the Quad. It should be noted that almost half of the contributing runoff volume is from the roof tops. The roof drains are connected to an underground system of conduit and run directly to the infiltration beds.
The original construction of the Pervious Concrete Infiltration BMP site began in May of 2002 and was completed in late August. Through the course of construction, there were a number of different elements that lead to the ultimate failure of the surface of the pervious concrete. The failed surface is illustrated in Figures 11 & 12. The site was redesigned and reconstructed in May of 2003.
Many lessons were learned from the initial construction which were taken into consideration in the redesign and construction. The key areas that led to the eventual failure of the surface were: environmental factors, material inconsistencies, and inadequate finishing methods. Environmental factors made the installation extremely difficult. High temperatures made the mixing process unpredictable and caused the curing process to occur too quickly. Inconsistencies in the concrete mixture from one truck to truck became an issue. Mixing time in the trucks varied as did travel time to the site. This increasing the amount of time the concrete spent sitting in the trucks. This worked to reduce the already small workability window of the material. The finishing technique used was also found to be inadequate. The use of a vibratory screed to spread, level, and compact the material proved to be insufficient. A modified plate tamper was tried next but was also found to do a poor job. The finished surface was uneven and rutted in some spots. Attempts to fix the bad spots often resulted in making the areas worse and were quickly abandoned.
During the reconstruction, many issues were rectified. The extremely high temperatures were combated by starting the project earlier in the summer when the conditions and temperatures were more favorable. Removal of the failed material began in late-May with construction being completed in early June, before it started getting hot. To eliminate inconsistencies in the material, it was determined that water would be added on site once the trucks arrived. Mixing would also take place on site so that it could be closely monitored. For compaction and finishing it was found that a 50 gallon plastic drum filled with water gave the best results.
From initial data and observations of the site it was determined that the original design had more than enough pervious concrete area and this area could be decreased without effecting the site’s performance. A new layout was designed which included small strips of pervious concrete around the perimeter, with standard concrete replacing the pervious concrete in the middle of the walkway. The regular concrete was crowned to drain toward the pervious strips on the perimeter.
In addition to the surface water hydrology, there is an interest in seeing what happens to the water once it infiltrates through the pervious concrete and enters the infiltration beds. To help understand what takes place, a number of different instruments have been installed. Figure 14 below is an illustration of a lysimeter. This instrument is used to extract water samples, representative of the respective storm events, from the soil beneath the infiltration beds. Figure 15 shows a lysimeter prior to installation and Figure 16 shows the lysimeters already installed. Note the tubes extending from the three lysimeters. The tubing runs through conduit to a utility box parallel to the site where the samples can be pumped out into sample containers.
Due to the size of the drainage area and lack of point source pollution, very little in the way of contaminants are expected. Villanova has always taken pride in maintaining the most beautiful landscape possible. To accomplish this, fertilizers containing Nitrogen and Phosphorous are used. Testing has begun to look for the presence of these two elements. The industry standard for septic tanks has always been that the waste water is cleansed after 4 feet of infiltration. This same approach was applied to the infiltration system in the Quad. The lysimeters were placed at depths of 1, 2 and 4 feet beneath the base of the infiltration beds at two locations, the opposite corners of the lower infiltration bed. Two lysimeters were also placed outside of the lower infiltration bed on the Sheehan side of the Quad. They were placed at depths corresponding to the 1 foot and 4 foot lysimeters in the bed. These lysimeters will be used to obtain "untreated" values. The results of the water quality tests should help to support the industry standard.
Tests are also being conducted for Hexavalent Chromium, Copper, and Zinc. It is believed that these values will be less prevalent than Total Nitrogen and Total Phosphorus. Once sufficient background testing has been completed, it is possible that some of these tests may be eliminated due to insignificant values.
The moisture fronts resulting from the infiltrated rainfall runoff are also being monitored as they pass through the soil strata. Moisture meters, as illustrated below in Figures 17, 18, and 19, have been installed in close proximity to the lysimeters. The layout of the moisture meters mirrors that of the lysimeters. They are spaced at 1, 2, and 4 foot depths at opposite corners of the lower infiltration bed. The data collected from the moisture meters will aid in a number of different aspects of the research. One of the benefits of these instruments is that they can help to determine the rate at which the water is being infiltrated.
A tipping bucket rain gauge was mounted on the roof of Bartley Hall. A v-notched weir was constructed in the storm drain on the Sullivan Hall side of the Quad directly below the lower infiltration bed. A pressure transducer probe was mounted upstream of the weir. The probe measures the depth of water behind the weir which can then be converted to a flow. This is an accurate measurement of the overflow from the infiltration beds.
To accurately measure the depth of water in the lower infiltration bed, another pressure transducer probe was placed in the junction box directly upstream of the overflow storm drain. The bottom of the junction box is at the same elevation as the bottom of the lower infiltration bed, therefore the depth of the water in the junction box is the same as in the infiltration bed. This data is helpful in determining infiltration rates from the lower bed.