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Authors: Haffner, Edward C.
Keywords: Cornell Structural Soil
Porous Asphalt
Stormwater management
Carolina Stalite Structural Soil
Turf and CU Soil
Issue Date: 7-Sep-2007
Abstract: CU Structural Soil? was invented in the mid 1990?s to address health and longevity issues of urban street trees. This 80 percent gravel and 20 percent soil mixture creates a gap graded, rigid stone matrix which conforms to standards for compaction underneath pavement, yet still allows a healthier rooting media than standard urban environments. Ecologically sensitive development and environmental legislation create unique needs for this soil mixture that requires additional research. Porous asphalt allows water to filter through the pavement profile and into a reservoir of NYSDOT Type 2 gravel (Appendix 1.4) underneath the pavement where it is held and available for groundwater recharge. While this approach addresses water quality and runoff mitigation issues as specified by National Pollution Discharge Elimination System (NPDES) legislation, replacement of the gravel reservoir with CU Structural Soil? allows for the incorporation of trees into the system to further remove water and add an inherently green aspect this technology. The target audience for this technology is urban and suburban big box parking lots. Additionally, this work examines replacing the asphalt surfaces with turfgrass sod. The use of turfgrass as a surface treatment for periphery parking spaces that receive infrequent traffic has potential benefits as well. These include runoff mitigation, lower surface temperatures and the appearance of green space, allowing large parking lots to appear smaller than they are. Laboratory studies undertaken to gain insight into the hydrological characteristics of both CU Structural Soil?, Carolina Stalite Structural Soil and their constituent components of gravel, expanded slate, and clay loam soil included porosity and macroporosity studies as well as infiltration and available water holding capacity trials. Our research found that total soil porosity at 95 percent Proctor Density was 31 percent for CU Structural Soil?, of which macropores comprised 57.4 percent of the total pores. Total soil porosity for the Carolina Stalite Structural Soil at 95 percent Proctor Density was 32.5 percent, of which macropores comprised 60.3 percent of the total pores. Comparably, total soil porosity for a clay loam soil compacted to 95 percent Proctor Density was 32.9 percent with only 2.3 percent of the pores being macropores. Infiltration for both structural soils at 95 percent Proctor Density was greater than 60 cm per hour, (24?/hour) while the clay loam soil resulted in 1.24 cm (0.5?) per hour. Plant available water for the CU Structural Soil? was 8.5 percent while the Carolina Stalite Structural Soil was 11 percent, classifying them as similar to a loamy sand. These results indicate that structural soil would be a strong viable alternative to the un-compacted NYSDOT Type 2 stone traditionally used in porous asphalt reservoirs. Field tests based on this research were carried out at test plots located on the Cornell University campus. Combinations of structural soils and both porous and traditional asphalt surfaces were tested as well as different turf surfaces. These experiments not only examined stormwater and runoff mitigation, but also the feasibility and durability of the different turfgrass surfaces on the different reservoir base course materials. Results indicated that tall fescue turf on both CU and Carolina Stalite Structural Soils consistently outperformed zoysiagrass in all traffic and wear tests, as well as in turf quality ratings. Neither grass performed well on a control base of straight gravel, going completely dormant during the summer months. In surface temperature studies, the fescue on both structural soils ranged from 15? to 20? C cooler than either of the asphalt surfaces. These results conclude that a properly specified turf on structural soil will not only lower surface temperatures and mitigate stormwater runoff, but also provide a wear tolerant surface. Though the zoysiagrass performed poorly in Ithaca?s cool-season climate, observation indicates it may be a suitable choice for warm-season locations in the southeast U.S.
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