Chicago is already home to the highest number of green rooftops of any city in the world, includingChicago’s City Hall’s Rooftop Garden. The next initiative being undertaken by the city is the Green Alley Program. This project is an expanded approach to Chicago Department of Transportation’s existing alley program and is designed to improve those alleyways in a sustainable way. The city contains one of the most extensive networks of alleyways in the world, covering 1,900 miles with over 3,500 acres of paved, impermeable surfaces. A number of improvements have been suggested for the Green Alley Program including the use of permeable surfaces, high albedo pavements, and recycled material.
1) Permeable Pavement- Permeable pavement has pores or openings that allow water to pass through the surface and percolate through the existing subsoil. Permeable pavement comes in the form of permeable asphalt, permeable concrete, and permeable pavers. In areas where soils do not drain freely, permeable pavement can be used in combination with subsurface drainage systems, like pipe underdrains or stormwater inﬁltration trenches to slow runoff and reduce stress on the combined sewer system.
• Reduces the rate and quantity of stormwater runoff
• Reduces stress on the sewer system
• Recharges ground water
• Filters silt, pollutants and debris
2) High Albedo Pavement- High albedo pavement material is light in color and reﬂects sunlight away from the surface. With less sunlight absorbed by pavement, less heat is radiated by the pavement. High albedo pavement therefore reduces the urban heat island effect. This reduces cooling costs, helps the survival of urban vegetation, and improves air quality, which can help reduce the symptoms ofsome respiratory diseases.
• Reduces the urban heat island effect
• Can be used under a wide variety of site conditions
• Conserves energy by reducing cooling costs
• Improves air quality
3) Recycled Construction Materials- Recycled construction materials can be incorporated in a variety of ways in green alleys. Recycled concrete aggregate can be used in the concrete mix and as a base beneath surface paving. Also, slag, a by-product of steel production, can be used as a component of the concrete mix, reducing industrial waste. Ground tire rubber can be used in porous asphalt and reclaimed asphalt pavement in non-porous asphalt.
• Reduces waste hauled to landﬁlls
• Reduces the need to extract virgin natural resources