| [College of ACES] | [University of Illinois] | [Illinois CES] |
Sounds like you've got the same problem that I've got here at A&M. Unfortunately, I don't have much helpful information.
Rice University recently won an award for convincing the university to install granite gravel instead of concrete for some sidewalks around their large live oaks. Unfortunately, I doubt this will meet ADA guidelines.
I've looked into some porous concrete that lets air and water through, but you still have to excavate to install the concrete. We have tried using brick pavers here (with some success), but you still have to excavate. Also, over time the pavers shift which can create a maintenance nightmare. I'm not aware of any other materials or techniques that would be relatively "safe" to use around existing trees.
My current strategy is to try to find ways to help the trees survive the stress of sidewalk installation. Since Dr. Alex Shigo says not to prune and Dr. Gary Watson recommends pruning for construction, I'm trying something that might satisfy both opinions. We've been soil injecting around some tree with growth regulators a few months before construction. This way, we reduce the crown without drastically reducing our carbohydrate stores and photosynthesis. The added benefit is that growth regulators have been shown to stimulated root growth in some species (as well as provide some fungicidal activity). It's too early to tell how this will work, but it's worth trying.
Good luck! I hope I've been some help.
W. Todd Watson
Certified Arborist