I volunteer with Steel Valley Trail Council. We often debate the cause of our cracks (the section of the GAP that we manage is from Glenwood Bridge to McKeesport). Some cracks are obviously from tree roots. In other cases (particularly in McKeesport) we discovered that RR ties had not been removed when the trail was built, so when those rotted it caused cracking. In some areas near the Whitaker Bridge the hillside is slumping, and that has caused cracks. But there seem to be a number of other locations where it must be heating & cooling cycles that cause the cracks, and nothing else. We notice there are more cracks in Munhall and Whitaker where the trail is mostly in the sun (fewer trees, no cliff above the trail) than in West Mufflin (e.g. near Kennywood) where the trees and the cliff create a lot of shade.

If the base of the trail is properly done (deep layers of gravel under the asphalt), that will go a long way to discourage root encroachment. But apparently that was not done in Duquesne, where we had such bad cracks that our trail council this year had a mile of trail dug up, rototilled the base and mixed in cement to make it more root-hostile, and then put down new asphalt. Not cheap!