Q: During a radio show, a caller mentioned his tree was pushing up the brick walk nearby. Walter suggested he might level the walk with “engineer soil”. What is this? I have an area around a long stone walk which I want to level and avoid pulling up and resetting all these stones.

A: “Engineered soil” is site-specified to provide two things: proper load bearing capacity for a walk or driveway and also enough pore space for tree roots to thrive without damaging the surface above. This is accomplished by mixing specific sizes of gravel or expanded slate with measured amounts of organic matter and other materials. Each site is different, so “engineered soil” isn’t sold in stores: it is specified by a landscape architect after examining a site. Perhaps you could exclude tree roots under your walk by putting a vertical root barrier, like aluminum flashing, between a problematic tree and your stone walk.