Roots can disturb pavement in their search for pockets of air and water. Flickr credit: elycefelizWe know a lot about the problems and headaches that pavement disturbed by roots can cause. For a long time, tree root barriers – mechanical barriers designed to direct roots down and away from hardscape – have been the backbone of our business.

Why do tree roots heave pavement, and will this be a problem with Silva Cells?

The short answer is that there have never been issues related to roots pushing up the pavement surface above the Silva Cells. There are two primary reasons why.

First, roots will go wherever they can find suitable growing conditions – that is, pockets of air and water within the soil. The reason that roots of urban trees tend to linger around the surface, where they may heave pavement, is because that is where those conditions are usually present. Since the Silva Cell is specifically designed to provide root-friendly growing conditions beneath paving, it dramatically reduces the likelihood that they will disturb the pavement surface.

Second, we guard against pavement heaving by specifying a couple of important design details. The first is that we recommend always using a root barrier in Silva Cell installations. These simple mechanical barriers direct tree roots down and away from paving once they reach the perimeter of the tree pit opening. Our standard details also recommend a turned down curb at the perimeter of the opening.These features not only prevent the buttress roots from adversely affecting the adjacent pavement, but also prevent the vascular roots from entering the aggregate base course beneath the hardscape. Once directed downward, roots can enter the lightly compacted soil within the Silva Cell system and be very happy there.

Just because you are using Silva Cells does not mean you throw out all the good rules of thumb associated with tree planting. Jim Urban has said to us many times that any tree opening less than 20’ x 20′ is a compromise. While those dimensions aren’t a reality on most sites, it is the correct dimension for mature shade trees and a good one to keep in mind when envisioning how much space a street tree will need to live a long, healthy life.

We’ve had Silva Cell installations in the ground since 2007, and I’m not aware of any that have had pavement heaving problems. If you are looking for longer-term evidence that pavement heaving is not a concern, we’ve noted that suspended pavement systems using custom poured-in-place suspended pavement systems also exhibit fewer instances of root/infrastructure conflicts. This is the case both at the Christian Science Center in Boston (where the trees are 45 years old) and at the multi-block installation at Tryon and Trade Streets in Charlotte, NC (where the trees are 25 years old).

Of course, design factors can influence the likelihood of conflicts between roots and paving. While we can’t guarantee that this will never happen when using Silva Cells, we can tell you that we’ve not seen it happen yet, and that we’ve created standard details specifically designed to minimize the chances of it ever happening in the future.