Lexington Center

What's Happening in Lexington Center? Silva Cell system and healthy tree roots

June 2022

If you’ve been through Lexington Center lately, you may have wondered about the extensive excavation that’s been happening in the sidewalk area on the north side of Massachusetts Avenue and puzzled over the rows of black pedestals being installed.

This part of the Lexington Center Streetscape project is designed to provide the Center’s trees with a better growing environment. You may have noticed the poor condition of many of the former trees lining Mass. Ave. through the Center, with stunted growth and extensive dieback. The Center did not present a hospitable environment for trees, with all of its impervious surface, winter salt load, and very little good soil with adequate air and water for tree roots.

The new construction is designed to improve on that. The key element is the “Silva Cells” being installed under the new sidewalk surface. These are modular units that provide an extensive area of good soil for tree roots to grow into, underneath the brick sidewalk on top.

Silva Cell module diagram

Here’s the sequence, illustrated by the accompanying photos taken over the past month:

1. The sidewalk area where the trees are going to be planted is excavated down to a depth of about four feet, and the base of the excavation is lined with geotextile fabric and compacted gravel.

2. The base and pedestals of the Silva Cells are installed.

3. Good soil is poured into the spaces between the pedestals and is compacted lightly by foot. Piping to provide air and drip irrigation to the tree roots is installed. Once the soil is in place, caps are put on the pedestals to support sidewalk construction.

4. Another layer of geotextile fabric is laid on top before the sidewalk is constructed.

5. Then, construction of the sidewalk – gravel base, concrete foundation, layer of sand, and bricks – can begin, essentially suspended over the lightly compacted soil within the Silva Cells.

6. The result: a streetscape with a much healthier environment for trees (and all the environmental benefits they provide) while also providing a smooth, attractive brick sidewalk that is more accessible and safer for all users.

To learn more, visit the Town of Lexington’s webpage with a more detailed explanation of the project’s trees and Silva Cells, and the Silva Cell webpage of the company (DeepRoot) that builds them.