Ask a Consulting Arborist Q&A: Root and Soil Concerns

Q: Can a retaining wall be built close to a redwood without damaging either the wall or tree?

Tuesday, March 15, 2016  

Q: We have a situation where two 50-year-old monument redwood trees are damaging a small, 5-foot retaining wall. The wall needs to be replaced and we want to avoid cutting the trees through a remedy. This is all next to a property line with a small parking lot with access for possible crane use. I understand there's a remedy of digging up to the root ball and pounding round steel I-beams (around the root ball) as an alternative to cutting. At that point a new retaining wall could be built around the remedy. We'd greatly appreciate any referrals or other ideas. We are in the Northern California area, outside of San Francisco. Thank you so much!

A: If the site has enough space to install the retaining wall around the tree trunk with strong inserted posts (similar to a large stake) and cross members designed to withstand the forces of soil on a 5-foot wall, it should be possible to install vertical posts outside the root crown of the tree and to install horizontal support strong enough to work. Hopefully, the design will provide enough space for the tree to continue to grow without the trunk pushing out on the new wall, or the new wall will be short-lived.

There was no information about the grade of the tree in the retained area or the space of the root plate area. If the new retaining wall will bury the base of the tree, an additional design element that can keep the soil away from the base of the tree should be considered. Methods of the vertical support installation can include pounding or using a drill or auger to insert a deep enough section to support the forces of a 5-foot high wall. Please contact a local Consulting Arborist and civil engineer to assist with your project design needs, as site specific issues usually need to be addressed.

Responder: Gordon Mann, RCA #480, Auburn, CA