Question
I am planning on expanding my garage and that will require that a foundation trench be dug along one side of a walnut tree. The tree is 26" in diameter at breast height and the trench will be centered 8' from the trunk and centered on the tree. I don't want to lose this great shade tree; will it survive?

Answer
Building foundations, driveway, sidewalk & road excavation are common events that can cause serious damage to a tree's root system. Construction damage may not be immediately noticeable, but over a period of years the health of the tree will decline as a result of root damage.

Generally you can cut 25% of a trees roots without causing serious damage to the tree. If you draw a circle around the tree at the outer edge of the branches and then draw the are where the trench will be and if this cuts more than 25% of the root system you can expect some dieback of foliage from the trenching. It will survive but the limbs on the side of the trench may die back. You may also need to prune some of the lower limbs that may rub or hit the building.

I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. This may help offset the roots that will be lost.

A prized tree may be inadvertently, but fatally injured when care is not taken during excavation or construction. If you are building near an existing tree, consider calling a certified  arborist to advise on the project during the planning phase, before any work has begun.
here is a web link to a contact list of certified arborist but if there is not one near you I would call the local office of the State Forestry agency and ask one of their Foresters to take a look.

http://www.isa-arbor.com/findArborist/findarborist.aspx