If you're about to have a trench dug on your land or have soil excavated for whatever reason, first look at your trees and make plans to protect them. You, the excavation company, and possibly an arborist need to evaluate each tree that the trench or hole will be near. Excavation can harm tree roots so thoroughly that trees can die or fall over, so careful planning now -- and not as the excavation is taking place -- is essential for your yard's health and safety.
Tree Root Systems
Tree roots can be lateral, or shallow -- extending out from the trunk just below the surface of the ground. Or, the tree can have a taproot, which is a big root extending far into the ground under the tree. Taproots do have some secondary lateral-type roots, but these don't extend that far away from the tree.
Digging trenches and holes near trees can be risky. If the tree has extensive roots stretching out from the trunk, the roots can end up cut and torn. Infectious pathogens can enter the wounds in the roots and eventually harm the tree. Or, if the cutting is extensive enough, the tree can essentially lose its balance -- it can fall over because the roots on one side are gone.
Another effect is much more insidious. Cut roots can have a reduced capacity for drawing nutrients and water up out of the soil. That can make the tree weaker over time. If the tree doesn't recover its capacity for nutrient intake, then the tree can get very sick or die.
If you're about to have excavation done, have the excavating company send a rep over early so you, the rep, and an arborist can discuss tree roots. Identify which trees along the trench or near the hole have lateral roots and how far out they might extend. See if the trench can be rerouted under the trees and roots instead. If the trench is going under plants, avoid those with taproots, of course.
You might actually have to relocate the tree, so the three of you should look for a suitable spot for it. The excavation company might be able to do the relocating for you, or you might have to call in a specialist tree company that can move larger trees. Be aware, though, that relocating large trees is difficult and not advisable if you can avoid it. In fact, in some cases, you might have to just cut down the tree instead.
After the excavation is done, monitor the trees in your yard for signs that something's wrong. The arborist can let you know how long the effects of the excavation might be felt by the trees.