Retaining Walls on Mercer Island, WA

Sometimes, with a modified approach, and it needs establishing before the line is fixed. Most of the roots keeping a large tree alive sit in the top foot or so of soil and spread well beyond the branch line, so a conventional footing trench through that area severs a substantial share of them — and the tree may take several years to show it. The workable options are shifting the alignment outside the critical root area, hand excavation with root pruning kept minimal, or a shallower founding detail. The city's tree regulations may also bear on significant excavation within a root zone, so we check rather than assume.

The ground is looser than the crown and considerably more sensitive to what you put on it. Adding load above a slope and changing where surface water runs are the two mechanisms that turn stable ground into moving ground, and both are exactly what a retaining wall does. On these edge lots the design starts with what the slope can carry and where water is going, and only then with the wall. On the level ground through Island Crest none of this applies and the work is straightforward — which is why the first question is always where on the island you are.

Because the ground under a dense canopy dries far more slowly than open ground, and that is the condition a wall has to stand in. A closed canopy keeps sun and wind off the soil, and heavy needle and leaf litter mats down and sheds water rather than absorbing it, so ground behind a wall on a wooded island lot can stay saturated for weeks after rain has stopped. Saturated soil is heavier and pushes harder. The counterintuitive result is that a shaded wall frequently wants the same drainage specification as a wall on an open hillside taking more direct rainfall.

Woodland planting, and it performs far better than the alternatives people usually try first. Behind a wall under an established island canopy you have low light, soil that dries in summer because the trees take the water, and acidic needle litter — so the planting has to tolerate all three rather than just the shade. Sword fern, salal, evergreen huckleberry, hellebores and woodland groundcover thrive in exactly that combination. What does not work is a lawn or a sun-loving border, and reseeding one repeatedly is money spent arguing with the canopy.