Permeable Pavers in Federal Way, WA — Infiltration Tested Against Dense Till and Perched Water.

A permeable interlocking concrete pavement system moves water through the joints into an open-graded reservoir base and, ideally, into the native soil below. That last step is the one that is not automatic. If the native soil will not take the water, the system is a very expensive puddle. Federal Way sits on glacial drift over dense till, with pockets of sandy loam near the Puget Sound shoreline.

Upland ground drains acceptably. The dense till underneath does not, so low spots develop a perched water table — the yard reads dry on install day and the subgrade never is. So we test infiltration before we design, and the design follows the answer. Where the soil accepts water, the reservoir infiltrates directly. Where it will not — which is common on the tighter ground in and around Federal Way — the reservoir is sized to store the design storm and release it through a perforated underdrain, so the system still performs and still counts. That distinction also drives whether the project earns impervious-surface credit under King County stormwater rules, which is the reason most Twin Lakes homeowners come to us about permeable in the first place. Designed correctly, it also neutralizes a perched water table under a base that looked fine on install day.