How Are Deep Foundations Designed in Seattle's Glacial Till?

The geotechnical investigation for a till-bearing foundation answers four questions. First, where is the till: borings locate the top of the dense stratum across the site and note where fill, outwash, or soft soils overlie it. Second, how strong is it: penetration tests, often reaching refusal in the till, and laboratory testing establish the bearing and side-resistance values the design can use. Third, what sits below: the engineer confirms that advance outwash or deeper strata are competent where the design relies on them. Fourth, where is the water: perched groundwater above the low-permeability till is common and shapes both the drilling method and the dewatering plan. The report translates these findings into design parameters: allowable end-bearing pressure, side friction or adhesion values, lateral load criteria for the upper shaft, and minimum socket depths. It also flags the exceptions that change the design: zones where the till is weathered or thin, buried channels of soft soil, and slopes where lateral soil pressure adds to the shaft's demands. The structural engineer cannot select shaft diameters, lengths, or reinforcement without these numbers.

Drilled shafts dominate Seattle's constrained sites because they install with minimal vibration and can carry very large axial loads on a small footprint. The design sizes the shaft diameter for the column loads, extends the socket into the till per the geotechnical report, and reinforces the cage for the combined axial, shear, and bending the shaft will see. Where lateral seismic demands are high, the engineer may enlarge the upper shaft, add reinforcement, or found a grade-beam system that distributes lateral loads across the group. Driven piles trade the drilled shaft's precision for speed and economy on open sites. Steel H-piles or pipe piles driven to refusal in the till develop capacity through end bearing with the driving criteria verified in the field. The trade-off is vibration and noise, which limits driven piles near existing structures and in dense neighborhoods. Both systems connect through pile caps or grade beams that the structural engineer designs for the column loads, overturning, and the seismic forces the code assigns, with special inspection of the concrete placement and reinforcement.