A commercial project along Ingersoll Avenue ran into a six-foot grade change between the parking level and the street right-of-way. The owner wanted a vertical wall to reclaim every square foot of usable space, but the upper four feet of the cut exposed weathered glacial till with lenses of sandy silt that sloughed within hours of exposure. We mobilized a crew that same week because the excavation was already open and a late-October rain event was forecast. The retaining wall design had to handle not just the lateral earth pressure of the native till but also construction-stage surcharge from a mobile crane set up six feet behind the wall line. In Des Moines, where winter freeze-thaw cycles reach depths of 36 to 42 inches, backfill drainage and frost protection govern long-term performance more than bearing capacity alone. Before finalizing reinforcement details, we correlated the till properties with data from a nearby SPT drilling program and confirmed the friction angle assumptions against site-specific grain-size curves.
A retaining wall in Des Moines lives or dies by its backfill drainage — freeze-thaw cycling demands a granular chimney drain and a positive outfall, no exceptions.
