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Core samples show poor subgrades under residential streets; staff say mill/overlay treatments failed
Summary
City engineers told council that core drilling revealed clay and inadequate subgrade under many residential streets, prompting a long‑term shift away from chip seals and mill/overlay toward full‑depth reconstruction on affected blocks.
Presenters at the work session described the results of several years of core sampling that altered the city’s approach to pavement maintenance.
"They milled that street off and hit clay," the Presenter (S4) said of an early mill‑and‑overlay candidate; the finding led staff to core hundreds of locations and conclude that 75–80% of residential streets had been constructed with inadequate subgrades. "If you just go with surface treatments, mill and overlays, they're just gonna fail quicker," S4 said.
Staff (S2,S5) said the coring work — some cores taken up to three feet deep — showed subgrade moisture and poor structural layers that make surface treatments brittle and short‑lived. That evidence is the primary reason the city shifted resources to full‑depth reconstruction on many residential streets, even though that work is more expensive upfront.
