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Council adopts microtrenching standard updates to engineering code after freeze‑thaw concerns addressed
Summary
The council adopted amendments to Flagstaff’s Engineering Design Standards to permit microtrenching under specified conditions. Staff said the standard limits trench depth to avoid conflicts with frost‑protected utilities and requires quality backfill and sealants to limit freeze‑thaw damage.
Flagstaff City Council on Oct. 21 adopted amendments to the city’s Engineering Design Standards to incorporate microtrenching as an allowed construction technique under defined specifications, including sealant and backfill requirements and placement guidance intended to minimize freeze‑thaw damage.
Development Engineer Dave Millis presented the draft amendments and responded to questions about microtrenching performance in a cold climate. He said the city examined peer‑community practice and spoke with municipal engineers in Utah and elsewhere, and that the proposed standard focuses on shallow microtrench depths to avoid conflicts with frost‑protected utilities.
“We really focus on the time when the construction is occurring,” Millis said, describing controls on the placement of flowable fill and temperature‑sensitive sealants. He noted the standard locates primary microtrench joints at curb interfaces and specifies that any secondary trench placement near bike lanes should be positioned just outside a bike‑lane stripe so pavement markings adhere to asphalt rather than sealant.
Council debate
Council member Garcia, speaking as a motorcyclist and cyclist, asked whether the sealant would leave a raised seam across pavement. Millis said the specification holds allowable vertical tolerance within approximately one‑eighth of an inch and that the primary location would be at the curb‑to‑asphalt joint where a joint already exists.
Council action
Council adopted Resolution 2025‑52 and Ordinance 2025‑20, updating Title 13 engineering standards; the ordinance was adopted during the Oct. 21 meeting and will be codified into city standards.
Ending
Staff said the updated engineering standards aim to allow lower‑impact broadband and conduit installations while protecting pavement performance and nonmotorized travel lanes; contracts and permits will require adherence to the new specification language.
