City details emergency downtown sewer repairs after flooding; crews used dewatering and a "burrito" bedding to stabilize pipe

City public works staff gave a technical briefing Jan. 21 on emergency downtown sewer repairs performed after recent flooding that produced sinkholes and heavy sediment in the system.

Director Hamlin told the Parks and Public Works Committee that crews discovered a sinkhole in a resident's yard and a larger sinkhole in Cedar Alley after the flood night (identified in the briefing as the 10th). The downtown sewer in that area consists of concrete bell-and-spigot pipe installed in the early 1960s and bedded in loose, sandy, lake-bed soils. Hamlin said the high groundwater and hydraulic pressure during the flood allowed water to carry sediment into the line, flushing material downstream and leaving voids that caused pipe segments to fail or sink out of sight.

The city located a contractor (named in the briefing as Fury) that was working nearby and put crews to work the next day. Crews drilled multiple dewatering wells and, after about two days of dewatering, accessed the excavations despite flowing sands and heavy muck. Hamlin described removing many yards of muck, moving wet sand and water into dump trucks for disposal, and stabilizing the excavations so crews could repair the pipe.

To stabilize the pipebed in soft soils, Hamlin said crews constructed a bedding system using geotextile fabric and rock folded over the pipe'a method he described on the record as, "we built what we call a burrito, out of rock material." After backfilling, staff flushed and TV-inspected the lines and reported retrieving about 40 cubic yards of sediment from the grit system alone; Hamlin also said additional sediment reached auxiliary ditches and that about 1,000,000 gallons of water will require later drainage work during summer changeover operations.

Hamlin reported the city will perform a complete inspection to identify remaining areas of concern and then return with proposals for in-place rehabilitation alternatives (cured-in-place liners, epoxy coatings or other approved techniques) or conventional open-cut repairs if required. He said in-place lining is generally much less expensive than cut-and-cover replacement, but in certain locations full excavation may be unavoidable.

Council members asked about the cost and cadence of draining oxidation ditches and whether other circa-1960s pipes downtown would need inspection; Hamlin said draining one set of oxidation ditches for sediment removal is routine, performed annually, and that most downtown mains have been replaced over the last eight years, though a few sections of older concrete mains remain and will be addressed as needed, usually in dry-season summer windows when groundwater levels are lower.

Hamlin confirmed the failed main runs through an easement across private property; because the emergency work extended beyond the easement, crews disturbed the homeowner's yard but staff said the city will restore the yard and will not charge the homeowner for repair to her lateral damaged during emergency operations. Mayor Mayhew noted that during an emergency declaration staff have authority under state law to act immediately to protect public safety; staff said they attempt to keep property owners informed during and after emergency work.

Next steps: staff will complete CCTV inspections of the lines, present rehabilitation options and cost estimates to the committee in a future meeting, and schedule larger cut-and-cover work (if required) for summer when dewatering is less expensive and groundwater is lower.