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Columbia Falls approves $5,000 change order after geotechnical review of 2nd Avenue East hillside failure
Summary
After an Alpine Geotech presentation showing a shallow slope failure driven by oversaturation and a buried fine-grained layer, the Columbia Falls City Council approved a $5,000 change order for additional lab and field tests and accepted recommendations for regrading and revegetation.
Columbia Falls — The City Council voted Feb. 17 to approve a $5,000 change order to Alpine Geotechnical Engineering to cover additional laboratory and field testing tied to a hillside failure on 2nd Avenue East.
Cliff of Alpine Geotech presented results from two sonic borings and a deeper drill hole that produced a continuous soil profile to roughly 85 feet. He said the upper 30–35 feet were dense gravel but that, unexpectedly, the team encountered fine-grained silt and clay starting around 35–40 feet. "We took a lot of samples, gathered a lot of data, put that data together," Cliff said, summarizing the field and lab work.
The firm ran direct-shear and triaxial strength tests to determine cohesion and friction-angle values used in slope-stability modeling. Alpine's models identified a shallow critical surface under existing conditions with a static factor-of-safety well below 1. Under Alpine's regrading recommendation — approximately 1.75 horizontal to 1 vertical — the static factor-of-safety would increase to about 1.7 and seismic-factor values would meet the industry target of 1.3, the consultant said.
Cliff told the council the likely triggers were torrential rainfall events combined with a long-running leaking water service that oversaturated materials and created a sliding plane at an interface between gravel and finer strata. He recommended defining lateral excavation bounds, sequencing work to allow revegetation, and retaining engineering oversight during construction. "We recommend having engineering oversight during this, preferably by us since we prepared the recommendations," he said.
Councilors asked whether Alpine would perform excavation; Cliff said the firm would not be the contractor for excavation but could map the bounds so the city can get accurate contractor volume estimates. The firm also installed PVC liners in boreholes to allow future inclinometer monitoring if warranted.
Following the presentation and questions, a councilor moved to approve Change Order No. 1 for an additional $5,000 to cover extra laboratory testing and staff/drilling costs. The motion passed on a roll call vote with Councilors King, Piper, Price, Robinson and Mayor Barnhart recorded as 'Aye.' The change order is intended to fund direct-shear and triaxial testing and other costs Alpine said were not anticipated in its original proposal.
City staff said most of the affected slope lies within the city right-of-way and that the work will be mapped to avoid unnecessary impacts to private property. The consultant and staff emphasized timing: regrading and revegetation would be scheduled to allow planting to establish before winter runoff.
Next steps described to the council include finalizing excavation bounds, developing an RFP or scope of work for contractors, retaining engineering oversight during construction, and using the additional lab results to refine remediation design.
