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Colfax approves infiltration trench to address groundwater seepage on Culver Street
Summary
City staff and the city engineer told council that groundwater was seeping up through recently paved Culver Street; after geotechnical borings, council approved moving forward with the lowest apparent bidder plus a 15% contingency to install an infiltration trench designed to relieve groundwater pressure.
Colfax City Council voted Dec. 10 to approve a construction contract (low apparent bidder plus a 15% contingency) to build an infiltration trench along a problematic section of Culver Street where groundwater has been ‘‘weeping’’ through new pavement.
City Engineer Carl Moore told the council that hand borings by Geocon indicate substantial sheet flow under the pavement and that an old concrete retention vault in front of the sheriff’s substation — whose piping has since failed and which had been paved over — is only intercepting part of the flow. "We think this infiltration trench should relieve some of that water pressure, but it might not relieve all of it," Moore said, adding that deeper borings to find the water's source could be expensive (SEG 1996–2005, SEG 2066–2076).
Moore explained the vault is about seven or eight feet deep and currently has a shallow 4-inch drain line; the new trench is intended to intercept more of the subsurface sheet flow and divert it before it forces water up through the pavement (SEG 2054–2062). Councilmembers acknowledged the possibility the trench might not fully solve the problem if the source is far upslope; they discussed alternatives including deeper borings, dye tracing, cisterns or pumping should the initial mitigation fail (SEG 2100–2110, SEG 2148–2156).
Councilmember Speaker 7 asked for clarification of the resolution language and was told staff had used a low apparent bidder plus 15% contingency figure to cover likely contingencies; councilmember Helberg moved approval. The motion passed by voice vote with councilmembers recording "Aye" (SEG 2220–2234).
What this means: the city will proceed with the proposed infiltration trench as the most cost-effective first step to reduce groundwater-related pavement damage. Staff said they will inspect and drain the nearby vault to check its condition and will monitor the outcome before deciding on additional engineering work.
