Colusa Wastewater Treatment Plant and Pipeline Improvements – Colusa, CA

Holdrege & Kull completed a geotechnical engineering investigation for the City of Colusa for improvements to the existing wastewater treatment system. The improvements included a new 55‑foot‑diameter by 22‑foot‑deep below-ground clarifier tank at the wastewater treatment plant; a new 26‑foot‑deep pump lift station on an existing sewer collector pipeline; replacement of approximately 15,000 linear feet of sewer pipeline; and design of perimeter earth berms and liner system for a 20-acre wastewater treatment pond.

The subsurface investigation entailed drilling and sampling 18 exploratory borings along the sewer pipeline alignment to a maximum depth of 20 feet, drilling one exploratory boring each at the clarifier tank and pump lift station to maximum depths of 50 feet, and drilling five exploratory borings at the treatment pond to a maximum depth of 20 feet. Drilling along the sewer pipeline alignment was performed on two-lane city residential and commercial streets, which required use of traffic controls to close off one lane. H&K characterized the soil and groundwater conditions at each exploratory boring location using field and laboratory test methods.

H&K’s engineering design services included estimates of seismic liquefaction induced settlements, foundation designs, and earthwork recommendations for the clarifier tank and pump lift station; sewer trench backfill recommendations; soil corrosion potential for structural design of the sewer pipeline; slope stability analysis of the wastewater treatment pond perimeter berms; and in situ permeability testing of the native soil exposed in the bottom of the wastewater treatment pond. H&K completed the project on-time and within budget.