Can You Dig It? Chimborazo Sewer And The Chamber Of Secrets

The Chimborazo Interceptor Sewer serves a portion of the Church Hill area to the northwest of historic Chimborazo Park in the City of Richmond, Virginia. The combined sewershed drains to the interceptor, stair stepping down the steep hillside adjacent to the park near the intersection of East Grace Street and North 32nd Street. Due to the torrential rainfall from Tropical Storm Gaston in August of 2004, the slope failed and caused a portion of the downstream combined sewer to collapse. The City was faced with a unique challenge to address structurally damaged portions of the 48-inch circular brick step sewer residing within and near the steep hillside that is prone to failure. An emergency sewer repair was completed in 2005, which involved reconstructing a portion of the step sewer upstream of the existing junction chamber and installation of a 42-inch ductile iron pipe within the existing distressed 48-inch brick sewer downstream of the junction chamber. Another emergency sewer repair was completed in 2017, which involved the installation of a special steel pipe with concrete encasement at a step failure. The emergency sewer repair work was a temporary solution, and the design and construction of a phased permanent solution to prevent future failures of this critical asset was incorporated into the City DPU Capital Improvement Plan. A Preliminary Engineering Report was developed, which included a holistic review of collected background information, field investigation, hydrologic and hydraulic review, topographical survey, subsurface geotechnical investigation, and a cultural resources study. Using this data, phased alternatives and recommendations for a comprehensive permanent solution were developed. Due to the location, geography, subsurface conditions, and hydraulics, an unconventional design approach was developed for the abandonment and replacement of Phase 1 of this original combined sewer which dates to the late 1800's. This presentation will showcase the final design, equipment selection, bid phase, construction phase, planning, and coordination efforts that went into the construction of this replacement sewer. Phase 1 features a 60-inch inlet sewer; a 10-ft diameter, 30-ft deep inlet chamber; an 18-ft diameter, 100-ft deep vertical drop shaft chamber, which houses a fabricated siphon spillway drop unit (with 30-inch inlet, 6-inch air switch, 30-inch overflow, and 24-inch drop pipes); and a 60-inch outlet sewer. It also includes 5 new manholes, 900-ft of 60-inch gravity sewer replacement via micro-tunneling and open-cut techniques, and upstream and downstream connections to the existing 60-inch brick sewer. From utilizing an innovative technology new to the City, encountering unknown utilities and subsurface conditions, and navigating coordination with all types of stakeholders (federal, state, and local agencies, neighborhood groups, civic associations, and nearby homeowners), this presentation will highlight the project successes, challenges, and lessons learned. This project is currently under construction with an estimated final completion in March 2024.