Digging Deep After its evaluation, McKim & Creed proposed several important changes to the project. The first was to “harden” the lift station to with - stand Category 3 hurricane storm surges. The more critical assessment, however, was digging eight feet farther down with the microtunnel under the Osprey Avenue Bridge. The mi - crotunneling process finally began in January of 2017. “This is where the rubber meets the road,’’ Mitt Tidwell, Sarasota’s Utilities Director at the time, told the Sarasota Herald-Tribune. “We scheduled this work first to protect the city’s investment and minimize risks.” The process included the closure of the Osprey Avenue Bridge in Sarasota in July of 2016. The bridge re-opened in June 2017. “The only choice was to go under the bridge, and microtuneling was the only option,” McKim & Creed Project Manager Robert Garland, PE, said in a radio interview . Microtunneling is a remotely controlled process that is frequently used in soft, unstable soils. Crews dig sending and receiving pits to install a new sewer pipe. A microtunnel boring machine is dropped into the sending pit and cuts a hole underground to the receiving pit, without disturbing the surface above. A jacking rig, functioning similar to a jack hammer, pushes the new sewer pipe in place. Garland said the process is slow, progressing about 20-40 feet per day. “You’re underground, chewing through rock, under a historic bridge, and in an environmentally sensitive area,’’ he said. “We went about this in a methodical way.” Wastewater is carried through a 36-inch PVC pipe in the microtunnel, and it is encased in a 60-inch casing. “We put the steel casing in first, which is the wall of the tunnel,’’ Garland said. “Inside that casing will be the pipe that carries the wastewater. It helps us in the construction, but if there is any failure of the pipe, it is contained within that casing.” New Lift Station While crews worked on the microtunnel, other teams started construc - tion of the lift station in October 2017. Work included the demolition of existing structures and the construction of a state-of-the-art odor control facility. The fully-enclosed building minimizes the impacts of routine operation and maintenance, according to the city. The building includes HVAC and equipment exhausts in the towers, and the two-story building was built over a new, 50-foot deep wet well. “We’ve spent the past year constructing the equivalent of a five-story building underground and below sea level,’’ Sarasota Utility Director Bill Riebe said in a press release in August 2019. “Construction of the wet well and connection of new, micro-tunneled 36-inch gravity sewer to the wet well piping has been technically complex.” PCL Construction installed 15 doors from BILCO in the lift station. The doors were manufactured in a wide range of sizes and allow ac- cess to vaults, pipes and pumps in the wet well. The floor doors are constructed from aluminum with stainless steel hardware for corrosion
The city estimated the cost of the new facility at $8.5 million. The city awarded Westra Construction Corporation a $9.6 million contract in February 2011, and work began in June with an estimated completion date in 2012. AECOM served as the project engineer. Construction began in June 2011, but another spill resulting from con - struction alarmed residents and city officials. In August, a contractor accidentally broke a 14-inch force main, causing an estimated 200-gal - lon sewage spill into the Hudson Bayou. A New Team In November of 2012, the city terminated its contract with engineering consultant AECOM. Westra was also removed as the contractor. The potential of another sewage spill if work had continued could have been catastrophic, as workers were on the verge of drilling into the northern support slab of the Osprey Avenue Bridge over the Hudson Bayou, according to a report . “We’re very lucky we didn’t have a ca - tastrophe with our utility department,’’ then-Mayor Shannon Snyder said in 2014. McKim & Creed replaced AECOM in August 2013, and construction began in 2015. With a budget of $32 million – about 4 times more the original plans – the company proposed a new above-ground structure that could withstand hurricanes. A bigger issue, however, remained with digging near the bridge. En- gineers decided a microtunnel below the bridge needed to be placed eight feet further down to avoid impacting the structural integrity of the bridge. Engineers believed the bridge might have collapsed in the soft silt base surrounding the bridge without the deeper microtunneling. Previous work done by contractors had to be done over. The project moved back to square one. “The invert elevation of the 36-inch main was lowered 8 feet to avoid interference with existing bridge abutment and to minimize potential impacts to the environment during drilling,’’McKim & Creed wrote on its website. “Phase 1 also included a review of the previous lift design and recommendations to increase reliability, improve worker safety and minimize operational issues.” The BILCO doors will allow access to pumps and vaults for workers at the lift station.
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csengineermag.com
June 2021
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