Drilling completed for sewage system
CORONADO – After boring a 3,200-foot-long hole 50 feet below the bottom of the San Diego Bay, workers are getting ready to replace Coronado’s 35-year-old sewer line – the city’s only link to the main sewage system in San Diego.
Simultaneous digging from both sides of the bay has been completed. On each end, workers hammered a round metal pipe, 48 inches in diameter, into the ground that allowed a large drill to bore a hole under the bay. The sides met Tuesday.
“We were concerned about the age and condition of the pipe,” said Ed Walton, a city engineer. “We tested it to determine its condition and found it was approaching the end of its useful life. At the pump station, we had a similar pipe failure so we decided to do this as a precaution.”
All of Coronado’s sewage, including that from North Island Naval Air Station and the Naval Amphibious Base, is collected at the Transbay Pump Station in the Ferry Landing Marketplace.
The city uses a single sewer pipe to transport daily about 3.5 million gallons of sewage under the bay. The pipe ties into the San Diego Metro Wastewater System near the intersection of Kettner Boulevard and Harbor Drive in downtown San Diego.
Coronado’s system is unique among county cities, Walton said, because of the city’s seclusion and water boundaries. The $8.5 million project has taken about 3½ years of planning. Work began in December. The new sewer line is expected to open in May.
At the Ferry Landing, workers behind a 14-foot sound wall began drilling late last month. They set up a construction site at a recently drained duck pond on First Street near B Avenue, next to the city’s pump station.
At the other end of the bay, at Embarcadero Park North near Seaport Village, workers also drilled, first diagonally until reaching a spot about 50 feet below the bottom of the bay, then straight across.
Using sensors attached to the head of the drill and underwater bay measurements, workers on the Coronado side found the hole bored from the San Diego side. The 8-inch-wide hole will be expanded until it reaches 45 inches across, which should take about a week.
Coronado’s sewer line has been in continual use since it was installed in the early 1970s. Over the years, the capacity of the pipe has been reduced because of sewage buildup on the inside, from 24 inches in diameter to 19.
The city studied different design options, including affixing the sewer pipe above the bay, along the San Diego-Coronado Bridge, but decided to use the same “directional drilling” method SDG&E used about two years ago to install a pipe that contains underwater cable lines.
The next step will be to install the 3,200 feet of high-density polyethylene pipe in the bored hole. The new pipe, which measures 30 inches in diameter, is stronger than the old pipe. City officials expect it to last 50-plus years. But first, the pipe must be welded together.
The pipe is being assembled in the parking and bike lanes along westbound First Street, from E Avenue to the Ferry Landing entrance between G and H avenues. “No parking” signs were installed Monday and will remain up until about Feb. 21. Access will be available to residents.
Assembly includes creating four 800-foot-long pieces of pipe by welding together smaller 50-foot sections. During one 24-hour period, tentatively set for Feb. 22, the four larger pieces will be rolled into the center of First Street and fused into one 3,200-foot-long string that will stretch from B Avenue to J Avenue.
Advertisement
Parking will be restricted on both sides of First Street. Special rollers will be used to move the pipe. Cranes will lift the pipe at First Street and Orange Avenue to allow traffic to pass underneath. During a second 24-hour period, tentatively set for Feb. 23, the pipe will be installed, rolled toward the Ferry Landing, then fed through the bored hole until it disappears.
The city will pay for the project using wastewater funds and is working to get federal funding.
The old main line will be inspected. City Engineer Jim Benson said the goal will be to clean out the line for use as a backup. If the city is able to repair the line, it will use an additional $1.5 million it has set aside for that project.
Source: www.signonsandiego.com