SAN DIEGO'S OCEAN OUTFALL CALLS FOR OUTSIZE COMPONENTS
U.S.$140 million project includes largest bonneted
knife-gate valves, grooved pipe couplings ever made
For many years the sewage treatment plant in Tijuana, Mexico, had been unable to process all the raw sewage it received, diverting the excess into the Tijuana River, which in turn flows into the Pacific Ocean. There the prevailing current carried it northward to San Diego's South Bay, forcing the city at one point to close 24 km of prime beach.
To solve this unpleasant problem, San Diego and the International Boundary and Water Commission decided to build a primary treatment plant to supplement the Tijuana facility, as well as a U.S.$140 million outfall pipeline to carry the treated effluent three miles out to sea. Effluent from the treatment plant will first enter the South Bay land outfall, a 3658-mm-diameter pipeline capable of handling 1.3 billion liters per day. It was built by Colich & Sons, Los Angeles, using reinforced concrete cylinder pipe from Ameron International Corp., San Diego.
Colich & Sons had used double-gasketed, spigot-and-bell joints to connect the more than 3600 meters of pipe, according to Thomas Bensfield, project manager. However, the company encountered an extraordinary challenge when it came time to make the final piping connections in a four-story deep underground valve vault situated 0.8 km from the treatment plant.
The San Diego Sewer Authority wanted the ability to remove the two 12-m bonneted knife-gate valves, the largest ever manufactured by Technaflow, for cleaning and maintenance. It was determined that the flanged valves would not come out even after loosening them; nor was it feasible to use sleeve couplings because of the space constraints of the vault. What was required were special couplings that could be taken apart to facilitate removal of the valves.
The 3658-mm couplings, the largest ever manufactured by Victaulic Company of America, Easton, Pa., U.S.A., were cast of ductile iron in 24 segments and machined to extremely close tolerances to assure proper alignment during installation. The couplings were supplied with continuous elastomer gaskets and two coats of tnemic paint to resist corrosion.
Under normal circumstances, using the Victaulic system simply involves grooving the pipe to receive a bolted coupling with a synthetic rubber gasket that seals on the outside diameter. The resilient C-shaped gasket provides a leak-tight seal that is further strengthened by compression when the coupling is tightened as well as by line pressure. The groove where the pipe and fitting engage forms a self-restraining joint capable of withstanding loads of up to 6897 kPa.
However, the circumstances of the South Bay project were anything but normal. "Usually we just cast our couplings," said Phillip Thomas, the Victaulic engineer who designed them.
"But in this case, there were so many mating surfaces that we had to machine the inside diameters and bolt pads of each segment. If you're off just minutes, the pieces won't align properly. Further complicating matters was the fact that the most precise measuring machines require more than 15 degrees of arc length to give you a good reading."
It took Victaulic only five months to design and produce the couplings. Notwithstanding the precision of the finished product, Colich & Sons had their work cut out for them. Pipe alignment required the use of jacks and a 12-ton crane. Workmen could not use the couplings to align the pipe because the stresses were too great, and pipe concentricity had to be controlled. In assembling the huge couplings, they tightened eight bolts on the top and bottom sections of the pipe, leaving about an inch gap between the bolt pads on the side segments. Tightening the bolts on these segments drew the bottom segments snugly into place.
"The Victaulic engineers did a great job," said Colich & Sons' Bensfield. "We were able to install the couplings in a single day. I was surprised how well everything went together considering the pipe and the couplings came from two different manufacturers. All we had to do on the second day was torque everything down."
At the end of the land outfall, the effluent will enter a 610-mm-thick, 11.4-m-diameter reinforced concrete drop shaft that will take it 50 meters below sea level. From the bottom of the shaft, a tunnel with a 3073-mm internal diameter will carry the effluent 1500 meters underground and another 4200 meters offshore. At that point it will join a 2743-mm-diameter, 44-m-long riser that will carry the effluent upward to a 3048-mm-diameter, 1500-m-long pipeline on the seabed that ends in a wye with two 600-m diffuser legs -- kilometers from the beaches of San Diego.