HOW TO CONNECT A NEW SEWER LINE TO AN EXISTING 72-INCH SEWER WHILE FLOWING AT 2/3 FULL AND OVER 13 FPS

The Rosewood Willoughby relief sewer project consisted of installing two miles of 45-inch sewer line to relieve a sewer line in Melrose Ave. that was flowing at 90% full. The new sewer line would intercept several sewers tributary to the Melrose sewer at the upstream end and discharge to the La Cienega San Fernando Valley Relief Sewer (LCSVRS) at the downstream end. The LCSVRS is 72-inch in diameter and is flowing at 2/3 full and at 13 fps.The flow in the LCSVRS was a changed condition to the contract. During design, the LCSVRS was gauged at less than 1/3 full at peak flow. Additional flows and a changed method of sewer operation caused a major change from the original concept and created a situation where the originally designed tie-in would not work.A change order was issued to the contractor modifying the method of the connection. The new method directed the contractor to excavate around the 72-inch pipe, support in place, form and build a cast in place concrete box around the pipe. A precast lined concrete insert, shaped like a half 72-inch pipe that would then serve as a continuation of the pipe was built. The idea was to saw-cut both ends of the pipe, lift the pipe, let the flow run in the box and then place the insert in the box. Once the insert was in the box, grout it securely and then place a precast cover on the new junction structure.The tie-in work happened over a 2-day period and was successful with minor delays. The actual tie-in was made at night. Odor controls, sewage spill mitigation, noise barriers and public relations efforts were put in place prior to the tie-in operation.The design of the junction structure has become the standard design for all major sewers where flow bypass is not an option. The method is cost effective, clean and minimally disruptive to nearby residents and businesses.