Planning and Design to Rehabilitate the Semielliptical Joint Outfall of a Unit 3C Trunk Sewer

The Los Angeles County Sanitation Districts (Districts) treats 400 million gallons of wastewater daily at one of their 11 treatment plants. Wastewater flows are transported to the plants through pumping stations and 1,400 mi of sanitary sewer collection systems. Constructed in the 1920s, a portion of the Joint Outfall A Unit 3C Trunk Sewer is composed of a 78-in. diameter semi-elliptical interior vitrified clay tile grouted reinforced concrete pipe sewer. The vitrified clay tiles have fallen into the invert of the sewer, resulting in one to three feet of accumulated debris, thus reducing the capacity of the sewer. The critical sewer reach extends approximately 7,400 ft and is located in an unincorporated area of Los Angeles County. The study evaluates the engineering analysis and design of various rehabilitation methods using hydraulic modeling. Those evaluated include replacement, rehabilitation with cured-in-place-pipe (CIPP), fiberglass reinforced plastic (FRP) liner pipe via sliplining, and internal rehabilitation methods with polyvinyl chloride (PVC) lining system. Critical site constraints include existing curves along the sewer alignment, roadway with congested utilities, railroad crossing, flow diversions to reduce depth and velocity, large bypass to remove all the flow, private properties, and permitting. As part of this evaluation, a decision matrix framework is generated to provide guidance to future engineers on selecting a sewer rehabilitation method. The non-circular FRP liner pipe, and internal rehabilitation method alternatives are selected for the project based on hydraulic modeling, site constraints, constructability, and total cost. The non-circular FRP liner pipe and internal rehabilitation alternatives will provide approximately 87% and 90% of the existing hydraulic capacity with a total cost of approximately $19 million and $25 million, respectively. The overall project is split into two separate phases to rehabilitate the most critical reaches first. Construction for the first phase of the project is tentatively scheduled to be completed in the Fall of 2024 with the second phase scheduled to be completed in the Fall of 2026.