Managing Risk While Dueling for a Dual Force Main

Project Description and IssuesThe City of Los Angeles (City), Department of Public Works (DPW), operates and maintains the largest wastewater collection system in the United States. The system serves a population of more than four million people within a 465 square mile service area that includes Los Angeles and 29 contracting cities and agencies. The DPW is responsible for the planning, engineering design, construction, renovation, operation and maintenance of more than 6,500 miles of public sewer collection and conveyance infrastructure and City wastewater system pumping and treatment facilities that process an average of 550 million gallons of wastewater each day of the year.The Venice Pumping Plant (VPP) is located within the City of Los Angeles coastal community of Venice. The VPP is the largest pumping plant in the City. It collects and then conveys sewage through an existing 48-inch pressurized force main pipeline to the Hyperion Treatment Plant (HTP) in the community of Playa Del Rey.Wastewater flows are pumped from the VPP and conveyed in the force main to a junction structure on the Coastal Interceptor Sewer in the community of Playa Del Rey. The majority of the VPP force main was constructed immediately adjacent to the Pacific Ocean coastline and lies within the beach sands of Venice Municipal Beach and the Dockweiler State Beach Park.An Environmentally Sensitive Habitat Area (ESHA) is located in close proximity to the force main. A large protected breeding ground for the Least Tern bird species exists on Venice Beach just north of the Marina Del Rey entrance channel. The Least Tern is both a state and federally listed endangered bird species. The colony is recognized as one of the most successful and important California Least Tern colonies on the west coast.Over the years, the existing pipeline has gradually approached maximum sewage conveyance capacity placing substantial strain on the system. During heavy wet-weather storms occurring in 1995 the City first identified that the VPP force main was serving as a bottleneck in the system. Sewage and infiltrated stormwater in the sewage system exceeded the capacity of the existing48-inch line, creating a potentially serious human and environmental health risk. Although the pumping plant had all five pumps running during peak rainfall, the existing downstream sewer force main that runs along the beach could only handle approximately 60 percent of the flows that would otherwise run through the pumps.The existing VPP force main was constructed in 1958. It has been in continuous operation since first being placed into service more than 50-years ago. The force main has no redundancy. Because of the lack of redundancy the pipeline has not been cleaned, inspected or rehabilitated in its lifetime.Redundant Force Main PlannedThe City elected to proceed with constructing and operating a new two-mile long, 54-inch diameter force main sewer pipeline. The City has carefully studied and evaluated a multitude of alternative alignments, engineering designs and construction methods for the installation of a new force main.The project’s intent is to construct a second force main sewer to be used in tandem with the existing force main sewer. The City’s three key project objectives include 1) Sewage Conveyance Capacity, 2) Pipeline Redundancy, and 3) Ability to Perform Maintenance.The project would serve to increase the VPP system sewage conveyance capacity to needed levels, provide systemredundancy and tandem operation of the existing pipeline and a new pipeline when needed, allow for bypass of the existing pipeline and provide the City with the ability to perform maintenance and rehabilitation. Importantly, the project would mitigate the risk and consequence of operating the aging existing pipeline until catastrophic failure and spillage from the system occurs and causes significant harm to the environment and potentially endangers property and human health.The City moved forward with project planning and development of an Environmental Impact Report (EIR) considering multiple new pipeline alignment alternatives. The Los Angeles City Council certified the EIR on January 12, 2010, thereby approving the preferred and recommended force main pipeline alignment route. Los Angeles County subsequently filed a lawsuit against the City, challenging the EIR and the selection of the proposed route for this pipe.On July 27, 2011, the LA County Superior Court issued a ruling on the case, upholding the certification of the EIR. However, on two other causes of action; namely violation of the Public Utilities Code and Declaratory Relief, the court ruled against the City, effectively preventing the City from proceeding with the construction of the project at this time. The City has appealed the court ruling and the case went before the Appellate Court beginning in October 2012.While the courts consider the legal issues concerning construction of a new, redundant VPP force main pipeline in the City’s preferred and recommended alignment route the City’s Department of Public Works, Bureau of Sanitation (BOS), Wastewater Engineering Services Division (WESD), initiated a project to investigate a perform a condition assessment of the highest risk of failure of the VPP force main at its high elevation points.Need for Existing Pipeline Investigation EstablishedThe VPP Force Main has been in continuous operation since its construction. The pipeline consists primarily of 48-inch unlined, reinforced concrete pipe (RCP). The structural condition and integrity of the pipeline were unknown owing to the lack of redundancy and inability to remove the pipeline from service for maintenance including interior cleaning, inspection, and rehabilitation.Pipelines constructed of RCP that convey se wage are subject to deterioration primarily owing to external environmental and loading factors and internal operating, performance, hydraulic and environmental factors. Sewer conveyance pipeline constructed of RCP are particular subject to interior materials degradation from hydrogen sulfide (H2S).Based upon the typical operational lives seen for similar pipelines in Southern California and elsewhere it was theorized that the pipe might be in a state of advanced deterioration and failing. With litigation postponing construction of a new force main pipeline the BOS WESD determined an inspection of the pipeline was needed to better understand the structural integrity and state of deterioration of the pipe materials and potential risks attributable to this pipeline’s anticipated poor condition.Project ExecutionBrown and Caldwell (BC) was retained by the City to evaluate inspection options, select suitable inspection technologies and then perform the inspection. BOS WESD and BC entered into the project acknowledging that inspecting the interior or exterior of this pipeline would carry significant and inherent risks. Based on the findings of the inspection, BC would recommend whether or not the City should pursue an additional phase for the project. This phase would include BC development of a conceptual plan to structurally stabilize the pipeline until such time as the new VPP force main is constructed and placed into service.Structures providing access to the interior of the pipeline were not constructed in 1958 when the pipeline was built. Constructing new access structures on aged, existing pipelines has the potential to causes failure of the pipeline and require flow bypass. It was already known that bypass pumping of flows from the VPP pipeline was neither practical nor feasible.BC reviewed and evaluated a variety of non-destructive force main inspection technologies that could be used on the exterior surfaces of the pipeline which would yield pipeline shell thickness measurements and other information to verify pipe conditions. It was determined that several non-destructive testing (NDT) inspection technologies were available that would provide sufficient inspection data to allow for a meaningful investigation and condition assessment evaluation to be completed.Air is commonly trapped in pockets in force mains at high elevation points. BC developed a plan for inspecting and testing two (2) specific high elevation sections of the VPP force main pipeline. The identified high elevation sections are located where the force main piping “dives down” to pass beneath the Marina Del Rey and Ballona Creek Channels. These pipe sections were selected for inspection and testing. The City obtained all necessary permits and coordinated all work activities with other stakeholder agencies such as the Department of Harbors and Beaches. On March 20 and 21, 2013, the City’s excavation contractor located, excavated and exposed theidentified sections.Scientific Construction Laboratories, Inc. (SCL) provided subcontractor services to BC for the NDT and visual inspection efforts. SCL conducted impact-echo testing to identify if concrete delamination was present; ground penetrating radar testing in order to determine the concrete pipe shell thickness and reinforcement layout within the concrete; Surface Ultrasonic Detector (SURFER) testing in order to determine the concrete thickness; Schmidt hammer testing to estimate concrete compressive strength; and visual examination for observable defects. A BC field inspector provided assistance to SCL during the NDT field inspections. The BC field inspector served as BC’s project site health and safety officer and was present during all aspects of the excavations, field inspections and backfill operations.Observations, Findings and Results–Risk UnderstoodThe FM pipe was found to be in good condition at both locations inspected as evidenced by both visual inspection of the pipe exterior and the non-destructive testing measurements and scans of the pipe shell. Deficiencies, defects, or appreciable loss of interior pipe wall was not identified that would lead to the conclusion that it is necessary for the City to pursue rehabilitation of the pipeline at these locations at this time. Recommendations were made for exercising the ARVs and re-inspection at intervals on the order of 8 to 10 years in length in order to observe and track the condition of the pipeline given present day observations and findings.The condition of the pipeline and the near-term risk of failure of the VPP force main are now better understood by the City. The City continues to pursue construction of a redundant force main to further mitigate risk of operating the existing pipeline in perpetuity without the ability to maintain and rehabilitate it.