The Venice Dual Force Main: Ensuring Resiliency through Redundancy in a Historically Large Scale Microtunneling Project

摘要

This paper presents resiliency through redundancy construction management considerations and techniques that were used in the Venice Dual Force Main project. The project's intent is to construct a second force main sewer to operate in tandem with the existing force main sewer for the purpose of fulfilling the three key objectives, (1) The new force main will operate as a parallel system in conjunction with the existing 48" force main to meet the existing Peak wet weather Flow demands experienced at Venice Pumping Plant. (2) Add operational flexibility and reliability. (3) Allow isolation of either force main during low flow conditions to perform necessary cleaning and maintenance. During the heavy storms in 1995, Venice Pumping Plant VPP experienced a tremendous amount of peak wet weather flow (PWWF) that was contributed by stormwater infiltration in the sewershed. At times, the PWWF exceeded the maximum capacity. As a result, sewage rose in the plant's equalization wet well, reaching a critical level. With this potential serious public health and environmental risk, the City of Los Angeles recognized the need to increase the output capacity in order to be resilient to the existing infrastructure. With this project, the current and proposed capacity concluded that the addition of this 54" force main sewer will increase the current capacity from 44,000 gpm to 57,000 gpm, with four pumps operating and one pump in standby mode. With the current PWWF of 45,000 gpm, the combined 54-inch/48-inch dual force mains can safely handle, in tandem, this capacity with three pumps in service and two pumps in standby mode. The project is in the City of Los Angeles in the communities of Venice and Playa Del Rey, which constructs approximately 10,800 feet of a new 54-inch-diameter force main sewer in the City of Los Angeles, California. The construction is being performed under a major 4-lane road in the neighboring City of Marina del Rey, adjacent to multi-million dollar residences and below the water table and a dense layer of existing utilities and then crosses under the Marina Entrance Channel and Ballona Creek. From there, the alignment continues south along Pacific Avenue and Vista del Mar and connects to the junction structure of the Coastal Interceptor Sewer Venice Dual Force Main (CIS) and North Outfall Sewer (NOS) next to high visibility beach areas. To manage this complex micro tunneling project in both politically and environmentally sensitive area, a team of Project, Engineering, Construction, and Public Outreach management were organized to plan the tasks of design, constructability review, geotechnical investigations, environmental assessment, right of way assessment, permitting evaluations, multiple solicitation/bid and award stages, and through construction and closeout. The trenchless construction method, which utilizes hydraulic jacks to push pipes through the ground behind a remotely operated Micro-tunnel boring machine (MTBM). Drive lengths are generally limited to about 1,000 feet, depending upon ground conditions and pipe size; but intermediate jacking stations can be used to extend the drive length. Unlike conventional trenching techniques that require excavation for the entire length of the pipeline, excavation for tunneling is limited to the endpoints of each drive at designated launching (jacking) and receiving pits. The launching pit contains the hydraulic jacks used to push the pipes, and the receiving pit is used to recover the TBM at the end of each drive. Tunneling can proceed intermittently; although, it is often necessary to proceed continuously, particularly on long drives through sticky soils, to prevent the pipe from getting stuck short of the receiving pit. Tunnel advance rates are typically between 30 and 50 feet per 8-hour work shift, depending on soil conditions and pipe size. The tunnel face is supported by a thick liquid ("slurry"), which is a mixture of the excavated soil ("muck") and bentonite (a natural clay mineral). Keeping the slurry pressurized in a closed chamber behind the cutter-head of the TBM prevents groundwater and excess soil material from entering the TBM. During construction, the Construction Management Team implemented processes to monitor and report on a daily basis any potential problems, including soil-related issues (i.e., heave, settlement, slurry consistency) and tunneling machine issues (i.e., misalignment, steering or stoppage). Other than the normal construction management factors, additional processes were implemented along with a comprehensive public outreach program to mitigate public nuisances and complaints (i.e., noise and vibration problems and traffic flow). This paper will provide guidance and lessons learned for planners, engineers, designers, construction managers on planning, designing, and construction management of large scale microtunneling projects to ensure sustainability and redundancy to their existing infrastructure.