Santa Monica's Coastal Interceptor Sewer System

“Upgrading an Aging Sewer System”

What’s the most effective way for a California coastal city to repair an earthquake-damaged sewer system and minimize future disruptions to busy streets?

MWH faced this challenge in Santa Monica, California.

In 1994, the Northridge earthquake caused significant damage to approximately 80 miles of Santa Monica’s aging sewer system. Much of the damage occurred in densely developed, highly visible and heavily traveled areas in the City. For that reason, officials wanted to minimize disruptions and ensure streets wouldn’t be torn up numerous times for different projects.

Instead of merely repairing earthquake damage, Santa Monica evaluated the capacity and condition of its entire sewer system. City officials decided to conduct all necessary repairs, upgrades and upsizing work in one five-year program.

Award-Winning Project

MWH was a key player in the program, as design and construction services engineers for replacing and upsizing the Coastal Interceptor Sewer System (CISS). This included approximately five miles of large diameter sewers and a $6 million major pumping system.

The result was honored by the Southern California Chapter of the American Public Works Association as “Project of the Year” in its water and wastewater category.

The award-winning project presented a number of challenges.

The first two segments involved constructing a 60-inch ID sewer. The sewer was located on busy roads and connected to the Moss Avenue Pumping Station, at depths up to 30 feet. The pumping station was located next to the Santa Monica Pier, a popular tourist destination.

One of the roads affected was Pacific Coastal Highway (PCH). This six-lane highway experiences heavy daily traffic from commuters and beachgoers. On one side of PCH, the Palisades Beach bluffs – which are prone to periodic sloughing and landslides -- line the highway. On the other, high-value beach clubs and residences border the beach front along PCH. Underneath PCH are electric, drainage, gas, communications and other utilities, including the primary water supply pipeline to neighboring Malibu.

Making matters worse, an old railroad bed, storm culverts originating in the City high above the bluffs, and abandoned building foundations, which were razed to make earlier highway expansion possible, underlay unknown portions of the highway.

On the project, MWH experts achieved the following objectives:

• No adverse construction impacts on the stability of the Palisades Bluffs
• Minimal relocation of existing utilities
• No damage to adjacent high-value residences and beach clubs
• Minimal disruption of traffic flows or access to clubs and residences
• Minimal inconvenience to tourists and beachgoers
• Prevention of sewage spills during construction and start-ups, including earthquake damaged-areas
• Prevention of construction-related runoff to storm drains or into ocean

Microtunneling was chosen as the construction method, because of the sewers’ depth and the predominant sand and cobblestone soils configuration. A number of unexpected problems arose, unfortunately, especially related to soil conditions.

Geotechnical borings along PCH only superficially alerted design engineers and workers to these situations. However, Caltrans officials required borings be conducted as much as 20 feet to the side of the proposed alignment. Soil conditions could only be estimated in several unexplored areas. Only careful and skilled tunneling prevented expensive damage to the houses, historic bluffs, highway or overlaying sewers and utilities.

Moss Avenue Pumping Station

A pumping station, located next to the Santa Monica Pier, was not damaged in the earthquake. However, the existing pumping station was more than 50 years old with inadequate capacity to handle future needs. The old station was also unattractive and unsuitable for its highly visible beachside location.

The team faced a number of challenges in planning, designing and constructing the new pumping station, including:

• Fitting the larger new facility into a small sloping site
• Making the station attractive and compatible with the beach and Pier’s amusement park area
• Dealing with difficult soils and groundwater conditions during construction
• Preventing noise and odor problems during and after construction
• Keeping the old station operational until flow could be diverted to the new station
• Minimizing inconvenience and disruption to nearby residents, restaurants, pier visitors and tourists
• Coordinating the construction with other area work, while maintaining maximum accessibility to tourist attractions
• Preventing sewage spills during construction, as well as runoff to storm drains or to the ocean

Sewage is conveyed downstream of the Moss Avenue Pumping Station by pipes located under several busy residential and commercial areas. Merchants were particularly sensitive to impacts on their businesses, already hurt by adverse economic conditions and a downturn in tourism. In response, special measures were taken to deal with noise, vibration, dust and potential odors.

Public Outreach

Throughout the project, a public outreach consultant worked closely with City officials.

A 24-hour hotline was established that allowed residents and merchants to report problems immediately. When construction lights and noise could not be mitigated, the City provided overnight hotel accommodations to affected residents. In some cases, the City agreed to clean carpets and wash exteriors of buildings located next to particularly dirty construction sites.

An independent outside specialist implemented pre- and post-construction building and property inspections. These precautions were taken to assure property owners their facilities had not been damaged during construction. They also resolved several questionable property damage claims that arose during the project.

The award-winning project has not only helped repair the earthquake damage. It also has prevented public nuisances, potential sewage spills and has provided Santa Monica with sewage capacity adequate for 100 years in the future.