The Loma Prieta Earthquake: Impact on Lifeline Systems
- Published on Monday, October 29, 2007
- Written by Dames and MooreÃ¢â‚¬â„¢s Earthquake Engineering Group
The estimated damages qualify this event, known as the Loma Prieta Earthquake, as one of the most costly natural disasters that the United States has experienced. From this perspective, the earthquake could be classified as the “Big One.” However, this was not the catastrophic earthquake that many have predicted would occur. In fact, the energy associated with the Loma Prieta Earthquake was 30 times less than that associated with the earthquake that devastated San Francisco in 1906.
The earthquake occurred along a portion of the San Andreas fault zone referred to as the Southern Santa Cruz Mountains segment. The U.S. Geological Survey has commented that rupture of this segment of the San Andreas fault has not affected their assessment that there remains a 50 percent chance that one or more magnitude 7.0 earthquakes will occur in the San Francisco Bay Area within the next 30 years. They have also noted that the probability of a magnitude 8.3 event occurring (comparable to the 1906 earthquake) is on the order of 10 percent over the same time period.
The Loma Prieta Earthquake is unique in that lifeline losses, particularly those related to transportation systems, account for a larger portion of the total losses than in other recent earthquakes. In the following sections, we have attempted to describe several significant incidents of lifeline damage.
The most significant effect of the Loma Prieta earthquake was the extensive damage and disruption to transportation lifelines throughout the San Francisco Bay Area. Catastrophic collapse of major bridge structures led to a significant loss of life, and the total repair cost for damaged bridges and other transportation lifelines are expected to exceed $1 billion. In addition, it is estimated that indirect losses resulting from time lost to traffic delays and delays in the transportation of goods vital to the economy of the affected areas will exceed several billion dollars.
Bridges and Roadways--San Francisco/Oakland
The most significant damage to bridges and roadways occurred in the cities of San Francisco and Oakland, both over 70 miles from the epicenter of the earthquake. It is certain that even greater damage to bridges and elevated viaducts within these cities would have resulted if the Loma Prieta Earthquake had been larger or centered closer to the Oakland/San Francisco area.
The most publicized freeway failure was the collapse of a 1.25-mile segment of the two-level Cypress Street Viaduct along the Nimitz Freeway (Interstate 880) just south of the San Francisco Bay Bridge. The collapse of this section, on which 170,000 vehicles traveled per day prior to the earthquake, caused over 40 fatalities and is estimated to cost $200 million to repair.
The two-level Embarcadero Freeway in downtown San Francisco, which has a similar design to the Cypress Street Viaduct in Oakland, exhibited signs of non-ductile (brittle) shear cracking at its concrete joints. Because of this damage and due to its similarity to the Cypress Street Viaduct, the entire Embarcadero Freeway was closed immediately after the earthquake. Plans to reopen the Freeway, on which 60,000 vehicles per day traveled prior to the earthquake, are currently the focus of ongoing discussions between elected officials, community representatives and people in the engineering profession.
Constructed during the mid-1930’s, the San Francisco Bay Bridge extends 8.5 miles and is built from double-decked steel. The Bay Bridge, which carried an average 250,000 vehicles per day, was the major transportation artery between San Francisco and the cities of the East Bay. About two miles west of the Oakland toll plaza, one of the 50-foot long upper deck sections collapsed into the lower deck, resulting in one fatality and the closure of the bridge for over one and one-half months.
Bay Area Rapid Transit (BART) System
The Bay Area Rapid Tansit (BART) system is a 75-mile long rail transit system that includes an underwater trans-bay tube that rests on the floor of the bay and was specially designed to accomodate earthquake motions and deformations. BART was virtually undamaged by the earthquake, and service was uninterrupted except for a short inspection period immediately following the quake. With the closure of the Bay Bridge and the Cypress Street Viaduct along the Nimitz Freeway, BART became the primary passenger transportation link between San Francisco and East Bay communities. Its average daily transport of 218,000 passengers before the earthquake increased to an average of 308,000 passengers per day during the first full business week following the earthquake.
BART includes 27 miles of elevated structures that are vulnerable to the same forces that had such a devastating effect on local highway bridges and viaducts. However, these structures, which were designed and constructed during the 1960’s, were not damaged by the earthquake. The seismic design criteria used for BART’s elevated structures have become standard practices in modern highway bridge design.
The major airports in San Francisco and Oakland suffered only moderate damage as a result of the earthquake. At the Oakland Airport, liquefaction of hydraulic fill materials caused movement and cracking of dike sections along the shoreline of the airport, as well as extensive cracking and settlement of the northernmost 3,000 feet of the airport’s 10,000-foot main runway. However, the dikes were quickly repaired and runways were usable following the earthquake.
The only reported damage at the San Francisco Airport occurred at an air cargo freight building, at the control tower (window damage) and at the North Terminal of United Airlines (extensive water damage due to breakage of sprinkler heads). The airport was closed for approximately 12 hours after the earthquake in order to repair the damage to the control tower and to check the airport for other major damage.
At the Port of Oakland, extensive damage resulted from earthquake-induced settlement and liquefaction of the fills and subsurface material. This damage included deformed rail lines, tilted container cranes, and cracked wharf piles. In spite of the reported damage, most of the port facilities were able to remain in operation following the earthquake.
WATER DISTRIBUTION SYSTEMS
Several private and public water districts were significantly affected by the Loma Prieta Earthquake. The San Francisco Water Department experienced roughly 70 water main breaks and 50 service line breaks in the Marina District north of the downtown area. No problems were reported at any of the department’s resevoirs or importation facilities, and the only problem at the pump stations was a temporary loss of power.
The East Bay Municipal Utilities District identified over 120 water pipeline breaks following the earthquake. Some of the significant damage included a break in a 60-inch raw water pipeline supplying the Sobrante filter plant and several breaks in a 20-inch cast iron pipeline near Laney College in downtown Oakland.
The San Jose Water Company reported 155 pipe breaks, 67 of which were repaired within the first 48 hours. Most appeared to be related to bell couplings at adjacent pipe segments. One 100,000-gallon anchored steel reservoir buckled peripherally (“elephant’s foot” buckling), and a 70,000-gallon reservoir leaked due to rupture of the inlet/outlet line.
The Santa Clara Water District also experienced major disruptions. The most substantial damaged occurred at the 20-year-old Rinconada water filtration plant. Three of the four AMCO clarifiers (each handling 20 million gallons per day) sustained major damage as a result of water sloshing. These units are expected to be out of service for three to four months.
Although there are about a dozen dams in the epicentral area, there were no major failures. Officials attribute this to the fact that most of the dams were designed to withstand earthquakes of higher intensity and longer duration than that experienced on October 17th. Furthermore, at the time of the earthquake most of the reservoirs were only filled to 10 to 50 percent of their full capacity.
The dam that was most significantly damaged was the 185-foot-high Austrian Dam (San Jose Water Works). The dam settled up to 2.8 feet and its end moved about 1.2 feet downstream. A 6-inch-wide, 12-foot deep crack at the spillway, and cracks in the reservoir (dewatered zone) and the downstream area, were also visible.
NATURAL GAS SYSTEMS
Earthquake damage to major gas transmission and large gas distribution lines appeared to be minimal. Most of the serious damage reported by Pacific Gas & Electric Company (PG&E) occurred to gas mains and service lines. Approximately 150,000 customers were reported to have been without gas service after the earthquake. According to PG&E officials, roughly 90 percent of these shutoffs were customer initiated. The most severely damaged area was the San Francisco’s Marina District, where over 5,000 customers were affected. Restoration of service in the Marina District involved replacement of much of the system at an estimated cost of $20 million and required several weeks to complete.
ELECTRIC POWER SYSTEMS
Pacific Gas & Electric reported damage to two power plants, Moss Landing and Hunter’s Point. Power at Hunter’s Point was restored within days after the earthquake, while restoration at Moss Landing took several weeks. In addition, three substations were damaged. There was no reported damage to transmission lines or towers, although there have been unverified reports of buckling in some of the tower legs.
SEWER AND WASTEWATER SYSTEMS
Damage to two large-diameter (96-inch and 84-inch) outfall lines has resulted in costly repairs which will be necessary for San Francisco’s clean water system. Within the city, most of the sewage and wastewater system damage involved frames around catch basins and manholes; in general, this damage was limited to areas with imported fill soil. There also appeared to be evidence of service lines being disconnected from the main sewer lines.
Power outages affected the operation of several sewage pump stations in the city of San Francisco. Sewage spillage into the bay was reduced by transport storage facilities which are intended to store the wastes temporarily when power fails.
The wastewater plant operated by the East Bay Municipal Utilities lost power, and the emergency generators malfunctioned. Consequently, 20 million gallons of raw sewage were reportedly dumped into the Oakland Estuary in a six-hour period after the earthquake.
This article adapted from Vol. 3, No. 2, p. 8.