Internal documents show two of the oldest Muni tunnels are riven with cracks, leaks and corrosion, but a spokesman said the transit agency has never seismically inspected them — even following the 1989 Loma Prieta earthquake.

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While San Francisco Municipal Transportation Agency officials say other inspections found the 82-year-old Sunset Tunnel and the 94-year-old Twin Peaks Tunnel “in good serviceable condition,” outside engineers question the wisdom of failing to inspect them for earthquake safety, particularly considering the defects that have been found.

A 2009 maintenance inspection of the 1917-built Twin Peaks Tunnel — which carries passengers on the K, L and M light-rail lines between Market Street and the West Portal station — documented defects with “the potential to affect train services if left unattended.”

Corroded steel beams surrounded by loose pieces of concrete held up the tunnel at its eastern end. A retaining wall was rotting. Further down the tunnel, corroded concrete beams and slabs were so deteriorated that inspectors said they “may be compromised.” And at the tunnel’s western end, the ceiling contained “extensive cracks,” one three-quarters of an inch wide and 10 feet long.

Major cracks also were found in the 1928-built Sunset Tunnel, which carries N-Judah light-rail riders between Duboce Street and Cole Valley. “One major concern is the longitudinal cracks beneath the tunnel roof closer to the west end,” a 2009 visual inspection found. It recommended repair as “the first order of business.”

Yet despite this urgency, no repairs have been made. SFMTA officials outlined the scope of work this week, saying the agency plans to invest about $1 million to fix the worst of the problems by January 2013. The remaining defects will be repaired after that, SFMTA spokesman Paul Rose said, although the $6 million needed has yet to be obtained.

Asked if the tunnels’ vulnerability to earthquakes had ever been studied, Rose said no.

“Our maintenance inspections have assured us the tunnels are in good, serviceable condition,” he said, adding that the known defects “do not jeopardize the safety of our riders.”

But University of Illinois tunnel expert Youssef Hashash said mere visual maintenance reports cannot be used to determine seismic safety. BART and Caltrans “have found it important” to seismically inspect their older tunnels, Hashash said.

When BART officials studied the seismic vulnerability of its tunnels last decade — tunnels built to a more rigorous standard than Muni’s older tunnels — the results were surprising.

“We believed our system was really rock-solid; it performed beautifully well during Loma Prieta,” BART spokeswoman Molly McArthur said. But viewed “through the lens of the modern code structure,” inspectors found some $1.1 billion worth of seismic vulnerabilities.

San Francisco State University engineering professor Elahe Enssani reviewed Muni’s inspections for The San Francisco Examiner. Because simple visual inspections revealed cracks, chips and corrosion, she said there is reason to do a more in-depth seismic stability study, which would likely involve testing soil and possibly computer modeling.

“Given what we know as of now, a seismic vulnerability study must be done,” Enssani said.

kworth@sfexaminer.com

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