Tunneling for what will be the first subway in the San Fernando Valley was temporarily halted Monday after the ground above the 50-foot-long excavation sank half an inch, according to the Metropolitan Transportation Authority.
"The situation is not serious at all. We are just being extra cautious in light of the Hollywood Boulevard problem," said MTA spokeswoman Andrea Greene, referring to the MTA construction debacle last July when Hollywood Boulevard sank a foot because of Metro Rail tunneling.
"We want to make darn sure this doesn't happen anywhere else," she said.
Greene said officials expect the tunneling to resume next week.
Mindful of the lessons learned in dealing with the Hollywood Boulevard problem, the MTA is now requiring work crews to use steel struts instead of wooden ones in the new tunnels and to keep watch for leaking water mains.
"The only similarity between the two areas is the soil in North Hollywood has some similarities to the soil under Hollywood Boulevard," Greene said.
The very fine soil, called young alluvium, was characterized as "collapse-prone" and is most troublesome when wet, according to a report by a private engineering firm last fall to the Federal Transit Administration.
To deal with excessive subsidence, MTA's contingency planners have stockpiled steel reinforcements and grouting material, which are now being used by construction crews in North Hollywood.
The contractor, Obayashi Corp. of San Francisco, has reinforced the completed tunnel segment to minimize further sinkage, Greene said.
MTA engineers also ordered the contractor to develop a plan to ensure soil stability and prevent excessive sinkage, which will include modification of the tunnel shield--the digging face of the excavating machine, Greene said.
So far, contractors have completed 50 feet of the easterly tunnel for the third segment of the Metro Red Line, which will eventually link the Valley with downtown Los Angeles. The digging began Feb. 13 under Lankershim Boulevard near Weddington Street and is expected to be completed by July, 2000.
Once completed, the twin tunnels will connect with tubes beneath Hollywood Boulevard to create a seamless ride from Union Station through the city's historic movie district and underneath the Santa Monica Mountains to the eastern Valley.