"This is actual retrofit work. It is the first work that will continue on the entire Bay Area Rapid Transit system over the next nine years" to make the system capable of withstanding a major earthquake, said Molly McArthur, a spokeswoman for the $1.3 billion project.

Work under the new contract, to be done on Port of Oakland property from the shoreline to the tube's end, is expected to begin in June, McArthur said. The retrofit work on the 3.6-mile underwater tube, estimated to cost about $300 million, could begin as early as fall of 2007, McArthur said.

Oakland-based Condon Johnson Associates Inc., won the bid over a second competing bid from Maryland-based Hayward Baker Inc., which had bid nearly $10 million for the project.

"It's an exciting job," said company president Gerard Condon. "It's some interesting technology. It's dealing with the Tube, which is a definite critical hub for keeping the Bay (Area) running."

Condon's firm has done similar work in the Seattle area on new subway tunnels and in the Bay Area has worked onBART tunnels in San Mateo County.

Directors also awarded a $9.6 million contract Thursday to test out a similar method of compaction for the underwater retrofit. That contract went to a consortium of firms led by Hayward Baker.

On both the underwater and above-ground retrofitting work, experts have determined that the most cost-effective way to keep the tube from shaking loose in a major earthquake — even one stronger than the 1989 Loma Prieta quake — is to vibrate the sand and rock covering the tube so that it compacts and becomes more stable, McArthur said.

Other methods, such as installing giant "staples" of piles connected by a cross beam across the Tube or "micropiles" driven through the floor of the tube were ruled out. Both of the alternate methods were considered too costly, and the micropile method also risked damage to the Tube and service disruptions, she said.

"One of the most important features of this particular approach is that we can perform this work safely while the trains are still running," McArthur said. And BART's 325,000 weekday riders? McArthur said they won't even notice as they roll by.

The vibro-compaction method involves inserting a car-sized cylindrical probe, which vibrates until surrounding sand and rocks have compacted. Vibro-replacement will do the same thing, but fill the hole made by the probe with a column of softball-sized rocks.

The area covered by the Condon Johnson contract is not underwater, but it still needs work.

"That area was all landfill," McArthur said, "and while it is on land, it is still subject to liquefaction and it needs densification as well."

To see a depiction of the vibro-compaction and vibro-replacement method, visit http://www.bart.gov/animation.