In both phases, the “dig down” involves excavating, jacking the existing structures, extending columns and seismic upgrades. The seismic upgrades include a combination of shear walls, column jacketing and column reinforcement using carbon fibre reinforced polymer (CFRP).
Now, as Saffarini explains, they are moving into the most complex stage. “To date [stage I] all the dig-down has taken place underneath the one-storey viaduct structure. A significant portion of the stage II dig-down occurs under the five-storey east wing of the head house. Removing the columns of this steel framed building cannot be done by simply shoring the lowest floor. In fact, each floor would need to be shored down to bedrock before the columns could be cut safely.”
The alternative, which was selected, he says, is to “grip the columns from the bottom, jack the entire building and cut the columns below the grip point.” The approach was used 15 years ago in San Francisco City Hall, “a building of the same vintage and almost identical structural system.” But, he points out, “The objective was different there, as 530 columns were cut and temporarily jacked to install seismic base isolators.”