Some of the deepest caisson's ever drilled on a building in San Francisco will soon be completed for 181 Fremont Street, an 800-ft, 55-story high-rise in the South of Market District of the city, adjacent to the new Transbay Transit Center. Plunging an average of 262 ft into the ground, the 44 caisson shafts measure five to six ft in diameter and are drilled at such depth because 181 Fremont sits on landfill where the San Francisco Bay used to encroach.

About 23 of the caissons are currently drilled, with all of them expected to be completed by July 1st.

Steve DeWees, project manager for Sunnyvale, CA-based Level 10 Construction, the general contractor for the project, said in a recent news release that his crew first drills the caissons into “hardpan” or hard rock and then drills a rock socket for the caisson to be seated. Each caisson shaft takes three days to complete. On the first day, the shaft is drilled; on the second day, debris is cleared from the hole and the rebar cage is set; on the third day, concrete is poured.

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The Structural Engineer for the caisson drilling is ARUP; the drilling subcontractor is Malcolm Drilling Company of Hayward, CA.

To drill the deep caissons, Malcolm employs a partially cased polymer drilling method during the process. The six-ft diameter shaft is reinforced with a full length rebar cage and has a concrete strength of 6,000 pounds per sq-inch. To transfer high vertical loads into competent bearing stratum and to limit building settlements, the shaft was drilled 30 ft into a mixture of sandstone and shale, locally known as Franciscan Complex.

After the caissons are in next month, excavation of the project's basement will begin. To help complete this section of the work, crews will employ a massive 700B Luffing Tower Crane, which will be brought to the site in October.

The 700B Luffing Tower Crane, the only crane of its kind on the West Coast, will remain on site for about a year. With a full height of about 1,000 ft, the crane is being used for its extremely heavy load capacity at a very fast line speed. The monstrous machine can move more than 40,000 pounds in a load and has a line speed of 623 ft per minute. There is one other 700B Luffing Tower Crane in the U.S., in New York City.

The crane is manufactured by WOLFFKRAN AG, German, and is distributed in the U.S. by Maxim Crane Works, L.P.

 When complete in early 2016, 181 Fremont, rumored to cost upwards of $375 million, will be the second tallest building in San Francisco, after the 853-foot Transamerica Pyramid. It is owned and developed by Jay Paul Company, and includes 404,000 sq-ft of Class A office space on the lower 36 floors, and 67 condos on the top 17 floors. It is pre-certified LEED Platinum. The project was designed by architectural firm Heller Manus Architects of San Francisco.