Becho Inc. - Drill Shaft Extraction
The contractor was digging multiple vertical shafts to a depth of 250' in a vacant city lot in downtown San Francisco. These shafts are to form a 'bulkhead' to prevent soil movement, while excavation for a subway station and California's future 'high speed rail line' is completed on the work site. The casings are filled with water during the excavation to resist surrounding ground pressure. During the extraction of a casing, the sections separated at three locations, 120, 145, and 170 feet.
Divers were lowered down the 84 inch diameter excavated casing in a man basket to a water depth of 170 feet. Repair options included welding the casing sections together or drilling through the section overlap and installing steel 'plugs' to lock the sections together.
The repair was successful and the casing was removed.