Tanana River Bridge #505

Steel Casings & Deep Shafts in Alaska

For this bridge project in Alaska, Malcolm Drilling installed 10-foot-diameter permanent steel casing at five shaft locations with a Leffer 3050 oscillator: one pier location was on land, two were installed on a sand island constructed in the river, and two were installed from a work trestle and finger piers constructed over the river. Permanent steel casing segments were welded together vertically over the shaft location, with the deepest casing reaching 176 feet—the deepest shaft in Alaska’s history. A Liebherr 885 dig crane excavated the shafts utilizing a 3-meter spherical grab, dumping shaft sands, and gravel into waiting dump trucks. Four of the shaft locations feature rock sockets up to 39 feet deep below the permanent casing. Malcolm placed all reinforcing steel cages as single units and then placed 4000P concrete in all shafts.

Malcolm also installed four (4) 4-foot-diameter permanent steel casings at Abutment 1 that were slipped inside the temporary oscillator casing. A 1.5-meter oscillator was hard coupled to a Bauer BG-40 top drive drill rig that was utilized to remove the shaft material from within the temporary casing. After the casings were excavated, permanent casing segments were hoisted and welded over the shaft, the temporary oscillator casing was extracted, the reinforcing steel cages were placed, and concrete was pumped into the permanent casings. In addition, six (6) 30-inch-diameter shafts were drilled in rock at Abutment 7 with the Bauer BG-40 drill rig to a depth of 52 feet.

Malcolm Drilling