Cushman Dam No. 2 right abutment concrete buttress

Location: Mason County

Owner/developer: N/A

Project team: Boss Construction, general contractor; Miles Sand & Gravel Co., ready-mix supplier

A mass concrete fill of a cleft located in the west abutment of Cushman Dam No. 2 was needed to encase and protect a fragile seam of native material.

The only way to access the abutment was from the dam’s east side, through an 11-foot-wide deck that runs across the top of the arched structure. Two sets of post-tensioned crane pads were installed so that the bottom of the cleft, 180 feet below the dam’s deck, could be accessed.

A 100-ton crawler crane with a modified anchor structure was secured to the pads. It was used to transport 2,350 cubic yards of loose material excavated from the cleft so that concrete could be placed against a firm bearing surface.

Lifts were formed and placed in 7.5-foot heights; stacked on top of each other. They ranged from 30 to 520 cubic yards, with the larger ones needing up to 20 consecutive hours of concrete pouring. Lifts were created with a one-sided gang form that was anchored back into the previously placed lift.

In all, 16 lifts using 4,882 cubic yards of concrete were placed over 15 weeks. The crane used a 3-cubic-yard bucket to place the concrete.

Low-slump and larger aggregate concrete was used to counteract shrinkage and the heat of hydration.