All about drilling for the bridge
This spring we are finishing the last of the drilling operations.
So what exactly are we doing with those large tubular steel casings? The steel casings are in place to prevent the earth from caving in while drilling. The rebar steel elements are critical reinforcement structures, to add strength, used during the construction process. The tubular steel casings, or “cans,” are used for bridge foundation work to add strength and durability to the pier foundations.
Here’s how we arrived at the drilled shaft construction process for bridge foundation work:
During the design and planning process for Grand Avenue Bridge project, different foundation types were considered. Large spread footings as well as driven piles (pile driving) were analyzed but dismissed in favor of drilled shafts. Drilling creates less noise and provides a low risk for scour during large floods. The drilled shafts also provide for very high load capacity levels, and helps the bridge structure have a long lifespan of 75 to 100 years.
Drilled shaft construction process (oscillation method):
Drilled (oscillation) shaft construction originated in Europe, and in the last 25 years has become a leading type of deep foundation drilling here in the West.
The large equipment for drilled shaft construction is manufactured in Europe, while most of the wear items, tooling buckets, augers and core cans, are manufactured in the United States.
The system consists of two separate devices: an oscillator unit and a drill rig. The oscillator unit is operated independently of the drill rig and produces a very large, downward force. This, coupled with the weight of the drill crane and oscillator, produces the downward force needed to twist the core cans into the earth. This method is designed to cut through numerous geological conditions.
The oscillator twists the drill casing cans into place. The cans are designed with multiple joints to allow operators to continually drill farther into the earth as the can is twisted to its required depth. As drilling continues, water is introduced inside of the shaft to keep the water level greater than or equal to the surrounding groundwater.
Once the bottom of the shaft has been reached, the reinforced steel cage is lowered into position. A secondary crane holds the cage in place while concrete is pumped from the bottom up.
The drilling cans help the shafts maintain good concrete quality, keeping all loose soils and contaminates from entering the wet concrete while being pumped. Concrete then displaces the water in the cans to holding tanks, while maintaining the appropriate water levels. As the concrete is placed, the oscillator is moving in reverse, and the drilling cans are pulled up by the oscillator.