frequently asked questions

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How will the drilled shafts be installed under the water?

To help explain the process of the drilled shaft installation, the first minute of this video is shared in order to visually represent the plan Jensen Construction and SDDOT has approved for this project. 

An oversized steel casing, 13.5’ in diameter, will be installed from a floating barge in the river down through the water into the river bottom. This is put in place to allow for a permanent 10-foot diameter steel casing to be installed inside the oversized casing and allow for the concrete to be placed inside the 10-foot casing without the need to worry about fresh concrete being spilled into the river. The permanent casings are approximately 49 feet long. 

The permanent casing is installed into the bed of the river and turned into the shale, creating a solid foundation. 

Credit for video belongs to Kalloc Tech

The “shaft” will then be excavated using an auger to drill deep into the shale below the river’s bottom.  This will occur in a two-step auguring process. 

Once the “shaft” is excavated and cleaned out, the steel reinforcing (rebar) is inserted into the drilled shaft. These are referred to as “cages” typically as can be seen on the video. 

These cages are fabricated offshore and taken by barge and then lowered into the drilled shaft using a crane on the barge. Care must be taken and special spacers are used to make sure the rebar does not come into contact with the bottom or sides of the drilled shaft to ensure that the rebar will be completely encased by the concrete. 

Upon final inspection of the placement of the rebar, concrete can then be placed in the drilled shaft. 

A concrete pump will be used to fill the drilled shafts with concrete. This is done by displacing the water in the drilled shaft with concrete by using a device called a tremie tube. A tremie tube is a long pipe with a length greater than the depth of the drilled shaft. One end of the tremie is lowered to the bottom of the drilled shaft and concrete is placed through the use of the tremie tube. Before starting to place the concrete, a foam ball is inserted into the top end of the tremie tube. Once the ball is in place, concrete is placed in the tremie tube above the ball. By the weight of the concrete on top of the ball, the ball is pushed down the tremie tube which is acting as a squeegee pushing the water down and out of the bottom of the tremie. When the ball exits the tremie’s bottom, the concrete then quickly begins to displace the water in the bottom of the shaft in an upward direction. The tremie always remains embedded into the concrete by 5’ or more and is lifted at the same rate that the concrete comes up in the shaft until the tremie reaches the top of the permanent casing.  

There are numerous testing procedures used throughout this process to ensure the highest quality in the materials used and ensure the installation is correct. 

Of additional note each drilled shaft will contain approximately 365 cubic yards of concrete and nearly 65,000 pounds of steel. The concrete will obtain a strength in excess of 4,500 pounds per square inch. 

**Disclaimer, please note this video was created by Kalloc Tech.**