Oscillation-Drilled Shafts

Sending Up Good Vibrations on UDOT Region 2 Projects

The not-so-common oscillation method of drilling holes for enormous underground pillars of concrete proved to be an excellent solution in a construction area that called for careful mitigation.

By Catherine Higgins

(Photo Courtesy UDOT)

Drilled shafts are nothing new in the bridge construction industry; along with pile foundations, drilled shafts are one of the two most common foundations used to secure large structures like bridges. In Utah, however, the not-so-common oscillation method of drilling holes for enormous underground pillars of concrete proved to be an excellent solution in a construction area that called for careful mitigation.

The Utah Department of Transportation Region 2 EXPRESSLink project is building three bridges along I-15 between Salt Lake County and Davis County in Utah. Drilled shafts are being used at the bent foundations of each of the bridges. The bridges, located at Beck Street, U.S Highway 89 (U.S. 89) and 1100 North in Salt Lake City, are in areas replete with conflicts including silty soil, a high water table, pressurized utility lines and close proximity to rail road tracks and residential and commercial neighbors. The oscillation method of drilling neatly addresses each of these conditions.Additionally, the drilling method can save time by allowing the project to progress without encountering delay due to underground obstructions.

UDOT Region 2 is having success with oscillation-drilled shafts on projects like EXPRESSLink. (Photo Courtesy UDOT)

How it’s done

The drilling operation on the EXPRESSLink project was conducted by Malcolm Drilling of Seattle, Washington. Malcolm uses an oscillator to push casing into the ground, and then excavates soil from within the casing.

The UDOT Region 2 job at Beck Street required a three meter casing oscillator to drill a 2.8 meter diameter shaft. Malcolm used reusable casing segments bolted together to reach depths of 115 to 120 feet.

This unique drilling method has thus far saved UDOT time and money on this project. (Photo Courtesy UDOT)

A Kobelco 2500CK 250 ton crane was used to lift and place the casings. A Liebherr dual cycle crane was used to suspend a hammer-grab clamming shell with a jaw that opens and closes. As the oscillator displaced soil, the clamming shell removed soil that was then loaded directly onto trucks for quick and clean removal from the construction site.

After the casing was fully excavated, a steel rebar cage was placed in the casing as reinforcement for the concrete. The Kobelco service crane also placed the cages. The casings were removed as the concrete was poured. It took several days per shaft, with drilling and pouring happening on different shafts simultaneously at the each site.

Oscillation means mitigation

Learning this method of drilling was expensive and time consuming but after 10 years of experience, Malcolm officials believe the time and expense was well worth the investment. Few contractors use this method, according to Lance Rasband, project manager at Malcolm drilling. “Pile driving or conventional drilling would have been risky at this site considering ground conditions,” says Rasband.

Soil conditions at the U.S. 89, Beck St., and 1100 North sites were problematic due to loose silts and sandy soils that are often typical in areas with a high water table. The oscillation method advantage over other drilled shaft methods was three fold: First, having a fully cased shaft controlled for soil conditions by preventing the shaft from caving in. Second, the oscillation method prevents soil from sinking around the excavation area. Third, pile driving or uncased shafts in silty soil conditions can limit the length of the pile or drilled shaft. The fully-cased shaft provides a “good concrete placement environment with the side walls of the drilled shaft always protected by casing,” says Rasband. Shafts lengths are typically not limited in unfavorable soil conditions with the oscillator method.

UDOT is continually looking for ways to reduce the price road users pay in inconvenience and delay. The oscillator drilled shafts approach, along with a design-build construction method and other construction techniques, work together to shorten the overall impact of construction on road users.
	Large reinforced rebar columns provide added stability. (Photo Courtesy UDOT)

Soil conditions were not the only factors calling for mitigation – the oscillation method of drilling shafts helped UDOT Region 2 to be a good neighbor to residents, businesses and utilities in the area. Oscillator drilling causes minimal vibration, as opposed to pile driving and other drilled shaft methods. The Beck St. Bridge is located near pressurized utility lines and above ground rail road lines. Vibration from oscillation is far less likely to impact utilities and railroads. This advantage, allowed for safe, low risk progression of the construction process. Additionally, pile driving is loud and can be disruptive to businesses and residents.

Other advantages

The risk of slowing down or stopping a project due to underground obstructions is usually fully eliminated using a fully cased shaft. “We don’t typically have a hard time removing natural obstructions which can stop the progression of the drilling, “says Rasband who also reports that the State of Washington has tracked obstruction delay-cost dollars at zero due to use of the oscillator method in the past five years.

(Photo Courtesy UDOT)

Malcolm’s approach can be better for populated areas. “Our job site is cleaner, says Rasband. The excavated soil is placed in the back of a truck and hauled away which is better in an urban environment.”

Work on the EXPRESSLink project happened quickly. Shafts typically take three to four days per shaft for the diameters and depths of shafts used at both bridge sites. The U.S. 89 portion took 12 shafts with three to four days per shafts just in drilling and pouring. Malcolm will return in the next phase of construction to build more drilled shafts at the Beck Street site.

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What does this mean for UDOT?

Time saved means shorter project duration – the best way to limit impact to road users is to shorten the duration of construction. UDOT is continually looking for ways to reduce the price road users pay in inconvenience and delay. The oscillator drilled shafts approach, along with a design-build construction method and other construction techniques, work together to shorten the overall impact of construction on road users.

Elimination of risk keeps the project moving forward – A proven method that provides structural integrity quickly with less risk is, of course, always preferable.

Less direct impact to the closest neighbors – Less is more when it comes to noise and potential delay caused by vibrations that could possibly damage utilities or rail road lines.

Project facts

Prime contractor: Kiewit-Clyde Joint Venture
Sub-contractor: Malcolm Drilling Co., Inc.
Contracting method: Design-Build
Project location: Interstate 15; 500 North in Salt Lake City to the Interstate 215 connection in Davis County.