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 provided an excellent solution in a construction area that called for careful mitigation and quick execution.
The Utah Department of Transportation (UDOT) Region 2 EXPRESSLink project is building three bridges on I-15 between Salt Lake County and Davis County. Oscillation-drilled shafts are being used at the bent foundations of each bridge. The bridges are in areas replete with construction conflicts, including silty soil, a high water table and proximity to businesses, residents, pressurized utility lines and railroad tracks. The oscillation method of drilling addressed each of these conflicts effectively and quickly, and allowed UDOT to keep a tight schedule.
The drilling operation on the EXPRESSLink project was conducted by Malcolm Drilling of Seattle. Malcolm’s method uses an oscillator to push casing into the ground, then soil is excavated from within the casing.
The UDOT Region 2 job at Beck Street required a 10-foot casing oscillator to drill a 9.25-foot diameter shaft. Malcolm used reusable casing segments bolted together to reach depths of 115 feet to 120 feet.
The job required two cranes. 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 and loaded the soil from the casing directly onto trucks for quick and clean removal from the construction site.
After excavation, the Kobelco service crane was used to place a steel rebar cage in the casing as concrete reinforcement. The casings were removed as the concrete was poured. The process took several days per shaft, with drilling and pouring occurring on different shafts simultaneously at each site.
Acquiring oscillation-drilling expertise was expensive and time-consuming, but after 10 years of experience, Malcolm’s officials believe the process was well worth the investment. “Few contractors use this method,” says Lance Rasband, project manager at Malcolm Drilling. “Pile driving or conventional drilling would have been risky at this site considering soil conditions.”
Soil at the U.S. 89, Beck St., and 1100 North sites contains loose silts and sandy soils that are typical in areas with a high water table. The oscillation method advantage over pile driving or other drilled shaft methods was three-fold. First, having a fully cased shaft controlled loose soil conditions by preventing the shaft from caving in. Second, the oscillation method prevented soil from sinking around the excavation area. Third, a fully cased shaft resulted in “good concrete placement,” according to Rasband, because the side walls of the drilled shafts always were fully protected. Driven pile or uncased shaft length can be limited in silty soil. The lengths of the fully-cased shafts were not limited, even in the presence of loose, sandy soil at the bridge sites.
Soil conditions were not the only factors calling for mitigation; oscillation-drilled shafts helped UDOT Region 2 to be a good neighbor to utilities, the railroad, businesses and residents in the area. Oscillation causes minimal vibration, as opposed to pile driving and other drilled-shaft methods. The Beck Street Bridge is located near pressurized utility lines and above ground railroad lines. Oscillation vibration is far less likely to damage utility lines and railroad tracks. This advantage allowed for safe, low-risk progression of construction. Also, compared to pile driving, oscillation drilling is quieter and less disruptive to businesses and residents.
The risk of slowing or stopping a project due to underground obstructions usually is 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. “The State of Washington has tracked and found obstruction delay-cost dollars were zero in the past five years due to use of the oscillation method.”
Malcolm’s approach also can be better in populated areas. “Our job site is cleaner,” explains Rasband. “The excavated soil is placed in the back of a truck and hauled away, which is better in an urban environment.”
Oscillation drilling progressed quickly on the EXPRESSLink project. Each shaft took 3 days to 4 days at all bridge sites. The U.S. 89 portion required 12 shafts. Malcolm will return in the next phase of construction to build more oscillation-drilled shafts at the Beck Street Bridge site.
What does the oscillation advantage mean for UDOT? First, it lessened impact to neighbors – the noise and vibrations associated with oscillation drilling is not bothersome to nearby businesses or residents. Second, reduced risk keeps the project moving forward – oscillation vibrations will not likely damage utilities or railroad lines, and underground obstructions usually are easy to remove. Lastly – and most importantly – a fast process helps reduce delay-costs for road users. A shorter project means drivers experience fewer traffic delays. UDOT continually is looking for ways to reduce tangible costs associated with traffic delay like higher gas expenditures, higher delivery prices and reduced income due to less time at work.
Oscillation-drilling is fast. Along with design-build contracting and other construction techniques, the oscillation-drilling method’s “good vibrations” helped UDOT shorten project duration and reduce delay-related costs for the driving public.