needs. The group determined that a bridge would provide the best balance between the needs of the two groups. Water model ing completed by the Utah Department of Natural Resources became critical in determining the length of the structure to be constructed. The group collaborated to determine that the structure would need to be about 180 ft (55 m) long. To maximize efficiency and utilize as many standard UPRR bridge specifications as possible, the bridge was designed to have six spans, each of about 29 ft 10 in (9.1 m) in length. The spans would be supported on driven piling, which would be founded in the lake bed. Each steel pipe pile would be 24 in (610 mm) in diameter and would be driven to a total resistance of at least 120-ton (1,070 kN). The lengths of the pipe piles ranged from about 110 to 145 ft (33.5 to 44.2 m), and were driven from grade. Subsequently, the causeway was excavated and eventually breached, as part of the construction sequence. Conventional Drilling Won’t Do Once a design had been determined and an installation plan in place, UPRR wanted to ensure that the new bridge would last as long as possible, given the saline environment within which the new bridge would be constructed. Steel pipe piles were Pile driving at the site selected and designed to support the new structure; however, conventional pile driving techniques would not be able install the piles to the required tip elevations because of the boulder fill in the lake bed. The size of the rock elements within the boulder fill varied from about 2 in (51 mm) to several feet in diameter. In addition, it was later discovered that railroad cars were present within the subsurface, which also prevented conventional pile installation. Installing the foundations through the causeway without causing damage to the steel piling became another challenge, as damage to the pile could result in premature corrosion of the element. Conventional drilling techniques gen- erally used to construct drilled shafts or for predrilling for driven piling could not be utilized because of the porous nature of the causeway. Any drilling fluids, whether traditional bentonite or newer polymers, would be ineffective and a waste of money because the drilling fluid would be lost due to flow through the large open spaces in the rockfill. Furthermore, the loss of drilling fluid through the rockfill would likely find its way out into the lake. Because of the various sizes of rock used to construct the causeway, through which the new foundations needed to be installed, the use of traditional open-hole drilling techniques would likely result in cave in or movement of the rock into the borehole, which could then trap or pin the tooling within the borehole. The risk of leaving tooling in a hole could result in a detrimental impact to railroad operations, depending on the location and orientation of the drill rig. After working with UPRR and other Class I railroads, the contractor understood the paramount concern expressed by the railroads regarding open, uncased drill holes, which eliminated this technique as an option to construct the foundations. In addition, the use of permanent casing in the upper zone within the rockfill, prior to installing piles deeper for support of the structure, would affect the hydraulic properties of the waterway and would result in not only an eyesore but also difficulty in excavating the causeway to open the channel to water flow. Ultimately, none of the proposed solutions were ideal because the resulting over-excavation could affect the causeway, the lake or the mainline railroad track that was still in service. Drilling the foundations using a casing oscillator seemed to be the only suitable solution. As such, the equipment and tooling were mobilized to the site, and the oscillator worked well to advance the casing through the rockfill and into the DEEP FOUNDATIONS • JULY/AUG 2018 • 79
