the western and eastern shores. Initially, Southern Pacific aimed to construct a causeway across the lake using rockfill; however, the material deposited beneath the water line kept settling away as quickly as it could be placed. After numerous unsuccessful attempts to construct a rock- fill causeway, the trestle was finally con- structed by using timbers obtained from the clearing of 2 sq mi (5 sq km) of forest or about 38,000 trees. The scope of work required to complete this project in 1902 was a testament to the rail engineers as well as the workers needed to physically construct the bridge. At that time, standard wood piling, about 40 to 50 ft (12 to 15 m) in length, was installed with steam or drop hammers. Because of the poor geotechnical conditions within the lake bed, Southern Pacific sought and found extra-long piles, which were of such length that the piling extended across three flat cars during transport. As railway travel in the U.S. became more popular, the Great Salt Lake was considered a tourist destination. Of course, every tourist location along one of the major rail routes in the U.S. required a tourist stop. Midlake Station was con- structed in the middle of the lake, which could be accessed from the trestle. Southern Pacific continued to operate and travel over the causeway until about 1956, when it was determined that main- tenance of the trestle was deemed unfeasible and uneconomic to sustain. A new plan was needed to traverse the lake and for the trestle, as the results of a survey of the structure predicted a short remaining life until replacement of the substructure would be required. In addition to repairs and/or replacement of superstructure components, the wooden piling was also found to be in a condition that warranted being replaced to keep the structure in service. Replacement of the Trestle In 1956, Southern Pacific completed an ambitious program of filling in the remaining 12 mi (19 km) to connect the two partial causeways from the western and eastern shores. The new route would be off- set from the existing trestle about 1,500 ft (457 m) to prevent damage to the existing trestle, which was just barely still in service. 98 • DEEP FOUNDATIONS • JULY/AUG 2018 New causeway, circa 1950 (photo credit: National Register of Historic Places) Engineers recognized the need to dredge the lake bed to remove the existing mud down to a layer of competent salt, resulting in a dredge volume of mud of about 15 million cu yd (11.5 million cu m). A total of about 45.5 million cu yd (34.8 million cu m) of rock and soil were excavated, transported and deposited into the lake to complete the filling for the new causeway. To prevent the embankment from functioning like a dam, which would prevent water and salt flows between the north and south sides of the lake, the causeway was intentionally built to be permeable. To enhance the flow further, two culverts were constructed through the causeway, which allowed small boats, wildlife and water to move and flow freely back and forth through the structure. During the 1980s, settlement of (Above) Waves crash against the causeway (Below) Train cars used to retain the rockfill (photos courtesy David Warnock) the causeway along with a rising lake level combined with periodic storm surges jeopardized rail traffic atop the causeway and impended the movement of salt and water through the causeway. Therefore, it was determined that the causeway would need to be lifted, which would result in even greater settlement but would allow the causeway to continue to be a viable route for the railroad. To raise the level of the causeway and to prevent loss of the rockfill into the lake bottom, Southern Pacific lined the causeway with old box cars that were filled with riprap. Time to Build a Bridge Working with third-party community groups and the Army Corps of Engineers, Union Pacific Railroad (UPRR) staff and consultants set out to find a solution that would balance the needs of industry, which relied on both the causeway and the Great Salt Lake, with the UPRR’s operational
