Research Helps Optimize Alaska Railroad Bridge

11/12/2010

A railroad bridge in the Alaskan wilderness will receive an optimal design thanks to the work of a hydrosystems research team.

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A railroad bridge in the Alaskan wilderness will receive an optimal design thanks to the work of a hydrosystems research team led by Professor Marcelo H. García, Siess Professor of Civil Engineering. 

The Alaska Railroad Corporation is planning and designing the Northern Rail Extension Project to extend from a location on the Eielson Branch near North Pole, Alaska, to Delta Junction, Alaska, a distance of approximately 80 miles. Near Salcha, Alaska , the alignment crosses the Tanana River, a large, glacially-fed, braided river capable of transporting large amounts of woody debris, silt, sand and gravel. At the location of the planned railroad crossing, the Tanana River is approximately 0.9 miles (1.44 km) wide.
 
Current design for the bridge support structure calls for 12-foot diameter piers constructed 165 feet apart. While local pier scour is to be expected, the relatively short span between piers has raised concern regarding potential erosion and scour of the foundation due to the pronounced lateral contraction of the flow, or the acceleration of the flow that occurs as the water passes through the narrower space between the piers. Large woody debris, for example 90-foot-long tree logs, could also contribute to bridge scour in the Tanana River.
 
The physical model shows scour around the pier.  The bundle of dowels is meant to simulate woody debris in the river.
The physical model shows scour around the pier. The bundle of dowels is meant to simulate woody debris in the river.
An underwater view of the bridge model shows scour around the piers.  The bundle of dowels is meant to simulate woody debris in the river.
 
Through an agreement with Hanson Professional Services Inc., the engineering firm responsible for the design of the bridge, García’s research group has constructed a movable-bed model of the river in the Ven Te Chow Hydrosystems Laboratory and is conducting testing of potential bridge scour.  A movable-bed model contains loose sediment that can be eroded and transported by a given water flow.  The 1:50 Froude scale physical model uses crushed walnut shells to model the sediment of the Tanana River. Wooden dowels have been used to simulate the river’s woody debris. 
 
García’s group will assess how much erosion might take place during floods at the bridge piers due to local scour, flow contraction and debris accumulation. Hanson water resources engineers, Bill Rice and Tony Comerio, will use this information to finalize the design of the railroad bridge foundation. 

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This story was published November 12, 2010.