The Port of Lewiston retained TD&H Engineering to improve one of its barge loading and unloading facilities to accommodate a new 240-ton crawler crane. The container dock facility consists of a 3-sided, anchored sheet pile structure encompassing granular fill and extending out into the Clearwater River. Numerical modeling and analyses performed by TD&H Engineering indicated that the sheet pile wharf structure should have experienced wall movements and partial failure under previous crane use assuming the structure was constructed as initially depicted on the design drawings. However, no knowledge of a previous failure or wall displacements had ever been observed or documented.
Coincidentally, the U.S. Corps of Engineers was making repairs to the downstream dam on the Snake River and water levels on the Clearwater River were planned to be lowered to near historic low levels. As a result, visual inspection of the normally submerged portion of the dock was performed and revealed that the sheet pile walls had in fact partially failed and displaced outward. This condition validated the numerical analyses and the structure was deemed unsafe for the new crane and the proposed dock improvements. Conventional repair and/or replacement of the anchored sheet pile wharf were considered cost prohibitive and the down time needed to make the necessary repairs would have been significantly detrimental to Port Authority operations.
The TD&H Engineering geotechnical-structural team developed an engineering solution for the Port of Lewiston which satisfied their financial and time constraints. Since the crawler crane generally traversed a limited area while loading and unloading the barges, TD&H Engineering designed reinforced concrete strip pads for the tracks of the crawler crane to travel. The critical loading scenario was during load picks and occurred on the outside track nearest the failing sheet-pile walls. For this concrete strip pad, TD&H Engineering transmitted the surface crane loads to the underlying basalt bedrock via 45-foot long, steel H-piles. This eliminated the additional lateral forces imposed by the new crane from behind the susceptible wall. To avoid severing the existing anchor tendons, they were exposed and located prior to pile-driving. The unanticipated modifications were completed prior to the resumption of barge traffic.