Rehabilitation with epoxy coated DYWIDAG Strand Anchors: The Railroad Canyon Dam
Construction of this concrete arch dam was completed in 1928. Important rehabilitation work had already been executed in 1965 to improve the stability of the structure. Nevertheless, new retrofitting measures became necessary in order to protect the dam from possible damage in a big seismic event.
During the 1965 rehabilitation effort, the 13m long thrust blocks located at each end of the arch had been strengthened by adding concrete. Old and new concrete was tied together by twelve anchors. However, in 1965 permanent anchors were relatively new in the US and the state of the art for corrosion protection at that time was not very advanced, and is considered inadequate by today’s standards. The anchors were in fact protected by cement grout in the bond zone and paper wrapping in the free length.
Following the National Dam Safety Inspection Program in the late 1970s, a series of investigations were carried out by Woodward Clyde Consultants (WCC) to evaluate the integrity of the dam under seismic and PMF (maximum probable flood) loads. The dam was found to be stable in an earthquake event but subject to overtopping during the PMFs. Due to concern that such overtopping could lead to erosion of the weak rock in the dam abutments, retrofitting measures were prepared to protect the abutments. DSOD (Department of Safety of Dams) and the owner retained WCC to develop the design concept which included lowering the dam crest to widen the existing overflow spillway and raising the thrust blocks and wing walls to direct flows away from the abutments. The survey also included an evaluation of anchors installed in 1965. Lift-off tests conducted on the anchors resulted in many wire break and examination of the failed wires revealed significant corrosion.
The final design incorporated a total of six 27-0.6" DYWIDAG strand tiedown anchors in the wing walls and nine 48-0.6" anchors in the thrust blocks. The new anchors have lengths up to 45m and a capacity of 12,512kN.
Due to the corrosion related deterioration of the 1965 anchors, corrosion protection of the strands became a top priority. Based on the success achieved with the Stewart Mountain Dam rehabilitation project and other similar projects across North America, WCC decided to specify FLO-BOND, FLO-FIL epoxy coated strand for the new anchors. As a matter of fact, the epoxy coated strand, with a complete filling of all interstices between the wires, provides continuous durable protection against corrosion during all project phases, as well as throughout the service life of the anchor. Each strand has a free length consisting of an HDPE sheathing over a layer of grease. Threaded restressable anchorages and permanent load cells allow for future periodic monitoring and subsequent load adjustment. Special wedges with longer teeth penetrate through the epoxy coating to grip the steel and ensure corrosion protection continuity in the anchorage zone.
The drilling contractor had to build working platforms to allow access for their drill rig. Despite limited access, each anchor was successfully installed and passed a performance and creep test. Total movements were within acceptable limits and final lockoff loads satisfied the requirements.