Borinquen Dam 1E is part of the new Pacific Access Channel (PAC) of the Panama Canal Expansion. The 2.3-km-long zoned rockfill dam forms the navigational channel providing navigation access from the Gaillard Cut to the new Post-Panamax Pacific Locks. A key geologic objective during construction was to confirm locations and activity of faults mapped at the dam during design, namely the Pedro Miguel Fault (PMF) and its suspected newly mapped “main trace.” The design allowed for core and filter widening at the anticipated location of the PMF at the south abutment and at a west branch of the PMF (believed to be the main active trace of the fault) mapped along the dam axis about one-third of the way north from the south abutment. As-built geologic mapping revealed complex faulting associated with the PMF crossing the southeast half of the foundation, the PAC, and the nearby Dam 1W foundation along a north-south trend. Trenching and age dating of alluvium overlying the faults crossing the Dam 1E foundation and overlying the PMF at Dam 1W indicated the unfaulted alluvium was latest Pleistocene to early Holocene age. At Dam 1E, the core and filters were widened to accommodate potential fault rupture on the PMF and a previously unrecognized fault revealed across the width of the dam foundation. The west branch of the PMF (trenched and mapped during design investigations) was determined to not exist at Dam 1E based on mapping the dam foundation and other extensive excavations created for the PAC.
ABSTRACT:A study was conducted to evaluate ongoing erosion of the upstream riverbank slope at the Pleasant Valley Dam. There is continued erosion concerns along the western shoreline abutment and potential future damage to dam structures due to slope failure caused by erosion. The dam is composed of compacted earth fill containing a mixture of gravel, sand, and silt; and the abutments are composed of Bishop Tuff. A geotechnical and geochemical investigation was performed consisting of core rock sampling, bulk sampling, and reservoir water sample testing. Geotechnical, geochemical, and wave runup and wind setup analyses were performed to provide mitigation recommendations. Geochemical analyses indicated that chemical dissolution of the earth materials is insignificant, and that clay swelling is also not a factor. The wave runup and wind setup analyses simulated wave action and erosive forces; indicating wave heights of 1.5 to 1.8 feet and strength testing along this zone did not indicate weakened rock. The freeze thaw mechanical weathering is considered one of primary causes for the erosion. The overall results indicate that mitigation should focus on controlling sequential wetting and drying near the airwater interface, and/or limit the penetration of water in the erosion area to avoid mechanical weathering.
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