Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment Central filters were installed along the centerline of all three dams. The central filter installed in the Powerline FRS extends along the entire length of the embankment (from Stations 17+83 to 150+70). The filter has an average depth of 18 feet and a maximum depth of 42 feet. The depth of the filter was established by the NRCS on the basis of its 1986 crack location investigation. The central filter extends through the embankment and into native soils along much, but not all of the dam. There are several areas where the filter does not extend through the entire embankment. The central filter in the Vineyard Road FRS extends along the entire length of the embankment (from Stations 85+00 to 360+00). As indicated on as-built plans of the filter, the central filter extends to depths of between 19 and 21 feet below the crest of the dam along most of its alignment, corresponding to depths of between 2.5 to 4.5 feet below the original ground surface. There are several areas where the filter does not extend through the entire embankment. The central filter in the Rittenhouse FRS extends from Stations 80+00 to 210+00, but it was not installed in the southern 5,000 feet of the embankment between Stations 30+00 and 80+00. The filter was designed to extend 12 feet below the crest of the dam, except where deeper cracks were observed in the trench by an onsite NRCS representative. In areas where cracks extended beyond a depth of 12 feet, the filter was deepened until there was no further evidence of the presence of the crack. The central filter has an average depth of 12.8 feet between Stations 80+00 and 109+72 and an average depth of 15.8 feet between Stations 109+72 and 200+00. The filter varies in depth from 14.6 to 6.9 feet between Stations 200+00 and 210+00. Forty outlet drains were constructed between Stations 80+00 and 200+00 of the Rittenhouse FRS at approximately 1,000-foot intervals along the dam or as directed by the Engineer. The outlet drains slope toward the downstream toe of the dam and are designed to provide positive drainage from the central filter. The central filters in the Powerline and Vineyard Road FRS are 3 feet wide, whereas the central filter in the Rittenhouse FRS has a nominal width of 2.6 feet. Unlike the central filters in the Powerline and Vineyard Road FRS, the filter in the Rittenhouse FRS is not continuous over the entire length of the dam nor was it designed to extend through the entire embankment into the underlying soils. Outlet drains were installed in the Rittenhouse FRS, but not the Powerline or Vineyard Road FRS. Collapsing Foundation Soils The Holocene soils beneath the dams have characteristics that are typical of collapsible soils, including depositional environment, low blow counts, low dry density (indicative of an open soil structure) and weak cementation. Some of the Late Pleistocene soils that underlie the Holocene soils have similar characteristics. The collapse potential of these soils was tested in the laboratory by performing one-dimensional consolidation tests using relatively undisturbed samples obtained during the current field investigation. The collapse potential of the samples tested ranged from 0.0 to 8.7 percent with an average collapse potential of 1.5 percent. The dry density of the 74 samples ranged from 88 to 124.2 pcf and averaged 104.5 pcf. The formation of cracks due to collapsing foundation soils is the result of differential collapse that occurs when soils that underlie the structures are not equally wetted. The amount of collapse also depends on the thickness of the collapsing soil layer. Water has been impounded behind all three dams, most notably during October 1972. Impoundment of this water likely resulted in partial collapse of the Holocene soils, and to a lesser extent the Late Pleistocene soils, USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 19
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment underlying the upstream section of the embankments. The rate of infiltration into the Holocene soils is estimated to range from 0.5 to 3 feet per day (Hansen and others 1989). The flood event of 1972 likely resulted in the wetting of Holocene soils beneath the upstream slope of the embankments to a distance of 5 to 30 feet downstream from the upstream toe of the embankments. It is probable that this wetting resulted in differential collapse of a variable thickness of soils, resulting in longitudinal cracks developing at various elevations in the upstream slope of the embankments. 4.1.6.2. DATA GAPS AND RECOMMENDATIONS FOR ADDITIONAL INVESTIGATIONS The following should be considered as part of planning and/or final design: No documentation of stability analysis is being performed for any of the structures, neither during the original design of the structures nor during subsequent studies. Depending upon the alternatives recommended a stability analysis should be considered for the structures. There is some indication that the materials in the central filters may have become cemented. Depending upon the alternatives recommended the condition of the filters and their susceptibility to cracking should be investigated. 4.2. Final Filter Alternatives Memorandum This memorandum presented the results of a comparison between a central filter and an upstream sloping filter for the rehabilitation of the Powerline, Vineyard Road and Rittenhouse (PVR) Flood Retarding Structures (FRSs). This memorandum includes a discussion of the project background, an overview of the existing PVR structures and filters, a discussion of potential failure modes associated with the existing filter conditions, a general “compare and contrast” of a central filter versus an upstream sloping filter, a summary of case studies, and recommendations for a filter design specific to the PVR structures. 4.2.1. Project Background 4.2.1.1. EMBANKMENT CRACKING Numerous cracks, both longitudinal and transverse to the dam alignment, were observed in the embankments in the early 1970s after water was first impounded behind the structures. Fugro, Inc. (1979) conducted a crack investigation of the Vineyard Road FRS and NRCS (1983, 1986) conducted crack investigations of the Vineyard Road and Powerline FRSs. Mapping of cracks was also performed during installation of the central filters (Fugro for the Rittenhouse FRS and NRCS (1983, 1991) for the Powerline and Vineyard Road FRSs. General results of these crack investigations are summarized in the “Final Geotechnical Summary and Analysis Report” (AMEC 2010). The cracking was evaluated by the NRCS, and it was concluded that the cracking did not pose an imminent danger to the structures. Ongoing surveillance of these dams and other PL-566 dams in Arizona revealed an increase in the intensity and severity of cracking over time and, as a result, an NRCS study team was appointed to study the magnitude and severity of cracking in PL-566 dams. Based on the results of these studies, the study team concluded that transverse cracks in the dams were primarily caused by tension release due to desiccation and shrinkage, and that these transverse cracks pose the greatest hazard to the integrity of the structures. USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 20
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DRAFT SUPPLEMENTAL WATERSHED PLAN A
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Powerline Flood Retarding Structure
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Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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