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 4.1.3. Regional Geologic Setting The PVR study area is near the junction of the Mexican Highland and Sonoran Desert sections of the Basin and Range physiographic province, just southwest of the boundary between the Basin and Range and Transition Zone physiographic provinces. The boundary between these two provinces is marked by the western edge of the Superstition and Goldfield mountains, which rise abruptly above the valley floor. These mountain ranges also mark the eastern margin of the down-dropped Phoenix Basin, which underlies much of the Phoenix metropolitan area and extends westward to the Sierra Estrella and White Tank Mountains. The Phoenix Basin formed between approximately 8 and 15 million years ago after cessation of the volcanic activity that formed the bulk of the Goldfield and Superstition mountains. The Phoenix Basin is subdivided into several sub-basins. The study area is within the Mesa- Chandler sub-basin. On the west, this sub-basin is partially separated from the central Phoenix Basin by the bedrock highs of the Tempe Butte area. The Mesa-Chandler sub-basin is bounded by the Goldfield and Superstition mountains on the north and northeast, by Mineral Mountain on the east, and by the Santan Mountains on the southwest. 4.1.4. Local Geomorphic Setting The Powerline FRS is underlain by deposits typical of a basin floor and identified as fine-grained distal alluvial fan/alluvial plain and terrace deposits with little soil development. These basin floor deposits are incised by younger Holocene piedmont and intramontane deposits identified as channel deposits of ephemeral streams consisting of stratified sand, silt, pebbles, cobbles and boulders with little or no soil development. The northern and central portions of the Vineyard Road FRS are underlain by an upper horizon of fine-grained alluvial fan and terrace deposits consisting of sand, silt and fine gravel. The southern portion of the Vineyard Road FRS and the Rittenhouse FRS are underlain by an upper horizon of piedmont and intramontane, low terrace and alluvial fan deposits consisting of poorly sorted silt, sand, pebbles, cobbles and boulders with weak soil development. These deposits have minimal soil development and are all incised by ephemeral streams containing channel deposits of stratified sand, silt, pebbles, cobbles and boulders with little or no soil development. These near-surface deposits, which are similar to the Holocene soils described throughout this report, are underlain by moderately to strongly cemented, predominantly fine-grained Late Pleistocene deposits. 4.1.5. Local Geologic Setting Surface Geology The Powerline FRS is underlain by four geologic units: Holocene Alluvial Channels Holocene Alluvial Surface; Undifferentiated Holocene Alluvium and Holocene to Late Pleistocene Alluvium. The Vineyard Road FRS is underlain by five geologic units: Holocene Alluvial Channels; Holocene Alluvial Surface; Undifferentiated Holocene Alluvium; Holocene to Late Pleistocene Alluvium; and Late Pleistocene Alluvium. The geologic units are the same units described above for the Powerline FRS, with the exception of Late Pleistocene Alluvium, which occurs at the far southern limit of the structure. USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 15
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment The Rittenhouse FRS is underlain by four geologic units: Holocene Alluvial Channels; Holocene Alluvial Surface; Late Pleistocene Alluvium; and Middle to Late Pleistocene Alluvium. The first three geologic units have been described above for the Powerline and Vineyard Road FRS. The fourth unit Middle to Late Pleistocene Alluvium is located beneath the southernmost portion of the FRS. Near-Surface Geologic Profile The FRSs are underlain by shallow low-density soils that were deposited at the distal end of coalescing alluvial fans during the Holocene era (the last approximately 11,000 years). The soils were deposited during semiarid climatic conditions similar to present-day conditions, and the depositional processes included sediment transport in channels and periods of flooding which resulted in avulsion of braided channels, debris flows and mudflows. These processes resulted in a high degree of stratification and variation in soils types. The Holocene soils typically vary from 10 to 20 feet in thickness and are dominated by sandy clay and clayey sand with subordinate amounts of silty sand, sandy silt and sandy gravel. The Holocene soils are uncemented or exhibit discontinuous Stage I calcareous (lime) cementation, and are predominantly moderately firm to firm with some soft and very firm zones. The Holocene soils have a high potential for collapse settlement, and typically experience from 2 to 6 percent of vertical strain upon wetting under their own weight or low superimposed loads. The Holocene soils are underlain by Late Pleistocene alluvium primarily consisting of silty sand, clayey sand, gravelly sand and sandy clay. These soils typically possess Stage II or III calcareous cementation, are characterized as being very firm to hard with some firm zones, and are essentially incompressible from the viewpoint of contributing to settlement of low embankments. Based on a review of geotechnical investigations conducted in the study area, it appears that the thickness of the Holocene soils varies considerably over short distances, suggesting that the contact between the Holocene and Pleistocene soils is a buried erosional surface with considerable past relief. The depth of the Holocene/Pleistocene contact below the original ground surface, the depths of the cutoff trenches along the upstream side of the dams, and the depths of the central drains within the embankments are important considerations when considering several of the potential failure modes for the structures. The potential presence of poor foundation conditions beneath the dams and how these conditions relate to potential failure modes are discussed in Section 5.5 of this section. 4.1.5.1. SOIL PROFILES UNDERLYING STRUCTURES BASED ON ALL GEOTECHNICAL INVESTIGATIONS The following descriptions of the geotechnical profiles at the structures are based on a review of published literature and available boring and test pit logs from previous geotechnical investigations and the results of subsurface investigations completed as part of this study. Powerline FRS The geotechnical profile underlying the Powerline FRS is best described as a three-layer profile. Unit P1 consists of Holocene soils with little to no cementation; Unit P2 is composed of Holocene to Late Pleistocene soils with little to no cementation; and Unit P3 soils are Pleistocene in age with Stage II to III cementation. The units are described below. USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 16
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Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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