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

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