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

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