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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
Vineyard Road FRS
The geotechnical profile at the Vineyard Road FRS is similar to the profile at the Powerline FRS,
but it is best described as a two-layer profile rather than a three-layer profile. Unit V1 consists of
Holocene soils with little to no cementation, and Unit V2 is composed of cemented Pleistocene
soils that are similar to Unit P3 of the Powerline FRS. The number of seismic lines completed
along the Vineyard Road FRS is not sufficient to characterize the subsurface seismic velocity
profile in the same detail as at the Powerline FRS.
Rittenhouse FRS
The geotechnical profile underlying the Rittenhouse FRS varies from north to south. Soils
beneath the northern portion of the dam (Stations 115+00 to 210+00) are similar to the soils
underlying the Vineyard Road FRS. Most of these soils classify as silty to clayey sand or silty
clay with some sand. These soils are best described by a two-layer profile similar to the profile
beneath the Vineyard Road FRS. Unit R1N consists of Holocene soils with little to no
cementation and Unit R2N is comprised of Pleistocene soils.
4.1.6. Discussions
This section contains discussions on the following topics:
Failure modes (with an emphasis on the impacts of the presence of Holocene soils
beneath the dams),
Data gaps and recommendations for additional investigations
4.1.6.1. FAILURE MODES (WITH AN EMPHASIS ON THE IMPACTS OF THE PRESENCE OF
HOLOCENE SOILS BENEATH THE DAMS
Site Specific Failure Modes.
Failure mode and consequence analyses (FMCA) were performed for each of the three FRS in
2001 and 2002. The objective of the FMCA was to gain an understanding of the most significant
site-specific potential failure modes for each of the structures and the consequences that could
result from these potential failures.
Several of the potential failure modes result from, or are exacerbated by, the presence of
Holocene soils (poor foundation conditions) beneath the structures or inadequate penetration of
the upstream cutoff trench or central filter into Late Pleistocene soils.
The most significant of these potential failure modes is seepage erosion or piping resulting from
flow through Holocene soils beneath the dam embankments or along the dam/foundation contact.
There are three potential pathways associated with this failure mode. The pathways are
described as follows:
• Path A – This failure mode is initiated by flow into transverse and longitudinal cracks
that do not extend through the central filter, but do extend into foundation materials
underlying the central filter and are in contact with Holocene soils.
• Path B – This failure mode is initiated by flow from the impoundment into differential
settlement cracks that extend below the central filter and are in contact with Holocene
soils.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 17