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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
4.2.1.2. INSTALLATION OF CENTRAL FILTERS
As a result of these findings, a phased approach was developed to further evaluate the existing
condition of the dams, to design remedial measures to mitigate the cracking, and to repair the
dams. Evaluations of the dams were performed by the NRCS and several geotechnical consulting
firms between 1979 and 1986 and, based on the results of these investigations central filters were
constructed in the three structures between 1978 and 1991. The central filters are designed to
create a filter cake at the upstream interface between the filter and the embankment to prevent
piping (internal erosion) along cracks that have developed in the embankment. The Powerline
and Vineyard Road FRSs have filters that typically extend through the embankments and into
native soils, except in a few isolated areas. In general, the central filters do not extend below the
elevation of the upstream cutoff trenches. The Rittenhouse FRS filter generally does not extend
through the embankment and into native soils or extend below the elevation of the upstream
cutoff trench. Additionally, a central filter is not present in the southern end of the Rittenhouse
FRS from Station 30+00 (the left abutment) to Station 80+00, a distance of 5,000 feet. It is the
opinion of the project geotechnical engineer that the filters should extend into soils that possess
Stage II cementation (or greater) and/or soils that are essentially incompressible and not
susceptible to internal seepage erosion.
4.2.1.3. PRESENCE OF COLLAPSIBLE HOLOCENE SOILS
The FRSs are underlain by low-density alluvial fan soils, which were deposited during the
Holocene (during the last approximately 11,000 years). These soils have a high potential for
collapse settlement and typically experience from 2 to 6 percent of vertical strain upon wetting
under self-weight or low superimposed loads. The Holocene soils are underlain by Late
Pleistocene alluvium consisting of dense or hard, highly stratified deposits. Unlike the Holocene
soils, which are uncemented or exhibit discontinuous Stage I calcareous cementation, the Late
Pleistocene soils typically possess Stage II or III cementation (the reader is referred to the “Final
Geotechnical Summary Report” for a presentation of the cementation stages) and are essentially
incompressible from the viewpoint of contributing to settlement of low embankments. It appears
that the thickness of the Holocene soils varies considerably over short distances, suggesting that
the top of the Pleistocene alluvium was an erosional surface with varying topography. The
orientation of drainage channels during deposition of the Late Pleistocene and Holocene deposits
may have differed from the orientation of the present-day drainage system.
Based on these profiles, it appears that not all of the soils that are susceptible to collapse and
internal erosion were removed from beneath the embankments or from the upstream cutoff
trenches during construction of the dams, nor do the existing central filters in the embankments
extend all the way through these soils and into incompressible soils that are not susceptible to
internal erosion. Compression (collapse) of the softer, uncemented soils beneath the
embankments may have resulted in differential settlement and cracking of the dams. Cracks that
extend to the bottom of the embankments may be in contact with erodible soils, resulting in the
potential presence of internal erosion pathways along the embankment-foundation interface
below the central filters.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 21