Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
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<strong>Powerline</strong> <strong>Flood</strong> Retarding Structure<br />
Pinal County, AZ<br />
Draft Supplemental Watershed <strong>Plan</strong><br />
<strong>and</strong> <strong>Environ</strong>mental <strong>Assessment</strong><br />
Failure Mode 3 (FM-3) – Internal erosion through cracks in the embankment that extend<br />
beneath the central filter.<br />
• Desiccation cracking creates voids within the embankment soils. Since the central filter is<br />
only a partially penetrating filter (along essentially all of the Rittenhouse FRS <strong>and</strong> only<br />
minor sections of the <strong>Powerline</strong> <strong>and</strong> Vineyard Road FRSs), moisture is able to migrate<br />
downstream below the bottom of the central filter, ultimately leading to a breach failure.<br />
Failure Mode 4 (FM-4) – Internal erosion through cracks in the embankment beneath the<br />
central filter along the erodible soils at the embankment-foundation interface (not in the<br />
embankment).<br />
• Desiccation cracking creates voids within the embankment soils allowing moisture to<br />
reach the erodible foundation soils. Since the central filter is only a partially penetrating<br />
filter, moisture is able to migrate downstream below the bottom of the central filter<br />
through the softer, uncemented, collapse-prone soils, ultimately leading to a breach<br />
failure.<br />
Failure Mode 5 (FM-5) – Internal erosion through cracks in the embankment (no central<br />
filter present).<br />
• Desiccation cracking creates voids within the embankment soils allowing moisture to<br />
migrate downstream ultimately leading to a breach failure.<br />
4.2.3.3. SUMMARY OF FAILURE MODES<br />
Based on the existing central filter conditions, FM-3 <strong>and</strong> FM-5 are only applicable to the<br />
Rittenhouse structure. The partially penetrating central filter does not provide adequate<br />
protection from cracking through the embankment as demonstrated in FM-3. In addition,<br />
approximately 5,000 feet of the Rittenhouse structure does not contain any central filter, making<br />
the structure susceptible to FM-5.<br />
The remaining failure modes, FM-1, FM-2 <strong>and</strong> FM-4, are applicable to all of the PVR structures.<br />
While more likely at the <strong>Powerline</strong> or Vineyard Road structures, these failure modes could<br />
potentially occur at any of the PVR structures. The likelihood is increased for the <strong>Powerline</strong> <strong>and</strong><br />
Vineyard Road structures because of their existing central filters. At these structures the central<br />
filters extend from the crest down to the erodible soils with an average of 5 <strong>and</strong> 7.5 feet,<br />
respectively, between the bottom of the central filters <strong>and</strong> competent, essentially incompressible<br />
soils.<br />
4.2.3.4. MITIGATION OF FAILURE MODES<br />
In order to mitigate the failure modes presented above the installation of a new filter system is<br />
being considered as part of the PVR Rehabilitation or Replacement Project. The new filter<br />
systems should extend from the crest of the dams into the moderately to strongly cemented soils<br />
along the entire length of each structure <strong>and</strong> should include a nonwoven geotextile on the<br />
downstream side of the filter. ADWR has informed the District <strong>and</strong> the District’s various dam<br />
rehabilitation project planning <strong>and</strong> design teams that District dam rehabilitation projects<br />
involving central filters must have a downstream geotextile (or much more costly alternative<br />
components or measures) for application approvals. To date, no feasible alternative has been<br />
identified to address ADWR’s extensive concerns with central filters as part of the District’s dam<br />
rehabilitation projects. The inclusion of a geotextile in the recommendation is to ensure any<br />
USDA- NRCS <strong>Jan</strong>uary <strong>2013</strong><br />
Kimley-Horn <strong>and</strong> Associates, Inc. Page 24