C150063 Feasibility Study

• Contract/PO #: C150063
• Contractor Name: Jackson Lake Reservoir & Irrigation Company
• Contract Type: Loan
• Water District: 1
• County: Morgan
• Bill Number: SB 94-29
• Loan Projects - Doc Type: Feasibility Study

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the stability of the embankment with an 8.0-foot wide soil cement facing

installed, again neglecting any contribution from the existing concrete slab

facing. The results indicate generally acceptable safety of the embankment

for a soil cement overlay.

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This study also evaluated the potential stability of the existing embankment

for liquefaction under earthquake loading. Using calibrated and assumed

soils parameters based on the data included the Smith report and a

preliminary evaluation of the earthquake potential at the site, a preliminary

determination was made that liquefaction does not pose a problem for the

dam.

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This report, dated December 21, 2000 is also included in Appendix D.

Discussion

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We have suspect that the lack of a significant seepage cutoff separating the
embankment from the reservoir and lakebed, the occasional open slab joint and holes,
and the relatively loose soils behind the slabs and in the embankment may have
contributed to the formation of sinkholes. Pressure fluctuations caused by wave action
may be transmitted behind the slabs via the shallow concrete toe and the open slab
joints. Cycling of the pressure during high-wave conditions are believed to produce
conditions whereby the outward and downward release of the hydraulic pressure from
behind the slabs may be transporting and/or piping the embankment materials from
behind the slabs, potentially creating localized voids and/or sinkholes. It is probable that
as the waves subside, the lower portion of the slab and toe areas is self-healed, hiding
evidence of voids or piping. The main evidence of the piping incident is the
development of sinkholes at the dam crest.

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With the exception of the 1999 slab failure event near station 103 + 00, we are
not aware'of any significant slab settlements, failures, or collapse, This would indicate
that any voids that may have been formed have been limited in extent, and the

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