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• <br /> elevation to the inside toe elevation. The membrane should be properly keyed into the face of <br /> the dam on all four sides. <br /> Advantages of this option are that minimal earthwork is required and that it uses conventional <br /> equipment. <br /> Disadvantages are that the reservoir must be fully drained and handling of saturated materials <br /> in the zone of keying will be difficult. Also if keying of the membrane is not adequate, water <br /> may get around or under the membrane and re-create the failure. <br /> Option 3: Placement of a slurry wall cutoff along one edge of the crest to a depth of about 35 <br /> feet below crest elevation and for the full length of the dam. <br /> Advantages include the creation of a new continuous horizontal cutoff. <br /> Disadvantages include high cost, quality control issues (i.e., difficulty ensuring proper mixing of <br /> clays with grout), creation of a soft zone of slurried materials to the ground surface that doesn't <br /> "harden"over time, and the need for specialized equipment. <br /> Option 4: Placement of a vertical impermeable membrane to about 20 feet below grade for <br /> the full length of the dam provided it can be verified by further exploration that the seepage is <br /> occurring within the top 10 or so feet of the dam embankment. <br /> Advantages include relative ease of construction, minimal need for imported material other than <br /> membrane, and positive horizontal flow cutoff. <br /> Disadvantages include quality control issues consisting mostly of verifying adequate seaming of <br /> the membrane, large size of the membrane, safety due to a deep trench, potential <br /> impracticality due to narrow working space, and the need for specialized equipment. <br /> Option 5: Complete removal of the top 30 feet of the dam followed by reconstruction without <br /> re-use of the organic-tainted sandy silty clay materials. <br /> Advantages include positive removal of the substandard embankment fill, positive cutoff of the <br /> seepage and positive, predictable long-term performance. <br /> Disadvantages include a long time frame for construction, requirement of significant earth <br /> moving equipment, the need for a large amount of imported material, reservoir downtime, and <br /> high cost. <br /> Repair Option Selection <br /> Based on discussions with the client and CDWR representatives, it was decided that the most <br /> appropriate alternative was excavation of the slumped area down to the toe of the dam <br /> followed by replacement with a chimney drain and new toe drain, and replacement of the <br /> embankment with compacted fill to current face grade (2.6H:1V). <br /> Buckhorn Geotech proposes a general process as follows: <br /> 1. excavate the width of the failed area and longitudinally from crest to toe of the <br /> downstream face, <br /> 2. install a chimney and toe drain system, <br /> 16 <br />