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CONSTRUCTION CONSIDERATIONS  R A.` ` Construction Timing ` ` Since all the conceptual design alternatives involve reconstructing the existing dam, the lake must be drained. Therefore, use of the reservoir will not be available during the construction.(#` XX` ` The Dulce Lake Dam area does not have a true year round construction season. However, the schedule should implemented such that construction can be ongoing from approximately February 1996 to February 1997. Prior to the spring floods which typically occur in April, the following major construction items should be completed; (1) control breach the dam to drain the lake, (2) construct the permanent outlet works, (3) extend the new permanent outlet upstream, beyond the cofferdam face, and the excavation limits, and (4) construct the cofferdam. The existing outlet works has very limited capacity and consequently the dam must be breached to drain the reservoir. In addition, the drainage which comes from the east side of Highway 64 downstream of the dam will require diversion channeling to preclude flooding the foundation excavation. (#` XX` ` After the cofferdam is in place, the major construction can begin. Consideration should be given to seasonal flood events in order to properly size the cofferdam. As a minimum, storage for a tenyear flood event should be provided. The reconstructed embankment should be in place before the major winter storms arrive, sometime around November 1996. If necessary, finished overtopping construction could be extended into the 1997 construction season to facilitate early filling of the reservoir.(#` (,))Ԍ R B.` ` Reservoir Operation During Construction XX` ` A cofferdam must be constructed far enough upstream of the existing dam to remove the dam, excavate the foundation and replace the dam. The cofferdam must also allow for diversion of low flow and small floods around the site. This will most likely be accomplished by constructing the permanent outlet works first and then extending it to the upstream toe of the future cofferdam to divert flow past the construction site. Also, irrigation water must be delivered downstream which could be accomplished by diversion of the perennial flow or water could possibly come from dewatering of the excavation area.(#` XX` ` Access to the west side of the lake does not have to be maintained during construction. Emergency access could be provided across the upstream cofferdam or around south end of lake.(#` C. Additional Exploration XX` ` A borrow pit must be located containing a dependable source of impervious fill for the embankment core and the foundation seepage cutoff. This borrow pit can also be used for random fill. The most likely source is an area on the northwest side of the lake upstream of the causeway. The borrow could be acquired form multiple small source locations; this would however, increase costs. To ensure that the material is adequate, some borrow exploration with backhoe trenches has to be completed. Also, it is believed necessary to perform additional exploration to identify the rock surface in the area where the outlet works is to be placed near the west abutment. Based on present information, the 12 6'x 6' concrete boxes will be part on rock and part on fill. This can be designed if necessary, but it is more desirable to place the entire outlet works on rock to eliminate potential differential settlement problems.(#` (,))ԌXX` ` The top of rock on the east end of the dams adjacent to Highway 64 is very deep, on the order of 100 feet below top of existing dam. For this reason, the excavation would have to be performed in two phases if the alignment for the new embankment follows the existing embankment and still ties into rock on the east side. The excavation would occur in two phases to keep Highway 64 open to traffic and still perform the necessary foundation modifications. In order to avoid this expensive problem, a new alignment is proposed for the embankment. The axis has been rotated to the south while holding the west abutment location. This is intended to take advantage of the much shallower rock in the upstream right abutment area and alleviate the need for a twophased excavation. This realignment is shown on Drawings DX220 and DX221.(#`  R D.` ` Foundation Excavation and Remediation(#` XX` ` Two foundation treatments have been investigated for conceptual design;(#` XX` ` (1) complete removal and replacement of foundation or (2) partial removal along with dynamic compaction. Removed material will be reutilized in the foundation. Therefore, a stockpile area located downstream of the excavation is needed to store excavated foundation material. The equipment yard and construction office will also most likely be located downstream of the excavation.(#` XX` ` Dewatering will be necessary on the upstream excavation slope and to some extent on the downstream slope. The number of stages of well points will depend on the depth of excavation that is selected. Deep wells may be feasible.(#` XX` ` Assuming that a consistent source of impervious borrow can be found, the foundation backfill should consist of a centerline impervious zone 10 feet wide and a 10 feet thick impervious blanket extending to the upstream(,)) slope of the excavation. The remainder of the foundation backfill can be random fill, primarily utilizing excavated foundation material. All of the material should be compacted to not less than 100% standard density. The variable nature of the random material will present special problems with construction control.(#`  R. E.` ` Embankment Materials(#` XX` ` Only minimal protection against upstream erosion is needed (maximum fetch is less than onequarter mile). Protection of the upstream face of the dam will be accomplished with a 2 foot thick layer of riprap on top of a 6 inch gravel bedding. The gravel bedding will act as a transition zone between the riprap and the random fill. Either massive stone or roller compacted concrete (RCC) will be used to protect the downstream face if the dam is overtopped. A 2 foot thick sand layer will act as a drain and a transition zone for the RCC protected slope. For the massive stone protected slope, 2 to 3 transition zones will have to be utilized to go from the random fill to the massive stone. All of the transition zones and/or drains must meet filter criteria for both stability and permeability.(#` XX` ` At least 25 percent of the material making up the gradation of the impervious fill should pass the No. 200 sieve and classify as a CL clay. Random fill can be any suitable material. The embankment fill materials should be compacted to not less than 100 percent standard density. (#` XX` ` The RCC protection for the downstream slope is specified in the conceptual design drawings as 12 inch thick by 8 feet wide lifts. A complete design of RCC protection will be included in final design. Most likely, the volume of RCC will not justify an on site batch plant.(#` XX` ` In conclusion, several types of materials will be required for construction:(,)) impervious material for central core of embankment and seepage cutoff in foundation, processed sands and gravels for filters and/or transition zones, riprap for upstream slope protection, and RCC or massive stone for downstream slope protection. The availability of a large amount of locally available impervious material will not be known until borrow exploration is completed. Therefore, the seepage cutoff in the foundation may not be as extensive as stated earlier. Potential sources for granular material, riprap and massive stone have been identified, however, their quality and extent are unknown. Their use may also not be permitted on the reservation. It is highly probable they will have to be imported from a commercial source off the Reservation. (#`