17 Professional Land Surveyors of Oregon | www.plso.org Featured Article water to seep around that. I was told that there was a great deal of water seeping under and through the dam because of the lack of an impervious membrane or a cutoff trench. The original dam was built by setting up a water cannon to allow material to be sluiced from a nearby canyon wall then transported by pipes as a slurry to the dam site. The material flowed into wooden forms, which were raised as the height of the dam increased. Literally, they ended up with a big pile of Central Oregon dirt holding back a lot of Central Oregon water. Later the Bureau of Reclamation (BOR) raised the height of the dam. The dam appeared to function well over the years, until the BOR determined that the dam was unsafe and subject to failure. A catastrophic failure would have devastated Prineville, a fewmiles downstream. As such, they restricted the amount of water the reservoir could contain, which reduced the availability of irrigationwater. The BOR and the irrigation district agreed that the dam had to be rebuilt to satisfy the safety concerns and to provide needed water. Since the dam, intake structure, and a spillway were in place it was not practical to remove the whole damand start over. The redesign called for reconstructing the upstreamfaceof thedam. It startedbydraining the lake, building a cofferdam, and piping the water coming down Ochoco Creek through the dam. Rock from the upstream portion of the dam was removed. A good deal of the existing soil was also stripped from that face, and a cutoff trench was dug below the base of the dam. The dam was then breached with a large terraced notch. The dam was full of water so they drilled wells on the benches in the notch and pumped the water out of the dam. It was literally a huge saturated sponge. Onceall thenecessarymaterialwas stripped away they began to back fill the upstream face of the dam. They placed two types of material. The first, or inner layer, was a thick layer of coarse river run rock that was trucked in from some nearby source. What theywere creatingwas a curtaindrain. This material was also placed through the breach. The second layer, or outer layer, was an impervious clay material that was trucked in from a pit upstream from the dam. The theory was that the impervious material would prevent most of the water from penetrating into the dam. What did make its way in would be picked up by the curtain drain and transported through and out of the dam. Our work had many facets. We cross- sectioned the dam and its environs three times. First was prior to any construction, second when all necessary material was removed, and third when everything was replaced. Apparently, the contractor was paid formaterial removed andmaterial replaced. As thematerial removal progressed, wewere calledupon to set a lot of construction stakes. We set slope stakes so proper cuts could be established as material was being removed. Grade stakes were also set as material was being brought back in to make sure everything was put back in its proper place. Fortunately, we had a good total station (before GPS). I had a qualified instrument man, so I ran the rod as I knew where I wanted various shots to be taken. Jim Perry in our Bend office produced all the digital terrain models and came up with the quantities. The dam project was the first of two improvements that were made. Two years later, the same contractor won the bid to build a stilling basin. The engineers in Pouring slurry onto the dam that is contained in the forms. continuesT
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