Cofferdam closure section at the UST'-Ilimsk hydroelectric project on the angara
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For rivers on rocky beds the preliminary shield method appears to be the most economical and technically advisable method for building closure sections. From the practical point of view such methods can be used on all rivers. However, the choice of the closure operation should be based on the overall evaluation of the various factors in a given case.
When the embankment link up has to be made in the face of increasing river resistance, joining boulders into chains appears to be the most effective measure. Large boulders or concrete blocks may be placed on the embankment to function as “anchors”.
Closure of the Angara River showed that under high heads (more than 3–3.5 m) the stream resistance does not subside almost until the last moment and needs special attention and measures till the embankment slopes meet above the water level.
No visible erosion of the rocky riverbed could be observed on the downstream side. Loss of materials through erosion was found to be 25% of the total volume deposited during the final stages, which agreed fully with the calculations. A major part of the lost material occurred in the final closure stage when the channel width is less than 25 m.
The materials prepared at the construction bases, in particular the boulders, appeared to be adequate for the proposed job. A factor of safety of 1.25 may be regarded as very modest and does not exceed the limits for similar coefficients used on well-known frontal closure projects.
The heads and velocities observed in the channel closely agree with the calculations. The dam linking the Pravyi Losenok island with the cofferdam in the final stages created an additional head of 0.3–0.4 m.
The total number of automobiles and machines directly participating in the closure operations should be decided upon after providing for adequate reserve strength.
KeywordsBoulder Concrete Block Large Boulder Closure Operation Final Closure
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