1. A increase of the height of the dam by stages as well as in construction time has a considerable effect on the SSS of the cofferdam, especially from stage II to VI. 2. Development of stages of construction of the dam and its construction time leads to the following changes in the SSS of the cofferdam: The levels of normal stressesσx,σy increase on average by more than 2 times; the level of the safety factors decrease by about 2 times, the minimum stabilized value Ksa=1.5; vertical movements increase, the maximum values reach 0.68 m in the middle of the downstream slope of the cofferdam, and the settlement of the crest bench marks is 0.4 m, horizontal movements change but insignificantly. 3. The rate of variation of the SSS is different for different points of the cofferdam body, decreasing on approaching the upstream slope. 4. After the VI stage of constructing the dam, the SSS of the cofferdam changes little, i.e., the SSS stabilizes. The causes of stabilization are: The effect of the dam design, i.e., the height of the dam and distance of the layer being built-up to the cofferdam; effect of three-dimensionality of the site, i.e., the overlying part of the dam is retained on the canyon walls and cannot transfer its weight to the lower part; the dam is constructed over the course of many years, and therefore creep subsequently after the end of construction develops very slowly with time and its amount is small. 5. For high earth-rockfill dams being constructed in a narrow site, the SSS of the lower part of the dam changes little, when the dam is brought to more than half the height, the load is transferred by friction to the canyon walls.