Permafrost

52
The Effect of Freeze-thaw Processes on Soil Properties in Earth Dam
Embankments on Permafrost
S. Guly
(North-Eastern Permafrost Research Station,Melnikov Permafrost Institute SB RAS, Magadan, Russia)
Abstract: The paper discusses soil expansion at the base of the seasonally frozen layer (SFL)
that occurs during the first years after the construction of thawed dams on permafrost. This
phenomenon has been found to be caused by the difference in settlement between the unfrozen
embankment soils and the seasonally frozen ground.
Investigations on the physicomechanical properties of the embankment soils in the
Arkagalinskaya electric power plant dam conducted in 2005 indicate that at the dam crest the
soil moisture content, density and porosity at a depth of 3.0 to 4.5 m from the surface are
10-25% different from those of the overlying and underlying soils. No changes were observed
in the soils of the downstream face. The comparison of the results of investigations carried out
in 1954 during the thawed embankment placement and in 2005 shows deterioration of the soil
engineering properties at the depth of the SFL. At a depth of 3.5 m from the surface, the soil
moisture content has increased by 13%, the soil density has decreased by 12%, and the porosity
has increased by 28%. For a depth of 1.5 m, the 2005 and 1954 values are absolutely identical.
Below the layer of seasonal freeze-thaw (at depths of 4.5 to 5.5 m), the engineering properties
of the soils, on the contrary, have improved: the moisture content has decreased by 50%, the
density has increased by 6%, and the porosity has decreased by 26%. Changes in the soil
physicomechanical properties within the SFL base are thus evident.
Based on the investigations, the following conclusions have been made.
1. The main cause of soil expansion at the SFL base is the different settlement rate of the frozen
soils within the SFL compared to the unfrozen soils of the dam embankment. The arching
effect is strongest during the first years of the dam construction.
2. The higher is the dam height and the longer is the frozen condition of the SFL, the greater is
soil expansion at the SFL base.
3. The arching effect at the SFL boundary affects soil expansion not only during the first year
after dam construction. If a dam is constructed during several years and only during the
summer season, there may be several zones of soil expansion in the dam. Further on,
increased seepage areas may develop in these zones during the dam operation.
4. Analysis of the soil physicomechanical properties in the dams constructed of unfrozen silty
soils indicates that they experience no such significant changes at the SFL boundary as do
those in the dams constructed of unfrozen sandy soils.
Key words: Seasonally frozen layer, hydraulic structure, settlement, soil expansion