Strength of sedimentary rocks in the annual heat cycle layer of discontinuous island permafrost in the town of Neryungri, Southern Yakutia

The purpose of this article is to summarize the data of engineering and geological surveys conducted in the 1970–1990s in the town of Neryungri in Southern Yakutia. According to the surveys, the foundation of engineering structures is composed of an unfrozen-frozen layer of coal-bearing sedimentary rocks, whose main lithotype is sandstones, which are also the object of study. The subject of the study is laboratory values of the uniaxial compressive strength of sandstone samples in the air-dry and water-saturated states. Variability of sandstone sample strength in the annual heat cycle layer was studied using probability theory and mathematical statistics. Using laboratory data, the strength of a sandstone massif at the depths of 2–11 m was estimated with 90 % reliability. The strength of the fractured part of sandstones was studied sporadically at depths of up to 22.5 m. The unbalanced probability distribution of strength near the mean values was interpreted as a natural defect of sandstones associated with their decompression by tectonic and exogenous-cryogenic processes. The defect is correctly approximated by the Weibull law.The strength of the sandstone massif in air-dry and water-saturated conditions varies from very low to very high. The proportion of rocky sandstones with the strength above 120 MPa is 78.9 %. The proportion of semi-rocky sandstones with the strength below 15 MPa is 12.1%. The background strengths of air-dry and water-saturated rocky sandstones range from 54.49 to 171.42 MPa and from 32.14 to 135.78 MPa, respectively. In the same conditions, the background strength values of semi-rocky sandstones are lower and range from 1.65 to 27.2 MPa and from 0.67 to 10.12 MPa. When the material is saturated with water, the variability of its strength decreases by a factor of 4. This pattern is explained by the loss of structural-petrographic, cryogenic-thermal, and temperature-moisture diversity in sandstones when they are soaked in water. The conducted studies led to the following conclusion: predicted based on laboratory data strength of the sandstone massif with the approximate reliability of 80 %, is sufficient for the safe operation of engineering structures in Neryungri.

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