Ninth International Conference on Permafrost ... - IARC Research

Massive Ground Ice in the Norilsk Basin: Evidence of Segregation OriginO.A. KazanskyIgarka Geocryological Laboratory, Melnikov Permafrost Institute SB RAS, Igarka, RussiaM.Y. KushchevPolar Division, MMC Norilsk Nickel, Norilsk, RussiaThe Norilsk Basin is situated in the northwestern part of thePutoran Plateau, between Mt. Lontokoisky Kamen and Mt.Kharaelakh. Surficial deposits which consist of glaciolacustrineclays contain massive beds of ground ice up to 15 m inthickness. As is the case with massive ice bodies elsewhere,the origin of the ice in the Norilsk Basin is controversial. Theuncertainty regarding this problem impedes understandingof the spatial distribution patterns of massive ground ice andreduces the accuracy of geocryological predictions.The results of our research suggest that the ice is ofsegregation origin. This conclusion is based on the field studyof sections, as well as on the experimental and theoreticalinvestigations that have demonstrated the possibility ofmassive ground ice formation by ice segregation during thedevelopment of epigenetic permafrost.The well-known thermal condition for continued ice lensgrowth is:q f= q w+ q i,where q fis the heat flow from the base of the growing lens tothe permafrost table, q wis the heat flow to the base of the lensfrom the underlying ground, and q iis the heat flow requiredfor removal of latent heat of migratory water.The physico-mechanical requirement for continued lensgrowth is that no subhorizontal (normal to the heat flow)low-density zones develop in the frozen fringe where newice lenses that capture the water flow could initiate.The hydro-physical condition is that the overburdenpressure (σ) must not exceed the maximum crystallizationpressure (σ n). Otherwise, migration of water, through theunfrozen water films, from the unfrozen soil connected withan aquifer will cease.Physically, the process of continued lens growth can bedescribed as follows. During the period when Т sdecreasesdue to a decrease in the permafrost surface temperature Т о(Fig. 1), the thermodynamic equilibrium in the adsorbedwater film between the ice lens and the soil particles isdisturbed, and part of the water is changed to the ice phase.As the unfrozen water film becomes thinner, thecrystallization pressure increases, pushing the frozen soilupward (Khaimov-Malkov 1959). At the same time, thechemical potential of the adsorbed water decreases in thesoil underlying the ice lens with a definite gradient. In orderto balance the chemical potential of the adsorbed water andbecause of the continuity of the films, water flows fromthe films of the lower-lying particles to the phase changeinterface.When the supply of water is matched by the latent heatFigure 1. Schematic of the freezing fine-grained soil (Konrad &Morgenstern 1982): 1 – frozen zone; 2 – frozen fringe; 3 – unfrozensoil; 4 – lens of segregated ice; 5 – freezing front.removal rate according to the thermal boundary condition,the ice lens will continue to grow. If the heat removal isreduced, the thermodynamic balance will be disturbed dueto increased Т sand the ice lens will start to melt, while thewater will be forced out to the unfrozen zone. If the heatremoval increases and a water deficit develops near thebottom of the lens, the freezing zone will be cooled and newcrystallization centres will develop below the lowest lens onwhich migratory water will subsequently settle, resulting ina new ice lens. In this way, a mineral layer forms betweenthe ice lenses.Any cold wave will be damped at the base of a growingice lens if it is provided with an adequate supply of water.The wave will not cool the underlying soil until work is donewith phase changes. Therefore, in the presence of confinedgroundwater, continued growth of an ice lens can occur overa long period of time sufficient for the lens to develop into abed of segregated ice several meters in thickness.To verify the theoretical concepts considered above,laboratory experiment #15 was conducted in the Igarkapermafrost tunnel. Its results corroborate the possibility ofcontinuous growth of a segregated ice lens under the thermaland stress conditions close to those in the field. In the test, a5.5 cm thick lens was grown in 55 days at a final overburdenpressure of 0.25 MPa. Ice lens growth was completelycontrolled by changing the temperature at the cold side of thesample, simulating the past and present climatic variations.Lowering of the temperature at the base of the ice lens to 0.2°Crelative the phase equilibrium temperature correspondingto the applied pressure was allowed. With greater loweringof this temperature, the growing ice lens incorporated soilfragments, resulting in ataxitic (irregular) cryostructure or a125