Model investigation of manifestation of rock pressure on underground hydraulic structures located in permanently frozen rocks

Abstract

1. In hydraulic tunnels located in permafrost rocks, the manifestation of rock pressure is characterized by its slow increase in relation to the long process of thawing of the enclosing rocks and by stabilization of the rock pressure after thawing ends (under ordinary mining-geologic conditions, the rock pressure most often increases more intensely in the initial period after excavation, and then its slow increase and stabilization occur). 2. Since in permafrost rocks the determination of the design magnitude of the rock pressure under natural conditions is difficult, the method of modeling by means of equivalent materials should be considered one of the principal methods of determining loads from rock pressure under conditions of permafrost rocks. 3. The method of modeling the behavior of permafrost rocks by means of equivalent materials provides a sufficient degree of similarity with the actual processes occurring in the rock stratum during thawing, and can be used successfully in further investigations of the manifestations of rock pressure in tunnels located in permafrostrocks. 4. To simulate thawing of ice or ice-saturated crack filler it is best to use crystalline calcium chloride (CaCl2·6H2O) or its mixture with other materials. 5. Rocks which are fractured during thawing around an unreinforced tunnel are inclined to arch. The shape of the excavation acquires a more regular arched outline as the depth of the thawing zone and degree of fracturing increase. 6. The development of deformations in the thawing zone and the magnitude of the load from rock pressure are effected by the depth of the thawing zone; degree of fracturing of the rocks (strength of the mass); volume of ice or ice-saturated material filling the cracks and cementing together individual blocks; and cross-sectional size of the underground structure. 7. In underground hydraulic structures located in permafrost rocks, the magnitude of the load from the rock pressure acting on the lining is determined as a function of the depth of thawing zone: a) upon thawing of the mass up to a certain limit, the load on the lining is determined by the weight of the rock column within the thawing zone and by the surcharge from the overlying stratum of frozen rock; the magnitude of the surcharge depends on the degree of stability of this stratum; b) upon thawing of the mass to a depth greater than this limit, the load on the lining approaches a certain value which does not exceed the total weight of the rock column in the thawed zone; the magnitude of this limit will depend on a number of factors, the most important being the size and shape of the rock blocks, degree of their cohesion during consolidation under their own weight, and degree of stiffness of the lining. 8. In the case of thawing of the entire mass of rocks overlying the tunnel, especially when the rock stratum over the tunnel is comparatively small (in the portal sections, etc.), one can expect some settling of the ground surface, which should be taken into account when locating the surface structure.