Notes: Conclusion The experience in constructing the underground machine hall in a frozen rock mass thawing during operation, just as of other underground structures at the Kolyma hydrostation, has great practical significance for consideration when designing and constructing underground structures under analogous conditions. The main requirements imposed on the creation of reliable underground structures having a large cross section under permafrost conditions are: Comprehensive consideration at the design stage of the engineering-geological, cryogenic, hydrogeological, temperature, and other conditions of the surrounding rocks, including the structure and initial stress state of the mass, established by specific experimental works; Substantiation of designs taking into account the change in the modulus of deformation and stress-strain state of the rock mass and conditions of performing the works; Creation of primarily yielding designs of the linings (support) able to absorb nonuniform loads without disturbing their continuity under conditions of specially constructed drainage systems for removing seepage waters; Thorough observations of the state of the rock mass during driving; in this case, a correct substantiation of the criteria making it possible to provide preservation of the enclosing rocks, structural members, and equipment from all types of effects with consideration of the stress-strain state of the structure is important. On-site observations, investigations, and calculations confirmed the sufficient effectiveness of the strengthening measured performed on the arch of the machine hall of the Kolyma hydrostation. The on-site observations should be continued until complete stabilization of movements in the arch and surrounding rocks over the entire contour of the hall chamber.

Notes: Conclusions 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.

Notes: Conclusions 1. The perennial frost condition of the rock has both a favorable and unfavorable effect on carrying out underground work. The facilitating conditions are: greater bearing capacity and stability of the frozen rock in the absence of water inflow within the frozen zone. Conditions impeding underground work are: the sharp drop in bearing capacity and stability of the rock on thawing difficulties in maintaining the concrete when erecting tunnel linings; and the danger of icing up with the entry of surface water through cracks or thawing; reaction envelope of the rocks. 2. In order to prevent work fallout the temperature in the works when driving tunnels and erecting the linings must be maintained near to the constant rock temperature or somewhat below (approximately −5 to −6°C). For this purpose heat-insulating screens should be installed at the entries preventing the entrance of hot air in summer and very cold air in the winter. In addition, air delivered to the works for ventilation should be heated to −5°–−6°C by means of heaters. 3. If the frozen rock may thaw, the arches in the rock must be supported by a temporary lining. The one most reliable, economincal, and easily installed is the expander type. 4. Blasting of headings is best performed using preliminary chipping around the contour. 5. Blasting of the surface in the excavations of structures near the tunnels must be carried out with a restricted simultaneous explosive charge. 6. The choice of method for concreting linings should be made in accordance with the frost geology conditions, the conditions under which the work will be performed, and on the basis of the results of an economic comparison of concreting methods. 7. Concreting of linings should be carried out by sections in a sequence without long interruptions in concreting adjacent sections. 8. Grout injection behind the lining should be carried out so as to achieve a 70% hardness grade concrete without allowing freezing at the contact between concrete and rock. 9. During concreting and hardening of the concrete there must be a constant check on the temperature conditions in the concrete, at the contact between the concrete and the rock, and in the adjacent rock layers.