For most underground hydraulic structures it is possible to determine only approximately by static calculations the necessary dimensions and safety factors; therefore, experimental investigations usually are carried out to substantiate the design. Along with field investigations model studies are widely used, the virtues of which lie in the possibility of controliing the loads, separately studying different factors, and repeating the experiments. The most common methods of model investigations of underground hydraulic structures at present are the polarization optical method of determining stresses (photoelastic method) and method of equivalent materials.Model investigations of underground structures at the OrgenergOstroi Institute showed that the best results can be obtained with the combined use of the methods of photoelasticity and equivalent materials. It is expedient to use the photoelastic method to investigate a structure in the elastic state in order to obtain relations between stresses and loads and other factors under conditions corresponding to the design scheme. When testing models of equivalent materials it is possible to control and correct the obtained results with maximum approximation to full-scale conditions and by loading the model to failure. Of particular interest are the studies presently underway at the Orgenergostroi Institute to develop test stands on which elastic models of tunnel linings are placed in a medium of equivalent materials simulating rock. Such modeling permits investigating the joint behavior of an underground structure and the rocklenclosing it under conditions close to natural.The methods and effectiveness of model investigations of underground structures of hydroelectric power stations carried out in recent years at the Orgenergostroi Institute can be illustrated by the following examples. Selection of Design and Investigation of the Mutual Influence of Pressure Tunnels of Hydroelectric PowerStations. The investigation by the photoelastic method was carried out to obtain comparative data on the stress state of round and horseshoe linings for a 15 m tunnel with a head of 10 gauge arm at a thickness of the reinforced concrete of I m in rocks with a modulus of elasticity of 150.000 kg/cm z. Another problem of the investigation was to check the correcmess of the designated size of the unexcavated block between parallel pressure tunnels (Fig. 1). As a result of the experiments it was concluded that it was expedient to adopt in this case a circular tunnel (Fig. 2) and it appeared possible to reduce the size of the block from that designed.Investigation of Problems of Selecting the Cross-Sectional Shape of Pressure Tunnels. Comparative experiments by the photoelastic method were carried out on models of round and horseshoe linings under different loads and a variable ratio of the moduli elasticity of the lining and rock. The area of application of horseshoe linings in pressure tunnels as a function of elastic characteristics of the lining and rock, thickness of the lining, dimen...
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