1. With respect to the length of the route, depth, engineering-geological conditions, presence of thick zones of faults, gas-dynamic phenomena, quite considerable inflows of geothermal waters with a high temperature, and especially large rock pressure, the Arpa-Sevan tunnel numbers among unique structures. 2. The main causes of the formation of damages are the engineering-geological conditions related mainly to the large depth of the tunnel and changes in the properties of the rocks when they are saturated with water, as well as to disturbance of the hydraulic regime of operation of the tunnel. 3. When designing the tunnel it was impossible to accurately predict changes in the physical and mechanical properties of the rocks, which entailed a decrease of their strength and deformability and an increase of movements and rock pressure on the lining. 4. Damage of the tunnel lining after operating for 5 years on a length of less than 10% of the total length (in this case the section of total reconstruction of the lining is only 5% of the tunnel length) is not an extreme phenomenon under the indicated conditions. The works performed on reconstructing the lining make it possible to evaluate the further operation of the tunnel as completely reliable, since the 5-yr operation, which can be regarded as a large-scale experiment, revealed the weakest places. 5. When designing analogous structures the value of the characteristics of the ground should be determined with consideration of a change in its properties upon saturation with water under pressure on the basis of investigations according to specially developed instructions. It is also necessary to develop a program of lining calculations taking into account the time factor and continuously varying properties of the medium during tunnel operation. 6. The experience of operating the Arpa-Sevan tunnel confirmed the recommendations of par. 1.5 of building code SNiP 2.06.09-84 “Hydraulic Tunnels,” that the main tunnels intended for constant passage of water during operation of hydrostations, reclamation systems, and water-supply systems should be equipped with monitoring and measuring instruments for evaluating the condition of the tunnel lining, ground surrounding it, and hydraulic and seepage regimes. The readings of these instruments should be led out from the tunnel to control boards allowing an evaluation of the state of the structure at any time. It is necessary to provide automation of the operating regime of long free-flow tunnels not calculated for pressure from the internal water pressure.