Outflow of volcanic rock and gas during opening of the Arpa-Sevan tunnel

Conclusions

• 1. All outflows occurred in porphyrites, which, according to geologic-exploratory studies, were classed as water- and gas-and-water-bearing rocks at a depth of more than 600 m. In opening excavations in volcanic rocks at depths of more than 400 m, therefore, the appearance of water and gas venting must be considered an indication of potential proneness to outflow.

• 2. The following can be classed as basic signs of porphyrite proneness to outflow: reduced strength and the extent to which the porphyrites are disturbed, increased venting of gas from blast holes during their drilling, the presence of circular macrocracks in the cores, and separation of the core into individual thin disks, or its disintegration to a state of sand. The formation of a milky-colored mud during the drilling of blast holes, and the distortion of blast holes and holes, which suggest the increased stressed state of the mass, as well as a change in the color of the porphyrite from dark-gray in segments not prone to outflow to a reddish-brown, light-gray, or green in zones prone to outflow may serve as additional criteria.

• 3. In developing methods that ensure safe working conditions in opening tunnels in rock prone to outflow, study of the effect of slot unloading on variation in the proneness of a mass to outflow, and also the creation of technological and technical facilities to realize the antioutflow measures that have been developed should be a high priority.