Hydraulic tunnel 48.3 Ian long has been constructed at the depths up to 1.5 Ian in extremely difficult rock conditions. During the first several years of operation severe damages to the lining occurred in some parts of the tunnel, obviously due to different phenomena in different regions. Numerical analysis has been performed to apprise the long term reliability of different types of the lining used at the reconstructed as well as at the undamaged parts of the tunnel.
Hydropower and irrigation water discharge from the lake Sevan in Armenia, Caucasus, USSR, led to reduction of the lake level by 18 m. To compensate this outflow and to preserve the Sevan lake being one of the most beautiful lakes in the world several tunnelling projects are now under construction. The first one, the Arpa-Sevan tunnel (Petrosian G.M. et.al.1982) has been put into operation several years ago.
This tunnel diverts the flow of the river Arpa to the lake Sevan. The tunnel consists of two parts 18.7 and 29.6 Ian long with crosssections 3.7×3.3 and 3.7×3.6 m and discharge capacity of 18 and 25 cubic meters per second respectively.
Along the tunnel mostly eocenic and to some extent quaternary rocks are represented by differently broken basalts, porphyrites, andesites, tuffites, tuff breocias, e.t.c. Jointed and scoriaoeaus basalts are distinguished with high water discharge which in some parts of the tunnelling sites was greater than 1000 cubic meters per hour. The rock 1s generally weathered, there are lots of clay intrusions - dark-brown clays and conglomerates. Within the described region intrusive rocks are presented by granodiarites. Volcanogenic- sedimentary eocenic rooks are highly dislocated and folded, with numerous cracks and crushed zones.
The first and most of the second part of the tunnel cross the relatively solid zones of mentioned rock types; approximately 5.5 km of the second part cross soft sandstones and tuff breccias with sandy and clay intrusions. Near the end of the second part quaternary andesites and basalts are jointed and watered.
One of the deepest sections of the second tunnel was driven in the extremely complicated rock conditions. About 1.2 Ian of the tunnel is located in the large fault zone, called Hankavan-Sunik fracture. Clay-like breccias, easily soaked, are typical here. Rock bursts up to 1680 m and gas-bursts up to 300000 cubic meters were encountered during construction 10 01978, inflow of thermal (up to 46 C) water had been met at several places. Rather severe swelling was observed and once a 32 m long section of reinforced concrete lining was destroyed by high rock pressure. The measurements (Petrosian G.M. et. ale 1982) showed 1.3 MPa and higher pressures on the lining, and it was already at that time proved that pure swelling can not explain large rock deformations and high pressure on the lining. The damaged part of the lining was replaced by the prefabricated structure with bearing capacity up to 4.0 MPa and finally the tunnel was put into operation.
