Tunnels , Galleries and Cavities

Predicting geologic formations and the hydrogeology to be encountered along a tunnel is the major task of the geologist. (1)Lotschberg ( Switzerland ) . The 14.6 km long away tunnel (1913) was designed to across the river Kander well beneath the normal water level. Construction ran straight into alluvium which filled the old river bed well below the present river;several workers were killed.The gallery was sealed off and was by-passed by a tunnel about 1 500 m long. Lack of systematic geologic prospection before the construction was the direct cause of this disaster. (2)Mauvoision ( Switzerland ) . The site of the proposed 237 m high arth dam of Mauvoisin , at that time the highest arch dam in the world , was examined most carefully from the geological point of view . The depth of alluvium filling the narrow gorge at the site was measured seismologically . To enable a direct geological survery to be made , it was decided to drive a gallery deep into the rock abutment on the left-hand side of the gorge . This gallery was planned to pass beneath the gorge and to extend into the right-hand abutment . In addition to a first-hand inspection of the rock , water from several springs was analyzed . Later , the geological gallery was used for grouting a deep grout curtain under the dam . When the narrow gorge was reached the gallery unexpectedly penetrated into

alluvium , which suddenly gave under the pressure of water . The gallery was flooded and four engineers were killed .

It is most likely that wrong interpretations were placed on the wave reflections in the narrow triangular-shaped gorge .

(3)Malgovert ( France ) . Immediately after the Second World War , France power stations were nationalized . Electricite de France was created to administer the whole French electric generating system , with the exception of the power stations on the river Rhone itself , which were entrusted to the Compagnie Nationale du Rhone , in Lyons . Electricite de France immediately realized that there was an acute shortage of power in post-war France , and possible schemes were hastily examined . A plan proposed for the upper valley of the Isere was found to be too modest .A new scheme was designed for a 180 m high arch dam at Tignes and the first power house at Les Brevieres . The Malgovert tunnel , 9.5 miles long , was driven from the portals and through twelve adits .

On the downstream section ( started in January 1948 ) , between adits 13 and 16 , it had been decided to try the full-face method suggested by the geologists . Two large jumbos , both carring 8 DA-35 Ingersoll-Rand drifters , were available and an American Conway 75 shovel . Forty-eight holes were needed for the full heading . As work proceeded ( 1949 ) , the quality of the ground

steadily deteriorated . Polygonal steel supports erected at 4ft centers along the heading . Between adits 13 and 14 , it became necessary to revert to the so-called “Belgian” tunneling method , using top heading and successive enlargements on part of the tunnel length . Between adits 14 and 16 a 100 ft2 invert gallery using drifters for full-face operation was started ;the widening of the small pilot gallery was carried out in either one operation or in a number of steps , depending on the type of ground . Heavy longitudinal reinforced concrete beams were used in places as a footing for the polygonal steel arches which maintained the crown . With these methods it took nine months to drive only 1 513 yd .

Difficulties were encountered all the way from adit 13 downwards ( Permian-Carboniferous and Carboniferous rock formations ) . The tunnel caved in with a consequent inflow of sand and water and collapse of the overburden .The groundpressure was so great that the steel ribs used to support the tunnel had to be spaced closer and closer together . When only one foot apart the ribs twisted and the reinforced concrete beams sheared through .Severe damage occurred in one adit where a lateral concrete wall moved inwards by 5 ft and the rail track was lifted by 4 ft in a few hours .

At one point , the tunnel ran into swelling ground , and a stratum of badly crushed shales was located near a fault . These