 The tunnels were the most difficult and time @-@ consuming part of construction . Of the twenty tunnels , machines were only used for the Nåli and Vatnahalsen Tunnels — the rest were built by hand . Hand @-@ construction was done by drilling up to 4 @.@ 2 meters ( 14 ft ) through the rock , filling the holes with dynamite and blasting . Construction of tunnels started in 1924 and the first tunnel was completed in 1926 ; the last tunnel was completed in 1935 . On average , tunneling proceeded at between 116 and 180 man @-@ hours per meter ( 35 – 55 hr / ft ) . The work caused death or lifelong respiratory problems through silicosis caused by inhaling the smoke . There were two fatal accidents , one in 1925 and one 1938 , both related to tunnel work . 
