Summit Tunnel
The Summit Tunnel, with a length of 1,659 ft, was the longest tunnel constructed by the Central Pacific Railroad and was also the longest on the Transcontinental Railroad. Due to the very hard granite at Donner Pass and the poor progress with black powder blasting, they decided to use nitroglycerine for blasting the rock, which dramatically increased the rate of advancing the tunnel’s four headings. This was the first use of nitroglycerine for construction in the United States.
The construction of the Central Pacific Railroad’s Summit Tunnel, Tunnel No. 6, began on October 14, 1865, but severe winter storms quickly shut down construction. The tunnel’s construction resumed in April 1866 with the Chinese tunnel crews working three 8-hour shifts, six days a week. The Irish construction foremen worked 12-hour shifts. By September 1866, the tunnel work was all inside the west and east portals, so work continued during the winter of 1866-67. Due to the slow progress with black powder blasting the granite rock, the Chinese started using nitroglycerine for blasting the tunnel in February 1867. This was the first use of nitroglycerine for construction in the United States. The nitroglycerine was much more effective with the hard granite, with the nitroglycerine’s blast pressure over 100 times greater than black powder. For safety reasons, the nitroglycerine was manufactured on-site by chemist James Howden.
To expedite the work, a central shaft to access the center of the tunnel was started on August 27, 1866. The central shaft measured 8 ft by 12 ft, and it was 72.9 ft deep to the tunnel. The tunnel’s construction broke through on August 29, 1867. This was a huge benefit for the workers in the tunnel, allowing fresh air to flow through the length of the tunnel. The tunnel was completed and tracks were installed on November 30, 1867. According to the tunnel’s construction engineer, John R. Gilliss, “Member of the Society” (ASCE), the tunnel’s two headings met with offsets “too small to measure.” The first locomotive traveled through the tunnel on December 1, 1867.
The completed Summit Tunnel was 1,659 ft long, with a maximum depth of 124 ft, and at an elevation of approximately 7,030 ft it was the highest tunnel in the world. It was constructed using drilling and blasting to drive the tunnel through the “medium quality” (hardness) granite rock, which was “crossed by seams in every direction.” The tunnel’s top arched heading had a diameter of 16 ft and the bottoms measured 16 ft wide by 11 ft high. Due to the hardness of the granite, the tunnel did not need to be timbered. The Summit Tunnel was last used by the Southern Pacific Company in 1993.
A significant challenge during the tunnel’s construction was with the elevation and dealing with the harsh winter in the high Sierra Nevada. The average snowfall at Donner Pass is over 400 inches, with an average depth of 140 inches. The winter of 1866-67 was the worst recorded in the Sierra Nevada up to that time, with a total snowfall of over 44 ft. The accumulated snow at the summit was not gone until July 1867, with snow depths at the site still at 15 ft in April and 11 ft in May. The snow accumulation at the two portals were so deep that snow tunnels from 50 ft to 200 ft long were required for construction access and removal of spoils.