Hydroelectric Tunnel 1925
THE BIGGEST HYDROELECTRIC TUNNEL
COLOSSAL UNIT WILL SOON BE ADDED to the water-power system of California through the completion of a conduit fifteen feet in diameter, bored through the solid granite of one of the loftiest mountain ranges in the High Sierras. The final blast, connecting the ends of the tunnel, has been fired; excavation of the thirteen-mile tunnel being completed a year earlier than estimatedâ€”a tremendous amount of time being saved by the establishment of an alinement whereby two “adits” were established and level shafts driven into the mountain to points where excavation should be started in two directions; enabling six crews of men all told, each crew in three shifts, to work continuously. A writer in General Contracting (Chicago), who gives data as to sundry technical details of value to engineers, gives also facts and figures of more general interest, some of which we quote:
“The Florence Lake tunnel, which has the greatest diameter of any tunnel of its length in the world, was constructed through solid gray granite. It follows the north contour of the Kaiser Range, which lies in the mountains 100 miles to the northeast of the City of Fresno, California, at an altitude of about 7,200 ft. The upper waters of the San Joaquin River, representing a drainage area of 175 square miles, will be impounded behind a 120-foot concrete dam, thereby creating a storage basin with a capacity of 60,000 acre feet. Water from this reservoir will be diverted under the mountains by way of the Florence Lake tunnel, down into Huntington Lake, and through the several power-houses which stretch for 20 miles down the Grand Canyon of the San Joaquin River.
“The operating portion of the tunnel is 67,640 feet long, more than twice as long as the Rogers Pass tunnel on the Canadian Pacific Railway, and about 2,000 feet longer than the Simplon tunnel through the Alps in Switzerland, which up to this time was the longest tunnel of its size in the world. Remarkable records were made on the Florence Lake tunnel by the use of modern equipment, high explosives and highly developed organization. An average of 22 feet per day was made for one month at one heading, and during one week a progress of 174 feet was made, which averages 24.9 feet per day, or over 1 foot per hour, through solid gray granite.
“During the period of construction on the tunnel an average of 2,500 men was constantly employed, and the average pay-roll amounted to over $375,000 per month. The total cost of the tunnel will amount to $17,000,000. It is worthy of note that fatalities here have been away below the average of one man per $1,000,000, which statistics show is the average under most favorable circumstances.”
Source: The Literary Digest for July 25, 1925