You’d think boring twin tunnels 5,889 feet through Mount Washington would be the most challenging aspect of building the Liberty Tunnels in the early 1920s.
You’d be wrong.
The tunnels themselves were a grand achievement, and although this one was the longest pair built only for automobiles when it opened, tunnels had been built before.
The bigger challenge for the design team led by civil engineer Almos D. Neeld was that the length of the tunnels called for a new kind of ventilation system to prevent the buildup of deadly carbon monoxide.
State and local officials celebrated Thursday the ventilation system’s selection in March as a National Historic Civil Engineering Landmark by the American Society of Civil Engineers. The only other site honored by the society in Allegheny County is the Smithfield Street Bridge, which was the first American bridge to use what’s called a “lenticular truss” design.
Storied beginning
Construction on the Liberty Bridge didn’t begin until 1919, but residents in Pittsburgh’s South Hills had been lobbying for a major connection to Downtown Pittsburgh for more than two decades. The belief among residents and business leaders was that their neighborhoods were primed for growth, but the missing ingredient was an easier city connection for residents and visitors.
There were projects proposed in several locations through and around Mount Washington, including a 1915 project also directed by Neeld where construction started only to stop when the state Supreme Court ruled that Allegheny County had no authority to build it. Four years later, the county committed to building the tunnels at their current location and constructing the Liberty Bridge as a companion project.
The public was so interested in the tunnels that a crowd showed up for the December 1919 groundbreaking. Contractor Booth & Flinn Ltd. turned the project into a competition with separate crews working 26 feet apart on both tubes at once, building from each end with a goal of meeting in the middle.
Crews were encouraged to bore at least 10 feet each day – considered extremely fast at that time — and were paid incentives. For example, they were paid for 11 hours of work if they bored 9 feet, 12 hours for 10 feet, even if they completed the distance in less time.
At that pace, the tunnels broke through in March 1922, when more than 300 officials watched Allegheny County Commissioner James Houlahen pull the switch on a charge of dynamite that created daylight. In early September, the county held an open house for the public to view the tunnels, and the push was on to open them in the next few months.
The tunnels themselves were ready in January 1923. The only problem was that the ventilation system was lagging behind.
Ventilation was considered a key part of the tunnel’s success, so the design team took extra pains to make sure it was done correctly. They brought in the U.S. Bureau of Mines and two universities to determine how much carbon monoxide vehicles would generate, what a safe level for people would be and how to control the air in the tunnel to keep the level low enough to for drivers and pedestrians to be safe.
The Bureau of Mines, based in the South Hills, computed the amount of carbon monoxide at its Bruceton Experimental Mine.
At Yale, scientists used volunteer staff and students who agreed to be exposed to polluted air to see what level would be safe.
The University of Illinois developed mathematical models for how much clean air would be needed to dilute the pollution to safe levels.
Working with those figures and projections for the traffic volume in the tunnels, designers determined they would need to create ventilation shafts to provide 280,000 cubic feet of fresh air each minute to keep the tunnels safe. Initially, they thought that vehicles moving through the tunnels would create enough circulation to reduce carbon monoxide to an acceptable level but then thought better of that when they considered headwinds could keep the air inside.
Instead, they proposed a series of ventilation shafts and huge fans to force clean air in and allow dirty air to leave. Each shaft would be designed to suck out polluted air through one half and force fresh air into the tunnel through the other half.
But concern about the effects of bad air was still controversial, and the county proposed cutting the cost of ventilation in half. In the end, the scientists prevailed, and plans called for four ventilation shafts near the center of the tunnels, two in each bore, that would move through the mountain and rise about 110 feet above a special fan house built on top of Mount Washington.
By the end of 1923, the public was clamoring for the tunnels to begin operation, but unexpected delays such as finding an abandoned coal mine while digging a hole for one of the shafts delayed ventilation. After renowned engineer Charles Holland, who would later build the Holland Tunnel in New York, reviewed the site and said he saw no reason why the tunnels couldn’t open without the ventilation system, the county commissioners gave in and opened the tunnels without ventilation on Jan. 30, 1924.
For more than three months, they operated relatively smoothly without the ventilation system, but that good fortune wouldn’t last.
With the county faced with a strike by transit workers who operated street cars in early May, commuters took to their personal vehicles to get to work in Downtown Pittsburgh. On May 10, a massive traffic jam occurred and dozens of vehicles were stuck in the tunnel, trapping dangerous levels of carbon monoxide that caused several drivers to pass out behind the wheel and more than a dozen to be taken to the hospital.
After that, police kept a close count of cars in the tunnel to control carbon monoxide until Sept. 1, when the ventilation system was put into operation after a month of testing. Crews in the fan house monitored printouts like those produced by an EKG for heart patients or a seismograph that checks for earthquakes to make sure the air was safe.
Modern efficiencies
Today, the ventilation system uses the same shafts and principles as the original system, but technology has changed some of the details.
During a tour of the fan house Thursday for PennDOT Secretary Mike Carroll, ASCE officials and reporters, tunnel supervisor Todd Caddy showed off computer-controlled fans that are about one-third the size of the originals.
“The building stands the same,” Caddy said. “Now, the computer system monitors the fans.”
Although the capacity for moving air is about the same, Caddy said the job is a little easier now because modern vehicles produce much less carbon monoxide. As a result, on many days the fans only run at full capacity during morning and evening rush hours.
During the late-morning tour, the fans were off. Had they been on, there wouldn’t have been much noise, Caddy said, but “it would be very windy” in the small rooms that house the fans.
Every two years, the shafts are inspected “literally by a guy on a rope” who is suspended inside to check their structural stability. That work is done by consultants for PennDOT in advance of a federal inspection.
These days, four electricians are assigned to the fan house each shift, part of about 70 crew members who oversee operations there and at the Fort Pitt and Squirrel Hill tunnels. Much of the monitoring is done below at the main Liberty Tunnel office at the south end of the tunnel.
The plaque hung at the entrance to the fan house honoring its historical significance says, in part, “The pioneering Liberty Tunnels Ventilation System incorporated a unique combination of induced air draft and forced air exhaust and intake. The research on exhaust emissions informed the design of subsequent vehicular tunnels.”
To Taylor DaCanal, president of ASCE’s Pittsburgh section, the ventilation system is “a permanent reminder” of the importance of innovation.
Ed covers transportation at the Pittsburgh Post-Gazette, but he's currently on strike. Email him at eblazina@unionprogress.com.