Once Upon A Tunnel

The Liberty Tubes were a first for automotive travel. Moving the rock was the easy part.
Boston Public Library; Universal Images Group North America LLC (Lake County Discovery Museum)/Alamy Long after their completion, the Liberty Tunnels were considered a wonder and a Pittsburgh landmark, as shown in the variety of vintage postcards from the 1930s and ’40s that can be found bearing the tubes’ image. Long after their completion, the Liberty Tunnels were considered a wonder and a Pittsburgh landmark, as shown in the variety of vintage postcards from the 1930s and ’40s that can be found bearing the tubes’ image.
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The calamity began at the stroke of midnight on May 10, 1924, when Pittsburgh Street Railway Company employees walked off the job. The streetcar strike threw commuters into a tizzy, and the following morning South Hills commuters jumped into their cars and headed for the recently opened Liberty Tunnels. Between 7:30 and 8 a.m., a record 649 cars entered the city-​bound tube. The surge brought traffic to a standstill all the way from downtown through the entire inbound tunnel.

The Liberty Tunnels had opened eight months earlier — before the ventilation system was complete— in response to public clamor for the longawaited shortcut to Downtown. As injudicious as allowing vehicles to drive through an unventilated tunnel may seem today, at that time, research indicated that the natural draft induced by steady traffic flow would render the tunnel air safe. The calculations, however, didn’t consider a streetcar strike causing a traffic jam that would fill the tunnel with idling cars and panicky drivers gasping for air while scurrying for the exits on foot and ignoring police orders to shut off their engines — all of which happened that morning.

As the situation became an emergency, County police closed the tunnel and turned traffic away. Thirty-​three people were hospitalized for carbon monoxide inhalation. After removing the standing vehicles and waiting for the air to clear, police ultimately reopened the tunnel that afternoon, with a limit of six entering vehicles per minute until the ventilation system was complete.

In the beginning

Ironically, the genesis of the streetcar-​strike traffic jam can be traced to the opening of a streetcar tunnel through Mount Washington 20 years earlier. In 1904, the Mount Washington Transit Tunnel (still in use today) brought streetcars to the South Hills, and that newfound accessibility to the city transformed the southern hinterlands into clear-​skied suburbs for those wishing to escape the smoke of Pittsburgh industry. Coincidentally, while the South Hills was evolving from rural backwater to residential retreat, transportation was evolving from horse-​drawn buggies and wagons to exhaust-​emitting cars and trucks.

Although coal mine and freight railway tunnels through Mount Washington had existed since the mid-​19th century, the debate about whether a roadway tunnel was necessary or even possible didn’t begin until 1907 and then continued for more than a decade. Proponents included vested South Hills real estate interests who saw a potential boom with South Hills property values catching up with those in the eastern suburbs. They and others also argued for improving the tedious and expensive means of supplying 100,000 South Hills residents— transporting 2,000 wagonloads of goods per day up and across Mt. Washington or, alternatively, waiting in line at the Castle Shannon wagon incline. Aggravating both options was the daily file of plodding horse-​drawn wagons, electric trolleys and automotive vehicles vying for passage to the South Side across the constantly congested Smithfield Street Bridge.

Debating the dig

A profusion of competing interests produced proposals for six different passageways at high and low elevations along the face of the escarpment. Ultimately, the Allegheny County Commissioners submitted a final plan, and in accordance with state law, the proposal was presented to a grand jury for pronouncement on the legitimacy of the public need for the tunnel and the justification of the associated expense. A series of legal brouhahas ensued. Objections ranged from improper presentation of evidence to the grand jury, to underestimated costs, undue influence of special interests, wasting taxpayer money, and the folly of attempting to build a tunnel the likes of which had never been built. Finally, on May 23, 1919 — after four years of legal tussling — the Commissioners’ proposal, called the Neeld Tunnel for its engineer designer Almos D. Neeld, was approved, and the “Low Tunnel Plan” closely resembles today’s Liberty Tunnel.

A new kind of tunnel for a new kind of vehicle

Although tunnels had been drilled through mountains, under rivers and beneath castle walls since the construction of the pyramids, the advent of the automotive age made the Liberty Tunnels the first of a new class of tunnel for a new kind of vehicle — a subsurface roadway built to accommodate exhaust-​emitting gasoline engines. Given the antiquity of tunnel excavation and the novelty of the internal combustion engine, digging the tunnel wouldn’t be the problem — ventilating it would be.

While the deadliness of carbon monoxide gas was well known at that time, the precise amount a person could breathe before suffering ill effects had never been established. As plans for vehicular tunnels began to emerge, the need to understand the odorless, colorless, noxious gas emitted by gasoline engines became urgent. Ventilation systems for roadway tunnels could not be designed without hard scientific data.

To get it, The New York, New Jersey Hudson River Tunnel Authority engaged Pittsburgh’s U.S. Bureau of Mines. Although the effort was made for designing the Holland Tunnel, the results of the investigation would apply to the new cohort of vehicular roadway tunnels being planned in New York, proposed in Boston, and under construction in Pittsburgh.

To solve the fundamental scientific problems, the Bureau of Mines enlisted researchers at Yale and the University of Illinois. They broke the problem into three component questions:

1) How much carbon monoxide did gasoline vehicles produce?
2) How much air was required to dilute the CO to a harmless level?
3) What methods and equipment would be necessary to achieve safe levels of dilution?

The Bureau of Mines answered the first question by conducting fuel consumption and exhaust emissions tests in its Bruceton Experimental Mine. Investigators determined that the average gasoline-​powered vehicle emitted 1.5 cubic feet of carbon monoxide per minute.

To answer the second question, Yale researchers subjected faculty and student volunteers to carefully metered doses of carbon monoxide. They determined that up to six parts carbon monoxide per 10,000 parts air was physically tolerable.

The University of Illinois was tasked with developing mathematical models of emissions flow under various tunnel conditions and configurations that could be applied to virtually any vehicular tunnel.

Compiling the experimental and calculated data, investigators determined that each car in a tunnel would require 2,500 cubic feet of fresh air per minute to dilute the carbon monoxide to a safe, breathable level.

While scientists worked out the physical theory, the Pittsburgh Department of Public Works conducted a traffic count on Bigelow Boulevard during a period of heavy use to find that the most likely scenario for the Liberty Tunnels during rush hour would be 114 vehicles traveling at 15 miles per hour. At that rate, each of Liberty’s twin tubes would require 280,000 cubic feet of fresh air per minute.

Early ventilation plans called for utilizing the prevailing westto– east winds to move contaminated air through the tubes. The idea was tabled when, upon close examination, planners realized that outbound traffic would always be bucking a headwind, thereby negating the ventilating force of the natural air current. At the same time, the draft created by inbound traffic would amplify naturally occurring winds to excessively high speeds.

Although the Bureau of Mines had conducted its research with customary scientific rigor, the novelty of running gasoline-​powered vehicles in an enclosed space allowed controversy about the tubes’ ventilation system to persist.

On March 13, 1923, more than two years after publication of the Bureau of Mines tunnel ventilation report and just six months before the tunnel opened to the public, engineering consultants for the Allegheny County Controller proposed to reduce ventilation system costs by more than half. The plan called for cutting the specified number of fans and motors in half and eliminating the proposed ventilation stacks atop Mt. Washington. The problem was that, based on the Bureau of Mines study, the proposed plan would not adequately ventilate the tunnels, especially under extreme conditions.

By taking advantage of the draft created by vehicles, air would flow in the same direction as traffic — each tube opposite the other.

In the end, the Planning Commission deferred to chief tunnel engineer Neeld, who had also designed and engineered several railway tunnels. Collaborating with the Bureau of Mines, Neeld opted for a ventilation system that met requisite air quality while minimizing the need for electric power to run the fans. By taking advantage of the draft created by vehicles, air would flow in the same direction as traffic — each tube opposite the other. At the same time, the amount of contaminated air requiring dilution would be minimized by splitting the tunnel air into two halves — entry and exit. In the entry half, the draft of the vehicles would drive contaminated air to a ventilation shaft in the tunnel ceiling midway through the tube. The massive shaft, which housed both exhaust and supply channels, rose 200 feet from the tunnel ceiling to a fan house atop Mt. Washington. While exhausted air from the first half of each tube was being vented into the atmosphere above Mt. Washington through 110-​foot exhaust stacks, fresh air would be drawn in through another set of stacks and blown into the exit half of each tube.

The fan house was equipped with four fans for each tube — two exhaust and two supply — each powered by a primary motor and a backup motor. All told, the fan house contained eight fans and 16 motors. At maximum output, the Liberty Tunnels ventilation system was capable of moving 560,000 cubic feet of air per minute. In the event of a catastrophe, such as a fire, each of the fans was capable of running at an additional 25 percent over specified capacity, giving the ventilation system the ability to move 700,000 cubic feet of air per minute in an emergency.

Blasting through

So great was anticipation of the tunnel that ground was broken before publication of the Bureau of Mines tunnel ventilation report. The official Dec. 20, 1919, ceremony followed a banquet at the Fort Pitt Hotel. Groundwork was begun on both tubes at both sides of the escarpment. The first explosive blast was detonated on April 8, 1920, by County Commissioner Addison C. Gumbert, who had championed the project.

The tunnel’s path bore closely to the horizontal rock strata. Layers of green and blue shale interspersed with courses of clay of varying thicknesses predominated. Engineers called the rock “good,” meaning it was unlikely to collapse into the void or break away after excavation. An especially hard course of limestone coincided roughly with the junction of the curved arch and vertical walls which, in engineering lingo, is called the springing line.

Each day the supervising engineer noted in a handwritten journal weather conditions, work performed, type of rock encountered, progress made and other noteworthy observations. The journals depict drillers and helpers boring holes in the tunnel face followed by blasters who would insert explosive powder in the holes and pack them with clay to direct the force of the blast into the rock. Blasting frequently took place on the night shift, between the hours of 2 and 5 a.m. The shattered remains would be mucked out by hand and steam shovel, hauled to the mouth of the tunnel in small rail cars called skips and loaded into trucks for dumping in a ravine atop Mt. Washington, which created the ground for McKinley Park.

Excavation was done in sequential steps beginning at the top and working down: blasting, mucking out and supporting the arched roof before simultaneously advancing the arch forward and the bench (the rock beneath the excavated arch) down. When the bench was fully blasted and mucked out, the walls were trimmed and the utility trench dug. Working each part of the excavation in tandem enabled workers to remove the rock as safely and quickly as possible.

The walls and ceiling of the tubes were lined with 24 inches of reinforced concrete and sealed with a waterproof barrier where water intrusion was a problem. Fortunately, groundwater intrusion was minimized by the drainage effect of abandoned coal mines about 150 feet above the excavation. The storm sewer beneath the floor ensured thorough drainage.

A cost-​accounting ledger for October 1920 shows a crew of foreman, drill runners, helpers, muckers, nippers, shovel runners, cranemen and pitmen working a total of 11,710 hours to advance 142.5 feet in the west (outbound) tunnel. Payroll came to $7,312.20. Steam shovel runners commanded the highest wage at $1.19 per hour. Pitmen and muckers received only 60 cents an hour.

Light at the end

On May 11, 1922, County Commissioner James Houlihan triggered the final blast connecting the two halves of the outbound tube. The official record shows the opposing excavations aligning within 38 of an inch. But an anecdote written years later by Neeld’s grandson reports an engineer peering through a survey scope and telling Neeld the sighting “split the string,” meaning the opposing excavations joined perfectly. Neeld is said to have told the engineer to report a slight misalignment because nobody would ever believe it was perfect.

Sixteen months later, after finishing the walls and concreting the roadway, the tubes were opened. Exactly two years and one day after the final blast, on May 10, 1924, the historic traffic jam occurred. And on Sept. 1, 1925, the ventilation system became operational. For the first few months, a sensor devised by the Bureau of Mines was employed to alert fan operators when increasing carbon monoxide levels necessitated boosting fan speed. However, it wasn’t long before traffic cycles were deemed predictable enough to dispense with the sensor. Thereafter, tunnel personnel would simply increase the fan speed an hour before the start of rush hour and decrease it when traffic slowed.

Originally estimated to cost $4.8 million, the added cost of the ventilation system brought the estimate to just over $6 million. But the project came in under budget at slightly under $6 million. The dig took 388 working days with an average of 200 men per day. There were no serious accidents while moving between 300,000 and 400,000 cubic yards of rock.

When finished, the Liberty Tunnel comprised the longest vehicular roadway tunnel in the world. The two parallel tubes, each 5,889 feet long and 26 feet 61÷2 inches in diameter, were set 59 feet apart on centers and separated by 20 feet of solid rock. Access passages every 500 feet connected the tubes. At each exit, a 66-​foot open-​air baffle chamber reduced turbulence outside the tunnel by deflecting the exiting air current skyward. Initially, a 4-​foot sidewalk flanked each roadway. Until 1932, horse-​drawn wagons were permitted in the tubes. The walkway remained until 1975.

Since their inaugural opening in 1923, the Liberty Tunnels have witnessed the construction of their river-​crossing sibling, the Liberty Bridge, completed in 1928, as well as five renovations — in 1932, 1939, 1975, 1996 and 2008.

Today, the Liberty Tunnel serves more than 63,000 vehicles a day. The roadway is wider due to the removal of the pedestrian walkway, and the fan-​house motors have been upgraded to a pair of 115 horsepower jobs for exhaust and 200 horsepower for intake. But the air still flows with traffic and the original fan rotors are still doing their job. And, just as when the tubes first opened, if you look up when you’re exiting, you can see the sky through the baffle chamber. At the time of its construction, the Liberty Tunnel was considered to be a marvel of civil engineering. During the excavation, representatives of the Japanese Imperial Government Railway came to witness the engineering prowess behind the world’s first and longest vehicular roadway tunnel. Since then, longer tunnels have been built, but the Liberty Tubes will always be the first of their kind.


Tom Imerito

Tom Imerito is president of Science Communications, a Pittsburgh technology communications consultancy.

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