The Shifting Tunnels
By the spring of 1906, the Pennsylvania Railroad Company’s (PRR) construction of the underwater tunnels into Manhattan was already well underway when General Charles W. Raymond first reported the deeply troubling problem of “The Behavior of the Subaqueous Tunnels.” According to Raymond, the Chairman of the Board of Engineers, the tunnels under the Hudson River were shifting.
The riverbed underneath the Hudson River consisted of soft, muddy silt, and Raymond’s data showed the tunnels regularly rising and falling as much as two feet within the silt. It weighed heavily on PRR Vice President Samuel Rea, who addressed his concerns in a confidential letter to President Alexander Cassatt on April 2, 1906. If the tunnels were indeed moving, Rea asserted, the matter needed to be taken on immediately.
The PRR's engineers considered several ideas to solve this potentially devastating problem. Cassatt ordered his engineers to increase the weight of the cast-iron ring segments that made up the tunnels from 9,272 pounds to 11,594 pounds, hoping that would settle the tunnels, but the tunnels were still moving.
Despite this problem, construction of the tunnels proceeded, and the two halves of the north tunnel met underneath the Hudson River on September 11, 1906. Though Chief Engineer Charles Jacobs led a celebratory inspection party through the tunnel, the mystery of the tunnels’ up and down movement vexed Cassatt. The next day he telegrammed Rea, telling him simply, "This whole subject should be exhaustively studied. The Board has nothing so important before it at this time." When the second Hudson River tunnel was linked on October 9, the press was invited to tour the completed tunnel; the oscillation issues were strategically not mentioned. Now that both tunnels were linked, Rea ordered that no further work be done, including the concrete reinforcement and installation of electric wiring and tracks, until the Board resolved the issue of the moving tunnels.
Measurements, experimental tests and debate went on for months, and by the spring of 1907, the explanation of the tunnels drifting in the Hudson River silt remained a mystery. Still not knowing why the movements were happening, the five members of the Board had to make an important decision. From the outset of the project, the Board had considered installing screw piles through the cast iron shells of the tunnels that would be secured to the solid bedrock underneath the silt, as a way to prevent any tunnel movement. Now they were unsure whether or not to move forward with their original plan. Jacobs and two of his fellow Board members strongly believed the screw piles were a necessary safety precaution. Rea and Raymond disagreed. On June 5, 1907, Rea weighed in on the subject: if the screw piles were attached directly to the bedrock and too much pressure was exerted by the tunnel movement, they “might rupture the shell” of the tunnel.
During the spring and summer of 1907 Jacobs installed Edson Recording Gauges, which would measure the pressure changes of the moving tunnels. During the process, he discovered a shockingly simple fact: the tunnels were rising and falling with the tide.
On October 3, 1907 Jacobs reported to the Board of Engineers that high tide corresponded exactly with the lowest depth measurements of the tunnel on the Edson Gauges: “The actual time and elevation of high and low tide in the river at this point are observed daily, and the time of high tide is found to agree identically with that at which the tunnel reaches its lowest position.” They all were aware that the Hudson was a tidal river, but they never dreamed that the tide was strong enough to move their massive tunnels.
On May 8, 1908, Rea informed the Board of his decision to not install screw piles: “The management... must decline to at this time... [to] approve of the installation of piles... I have, after careful consideration, reached the conclusion that piles, not being a necessity or advisable, we should not install them.”
Rea had studied the movements of the tunnels for two years and felt there was no convincing evidence that the “additional insurance” offered by installing screw piles was worth the pursuit. In fact, Rea felt the potential dangers of installing them was greater than not installing them, since proper testing with the screw piles attached had not been conducted.
Rea remained President of the Pennsylvania Railroad until September 30, 1925, and never once told the public about his decision not to install screw piles, concerned that it would incite unnecessary fear in the public. Time has proven his decision wise, as the tunnels, constantly surveyed over the years, continue to oscillate slightly, but remain completely safe.