Engineering Wonders of the World
Volume I

THE RIVER TUNNELS OF NEW YORK CITY. 105 years before—the old Morton Street Tunnel, now the upper Hudson Tunnel. That his- toric enterprise has the proud dis- tinction among New York’s river tunnels of being the first to be at- tempted, and the first of its soft- ground tunnels to be completed. However, during the long periods when it lay- idle, half-finished, paralyzed either by hopeless con- struction difficul- ties or by equally hopeless lack of money, two tun- nels were dug under the rock portions of the East and Harlem Fig. 3.—ONE PHASE OF THE SURVEY WORK IN A TUNNEL. The survey work, including constant checking of alignment, levels, distance out from shore, and shape of lining, is of the utmost importance. The accuracy of "tunnel-driving depends directly upon the frequency and exactness of the surveys. This shows a transit-man in an air- lock, engaged in prolonging the tube centre line into the compressed-air section, a difficult operation. Rivers, and in one shield method First Hudson Tunnel. of them the pneumatic found a notable application. We cannot better begin our study than by a brief glance at these two works, which present highly instructive similarities and contrasts. The Croton Aqueduct Tunnel (Har- lem River). The second Croton Aqueduct, instead of crossing the Harlem on a bridge like its pre- decessor of forty years before, was planned to cross by tunnel. The shores are rocky bluffs, and it appeared that at no great depth solid rock would be found across the whole width of the river-bed. Test borings were first made, of course, and on their showing the tunnel was planned to be dug 150 feet below water level. But when the contractor, after sinking his shore shafts and starting his tunnel riverward, had advanced but one-fourth, of the way, he encountered soft material occupying a depression in the rock surface. As this tunnel was to serve as a water - conduit under very high pressure, it was best to have it in solid rock ; and since the water could flow just as well at a lower level, it was permissible to go down deeper until sound rock was obtained. The tunnel was finally put through successfully at a depth of 300 feet, the partly - finished drift being aban- doned. At the great depth of the tunnel, any