Where the water-proofing was placed against the sand-wall, the method of securing the sheets at the top is shown in the small sketch on [Fig. 14] and by Figs. 3 and 4, [Plate XXIV]. Fig. 3, [Plate XXV], shows the laps of the sheets and the method of hanging. At the start an attempt was made to stick the water-proofing to the sand-wall, but this could not be done on account of its dampness and the overhang at the top.
The sand-wall water-proofing was kept about 35 ft. ahead of the finished bench-wall, as shown by Fig. 3, [Plate XXV]. As the bench-wall form was moved ahead and set, the mat was braced back against the sand-wall from the forms at a point just above the top of the finished bench, care being taken to avoid wrinkles, as, if these were once formed, it was practically impossible to straighten them out.
The completion of the bench-wall left the upper part of this water-proofing stretched taut across the curved top of the sand-wall, forming a chord of the arc. As the arch was built up, the top was gradually slackened so as to allow the concrete to press the mat back into place until the top of the sand-wall was reached, when the end was turned in, as shown at C, [Fig. 18], or the water-proofing was continued over the arch, if that was necessary.
The desire to obtain a dry tunnel, and the methods adopted to
secure it, were responsible in a great measure for the decision to build the arch in short lengths, as well as the reasons given under the head of arches. Had the tunnels been dry throughout, the method shown at A, [Fig. 18], could have been used exclusively, and, except for the fact that monolithic concrete might not have been obtained, there would have been no objection to building longer lengths.
The quantity of water reaching the tunnel drains and flowing out of their lower ends after the completion of the lining was about 100,000 gal. per day, or 75 gal. per min.; of this it is estimated that considerably less than 1% comes through the lining in the form of leaks. The very general distribution of this water over the roof is indicated by the fact that, during the excavation of the first 1,000 ft. of both tunnels from the Weehawken end, oilskins had to be provided for the laborers to induce them to work at all. The success, therefore, of the rock packing as a means of diverting this water to the side drains, is shown, especially in view of the fact that, excluding the cut-and-cover section, only 10% of the length of the arch, 1,189 ft., was water-proofed.
Considerable care was taken to make all joints in the concrete which were in such a position that water might follow through them to the inside of the tunnel lining, in such a manner that they would slope outward toward the rock. The top of the sand-wall is shown by Figs. [14] and [18]. The slope of the back of the foundation may be noted in Fig. 18, and the method of making the joint in the arch, in the few instances where a section was not completed at one operation, is shown at A, Fig. 18. These joints in the arch were not allowed to be made above a point 60° above the springing line.
