=Grouting.=--From the exterior of the tunnel already lined with cast-iron rings, grout was forced through the holes closed by screw-plugs, at a pressure of 90 lbs. per sq. in. The grout was composed of 1 Portland cement and 1 sand by volume and was forced in by a specially constructed machine, so it formed a shell of cement nearly 3 ins. thick around the exterior of the iron lining. The grouting began at the lower segment; the cement was forced in until it reached the hole above, then the hole was plugged, and the grouting was carried on from the consecutive hole and so on until all the tunnel was finally encased in grout, as it filled every crevice between the outside of the lining and the ground as excavated. The cast-iron rings of the tunnel were covered with a concrete lining which was placed in the following order: First, on the invert; second, on the duct benches; third, on the arch; fourth, on the ducts; fifth, on the face of the bench. Before any concrete was placed, the surface of the iron was cleaned by scrapers and wire brushes and by washing it with water. The invert was built in sections 30 ft. long and the duct benches were constructed soon after. These duct benches were built with several steps for the ducts to be laid later. They were built by means of a traveling stage on wheels which ran on tracks on the working platform of the tunnel. The arch was constructed soon after. First the portion from the duct benches to the haunches, then the arch proper, was built on traveling centers on tracks laid on the steps of the duct benches. The concrete was received in ³⁄₄-cu.-yd. dumping buckets, from the flat cars on which they were run; the buckets were hoisted to the level of the lower platform of the arch by a small Lidgerwood compressed air hoister. At this level the concrete was dumped on a traveling car or stage and moved in that to the point on the form where it was to be placed. For the lower part of the arch the concrete was thrown directly into the form from this traveling part of the stage. Fig. 140 shows the cross-section of the tunnel with the iron lining and concrete. [Illustration: Section in Sand and Gravel or Rock Section in Hudson River Silt, with foundations FIG. 140.--Typical Cross-Sections of One Tube of Pennsylvania Railroad Tunnel Under the Hudson River.]