Engineering Wonders of the World
Volume I

118 ENGINEERING WONDERS OF THE WORLD. of raising the air pressure without risk of blow-outs. When one considers that the tunnels at places have only seven feet of quicksand between them and the river above, the benefit of the clay blanket can be realized. It is admitted by all that the clay blanket deposited in advance of the shield, combined with continuous grouting over the iron lining with a quick-setting cement or lime, has taken away many of the dangers of tunnelling the East River. “ In material where the lower part of the face consisted of The difficulties of the East River work in quicksand are succinctly sketched in a mem- orandum of the con- tractors thus :— “ In trying to keep the air pres- sure sufficiently high to overcome the head of water and quicksand at the bottom of the face, the contractors were constantly con- fronted with the risk of blowing off the roof of quick- sand and flooding the tunnel ; while if they lowered the pressure sufficiently to prevent blow-outs, then the quicksand flowed from Fig. 19.—A SPECIMEN OF ROCK TUNNELLING I HORIZONTAL DRIFT FOR ERECTION OF SHIELD. (PENNSYLVANIA RAIL- ROAD EAST RIVER TUNNELS, MANHATTAN END.) Fig. 20.—REAR OF SHIELD, BATTERY TUNNEL. The drainage pump platform carries the segment erector, and serves also as staging for the men who bolt the segments together. The erector is just about to put a segment in place. The shield rams are plainly visible. rock and the upper part of quicksand, great care had to be taken in blasting out the rock to pre- vent the boards supporting the quicksand from be- ing shaken down. In a full quicksand face the quicksand was supported by timber poling- boards and breast- boards ; and when the breasting was carried down as low as the wetness of the sand allowed, th© shield was shoved (with pressure up to 6,000 lbs. per square inch in the rams, or a total of 5,000 tons). The maximum air pressure used was under the shield and between the joints of the lining, and allowed the tunnel to settle. E. W. Moir * foresaw and guarded against this danger by specifying that the Pennsyl- vania Railroad should obtain from the War Department permission for dumping clay in the river bed to a sufficient depth to permit * Mr. Moir had already distinguished himself in the Old Hudson River Tunnel. 36 lbs.” In these few words are suggested some of the many tedious and highly-skilled opera- tions which go into the building of such a tunnel. If the final achievement is a wonder before our eyes, we may well bestow our admiration upon the intricacies of its con- struction, and the courage, intelligence, pa- tience, and skill by which it is brought about.