Engineering Wonders of the World
Volume I

116 ENGINEERING WONDERS OF THE WORLD. construction. The doors in the cross wall, instead of being simple shutters, as in the other tunnels, were cylindrical cut-off Thp gates, which could be opened Shields. , or shut even while mud was flowing in, without having to displace or crowd back this material. At the front of the shield the horizontal working platforms were provided with extensible slides, pressed forward when desired by hydraulic jacks. These could be pushed out against the “ sheet- ing ” or timbering of the earth face, and would hold it in place even while the shield itself was being “ shoved ” forward. Such platforms were also used on the shields of the lower Hudson tunnels and on the Bel- mont shields, Under the main part of the river the two tubes encountered river silt only. But at and near the shores they had to pass through sand and gravel, which required the employ- ment of methods very different from those suitable for penetrating mud. In the mud it was found that, as in the case of the Hudson tunnels, the shield could push its way through the materials. But a Fig. 17.—A STEEL-PLATE CAISSON TO BE SUNK AS A SHAFT FROM WHICH TWO TUNNEL SHIELDS ARE TO BE STARTED. (LONG ISLAND END OF PENNSYLVANIA RAILROAD EAST RIVER TUNNELS.) The shields of the two tubes were started from this caisson, through the removable circular panels of wall seen in the picture. curious action developed. If the shield doors were all shut and no mud taken in, the act of shoving the shield would also make it rise, owing to the 5Ur*°us fact that the displaced mud necessarily had to flow upward, lifting the river bottom, which flow would carry the shield with it. Experience showed that, if the lowest door of th© shield was kept open during the “ shove,” allowing part of the mud to be squeezed inward into the tunnel, the rising tendency of the shield was neutral- ized. By due observance of this condition the progress through the silt was expeditious and satisfactory, the shield being kept very close to a mathematically correct alignment. The accuracy of junction of the shields, shown strikingly by the view (Fig. 15), de- monstrates the excellence of guidance attained. In the sand and gravel, however, the shield could not do its own excavating in this way, so the workmen had to go out in front of the shield and dig away the soil, keeping it tim- bered very carefully. The exceedingly treacherous nature of work in wet sand is well illustrated by an incident that occurred on the New Jersey shore— landward of the shield work, ~ . _ 4 . Quicksand. it is true, but under tint ber- ing methods like those used in front of a shield. Here one day the quicksand face broke through the timbering, and the run- ning, water-charged material poured in in such volume, that several hundred feet of the tunnel were filled up. What happened on the ground surface overhead during this break-in is demonstrated by the photograph (Fig. 11). There was a railway freight yard above, and when the ground sank down into the tunnel a great hole was formed, which, as the view shows, swallowed up several freight cars. The worst part of the shield work occurred at the New York shore, where the tunnel passed through the piling and stone filling