The Pilot Method of This method has not been extensively employed. It was introduced in 1880 by Anderson of Ander son and Barr, who used the method in the con struction of the main relief sewer tunnel in Brooklyn, N. Y., in 1891. About 700 linear feet of this sewer has a diameter of 15 feet, 4,700 linear feet has a diameter of 14 feet, and 3,940 linear feet has a diameter of 12 feet, making 9,340 linear feet in all. • The material penetrated was mostly sand and gravel, some of it being wet. The principal feature of the method is a wrought-iron cylinder of one-fourth inch plate and 6 feet in diameter, the axis of which corresponds with the axis of the tunnel. This small 6-foot heading or pilot is the first portion of the tunnel excavated. The front end of the pilot was carried about 30 feet ahead of the completed tunnel section, allowing a 10-foot length of four-ring brick masonry lining to be put in place, while 15 to 20 feet of the pilot rested in unexcavated material. As fast as excavation was made at the front end of the pilot, plates were taken off the rear end which projected into the finished work and put in place at the front end. In this manner the pilot was carried ahead as fast as excavation could be made. In this case the pilot was built of one-fourth inch iron plates carrying 3-inch by 3-inch angles on their edges. The exca vation then made around the outside of the pilot was closely followed by radial struts with one end supported against the pilot and the other end carrying iron plates and lagging to protect the exposel face of the excavation. Iron plates were used over the crown of the finished excavation, but were not carried down on the sides, although they could be so extended if desired. In this manner the complete tunnel was successfully excavated and the material held back until the brick lining used in this case was in place. All work was done at ordinary atmospheric pres sure. No other large work has been com pleted by this method, although it possesses advantages for many localities.
Baltimore Belt Line Tunnel.—This double track tunnel is a part of the double-track line of railway in the city of Baltimore, Md., built for the Baltimore and Ohio and Western Maryland railroads. It was driven through a great variety of materials, some of which were soft and saturated with water and gave much trouble. Rock was penetrated in some places and in others clay so hard and tough as to require blasting for its removal. For the greater part of its length, however, this tunnel was driven through soft material, some of which carried so much water that it was with difficulty that the work could he pros ecuted without danger of serious settlements at the surface above it. In one case the set tlement caused the destruction of a large building. The general plan of operations was that of the German method. Drifts along the sides of the tunnel were first driven, thus providing for the drainage of the upper part of the material to be removed. Por tions of the side walls were built in these drifts and subsequently extended upward to the elevation of the springing line of the arch. The top heading was then driven and the adjacent side portions were immediately thereafter removed, thus completing the ex cavation to the full width of the upper half of the section. All these drifts and other excavated portions were heavily timbered and strutted so as to protect the exposed surface to the greatest possible extent. The
illustrations show the method of strutting or bracing and the use of the poling boards employed. (Figs. 6 and 7). In laying up the arch of the tunnel both iron and timber centres were employed. As is shown in the illustrations the masonry of the arch and side walls was laid up inside of the exterior line of round struts and the poling boards outside of them. The voids outside of the tunnel masonry were filled with rubble masonry so that no vacant spaces were left unfilled. The lining of the arch in general consisted of five rings of brick work, but where the soil was unusually soft eight rings were employed. The large amount of water in the material under the tunnel caused considerable difficulty in build ing the invert. Enclosures, however, of short sheet piling transversely across the tunnel were formed, within which the material was excavated and the foundation course of con crete 8 inches thick was laid. On this con crete foundation the brick invert was placed. One of the unique features of this work was the 6-inch perforated pipes which were inserted radially about 10 feet into the soft wet material surrounding the upper part of the tunnel where it was deemed feasible to resort to this pro cedure. Through these perforated pipes thin Portland cement, mortar or grout was forced into the surrounding soft material, so that when the Portland cement set a solid stiff mass of material was produced instead of the saturated and easily flowing natural material. • The bot tom side drifts and the masonry walls built in them were ordinarily carried about 20 feet ahead of the arch, although at times this work was as much as 90 feet in advance. The arch was usually built in complete transverse sections about 18 feet in length.
General Features of Method Used in Quicksand.—The construction of the Baltimore Belt Line Tunnel illustrates some of the general features of method suitable for use in quick sands and other soft materials. Quicksands are simply fine sand mixed to some extent with fine clayey matter and saturated usually with running water. All wet material is soft, and in tunneling through such material the first effort is to drain that part of the ex cavation as effectively as possible. This is accomplished by driving either a centre bot tom drift or, as in the Baltimore Belt Line Tunnel, two bottom side drifts, all of which are well calculated to drain the material above them. If the water does not flow away from these drifts by gravity it may be led into sumps and removed from them by pumping. After this draining is accomplished the remaining material is taken out by some method such as those already described, timber bracing or strutting being introduced concurrently with the excavation. Poling boards are constantly used to keep the material from falling into the excavation and the joints between them are frequently packed with hay or material of similar character so as to permit water to enter without bringing with it the soft material. The lining is then put in place as al ready described in connection with the Balti more Belt Line Tunnel, frequently leaving the exterior line of strutting and the poling boards in place. The hardening of soft material may also be produced in other ways than by inject ing Portland cement grout, as by freezing or by stock ramming.