For tunnels in soft ground requiring the use of a shield, some difficulties in using a concrete lining are apparent. The principal one of these lies in the fact that the fresh concrete is not capable of taking the thrust of the jacks used in forcing the shield ahead. Attempts have been made to overcome this difficulty by so constructing the centers that the jacks may bear against them instead of on the fresh concrete.
Another difficulty is that in materials requiring almost continuous support, the temporary timbering is in the way of the centering for the concrete construction; and still another is the difficulty of properly tamping the arch at the crown where the tail of the shield confines the working space. Concrete blocks were tried in the construction of sewers in Melbourne, but without entire success. Such blocks were successfully employed in the underground road system of Paris, though attempts to use fresh concrete in shield tunneling for this work proved a failure.
In the construction of the East Boston Tunnel Extension of the Boston Subway, however, a monolithic concrete lining has been successfully built, the tunnel being excavated by shield.
This tunnel is about twenty by twenty-four feet for double track electric line. The arch ring and the walls are thirtythree inches in thickness, while the invert is twenty-four inches. Two side drifts, eight feet square, were first driven a certain distance and timbered. The bottoms of these drifts were then excavated, and the side foundations of concrete were placed in lengths of sixteen to twenty feet. When the foundations had set, the interior forms for the side walls were placed upon them, supporting the caps, the exterior plumb posts removed, and the concrete side walls, three feet thick, built up to within sixteen inches of the springing line of the arch. This work was kept about one hundred feet in advance of the shield.
The shield, provided with live rollers, rested upon these side walls, the rollers running in a flanged plate placed on top of the walls. The shield was forced ahead thirty inches at a time, and sections of the arch thirty inches in length were turned directly behind the shield.
685. The centers of the arch were of curved, ten inch steel channels spaced thirty inches apart, and the lagging, four inches thick, was placed from the bottom toward the key as the concrete was built up. Each section of arch is keyed with concrete pressed through two holes in the rear girder at the top of the shield, special rammers being used to tamp the concrete into the space at the crown of the arch, the concrete being directed into place by curved sheet-iron troughs.
In each section of arch sixteen cast iron bars, three and one-quarter inches in diameter and thirty inches long, are built into the concrete in position to receive the thrust of the shield jacks. Wooden bulkheads on the jack plungers serve to confine the fresh concrete, but the reaction is taken on the cast iron bars which, being butted end to end in successive sections of the arch, carry the stress back to concrete that is able to sustain it. As the shield advanced, the space left over the completed arch by the tailpiece of the shield was filled with grout under pressure. The centers remained in place thirty days. The invert was excavated and laid in ten-foot sections about twenty-five feet in the rear of the shield. The concrete was mixed at the bottom of the shaft and passed through the air lock on cars. The concrete cars ran on a higher level than the muck cars, in order not to interfere with the excavation.
If the rock through which a tunnel is driven is seamy and insecure, concrete is in most cases the cheapest and best lining. The Cost of the lining is, of course, less if it can be built in connection with the excavation, but it is frequently difficult to foresee how a given rock will stand exposure to the air and water, and it becomes an exceedingly nice question to determine at the time of building a tunnel whether lining is required. In many cases this question is settled in the affirmative by other considerations than the character of the rock, as the resistance to flow, in waterworks and sewers, or the ease of ventilation and the necessity of a good appearance, as in street or steam railway tunnels.
In the construction of portions of the rapid transit subways of New York, a traveling center which served also to support a working platform was carried on six wheels running on a track laid on the footing courses of the side walls. This center carried at the side, sections of lagging curved to the required form of the side walls. This lagging was adjusted in place, and braced from the platform or center by means of wedges. Directly behind this traveling center was a similar platform carrying a derrick; and behind this, the traveling center carrying the lagging for the roof. This third platform was jacked up to place the roof lagging at the correct elevation, and firmly supported by wedges.
The concrete was brought in skips on cars that ran on the floor level and stopped beneath the derrick platform. The derrick hoisted the skips through a hole in the platform and placed them on cars on either the side wall or the roof platform, so that the concrete was delivered either to the side wall forms in advance, or the roof forms in the rear as required. The concrete was rammed in a direction transverse to the tunnel axis until the roof was completed, except for a space about five feet wide at the crown. The arch was then keyed by tamping the concrete in from the end of the form. The two platforms carrying the forms were each forty feet long, and the derrick platform was eighteen feet.
688. The excavated rock was crushed for the concrete on a working platform erected over and around the shaft head. Cars delivered the excavated material at the shaft in steel skips, which were hoisted to the working platform, set on push cars and dumped into bins, from which stone was delivered to the crusher; these cars then passed under the crushed stone bins, were loaded with broken stone, run back to the shaft head, and the broken stone dumped into bins mounted over the mixer. The skips were then lowered into the shafts by the derricks, to be run to the headings and reloaded. The stone and sand were fed to a measuring box by means of a hopper, the measuring box discharging directly into a cubical mixer, which was high enough above the tunnel floor to dump directly into skips on the cars.