In the construction of the Cascade Tunnel of the Great Northern Railway a somewhat different arrangement was used.1 The working platform in the tunnel was erected five hundred feet in length, and cars hauled by cable up an incline to the platform. The side walls were built in alternating sections, eight to ten feet in length, the support of the arch timbering being thus gradually transferred from the plumb posts to the concrete of the side walls. Arch sections were built in twelve foot lengths, the centers being made of four by sixteen inch plank without radials, so as to leave a clear way for concrete cars on the working platform. The latter were high enough to allow the material cars to run beneath them.
There are frequent instances in engineering construction where brick masonry might well have been replaced by concrete, and the use of brick for tunnel lining is still adhered to in many cases. This is partly because somewhat less elaborate centers can be used for brick arches, and the centers may be struck somewhat earlier, and partly because of extreme conservatism on the part of the designer, although without doubt there are cases where the use of brick is entirely warranted.
An interesting instance of the greater adaptability of concrete under unforeseen conditions, however, is presented by the Third Street concrete and brick lined tunnel at Los Angeles, Cal.2 This tunnel was excavated mostly through an argillaceous sandstone. The side walls were of concrete up to the haunches, the upper part of the arch being of six courses of brick. A streak of yellow clay was encountered, and it "was soon demonstrated that the six ring brick arch, which occupies the central portion of the roof, was not strong enough to hold up the immense weight above it, and the temporary timbering was crushed and broken in a most alarming way." The strength of the arch was increased by using nine rows of brick instead of six until the clay seam was passed. In such portions of the six ring arch as had cracked, it was found that the inner ring of brickwork had separated from the second ring, and in places the second ring had separated from the third. The concrete walls had shown no evidence of weakness.
1 Mr. John F. Stevens, M. Am. Soc. C. E., Engineering News, Jan. 10, 1901.
2 J. H. Quinton, M. Am. Soc. C. E., Engineering News, July 18, 1901.
To repair the brickwork, steel concrete beams or arches were inserted in the brickwork at intervals of four feet, and extending from one concrete wall to the other. These beams were twelve inches wide and eight to twelve inches deep, made of rich concrete, and had imbedded in each beam two pieces of three inch by three-quarter inch steel. The steel ribs were set in recesses cut out of the brickwork, and rested at the ends upon the concrete of the side walls. Substantial centers were used for building the concrete beams, and when the latter had set, the defective brickwork between adjacent beams was cut out and replaced by rich concrete.
Another illustration of the adaptability of concrete when unexpected difficulties arise, is furnished by the construction of the Aspen Tunnel on the Union Pacific Railroad.1 The original design provided for sets of timbers to support the excavation, spaced about three feet, center to center, but for nine hundred feet of the tunnel such pressures were encountered that in places a solid wall of twelve by twelve inch timber was forced in. For a portion of this section the lining was builfr of a combination of concrete with steel ribs. The latter were 12-inch, 55-pound I-beams spaced from twelve to twenty-four inches, center to center, curved to conform to the interior of the tunnel. The concrete was built up around and between the beams, the inner flange being covered by from four to seven inches, and the total thickness of the walls two to three feet.
The Perkasie Tunnel of the Philadelphia and Reading Railroad was constructed through a firm rock, which, however, was intersected by several strata of seamy rock. As trouble was experienced from rock falling from these strata, it was decided to line the tunnel at such places. This lining had a minimum thickness of eighteen inches at the crown and twelve inches at the sides. Traffic through the tunnel was not obstructed during the work of placing the lining. In laying about five hundred cubic yards of concrete, the Cost was about ten dollars and eighty cents per cubic yard, exclusive of Cost of centering and dry filling.1
1 W. P. Hardesty, Engineering News, March 6, 1902.
The lining of portions of the Beacon Street Tunnel of the Sudbury River Aqueduct was undertaken some fourteen years after its excavation, and at a time when it was necessary to use the tunnel intermittently to supply water to the city of Boston. The methods employed are described by Mr. Desmond FitzGerald in Transactions American Soc. C. E. for March, 1894.
A substantial track of 2 feet 1 1/4 inch gage was laid from a manhole furnishing access to the sewer to the portion of the tunnel to be lined. The rails, weighing thirty-six pounds to the yard, were supported on small but substantial trestles, built of three by four inch spruce joists, and placed eight feet between centers. Every third trestle was braced from the sides and roof of the tunnel to prevent the track being floated when the tunnel was in use. The trestles also carried five rows of planks for the workmen to walk on in pushing the cars. The track was elevated by these trestles, so the work was not seriously interfered with by a small amount of water in the tunnel. The track Cost about eighty-seven cents a foot.
Cars to run on these tracks to deliver materials and concrete had frames five feet by one foot nine inches, with twenty inch wheels, and Cost about fifty-six dollars each.
The centers were in three parts, two for side walls and one for roof. The ribs were of three thicknesses of two by ten spruce plank, without interior bracing for the roof section. The side sections had each an inclined brace. Wedges were inserted between the tops of the side sections and the bottoms of the roof ribs to hold the latter in place. The lagging was two by four inch spruce, in eight foot lengths, with beveled edges and planed both sides. The centers were spaced four feet apart, and seventy-five full centers were built; these, with the lagging, contained 14,000 feet B. M. of lumber, and Cost $1,460.55, or $104.30 per thousand feet B. M.
1 P. D. Ford, M. Am. Soc. C. E., Trans. A. S. C. E., March, 1894.
Broken stone, sand and cement were stored in shanties over and around the manhole leading to the tunnel, and arrangements made by which the materials could be delivered through chutes down the manhole to the cars. As it was found more convenient to work in winter, special provision was made for storing large quantities of material in the shanties. The sand was piled around an iron lined, wooden bulkhead, in the center of which was a large stove.
The concrete was mixed within the tunnel as close to the work as possible, and in places where the cross-section had been sufficiently enlarged by falls of rock to permit easy working. The materials, delivered to the material cars down the chutes already mentioned, were pushed to the mixing platforms and combined in the proportions of 18.56 cubic feet of crushed stone and 7.35 cubic feet of sand to one barrel of Portland cement, being approximately 1 to 2 to 5 1/2. The above quantities of materials made 20 to 21 cubic feet of concrete. When mixed, the concrete was shoveled into cars, conveyed to the work and then shoveled into place.
The tamping was done principally with oak rammer five inches square, twelve inches long, with a short wooden handle in one end. In tamping the key of the arch, long-handled iron rammers were used. Much care was requisite here to prevent the aggregate separating from the mortar and lodging next the lagging, as it always has a tendency to do, thus resulting in voids in the face of the work when the lagging is removed. The concrete was built up on the sides in horizontal layers and stepped back by inserting bulkheads, so that the adjacent sections bonded together.
The Cost of this concrete lining, which was built under great disadvantages, amounted to $16.15 per cubic yard. This Cost must be considered reasonable in view of the fact that the materials had to be transported an average distance of more than one-half mile on small push cars, and the work in the tunnel was suspended for three days of each week to allow the tunnel to be used to maintain the water supply of the city.