Tunneling

“ The guiding precept of the exhibit has not been to outline exhaustively the entire history of tunneling, but rather to show the fundamental advances which have occurred between primitive man’s first systematic use of fire for excavating rock in mining, and the use in combination of compressed air, iron lining, and a movable shield in a subaqueous tunnel at the end of the 19th century. ”

“ There are two important secondary techniques for opening subterranean and subaqueous ways, neither a method truly of tunneling. One of these, of ancient origin, used mainly in the construction of shallow subways and utility ways, is the “cut and cover” system, whereby an open trench is excavated and then roofed over. The result is, in effect, a tunnel. The concept of the other method was propounded in the early 19th century but only used practically in recent years. ”

“ SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD; QUICKSAND TUNNELING; PILOT METHOD. ITALIAN METHOD.
The Italian method of tunneling was first employed in constructing the Cristina tunnel on the Foggia & Benevento R.R. in Italy. This tunnel penetrated a laminated clay of the most treacherous character, and after various other soft-ground methods of tunneling had been tried and had failed, Mr. Procke, the engineer, devised and used successfully the method which is now known as the Italian or Cristina method. The Italian method is essentially a treacherous soil method. ”

“ The machine made one and a half to two revolutions a minute, and Sir Edward estimated for his stockholders that with simultaneous tunneling with the use of similar equipment from the French shore—the French Tunnel Company had already sunk a 280-foot shaft of its own at Sangatte and was preparing to drive a gallery toward England—the Channel bottom would be pierced from shore to shore by a continuous single pilot tunnel, twenty-two miles long, in three and a half years. ”

“ The principal advantages claimed for the Austrian method of tunneling are: (1) The excavation being conducted by driving a large number of consecutive small galleries, which are immediately strutted, there is little disturbance of the surrounding material; (2) the polygonal type of strutting adopted is easily erected and of great strength against symmetrical pressures; (3) the masonry, being built from the foundations up, is a single homogeneous structure, and is thus better able to withstand dangerous pressures ”

“ Thus the English and Austrian methods of tunneling, where the full section is excavated before any of the lining is placed, and where, as the consequence, the strutting has to sustain all of the pressures, present more likelihood of the roof caving in than any of the other common methods.
The character and structure of the material penetrated also influence the danger of a collapse. A loose soil with little cohesion is of course more likely to cave than one which is more stable. ”

“ Whatever the opinion may be in regard to this point, however, there is no chance for dispute in the statement that the difficulty of tunneling the softer and more treacherous clays, peats, and sands is greater than that of tunneling firm soils and rock; and if we describe the methods which are used successfully in tunneling very unstable materials, no difficulty need be experienced in modifying them to handle stable materials. ”

“ The observation of any tunneling operations in soft normally dry ground, or the examination of existing structures, will convince any one that, after a depth of approximately twice the greatest diameter of opening has been reached, it is impossible to tell, by any difference in the pressure, the depth of the tunnel. ”

“ The first man to perceive and seize upon the benefits to be gained by combining the two systems was, most fittingly, Greathead. Although he had projected the technique earlier, in driving the underground City and South London Railway in 1886, he brought together for the first time the three fundamental elements essential for the practical tunneling of soft, water-bearing ground: compressed-air support of the work during construction, the movable shield, and cast-iron, permanent lining. ”

“ The art of mining was first developed highly in the Middle Ages in the Germanic states; the tunnel shield was invented by a Frenchman residing in England, and the use of compressed air to exclude the water from subaqueous tunnels was first introduced on a major work by an American. In addition, the two main subdivisions, rock and soft-ground tunneling, are each introduced by a model not of an actual working, but of one typifying early classical methods which were in use for centuries until the comparatively recent development of more efficient systems of earth support and rock breaking. ”

“ A tunnel, defined as an engineering structure, is an artificial gallery, passage, or roadway beneath the ground, under the bed of a stream, or through a hill or mountain. The art of tunneling has been known to man since very ancient times. A Theban king on ascending the throne began at once to drive the long, narrow passage or tunnel leading to the inner chamber or sepulcher of the rock-cut tomb which was to form his final resting-place. Some of these rock-cut galleries of the ancient Egyptian kings were over 750 ft. ”

“ Tunneling in Compressed Air Without a Shield.
When it became evident that there would not be time for one shield to do the soft ground portions of all four tunnels under the Long Island Railroad station, a plan was adopted and used in Tunnel B which, while not as rapid, turned out to be as cheap as the work done by the shields. Figs. 6 and 7, Plate LXIII, and Fig. 1, Plate LXIV, illustrate this work fairly well. The operation of this scheme was about as follows: Having the iron built up to the face of the full-sized excavation, a hole or top heading, about 3 ft. wide and 4 or 5 ft. ”

“ The general method of directing the excavation in enlarging the profile by referring all points of the profile to the vertical axis is the one usually employed in tunneling, and gives good results. It is considered better in actual practice to have the excavation exceed the profile somewhat than to have it fall short of it, since the voids can be more easily filled in with riprap than the encroaching rock can be excavated during the building of the masonry. In tunnels where strutting is necessary the excavation must be made enough larger than the finished section to provide the space for it. ”

“ While the engineering of tunnels has been comprehensively treated in this paper from the historical standpoint, it is well to still reflect that the advances made in tunneling have not perceptibly removed the elements of uncertainty but have only provided more positive and effective means of countering their forces. ”

“ The material through which the tunnel was driven was chiefly hard granitic gneiss, conglomerate, and mica-schist rock. The excavation was conducted from the entrances and [94] one shaft, the wide heading and single-bench method being employed, with the center-cut system of blasting which was here used for the first time. The tunnel was begun in 1854, and continued by hand until 1866, when the mechanical plant was installed. Most of the particular machines employed have now become obsolete, but as they were the first machines used for rock tunneling in America they deserve mention. ”

“ Except for the development in steel apparatus and the introduction of electricity as a motive agent, there has not been such a great improvement on the Greathead shield as one would naturally expect in thirty years.
The method of excavating a tunnel depends altogether on the nature of the obstruction to be removed for the passage. In the case of solid rock the work is slow but simple; dry, hard, firm earth is much the same as rock. The difficulties of tunneling lie in the soft ground, subaqueous mud, silt, quicksand, or any treacherous soil of a shifting, unsteady composition. ”

“ Open-cut tunnels are those driven at such a small depth under the surface that it is more convenient to excavate an open cut, build the tunnel masonry inside it, and then refill the open spaces, than it is to carry on the work entirely underground. In firm soils the usual mode of operation is to excavate first two parallel trenches for the side walls, then remove the core, and build the arch and the invert. In unstable soils, since the invert must be built first, it is usual to open up a single wide trench. ”

“ The time required to excavate a tunnel depends upon the character of the material penetrated and upon the method of work adopted. Tunnels driven through soft ground by hand require about the same time to construct as tunnels driven through hard rock by the aid of machinery. Tunnels can be driven through hard rock at about as great a speed as through soft or fissured rock, chiefly because the work of blasting is more efficient in hard rock, and because no time is required in timbering. ”

“ Therefore, to avoid tedious and uncertain calculations, the engineer adopts dimensions which experience has proven to be ample under similar conditions in the past. Thus we have all gradations in thickness, from hard-rock tunnels requiring no lining, and tunnels through rocks which simply require a thin shell to protect them from the atmosphere, to soft-ground tunnels where a masonry lining 3 ft. or more in thickness is employed. ”

“ During construction, however, special means will have to be provided for removing the water from the excavation, their character depending upon the method of excavation and upon the grades of the tunnel bottom. When the excavation is carried on from the entrances only, unless the tunnel has a descending grade from the center toward each end, the tunnel floor in one heading will be below the level of the entrance, or, in other words, the descending grade will be toward the point where work is going on, while at the opposite entrance the grade will be descending from the work. ”

“ Great care is taken that the miners and men working in the tunnel shall not suffer from the sudden change from the warm headings to the cold Alpine air outside; and for this purpose a large building is in course of erection, where they will be able to take off their damp working clothes, have a hot and cold douche, put on a warm dry suit, and obtain refreshments at a moderate cost before returning to their homes. Instead of each man having a locker in which to stow his clothes, a perfect forest of cords hangs down from the wooden ceiling, 25 ft. ”

“ There are cross passages in the tunnel every 200 ft., and also various niches for the different equipment needed in connection with the signaling, telephone and fire alarm system. The tunnel is lighted with 800 16-candle-power incandescent lights.
The track construction is new. There is no ballast used, the ties being laid in concrete. A ditch in the center of each track carries the rainfall that will flow down from the summits to sumps which are drained by centrifugal pumps. ”

“ The material, dislodged by the cutting edge of the shield, is forced through the doors and falls on the floor whence it is removed in small cars. In very loose soils the excavation has been made in a still more economic way; the shield with closed doors is simply squeezed through the soil. This method is financially convenient, because all the excavating and hauling operations are eliminated and the tunnel progresses from 40 to 50 ft. per day, but clearly indicates a lack of stability. ”