For centuries there was no progress in the art of tunneling. On the contrary there was a decline from the earlier construction until late in the 17th century when gunpowder came into use as an explosive in blasting rock. The first application of gunpowder was probably at Malpas, France, 1679-1681, in the construction of the tunnel on the line of the Languedoc Canal 510 feet long, 22 feet wide and 29 feet high.
It was not until the beginning of the nineteenth century that the art of tunnel construction, through sand, wet ground or under rivers was undertaken so as to come rightly under the head of practical engineering. In 1803 a tunnel was built through very soft soil for the San Quentin Canal in France. Timbering or strutting was employed to support the walls and roof of the excavation as fast as the earth was removed and the masonry lining was built closely following it. From the experience gained in this tunnel were developed the various systems of soft ground subterranean tunneling in practice at the present day.
The first tunnel of any extent built in the United States was that known as the Auburn Tunnel near Auburn, Pa., for the water transportation of coal. It was several hundred feet long, 22 feet wide and 15 feet high. The first railroad tunnel in America was also in Pennsylvania on the Allegheny-Portage Railroad, built in 1818-1821. It was 901 feet long, 25 feet wide and 21 feet high.
What may be called the epoch making tunnel, the construction of which first introduced high explosives and power drills in this country, was the Hoosac in Massachusetts commenced in 1854 and after many interruptions brought to completion in 1876. It is a double-track tunnel nearly 5 miles in length. It was quickly followed by the commencement of the Erie tunnel through Bergen Hill near Hoboken, N.J. This tunnel was commenced in 1855 and finished in 1861. It is 4,400 feet long, 28 feet wide and 21 feet high. Other remarkable engineering feats of this kind in America are the Croton Aqueduct Tunnel, the Hudson River Tunnel, and the New York Subway.
The great rock tunnels of Europe are the four Alpine cuts known as Mont Cenis, St. Gothard, the Arlberg and the Simplon. The Mont Cenis is probably the most famous because at the time of its construction it was regarded as the greatest engineering achievement of the modern world, yet it is only a simple tunnel 8 miles long, while the Simplon is a double tunnel, each bore of which is 12-1/4 miles. The chief engineer of the Mont Cenis tunnel was M. Sommeiler, the man who devised the first power drill ever used in such work. In addition to the power drill the building of this tunnel induced the invention of apparatus to suck up foul air, the air compressor, the turbine and several other contrivances and appliances in use at the present time.
Great strides in modern tunneling developed the "shield" and brought metal lining into service. The shield was invented and first used by Sir M. I. Brunel, a London engineer, in excavating the tunnel under the River Thames, begun in 1825 and finished in 1841. In 1869 another English engineer, Peter Barlow, used an iron lining in connection with a shield in driving the second tunnel under the Thames at London. From a use of the shield and metal lining has grown the present system of tunneling which is now universally known as the shield system.
Great advancement has been made in the past few years in the nature and composition of explosives as well as in the form of motive power employed in blasting. Powerful chemical compositions, such as nitroglycerine and its compounds, such as dynamite, etc., have supplanted gunpowder, and electricity, is now almost invariably the firing agent. It also serves many other purposes in the work, illumination, supplying power for hoisting and excavating machinery, driving rock drills, and operating ventilating fans, etc. In this field, in fact, as everywhere else in the mechanical arts, the electric current is playing a leading part.
To the English engineer, Peter Barlow, above mentioned, must be given the credit of bringing into use the first really serviceable circular shield for soft ground tunneling. In 1863 he took out a patent for such a shield with a cylindrical cast iron lining for the completed tunnel. Of course James Henry Greathead very materially improved the shield, so much so indeed that the present system of tunneling by means of circular shields is called the Greathead not the Barlow system. Greathead and Barlow entered into a partnership in 1869. They constructed the tunnel under the Tower of London 1,350 feet in length and seven feet in diameter which penetrated compact clay and was completed within a period of eleven months. This was a remarkable record in tunnel building for the time and won for these eminent engineers a world wide fame. From thenceforth their system came into vogue in all soft soil and subaqueous tunneling. 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.
When the rock is to be removed it is customary to begin the work in sections of which there may be seven or eight. First one section is excavated, then another and so on to completion. The order of the sections depends upon the kind of rock and upon the time allotted for the job and several other circumstances known to the engineer. If the first section attacked be at the top immediately beneath the arch of the proposed tunnel, next to the overlying matter, it is called a heading, but if the first cutting takes place at the bottom of the rock to form the base of the tunnel it is called a drift.