CHAPTER XVII. SUBMARINE TUNNELING: GENERAL DISCUSSION.--THE SEVERN TUNNEL. GENERAL DISCUSSION.

Submarine tunnels, or tunnels excavated under the beds of rivers, lakes, etc., have been constructed in large numbers during the last quarter of a century, and the projects for such tunnels, which have not yet been carried to completion, are still more numerous. Among the more notable completed works of this character may be noted the tunnel under the River Severn and those under the River Thames in England, the one under the River Seine in France, those under the St. Clair, Detroit, Hudson, Harlem and East Rivers, and the one under the Boston Harbor for railways, that under the East River for gas mains, that under Dorchester Bay, Boston, for sewage, and those under Lakes Michigan and Erie for the water supply of Milwaukee, Chicago, Buffalo, and Cleveland in America. For the details of the various projected submarine tunnels of note, which include tunnels under the English and Irish Channels, under the Straits of Gibraltar, under the sound between Copenhagen in Denmark and MalmÖ in Sweden, under the Messina Straits between Italy and Sicily, and under the Straits of Northumberland between New Brunswick and Prince Edward Island, and those connecting the various islands of the Straits of Behring, the reader is referred to the periodical literature of the last few years.

Previous to attempting the driving of a submarine tunnel it is necessary to ascertain the character of the material it will penetrate. This fact is generally determined by making diamond-drill borings along the line, and the object of ascertaining it is to determine the method of excavation to be adopted. If the material is permeable and the tunnel must pass at a small depth below the river bed, a method will have to be adopted which provides for handling the difficulty of inflowing water. If, on the other hand, the tunnel passes through impermeable material at a great depth, there will be no unusual trouble from water, and almost any of the ordinary methods of tunneling such materials may be employed. Shallow submarine tunnels through permeable material are usually driven by the shield method or by the compressed air method, or by a method which is a combination of the first and second.

It is not an uncommon experience for a submarine tunnel to start out in firm soil and unexpectedly to find that this material becomes soft and treacherous as the work proceeds, or that it is intersected by strata of soft material. The method of dealing with this condition will vary with the circumstances, but generally if any considerable amount of soft material has to be penetrated, or if the inflow of water is very large, the firm-ground system of work is changed to one of the methods employed for excavating soft-ground submarine tunnels. The Milwaukee water supply tunnel, described elsewhere, is a notable example of submarine tunnels, began in firm material which unexpectedly developed a treacherous character after the work had proceeded some distance. Occasionally the task of building a submarine tunnel in the river bed arises. In such cases the tunnel is usually built by means of cofferdams in shallow water, and by means of caissons in deep water.

Submarine tunnels under rivers are usually built with a descending grade from each end which terminates in a level middle position, the longitudinal profile of the tunnel corresponding to the transverse profile of the river bottom. Where, however, such tunnels pass under the water with one end submerged, and the other end rising to land like the water supply tunnels of Chicago, Milwaukee, and Cleveland, the longitudinal profile is commonly level, or else descends from the shore to a level position reaching out under the water.

The drainage of submarine tunnels during construction is one of the most serious problems with which the engineer has to deal in such works. This arises from the fact that, since the entrances of the tunnel are higher than the other parts, all of the seepage water remains in the tunnel unless pumped out, and from the possibility of encountering faults or permeable strata, which reach to the stream bed and give access to water in greater or less quantities. Generally, therefore, the excavation is conducted in such a manner that the inflowing water is led directly to sumps. To drain these sumps pumping stations are necessary at the shore shafts, and they should have ample capacity to handle the ordinary amount of seepage, and enough surplus capacity to meet probable increases in the inflow. For extraordinary emergencies this plant may have to be greatly enlarged, but it is not usual to provide for these at the outset unless their likelihood is obvious from the start. The character and size of the pumping plants used in constructing a number of well-known tunnels are described in Chapter XII.

In this book submarine tunnels will be classified as follows: (1) Tunnels in rock or very compact soils, which are driven by any of the ordinary methods of tunneling similar materials on land; (2) tunnels in loose soils, which may be driven, (a) by compressed air, (b) by shields, or (c) by shields and compressed air combined; (3) tunnels on the river bed, which are constructed, (a) by cofferdams, or (b) by caissons. To illustrate tunnels of the first class, the River Severn tunnel in England has been selected; to illustrate those of the second class, the several tunnels discussed in the chapter on the shield system of tunneling and the Milwaukee tunnel is sufficient; to illustrate those of the third class, the Van Buren Street tunnel in Chicago, the Harlem, the Seine and the Detroit River tunnels are selected.

THE SEVERN TUNNEL.

The Severn tunnel, which carries the Great Western Railway of England, beneath the estuary of a large river, is 4 miles 642 yards long. It penetrates strata of conglomerate, limestone, carboniferous beds, marl, gravel, and sand at a minimum depth of 44³/₄ ft. below the deepest portion of the estuary bed. The following description of the work is abstracted from a paper by Mr. L. F. Vernon-Harcourt.[12]

The first work was the sinking of a shaft, 15 ft. in diameter, lined with brickwork, on the Monmouthshire bank of the Severn, 200 ft. deep. After the Monmouthshire shaft had been sunk, a heading 7 ft. square was driven under the river, rising with a gradient of 1 in 500 from the shaft on the Monmouthshire shore, so as to drain the lowest part of the tunnel. Near to the first, a second shaft was sunk and tubbed with iron, in which the pumps were placed for removing the water from the tunnel works, connection being made by a cross-heading with the heading from the first shaft. There was also a shaft on the Gloucestershire shore; and two shafts inland from the first on the Monmouthshire side, to expedite the construction of the tunnel. Headings were then driven in both directions along the line of the tunnel, from the four shafts; and the drainage heading from the old shaft was continued, in the line of the tunnel, under the deep channel of the estuary, and up the ascending gradient towards the Gloucestershire shore, till, in October, 1879, it had reached to within about 130 yards of the end of the descending heading from the Gloucestershire shaft. During this period, though the work had progressed slowly, no large quantity of water had been met with in any of the headings, in spite of their already extending under almost the whole width of the estuary. On October 18, 1889, however, a great spring was tapped by the heading which was being driven landwards from the old shaft, about 40 ft. above the level of the drainage heading; and the water poured out from this land spring in such quantity that, flowing along the heading, falling down the old shaft, and thus finding its way into the drainage heading and the long heading of the tunnel under the estuary in connection with it, it flooded the whole of the workings in communication with the old shaft, which it also filled within twenty-four hours from the piercing of the spring.

To obtain increased security against any influx of water from the deep channel of the estuary into the tunnel, the proposed level portion of the tunnel, rather more than a furlong long under this part, was lowered 15 ft. by increasing the descending gradient on the Monmouthshire side from 1 in 100 to 1 in 90, and lowering the proposed rail level on the Gloucestershire side 15 ft. throughout the ascent, so as not to increase the gradient of 1 in 100 against the load. A new shaft, 18 ft. in diameter, was sunk slightly nearer the estuary on the Monmouthshire shore than the old one; two shafts also were sunk on the land side of the great spring for pumping purposes; and additional pumping machinery was erected. The flow from the spring into the old shaft was arrested by a shield of oak fixed across the heading; and at last, after numerous failures and breakdowns of the pumps, the headings were cleared of water, after a diver, supplied with a knapsack of compressed oxygen, had closed a door in the long heading under the estuary; and the works were resumed nearly fourteen months after the flooding occurred. The great spring was then shut off from the workings by a wall across the heading leading to the old shaft; and, owing to the lowering of the level of the tunnel, a new drainage heading had to be driven from the bottom of the new shaft at a lower level, which was made 5 ft. in diameter, and lined with brickwork, whilst the old drainage heading was enlarged to 9 ft. in diameter, and lined with brickwork, so as to aid in the permanent ventilation of the tunnel. The lowering of the level, moreover, converted the bottom tunnel headings into top headings, so that along more than a mile of the tunnel the semicircular arch, 26 ft. in diameter, was built first, and then, after lowering the headings, the invert was laid and the side walls were built up. Bottom headings were driven along the remainder of the tunnel, and the work was expedited by means of break-ups. Ventilation was effected in the works by a fan 18 inches in diameter and 7 ft. wide, fixed at the top of the new deep shaft; the rock was bored by drills worked by compressed air; the blasting was eventually effected exclusively by tonite, owing to its being freer from deleterious fumes than any other explosive; and the workings were lighted by Swan and Brush electric lamps. The tunnel is lined throughout with vitrified brickwork, between 2¹/₄ ft. to 3 ft. thick, set in cement, and has an invert 1¹/₂ ft. to 3 ft. in thickness; the lining was commenced towards the end of 1880, the headings under the river were joined in September, 1881, and the last length of the tunnel, across the line of the great spring, was completed in April, 1885.

Water came in from the river through a hole in a pool of the estuary, close to the Gloucestershire shore, in April, 1881, during the lining of a portion of the tunnel, but fortunately before the headings were joined. This influx was stopped by allowing the water to rise in the tunnel to tide-level, to prevent the enlargement of the hole, which was then filled up at low water with clay, weighted on the top with clay in bags. The great spring broke out again in October, 1883, and flooded the works a second time; but within four weeks the water had been pumped out and the spring again imprisoned. During this period an exceptionally high tide, raised still higher by a southwesterly gale, inundated the low-lying land on the Monmouthshire side of the estuary, and, flowing down one of the inland shafts, flooded a section of the tunnel, but the pumps removed this water within a week.

In order to construct the portion of tunnel traversing the line of the great spring, the water was diverted into a side heading below the level of the tunnel, leading to the old shaft, whence it was pumped, and the fissure below the tunnel was filled with concrete, over which the invert was built. An attempt to imprison the spring, on the completion of this length of tunnel, having resulted in imposing an excessive pressure on the brickwork, leading to fractures and leakage, a shaft, 29 ft. in diameter, was sunk at the side of the tunnel at this point in 1886, and pumps were erected powerful enough to deal with the entire flow of the spring.

The tunnel was opened for traffic in December, 1886, and gives access to a double line of railway, connecting the lines converging to Bristol with the South Wales railway and the western lines. The pumping power provided at the shaft connected with the great spring, and at four other shafts, is capable of raising 66,000,000 gallons of water per day, the maximum amount pumped from the tunnel being 30,000,000 gallons a day. The ventilation of the tunnel is effected by fans placed in the two main shafts on each bank of the estuary, and the fan in the Monmouthshire shaft is 40 ft. in diameter, and 12 ft. wide. The tunnel gives passage to a large traffic, numerous through-trains between the north and southwest of England making use of it.

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