Fig. 115.—Wide Arch Section, Boston Subway.
Fig. 115.—Wide Arch Section, Boston Subway.
—The subway being built for two tracks in some places and for four tracks in other places, it was necessary to vary the form and dimensions of the cross-section. The cross-sections actually adopted are of three types.Fig. 115shows the section known as the wide-arch type, in which the lining is solid masonry. The second type was known as the double-barrel section, and is shown byFig. 116. The third type of section is shown byFig. 117. The lining consists of steel columnscarrying transverse roof girders, the roof girders being filled between with arches, and the wall columns having concrete walls between them. The wide-arch type and the double-barrel type of sections were employed in some portions of the Tremont St. line, where the traffic was very dense, since it was possible to construct them without opening the street. Much of the wide-arch line was constructed by the use of the roof shield, which is described in thesucceeding chapteron the shield system of tunneling.
Fig. 116.—Double-Barrel Section, Boston Subway.
Fig. 116.—Double-Barrel Section, Boston Subway.
—Several different methods were employed in constructing the subway. Where ample space was available, the single wide trench method of cut-and-cover construction was employed, the earth being removed as fast as excavated. In the streets, except where regular tunneling was resorted to, the parallel trench or transverse trench cut-and-cover methods were employed.
In the transverse trench method, trenches about 12 ft. widewere excavated across the street, their length being equal to the extreme transverse width of the tunnel lining, and their depth being equal to the depth of the tunnel floor. These trenches were begun during the night, and immediately roofed over with a timber platform flush with the street surface. Under these platforms the excavation was completed and the lining built. As each trench or “slice” was completed, the street above it was restored and the platform reconstructed over the succeeding trench or slice. During the construction of each slice the street traffic, including the street cars, was carried by the timber platform.
Fig. 117.—Four-Track Rectangular Section, Boston Subway.Larger illustration
Fig. 117.—Four-Track Rectangular Section, Boston Subway.
Larger illustration
Fig. 118.—Section Showing Slice Method of Construction, Boston Subway.Larger illustration
Fig. 118.—Section Showing Slice Method of Construction, Boston Subway.
Larger illustration
In the parallel trench method, short parallel trenches were dug for the opposite side walls, and also for the intermediate columns, and completely roofed over during the night. Underthis roofing the masonry of the side walls and column foundations and the columns themselves were erected. When the side walls and columns had been erected, the surface of the street between them was removed, the roof beams laid, and a platform covering erected, as shown byFig. 118. This roofing work was also done at night. The subsequent work of building the roof arches, removing the remainder of the earth, and constructing the invert, was carried on underneath the platform covering which carried the street traffic in the meantime. The successive repetition of the processes described constructed the subway.
Where the traffic was very dense on the street above, tunneling was resorted to. For small portions of this work the excavation was done in the ordinary way, using timber strutting, but much the greater portion of the tunnel work was performed by means of a roof shield. In the latter case, the side walls were first built in small bottom side drifts and were fitted with tracks on top to carry the roof shield. The construction and operation of this shield are described fully in thesucceeding chapteron the shield system of tunneling.
—The masonry of the inclined approaches to the subway consists simply of two parallel stone masonry retaining walls. In the wide-arch and double-barrel tunnel sections, the side walls are of concrete and the roof arches are of brick masonry. In the other parts of the subway the masonry consists of brick jack arches sprung between the roof beams and covered with concrete, of concrete walls embedding the side columns, and of the concrete invert and foundations for the columns.Figs. 115to118inclusive show the general details of the masonry work for each of the three sections. The inside of the lining masonry is painted throughout with white paint.
—The design and construction of the stations for the Boston Subway were made the subjects of considerable thought. All the stations consist of two island platforms of artificial stone having stairways leading to the street above.The platforms are made 1 ft. higher than the rails. The station structure itself is built of steel columns and roof beams with brick roof arches and concrete side walls. Its interior is lined with white enameled tiles. The intermediate columns are cased with wood, and have circular wooden seats at their bottoms. Each stairway is covered by a light housing, consisting of a steel framework with a copper covering and an interior wood and tile finish.
—The subway is ventilated by means of exhaust fans located in seven fan chambers, some of which contain two fans, and others only one fan. Each of the fans has a capacity of from 30,000 to 37,000 cu. ft. of air per minute, and is driven by electric motor, taking current from the trolley wires. This system of ventilation has worked satisfactorily.
—The rain water which enters the subway from the inclined entrances, together with that from leakage, is lifted from 12 ft. to 18 ft. by automatic electric pumps to the city sewers. The subway has pump-wells at the Public Garden, at Eliot St., Adams Square, and Haymarket Square. In each of these wells are two vertical submerged centrifugal pumps made entirely of composition metal. In each chamber above, are two electric motors operating the pumps. Each motor is started and stopped according to the height of water by means of a float and an automatic release starting box. The floats are so placed that only one pump is usually brought into use. The other, however, comes into service in case the first pump is out of order or the water enters more rapidly than one pump can dispose of it. In the latter case, both motors continue to run until the same low level has been reached.
Very little dampness except from atmospheric condensation is to be found on the interior walls or roof of the subway, although numerous discolored patches, caused by dampness and dust, may be seen on some parts of the walls. Substantially all of the leakage comes through the small drains in the invert leading from hollows left in the side walls. Careful measurementwas taken at the end of an unusually wet season to determine the actual amount of leakage, and the total amount for the entire subway was found to be about 81 gallons per minute.
—The estimated quantities of material used in constructing the subway were as follows:
—The estimated cost of the subway made before the work was begun was approximately $4,000,000, and the cost of construction did not exceed $3,700,000. This includes ventilating and pump chambers, changes of water and gas pipes, sewers and other structures, administration, engineering, interest on bonds, and all cost whatsoever. Dividing this number by the total length we obtain a cost per linear foot of $342.30.
—The project of an underground rapid transit railway to run the entire length of Manhattan Island was originated some years previous to 1890. In 1894, however, a Rapid Transit Commission was appointed to prepare plans for such a road, and after a large amount of trouble and delay this commission awarded the contract for construction to Mr. John B. McDonald of New York City, on Jan. 15, 1900.
—The road starts from a loop which encircles the City Hall Park. Within this loop the tunnel construction is two-track; but where the main line leaves the loop, all four tracks come to the same level, and continue side by side thereafterexcept at the points which will be noted as the description proceeds. Proceeding from the loop, the four-track line passes under Center and Elm Streets. It continues under Lafayette Place, across Astor Place and private property between Astor Place and Ninth St. to Fourth Ave. The road then passes under Fourth and Park Avenues until 42d St. is reached. At this point the line turns west along 42d St., which it follows to Broadway. It turns northward again under Broadway to the boulevard, crossing the Circle at 59th St. The road then follows the boulevard until 97th St. is reached, where the four-track line is separated into two double-track lines.
At a suitable point north of 96th St. the outside tracks rise so as to permit the inside tracks, on reaching a point near 103d St., to curve to the right, passing under the north-bound track, and to continue thence across and under private property to 104th St. From there the two-track tunnel goes under 104th St. and Central Park to 110th St., near Lenox Ave.; thence under Lenox Ave to a point near 142d St.; thence across and under private property and the intervening streets to the Harlem River. The road passes under the Harlem River and across and under private property to 149th St., which street it follows to Third Ave., and then passes under Westchester Ave., where, at a convenient point, the tracks emerge from the tunnel and are carried on a viaduct along and over Westchester Ave., Southern Boulevard, and Boston Road to Bronx Park. This portion of the line, from 96th St. to Bronx Park, is known as the East Side Line.
From the northern side of 96th St. the outside tracks rise and after crossing over the inside tracks they are brought together on a location under the center line of the street and proceed along under the boulevard to a point between 122d and 123d Streets. At this point the tracks commence to emerge from the tunnel, and are carried on a viaduct along and over the boulevard at a point between 134th and 135th Streets, where they again pass into the tunnel under and along the boulevard and Eleventh Ave. to a point about 1350 ft. north of the centerline of 190th St. There the tracks again emerge from the tunnel, and are carried on a viaduct across and over private property to Elwood St., and over and along Elwood St. to Kingsbridge St. to Kingsbridge Ave., private property, the Harlem Ship Canal and Spuyten Duyvil Creek, private property, Riverdale Ave., and Broadway to a terminus near Van Cortland Park. That portion of the line from 96th St. to the above-mentioned terminus at Van Cortland Park is known as the West Side Line.
The total length of the rapid transit road, including the parts above and below the surface ground of the streets, as well as both the East and West Side Lines, is about 221⁄2miles.
—The soil through which the road was excavated was a varied one. The lower portion of the road, or the part including the loop up to nearly Fourth St., was excavated through loose soil, but from Fourth St. to the ends it was excavated in rock. The loose soil forming the southern part of Manhattan Island is chiefly composed of clay, sand, and old rubbish—a soil very easy to excavate. Water was met at some points, but not in such quantities as to be a serious inconvenience. From Fourth St. to the ends of both the east and west side lines, the soil was chiefly composed of rock of gneissoid and mica-schistose character, these rocks prevailing nearly throughout the whole of Manhattan Island. The rock, as a rule, was not compact, but full of seams and fissures, and at many points it was found disintegrated and alternated with strata of loose soils, and even pockets of quicksand were met with along the line of the road.
—The section of the underground road is of three different types,—the rectangular, the barrel-vault, and the circular. The rectangular section.Fig. 119, is used for the greater part of the road, of which a portion is for four tracks and a portion for two tracks. The dimensions adopted for the four tracks are 50 × 13 ft., and for the double tracks 25 × 13 ft. The barrel-vault section, composed of a polycentric arch, having the flattest curve at the crown, has been adopted for the tunnelsunder Park Avenue—while the semicircular arch is used for all the other portions of the road to be tunneled. The circular section of 15-ft. diameter is used under the Harlem River, and being for single track, two parallel tunnels were built side by side.
Fig. 119.—Double-Track Section, New York Rapid Transit Railway.Larger illustration
Fig. 119.—Double-Track Section, New York Rapid Transit Railway.
Larger illustration
The main line from the City Hall loop to about 102d St. consists of four tracks built side by side in one conduit, except for that portion under the present Fourth Ave. tunnel where two parallel double-track tunnels are employed. The West Side Line will consist of double tracks laid in one conduit, except across Manhattan St. and beyond 190th St., where it is carried on an elevated structure. The East Side Line consists of a double-track tunnel driven from 102d St., and the boulevard under Central Park to 110th St. and Lenox Ave., and two parallel circular tunnels excavated under the Harlem River,—the other portions of the road being double-track, subway and elevated structure.
—Both the double-and four-track subway were built by using the different varieties of the cut-and-cover method. The single wide-trench method was used for the construction of the double-track line and also for the construction of the four-track line where the local conditions allowedit. The single narrow-trench method was used for the construction of the four-track subway at 42d St., to meet with the peculiar conditions of the traffic. Almost the total length of the four-track line of the subway was built by means of the two parallel side trenches. The slice method, so successfully employed in the Boston Subway, was used only on 42d St. west of 6th Avenue.
—The lining of the subway is of concrete, carried by a framework of steel. The floor consists of a foundation layer of concrete at least eight inches thick on good foundation, but thicker, according to conditions, where the foundation is bad. On top of this is placed another layer of concrete, with a layer of waterproofing between the two. In this top layer are set the stone pedestals for the steel columns, and the members making up the tracks.
In the four-track subway, the steel framework consists of transverse bents of columns, and I-beams spaced about five feet apart along the tunnel. The three interior columns of each bent are built-up bulb-angle and plate columns of H-section. The wall columns are I-beams, as are also the roof beams; between the I-beams, wall columns, and roof beams there is a concrete filling, so that the roof of the subway will be made up of concrete arches resting on the flanges of the I-beams of the roof. The concrete used is of one part Portland cement, two parts sand, and four parts broken stones. The double-track subway is built in the same way, except that only one column is placed between the tracks for the support of the roof.
All the concrete masonry of the roof, foundations, and side walls contains a layer of waterproofing, so as to keep perfectly dry the underground road, and prevent the percolation of water. This waterproofing is made up as follows: On the lowest stratum of concrete, whose surface is made as smooth as possible, a layer of hot asphalt is spread. On this asphalt are immediately laid sheets or rolls of felt; another layer of hot asphalt is then spreadover the felt, and then another layer of felt laid, and so on, until no less than two, and no more than six, layers of felt are laid, with the felt between layers of asphalt. On top of the upper surface of asphalt the remainder of the concrete is put in place so as to reach the required thickness of the concrete wall.
Fig. 120.—Park Avenue Deep Tunnel Construction, New York Rapid Transit Railway.
Fig. 120.—Park Avenue Deep Tunnel Construction, New York Rapid Transit Railway.
—When the distance between the roof of the proposed structure and the street was 20 ft. or over, the Standard Subway construction was replaced by tunnels. Three important tunnels have been constructed along the line of the New York Rapid Transit and these are located between 33d and 42d Streets on Park Ave., under Central Park northeast of 104th St. and under Broadway north of 152d St. The Park Ave. construction (Fig. 120) consists of two parallel double-track tunnels, located on each side of the street, and about 10 ft. below the present tunnel. The soil being composed of good rock, the tunnels were driven by a wide heading, and one bench, since no strutting was required, and the masonry lining, even of the roof, was left far behind the front of the excavation. The masonry lining consists of concrete walls and brick arches. The tunnels under Central Park and under Broadway being driven through a similar rock, the same method of excavation and the same manner of lining was used.
The tunnel under the Harlem River was driven through soft ground; and it was constructed as a submarine tunnel, according to the caisson process. The tunnels were lined with iron madeup of segments, with radial and circumferential flanges. Concrete was placed inside and flush with the flanges.
Fig. 121.—Harlem River Tunnel, New York Rapid Transit Railway.Larger illustration
Fig. 121.—Harlem River Tunnel, New York Rapid Transit Railway.
Larger illustration
The tracks, both in the subway and tunnels, are an intimate part of the concrete flooring. The rail rests on a continuous bearing of wooden blocks, laid with the grain running transversely with respect to the line of the rail, and held in place by two channel iron guard rails. The guard rails are bolted to metal cross-ties embedded in the concrete.
—A considerable portion of the double-track branch lines north of 103d St. is viaduct, or elevated structure. The viaduct construction on the West Side Line extends, including approaches, from 122d St. to very near 135th St. Of this distance, 2018 ft. 8 ins. are viaduct proper, consisting of plate girder spans carried by trestle bents at the ends, and by trestle towers for the central portion. The columns of the bents and towers are built-up bulb-angle and plate columns of H-section of the same form as those used in the bents inside the subway. The approaches proper are built of masonry. The elevated line proper consists of plate girder spans, supported on plate cross girders carried by columns.
—Many stations are built along the line. These are located on each side of the street. The entrances at the stations consist of iron framework, with glass roofs covering the descending stairways. The passageways leading down are walled with white enameled bricks and wainscoted with slabs of marble. The stations for the local trains are located on each side of the road close to the walls, since the outside tracks are reserved for the local trains, while the middle ones are reserved for the expresses. The few stations for the express trains are located between the middle and outside tracks. Stations are provided with all the conveniences required, having toilet rooms, news stands, benches, etc., and are lighted day and night by numerous arc lamps.
—The contractor completed the work in four years. No difficulty was encountered in doing this, since the great extension of the road and the great width of the avenues under which it runs allowed work all along the line at the same time. The work, briefly summarized, comprises the followingitems:—
In addition to the construction of the railway itself, it was necessary to construct or reconstruct certain sewers, and toadjust, readjust, and maintain street railway lines, water pipes, subways, and other surface and subsurface structures, and to relay street pavements.
The total cost of the work, according to the contract signed by Mr. McDonald, was $35,000,000. Dividing this amount by the total length of the road, which is 109,570 lineal feet, we have the unit cost a lineal foot $315, or a little less than unit of cost of the Boston Subway, which was $342 per lineal foot.
The road belongs to the city. The contractor acts as an agent for the city in carrying out the work, and he is the leaser of the road for fifty years. The work was paid for as soon as the various parts of the road were completed, and the money was obtained from an issue of city bonds. During the fifty years’ lease the contractor will pay the interest plus 1% of the face value of the bonds. This 1% goes to the sinking-fund, which within the fifty years at compound interest forms the total sum required for the redemption of bonds.
This first New York Subway has been extended to Brooklyn, and more lines will be built so as to form a complete underground railway system to accommodate the ever-increasing traveling crowd of the American metropolis. No new method of construction has been devised yet. The only variation from the illustrated methods has been where the subway is built underneath the Elevated Road which had to be strongly supported during the construction of the subway. This has been done in two different ways, either by supporting the columns of the Elevated Road by means of two wooden A-frames abutting at the top and leaving a large space close to the foot of the column where a pit was excavated to the required depth of the subway, or by attaching the columns to long iron girders placed longitudinally and resting with both ends in firm soil.
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 willpenetrate. 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, describedelsewhere, 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, andthe 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 inChapter 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, 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 443⁄4ft. 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]
[12]Proceedings Inst. C. E., vol. cxxi.
[12]Proceedings Inst. C. E., vol. cxxi.
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 drainageheading; 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 21⁄4ft. to 3 ft. thick, set in cement, and has an invert 11⁄2ft. 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 withconcrete, 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.
Tunnels excavated at shallow depth from the bed of the river are liable to cave in under the great weight of the water and material above the roof. Besides, the progress of the work will be greatly interfered with by the water which may reach the tunnel passing through the loose soil in large quantities. To contend with these two sources of trouble, different methods of constructing subaqueous tunnels have been devised; they are: by compressed air, by shield, and finally by a combination of these two methods, viz., by shield and compressed air.
The compressed air method was suggested by Mr. Haskin, the promoter and the first builder of the Hudson River tunnel. In 1874, when he began to sink the shaft for the construction of his tunnel, several subaqueous tunnels had already been driven by means of shields. Mr. Haskin had ideas of his own, and thought he could dispense with the shield and could trust to compressed air, since he was firmly convinced that compressed air alone could expel the water and temporarily support the roof of the excavation prior to the building of the lining masonry. In other words, he expected to substitute a core of compressed air for the core of earth removed. In the patent granted him for this method of tunneling, he expresses himself as follows: “The distinguishing feature of my system is that, instead of using temporary facings of timber or other rigid material, I rely upon the air pressure to resist the caving in of the wall and infiltration of water until the masonry wall is completed. The pressureis, of course, to be regulated by the exigencies of the occasion. The effect of such a pressure has been found to drive water in from the surface of the excavation, so that the sand becomes dry.”
The compressed air method was soon found to be inefficient, even in the construction of the Hudson tunnel where the roof of the excavation was supported by timbering in the manner indicated in the pilot system. Thus large subaqueous railway tunnels cannot be driven exclusively by the compressed air method; still it has been successfully employed in the construction of small tunnels driven for aqueduct purposes. But the use of compressed air marked a great progress in the art of submarine tunneling.
The following description of the Milwaukee Water-Works Tunnel is an example of subaqueous tunnels driven through good soil in the usual manner employed in land tunnels; but afterward when treacherous material was encountered, the work was continued by means of compressed air.
The new water supply intake tunnel for the city of Milwaukee, Wis., is one of the most difficult examples of tunnel construction which American engineering practice has afforded. The difficulties were in a large measure unexpected when the work was decided upon and put under way. The tunnel began and ended in a hard, impervious clay, practically a rock, and all the preliminary investigations led to the conclusion that the same favorable material would be encountered for its entire length. With such material a brick-lined tunnel 71⁄2ft. in diameter presented no unusual problems; but after about 1640 ft. had been excavated from the shore end the tunnel ran out of the hard clay, and for the next 600 ft. or more a variety of water-bearing material was encountered, which tried the skill and patience of the engineer to their utmost. Other difficulties were indeed met with, but these were of minor importance in comparison with that of safely and successfully penetrating the water-bearing drift.
The work of sinking the shore shafts and excavating the first 1600 ft. of tunnel did not prove especially difficult. A hard, compact, and rock-like clay, bearing very little moisture, was encountered all along, and was blasted and removed in the ordinary manner. The only mishap which occurred with this portion of the work was the destruction of the contractor’s boiler plant by fire on Jan. 12, 1891, which allowed the tunnel to fill with water, and delayed work about a month. By Oct. 21, 1891, 1640 ft. had been driven, averaging about 62⁄3ft. per day, all in the hard clay. No timbering had been necessary, and except for the first 100 ft. of the tunnel there was very little seepage. On the afternoon of Oct. 21 water was observed coming out from one of the drill holes in the heading, but no attention was paid to it. Shortly after a blast was fired, and was immediately followed by a rush of water from the heading. An unsuccessful attempt was made to check the flow, and the pumps were started; but they were unable to keep the water down, and after seven hours’ hard work the tunnel was abandoned. By the next morning the tunnel and shaft were full of water.
Several attempts were made to empty the tunnel; but the limited pumping capacity was not equal to the task, and it was finally decided to install larger pumps. The pumping had, however, shown that about 1000 gallons of water a minute was coming through the leak. With the increased pumping plant the tunnel was finally laid dry Feb. 13, 1892. Upon examination the head of the drift was found to be in the same undisturbed condition in which it was left when the water broke in three months before.
A brick bulkhead was built into the end of the brickwork of the tunnel, and provided with a timber door for passage, and two 10-in. pipes for the outlet of the water. With these openings closed, the flow was checked sufficiently to allow the placing of pumps at the bottom of the shore shaft. Meanwhile the pressure of the water against the bulkhead caused dangerousleakage, and so after the pumps were in position the 10-in. pipes were opened, relieving the pressure and allowing the water its normal rate of flow. Trouble with the pumps now arose, and after various stoppages and breaks the discharge pipe finally fell, disabling the whole plant. It became necessary to close the 10-in. pipes in the bulkhead and draw up the pumps. This allowed the tunnel to again fill with water.
After thoroughly overhauling the pumping machinery, the contractor again laid the tunnel dry on March 19; and after the pumps had been permanently placed so as to take care of the water, an examination of the work was made. It was found that the water was coming from the north, and with the hope of avoiding the difficulties of the old heading, it was decided to make a détour of the south. On April 16 work was begun at a point about 90 ft. back from the face, and deflecting the line about 38° toward the south. About 38 ft. from the angle of junction a brick bulkhead with two 8-in. openings was built into the new bore. The work progressed successfully for about 75 ft., when water was again encountered; and upon pushing forward the heading, gravel and sand came in such quantities that it was found impracticable to continue the work further. On June 1 the bulkhead was permanently closed, and the work in this direction was abandoned.
A further and closer examination was now made of the heading first abandoned. Upon breaking through the rock-like clay it was found that the water came from an underground stream flowing from the north through a well defined channel in red clay. This channel was about 13 ft. above the grade of the tunnel; and above it in every direction visible was a bed of hard, dry, red clay, while immediately in front of the face of the work was a bank of coarse gravel.Fig. 122is a sketch of the channel and stream where they entered the work. In this last drawing the photograph has been followed exactly, no particular being exaggerated in the slightest. The water from this stream was clear and pure; and a chemical analysis showedthat it was not lake water, but must come from some separate source.