CHAPTER XIX. SUBMARINE TUNNELING (Continued). THE SHIELD SYSTEM. Historical Introduction.

—The invention of the shield system of tunneling through soft ground is generally accredited to Sir Isambard Brunel, a Frenchman born in 1769, who emigrated to the United States in 1793, where he remained six years, and then went to England, in which country his epoch-making invention in tunneling was developed and successfully employed in building the first Thames tunnel, and where he died in 1849, a few years after the completion of this great work. Sir Isambard is said to have obtained the idea of employing a shield to tunnel soft ground from observing the work of ship-worms. He noticed that this little animal had a head provided with a boring apparatus with which it dug its way into the wood, and that its body threw off a secretion which lined the hole behind it and rendered it impervious to water. To duplicate this operation by mechanical means on a large enough scale to make it applicable to the construction of tunnels was the plan which occurred to the engineer; and how closely he followed his animate model may be seen by examining the drawings of his first shield, for which he secured a patent in 1818. Briefly described, this device consisted of an iron cylinder having at its front end an auger-like cutter, whose revolution was intended to shove away the material ahead and thus advance the cylinder. As the cylinder advanced the perimeter of the hole behind was to be lined with a spiral sheet-iron plating, which was to be strengthened with an interior lining of masonry. It will be seen that the mechanical resemblance of this device to the ship-worm, on which it is alleged to have been modeled, was remarkably close.

In the same patent in which Sir Isambard secured protection for his mechanical ship-worm he claimed equal rights of invention for another shield, which is of far greater importance in being the prototype of the shield actually employed by him in constructing the first Thames tunnel. This alternative invention, if it may be so termed, consisted of a group of separate cells which could be advanced one or more at a time or all together. The sides of these cells were to be provided with friction rollers to enable them to slide easily upon each other; and it was also specified that the preferable motive power for advancing the cells was hydraulic jacks. To summarize briefly, therefore, the two inventions of Brunel comprehended the protecting cylinder or shield, the closure of the face of the excavation, the cellular division, the hydraulic-jack propelling power, and cylindrical iron lining, which are the essential characteristics of the modern shield system of tunneling. The next step required was the actual proof of the practicability of Brunel’s inventions, and this soon came.

Those who have read the history of the first Thames tunnel will recall the early unsuccessful attempts at construction which had discouraged English engineers. Five years after Brunel’s patent was secured a company was formed to undertake the task again, the plan being to use the shield system, under the personal direction of its inventor as chief engineer. For this work Brunel selected the cellular shield mentioned as an alternative construction in his original patent. He also chose to make this shield rectangular in form. This choice is commonly accounted for by the fact that the strata to be penetrated by the tunnel were practically horizontal, and that it was assumed by the engineer that a rectangular shield would for some reason best resist the pressures which would be developed. Whatever the reason may have been for the choice, the fact remains that a rectangular shield was adopted. The tunnel as designed consisted of two parallel horseshoe tunnels, 13 ft. 9 ins. wide and 16 ft. 4 ins. high and 1200 ft. long, separated from each other by a wall 4 ft. thick, pierced by 64 arched openings of 4 ft. span, the whole being surrounded with massive brickwork built to a rectangular section measuring over all 38 ft. wide and 22 ft. high.

The first shield designed by Brunel for the work proved inadequate to resist the pressures, and it was replaced by another somewhat larger shield of substantially the same design, but of improved construction. This last shield was 22 ft. 3 ins. high and 37 ft. 6 ins. wide. It was divided vertically into twelve separate cast-iron frames placed close side by side, and each frame was divided horizontally into three cells capable of separate movement, but connected by a peculiar articulated construction, which is indicated in a general way by Fig. 124. To close or cover the face of the excavation, poling-boards held in place by numerous small screw-jacks were employed. Each cell or each frame could be advanced independently of the others, the power for this operation being obtained by means of screw-jacks abutting against the completed masonry lining. Briefly described, the mode of procedure was to remove the poling-boards in front of the top cell of one frame, and excavate the material ahead for about 6 ins. This being done, the top cell was advanced 6 ins. by means of the screw-jacks, and the poling-boards were replaced. The middle cell of the frame was then advanced 6 ins. by repeating the same process, and finally the operation was duplicated for the bottom cell. With the advance of the bottom cell one frame had been pushed ahead 6 ins., and by a succession of such operations the other eleven frames were advanced a distance of 6 ins., one after the other, until the whole shield occupied a position 6 ins. in advance of that at which work was begun. The next step was to fill the 6-in. space behind the shield with a ring of brickwork.

The illustration, Fig. 124, is the section parallel to the vertical plane of the tunnel through the center of one of the frames, and it shows quite clearly the complicated details of the shield construction. Two features which are to be particularly noted are the suspended staging and centering for constructing the roof arch, and the top plate of the shield extending back and overlapping the roof masonry so as to close completely the roof of the excavation and prevent its falling. Notwithstanding its complicated construction and unwieldy weight of 120 tons, this shield worked successfully, and during several months the construction proceeded at the rate of 2 ft. every 24 hours. There were two irruptions of water and mud from the river during the work, but the apertures were effectually stopped by heaving bags of clay into the holes in the river bed, and covering them over with tarpaulin, with a layer of gravel over all. The tunnel was completed in 1843, at a cost of about $5600 per lineal yard, and 20 years from the time work was first commenced, including all delays.

The next tunnel to be built by the shield system was the tunnel under London Tower constructed by Barlow and Greathead and begun in 1869. In 1863 Mr. Peter W. Barlow secured a patent in England for a system of tunnel construction comprising the use of a circular shield and a cylindrical cast-iron lining. The shield, as shown by Fig. 125, was simply an iron or steel plate cylinder. The cylinder plates were thinned down in front to form a cutting edge, and they extended far enough back at the rear to enable the advance ring of the cast-iron lining to be set up within the cylinder. In simplicity of form this shield was much superior to Brunel’s; but it seems very doubtful, since it had no diametrical bracing of any sort, whether it would ever have withstood the combined pressure of the screw-jacks and of the surrounding earth in actual operation without serious distortion, and, probably, total collapse. It should also be noted that Barlow’s shield made no provision for protecting the face of the excavation, although the inventor did state that if the soil made it necessary such a protection could be used. The patent provided for the injection of liquid cement behind the cast-iron lining to fill the annular space left by the advancing tail-plates of the shield. Although Barlow made vigorous efforts to get his shield used, it was not until 1868 that an opportunity presented itself. In the meantime the inventor had been studying how to improve his original device, and in 1868 he secured additional patents covering these improvements. Briefly described, they consisted in partly closing the shield with a diaphragm as shown by Fig. 126. The uninclosed portion of the shield is here shown at the center, but the patent specified that it might also be located below the center in the bottom part of the shield. The idea of the construction was that in case of an irruption of water the upper portion of the shield could be kept open by air pressure, and work prosecuted in this open space until the shield had been driven ahead sufficiently to close the aperture, when the normal condition of affairs would be resumed. This was obviously an improvement of real merit. The partial diaphragm also served to stiffen the shield somewhat against collapse, but the thin plate cutting-edges and most of the other structural weaknesses were left unaltered. To summarize briefly the improvements due to Barlow’s work, we have: the construction of the shield in a single piece; the use of compressed air and a partial diaphragm for keeping the upper part of the shield open in case of irruptions of water; and the injection of liquid cement to fill the voids behind the lining.

Turning now to the London Tower tunnel work, it may first be noted that Barlow found some difficulty in finding a contractor who was willing to undertake the job, so little confidence had engineers generally in his shield system. One man, however, Mr. J. H. Greathead, perceived that Barlow’s device presented merit, although its design and construction were defective, and he finally undertook the work and carried it to a brilliant success. The tunnel was 1350 ft. long and 7 ft. in diameter, and penetrated compact clay. Work was begun on the first shore shaft on Feb. 12, 1869, and the tunnel was completed the following Christmas, or in something short of eleven months, at a cost of £14,500.

The shield used was Barlow’s idea put into practical shape by Greathead. It consisted of an iron cylinder, or, more properly, a frustum of a cone whose circumferential sides were very slightly inclined to the axis, the idea being that the friction would be less if the front end of the shield were slightly larger than the rear end. The shell of the cone was made of ¹/₂-in. plates. The thinned plate cutting-edge of Barlow’s shield was replaced by Greathead with a circular ring of cast iron. Greathead also altered the construction of the diaphragm by arranging the angle stiffeners so that they ran horizontally and vertically, and by fastening the diaphragm plates to an interior cast-iron ring connected to the shell plates. This was a decided structural improvement, but it was accompanied with another modification which was quite as decided a retrogression from Barlow’s design. Greathead made the diaphragm opening rectangular and to extend very nearly from the top to the bottom of the shield, thus abandoning the element of safety provided by Barlow in case of an irruption of water. Fortunately the material penetrated by the shield for the Tower tunnel was so compact that no trouble was had from water; but the dangerous character of the construction was some years afterwards disastrously proven in driving the Yarra River tunnel at Melbourne, Australia. To drive his shield Greathead employed six 2¹/₂-in. screw-jacks capable of developing a total force of 60 tons. The tails of the jack bore against the completed lining, which consisted of cast-iron rings 18 ins. wide and ⁷/₈ in. thick, each ring being made up of a crown piece and three segments. The different segments and rings were provided with double (exterior and interior) flanges, by means of which they were bolted together. The soil behind the lining was filled with liquid cement injected through small holes by means of a hand pump.

The remarkable success of the London Tower tunnel encouraged Barlow to form in 1871 a company to tunnel the Thames between Southwark and the City, and Greathead, in 1876, to project a tunnel under the same waterway known as the North and South Woolwich Subway. Barlow’s concession was abrogated by Parliament in 1873, without any work having been done. Greathead progressed far enough with his enterprise to construct a shield and a large amount of the iron lining when the contractors abandoned the work. From the brief description of his shield given by Greathead to the London Society of Civil Engineers, it contained several important differences from the shield built by him for the London Tower tunnel, as is shown by Fig. 127. The changes which deserve particular notice are the great extension of the shield behind the diaphragm, the curved form of the diaphragm, and the use of hydraulic jacks. Greathead had also designed for this work a special crane to be used in erecting the cast-iron segments of the lining.

While these works had been progressing in England, Mr. Beach, an American, received a patent in the United States for a tunnel shield of the construction shown by Fig. 128, which was first tried practically in constructing a short length of tunnel under Broadway for the nearly forgotten Broadway Pneumatic Underground Railway. This shield, as is indicated by the illustration, consisted of a cylinder of wood with an iron-cutting-edge and an iron tail-ring. Extending transversely across the shield at the front end were a number of horizontal iron plates or shelves with cutting-edges, as shown clearly by the drawing. The shield was moved ahead by means of a number of hydraulic jacks supplied with power by a hand pump attached to the shield. By means of suitable valves all or any lesser number of these jacks could be operated, and by thus regulating the action of the motive power the direction of[247]
[248] the shield could be altered at will. Work was abandoned on the Broadway tunnel in 1870. In 1871-2 Beach’s shield was used in building a short circular tunnel 8 ft. in diameter in Cincinnati, and a little later it was introduced into the Cleveland water-works tunnel 8 ft. in diameter. In this latter work, which was through a very treacherous soil, the shield gave a great deal of trouble, and was finally so flattened by the pressures that it was abandoned. The obviously defective features of this shield were its want of vertical bracing and the lack of any means of closing the front in soft soil.

With the foregoing brief review of the early development of the shield system of tunneling, we have arrived at a point where the methods of modern practice can be studied intelligently. In the pages which follow we shall first illustrate fully the construction of a number of shields of typical and special construction, and follow these illustrations with a general discussion of present practice in the various details of shield construction.

Mr. Raynald LÉgouez, in his excellent book upon the shield system of tunneling, considers that tunnel shields may be divided into three classes structurally, according to the character of the material which they are designed to penetrate. In the first class he places shields designed to work in a stiff and comparatively stable soil, like the well-known London clay; in the second class are placed those constructed to work in soft clays and silts; and in the third class those intended for soils of an unstable granular nature. This classification will, in a general way, be kept by the writer. As a representative shield of the first class, the one designed for the City and South London Railway is illustrated in Fig. 129. The shields for the London Tower tunnel, the Waterloo and City Railway, the Glasgow District Subway, the Siphons of Clichy and Concorde in Paris, and the Glasgow Port tunnel, are of the same general design and construction. To represent shields of the second class, the St. Clair River and Blackwall shields are shown in Figs. 130 and 131. The shields for the Mersey River, the Hudson River, and the East River tunnels also belong to this class. To represent shields of the third class, the elliptical and semi-elliptical shields of the Clichy tunnel work in Paris are shown by Figs. 132 and 133. The semi-circular shield of the Boston Subway is illustrated by Fig. 134.

Prelini’s Shield.

—In closing this short review mention will be made of a new shield designed and patented by the Author and shown in Fig. 135. It is an articulated shield composed of two separated shields whose outer shells overlap each other. The shields are connected together by means of hydraulic jacks attached all around the two diaphragms. Between these diaphragms is a large inclosed space called a safety chamber, where the men can withdraw in case of accidents and where the air can be immediately raised to the required pressure. This is an advantage in case of blow-outs, because the flooding of the tunnel is prevented, while the accident is limited to only a few feet from the front. On account of the shield being advanced half at a time it is always under control and is thus better directed through grade and alignment. Besides, this shield will not rotate around its axis and consequently it can be built of any shape, thus permitting the construction of subaqueous tunnels of any cross-section and even with a wider foundation, which is impossible to-day with the ordinary rotating shields of circular cross-section.

SHIELD CONSTRUCTION.

General Form.

—Tunnel shields are usually cylindrical or semi-cylindrical in cross-section. The cylinder may be circular, elliptical, or oval in section. Far the greater number of shields used in the past have been circular cylinders; but in one part of the sewer tunnel of Clichy, in Paris, an elliptical shield with its major axis horizontal, was used, and the German engineer, Herr Mackensen, has designed an oval shield, with its major axis vertical. A semi-elliptical shield was employed on the Clichy tunnel, and semi-circular shields were used on the Baltimore Belt Line tunnel and the Boston Subway in America. Generally, also, tunnel shields are right cylinders; that is, the front and rear edges are in vertical planes perpendicular to the axis of the cylinder. Occasionally, however, they are oblique cylinders; that is, the front or rear edges, or both, are in planes oblique to the axis of the cylinder. One of these visor-shaped shields was employed on the Clichy tunnel.

The Shell.

—It is absolutely necessary that the exterior surface of the shell should be smooth, and for this reason the exterior rivet heads must be countersunk. It is generally admitted, also, that the shell should be perfectly cylindrical, and not conical. The conical form has some advantage in reducing the frictional resistance to the advance of the shield; but this is generally considered to be more than counterbalanced by the danger of subsidence of the earth, caused by the excessive void which it leaves behind the iron tunnel lining. For the same reason the shell plate, which overlaps the forward ring of the lining, should be as thin as practicable, but its thickness should not be reduced so that it will deflect under the earth pressure from above. Generally the shell is made of at least two thicknesses of plating, the plates being arranged so as to break joints, and, thus, to avoid the use of cover joints, to interrupt the smooth surface which is so essential, particularly on the exterior. The thickness of the shell required will vary with the diameter of the shield, and the character and strength of the diametrical bracing. Mr. Raynald LÉgouez suggests as a rule for determining the thickness of the shell, that, to a minimum thickness of 2 mm., should be added 1 mm. for every meter of diameter over 4 meters. Referring to the illustrations, Figs. 128 to 132 inclusive, it will be noted that the St. Clair tunnel shield, 21¹/₂ ft. in diameter, had a shell of 1-in. steel plates with cover-plate joints and interior angle stiffeners; the shell of the East River tunnel shield, 11 ft. in diameter, was made up of one ¹/₂-in. and one ³/₈-in. plate; the Blackwall tunnel shield, 27 ft. 9 ins. in diameter, had a shell consisting of four thicknesses of ⁵/₈-in. plates; and the Clichy tunnel shield, with a diameter of 2.06 meters, had a shell 2 millimeters thick.

Front-End Construction.

—By the front end is meant that portion of the shield between the cutting-edge and the vertical diaphragm. The length of this portion of the shield was formerly made quite small, and where the material penetrated is very soft, a short front-end construction yet has many advocates; but the general tendency now is to extend the cutting-edge far enough ahead of the diaphragm to form a fair-sized working chamber. Excavation is far more easy and rapid when the face can be attacked directly from in front of the diaphragm than where the work has to be done from behind through the apertures in the diaphragm. So long as the roof of the excavation is supported from falling, experience has shown that it is easily possible to extend the excavation safely some distance ahead of the diaphragm. In reasonably stable material, like compact-clay, the front face will usually stand alone for the short time necessary to excavate the section and advance the shield one stage. In softer material the face can usually be sustained for the same short period by means of compressed air; or the face of the excavation, instead of being made vertical, can be allowed to assume its natural slope. In the latter case a visor-shaped front-end construction, such as was used on some portions of the Clichy tunnel, is particularly advantageous. The following figures show the lengths of the front ends of a number of representative tunnel shields.

City and South London	1		ft.
St. Clair River	11	.25	„
Hudson River	5	2/3	„
Mersey River	3		„
East River	3	2/3	„
Blackwall	6	.5	„

Two general types of construction are employed for the cutting-edge. The first type consists of a cast-iron or cast-steel ring, beveled to form a chisel-like cutting-edge and bolted to the ends of the forward shell plates. This construction was first employed in the shield for the London Tower tunnel, and has since been used on the City and South London, Waterloo and City, and the Clichy tunnels. The second construction consists in bracing the forward shell plates by means of right triangular brackets, whose perpendicular sides are riveted respectively to the shell plates and the diaphragm, and whose inclined sides slant backward and downward from the front edge, and carry a conical ring of plating. The shields for the St. Clair River, East River, and Blackwall tunnels show forms of this type of cutting-edge construction. A modification of the second type of construction, which consists in omitting the conical plating, was employed on some of the shields for the Clichy tunnel. This modification is generally considered to be allowable only in materials which have little stability, and which crumble down before the advance of the cutting-edge. Where the material is of a sticky or compact nature, into which the shield in advancing must actually cut, the beveled plating is necessary to insure a clean cutting action without wedging or jamming of the material.

Cellular Division.

—It is necessary in shields of large diameter to brace the shell horizontally and vertically against distortion. This bracing also serves to form stagings for the workmen, and to divide the shield into cells. The following table shows the arrangement of the vertical and transverse bracing in several representative tunnel shields.

Name of Tunnel.	Diameter.			Hori- zontal.	Plates, Dist. Apart.	Vert. Braces.
	Ft.	In.		No.	Ft.	No.
Hudson River	19	11		2	6.54	2
Clichy	19.4	0		2	6.54	None
St. Clair River	21	6		2	6.98	3
Waterloo (Station)	24	10	1/2	2	7.12	None
Blackwall	27	8		2	6.0	3
East River	11		3/4	None	...	1

Referring first to the horizontal divisions, it may be noted that they serve different purposes in different instances. In the Clichy tunnel shield the horizontal divisions formed simply working platforms; in the Waterloo tunnel shield they were designed to abut closely against the working face by means of special jacks, and so to divide it into three separate divisions; in the St. Clair tunnel they served as working platforms, and also had cutting-edges for penetrating the material ahead; and in the Blackwall tunnel shield they served as working platforms, and had cutting-edges as in the St. Clair tunnel shield, and in addition the middle division was so devised that the two lower chambers of the shield could be kept under a higher pressure of air than the two upper chambers. Passing now to the vertical divisions, they serve to brace the shell of the shield against vertical pressures, and also to divide the horizontal chambers into cells; but unlike the horizontal plates they are not provided with cutting-edges. The St. Clair, Hudson River, and Blackwall tunnel shields illustrate the use of the vertical bracing for the double purpose of vertical bracing and of dividing the horizontal chambers into cells. The Waterloo tunnel shield is an example, of vertical bracing employed solely as bracing. The vertical division of the East River tunnel shield was employed in order to allow the shield to be dissembled in quadrants.

The Diaphragm.

—The purpose of the shield diaphragm is to close the rear end of the shield and the tunnel behind from an inrush of water and earth from the face of the excavation. It also serves the secondary purpose of stiffening the shell diametrically. Structurally the diaphragm separates the front-end construction previously described from the rear-end construction, which will be described farther on; and it is usually composed of iron or steel plating reinforced by beams or girders, and pierced with one or several openings by which access is had to the working face. In stable material, where caving or an inrush of water and earth is not likely, the diaphragm is omitted. The shield of the Waterloo tunnel is an example of this construction. In more treacherous materials, however, not only is a diaphragm necessary, but it is also necessary to diminish the size of the openings through it, and to provide means for closing them entirely. Sometimes only one or two openings are left near the bottom of the diaphragm, as in the St. Clair and Mersey tunnel shields; and sometimes a number of smaller openings are provided, as in the East River and Hudson River tunnel shields.

In highly treacherous materials subject to sudden and violent irruptions of earth from the excavation face, it sometimes is the case that openings, however small, closed in the ordinary manner, are impracticable, and special construction has to be adopted to deal with the difficulty. The shields for the Mersey and for the Blackwall tunnels are examples of such special devices. In the Mersey tunnel a second diaphragm was built behind the first, extending from the bottom of the shield upward to about half its total height. The aperture in the first diaphragm being near the bottom, the space between the second and first diaphragms formed a trap to hold the inflowing material. The Blackwall tunnel shield, as previously indicated, had its front end divided into cells. Ordinarily the face of the excavation in front of each cell was left open, but where material was encountered which irrupted into these cells a special means of closing the face was necessary. This consisted of three poling-boards or shutters of iron held one above the other against the face of the excavation. These shutters were supported by means of strong threaded rods passing through nuts fastened to the vertical frames, which permitted each shutter to be advanced against or withdrawn from the face of the excavation independently of the others. Various other constructions have been devised to retain the face of the excavation in highly treacherous soils, but few of them have been subjected to conclusive tests, and they do not therefore justify consideration.

Rear-end Construction.

—By the rear end of the shield is meant that portion at the rear of the diaphragm. It may be divided into two parts, called respectively the body and the tail of the shield. The chief purpose of the body of the shield is to furnish a place for the location of the jacks, pumps, motors, etc., employed in manipulating the shield. It also serves a purpose in distributing the weight of the shield over a large area. To facilitate the passage of the shield around curves, or in changing from one grade to another, it is desirable to make the body of the shield as short as possible. In the Mersey, Clichy, and Waterloo tunnel shields, and, in fact, in most others which have been employed, the shell plates of the body have been reinforced by a heavy cast-iron ring, within and to which are attached the jacks and other apparatus. The latest opinion, however, seems to point to the use of brackets and beams for strengthening the shell for the purpose named, rather than to this heavy cast-iron construction. In the Hudson River, St. Clair River, and East River tunnel shields, with their long and strongly braced front-end construction to carry the jacks, the body of the shield, so to speak, is omitted and the rear-end construction consists simply of the tail plating. In the Blackwall shield, the body of the shield shell provides the space necessary for the double diaphragms and the cells which they inclose. In a general way, it may be said that the present tendency of engineers is to favor as short and as light a body construction as can be secured.

The tail of the shield serves to support the earth while the lining is being erected; and for this reason it overlaps the forward ring of the lining, as shown clearly by most of the shields illustrated. To fulfill this purpose, the tail-plates should be perfectly smooth inside and outside, so as to slide easily between the outside of the lining plates and the earth, and should also be as thin as practicable, in order not to leave a large void behind the lining to be filled in. In soils which are fairly stable, the tail construction is often visor-shaped; that is, the tail-plates overlap the lining only for, say, the roof from the springing lines up, as in one of the shields for the Clichy tunnel. In unstable materials the tail-plating extends entirely around the shield and excavation. The length of the tail-plating is usually sufficient to overlap two rings of the lining, but in one of the Clichy tunnel shields it will be noticed that it extended over three rings of lining. This seemingly considerable space for thin steel plates is made possible by the fact that the extreme rear end of the tail always rests upon the last completed ring of lining.

In closing these remarks concerning the rear-end construction, the accompanying table, prepared by Mr. Raynald LÉgouez, will be of interest, as a general summary of principal dimensions of most of the important tunnel shields which have been built. The figures in this table have been converted from metric to English measure, and some slight variation from the exact dimensions necessarily exists. The different columns of the table show the diameter, total length, and the length of each of the three principal parts into which tunnel shields are ordinarily divided in construction as previously described:

Name of Shield.	Length in Feet.
	Diameter.	Tail.	Body.	Front.	Total.
Concorde Siphon	6.75	2.51	2.55	1.16	6.67
Clichy Siphon	8.39	2.51	2.55	1.16	6.16
Mersey	9.97	5.61	2.98	2.98	11.58
East River	10.99	3.51	0.32	3.67	7.51
City and South London	10.99	2.65	2.82	1.01	6.49
Glasgow District	12.07	2.65	2.82	1.01	6.49
Waterloo and City	12.99	2.75	2.98	1.24	6.98
Glasgow Harbor	17.25	2.75	2.98	1.08	8.49
Hudson River	19.91	4.82	2.98	5.67	10.49
St. Clair River	21.52	4.00	2.98	11.25	15.25
Clichy Tunnel	23.7-19.8	4.00	2.98	6.88	17.22
Clichy Tunnel	23.8-19.4	7.44	11.90	4.46	23.65
Blackwall	27.00	6.98	5.90	6.59	19.48
Waterloo Station	24.86	3.34	5.51	1.14	10.00

A shield of 60 or 100 tons weight can hardly be directed along the line of the proposed tunnel and also through curves and grades, especially when driven through loose or muddy soils. The tunnels of the New York and Hudson River Railroad under the Hudson, and the tunnel of the New York Rapid Transit Railway under the East River, show marked evidence of how troublesome this work is. To avoid these and other inconveniences encountered in every shield, the Author has designed a new shield which was briefly described at page 251.

Fig. 136.—Elevation and Section of Hydraulic Jack, East River Gas Tunnel.