Tunnel Engineering. A Museum Treatment Part 2

The Thames Tunnel was used for foot and light highway traffic until about 1870 when it was incorporated into the London Underground railway system, which it continues to serve today. The roofed-over top sections of the two shafts may still be seen from the river.

A number of contemporary popular accounts of the tunnel exist, but one of the most thorough and interesting expositions on a single tunnel work of any period is Henry Law's _A Memoir of the Thames Tunnel_, published in 1845-1846 by John Weale. Law, an eminent civil engineer, covers the work in incredible detail from its inception until the major suspension in late 1828 when slightly more than half completed.

The most valuable aspect of his record is a series of plates of engineering drawings of the shield and its components, which, so far as is known, exist nowhere else. These formed the basis of the enlarged section of the shield, shown to the right of the model of the tunnel itself. A vertical section through the shield is reproduced here from Law for comparison with the model (figs. 21 and 23).

[Ill.u.s.tration: Figure 17.--SOFT-GROUND TUNNELING. The support of walls and roof of mine shaft by simple timbering; 16th century.

MHT model--3/4" scale. (Smithsonian photo 49260-J.)]

[Ill.u.s.tration: Figure 18.--SOFT-GROUND TUNNELING. The model of a 16th century mine in the Museum of History and Technology was constructed from ill.u.s.trations in such works as G. E. von Lohneyss' _Bericht vom Bergwerck_, 1690, as well as the better known ones from _De re Metallica_.]

[Ill.u.s.tration: Figure 19.--THE SUCCESSIVE STAGES in the enlargement of a mid-19th century railroad tunnel, using the Austrian system of timbering. MHT model.]

[Ill.u.s.tration: Figure 20.--M. I. BRUNEL'S THAMES TUNNEL, 1825-1843, the first driven beneath a body of water. MHT model--1/4" scale.

(Smithsonian photo 49260-F.)]

THE TOWER SUBWAY

Various inventors attempted to improve upon the Brunel shield, aware of the fundamental soundness of the shield principle. Almost all bypa.s.sed the rectangular sectional construction used in the Thames Tunnel, and took as a starting point a sectional shield of circular cross section, advanced by Brunel in his original patent of 1818.

James Henry Greathead (1844-1896), rightfully called the father of modern subaqueous tunneling, surmised in later years that Brunel had chosen a rectangular configuration for actual use, as one better adapted to the sectional type of shield. The English civil engineer, Peter W. Barlow, in 1864 and 1868 patented a circular shield, of one piece, which was the basis of one used by him in constructing a small subway of 1350 feet beneath the Thames in 1869, the first work to follow the lead of Brunel. Greathead, acting as Barlow's contractor, was the designer of the shield actually used in the work, but it was obviously inspired by Barlow's patents.

The reduction of the multiplicity of parts in the Brunel shield to a single rigid unit was of immense advantage and an advance perhaps equal to the shield concept of tunneling itself. The Barlow-Greathead shield was like the cap of a telescope with a sharpened circular ring on the front to a.s.sist in penetrating the ground. The diaphragm functioned, as did Brunel's breasting boards, to resist the longitudinal earth pressure of the face, and the cylindrical portion behind the diaphragm bore the radial pressure of roof and walls. Here also for the first time, a permanent lining formed of cast-iron segments was used, a second major advancement in soft-ground tunneling practice. Not only could the segments be placed and bolted together far more rapidly than masonry lining could be laid up, but unlike the green masonry, they could immediately bear the full force of the shield-propelling screws.

Barlow, capitalizing on Brunel's error in burrowing so close to the riverbed, maintained an average cover of 30 feet over the tunnel, driving through a solid stratum of firm London clay which was virtually impervious to water. As the result of this, combined with the advantages of the solid shield and the rapidly placed iron lining, the work moved forward at a pace and with a facility in startling contrast to that of the Thames Tunnel, although in fairness it must be recalled that the face area was far less.

The clay was found sufficiently sound that it could be readily excavated without the support of the diaphragm, and normally three miners worked in front of the shield, digging out the clay and pa.s.sing it back through a doorway in the plate. This could be closed in case of a sudden settlement or break in. Following excavation, the shield was advanced 18 inches into the excavated area by means of 6 screws, and a ring of lining segments 18 inches in length bolted to the previous ring under cover of the overlapping rear skirt of the shield.

The small annular s.p.a.ce left between the outside of the lining and the clay by the thickness and clearance of the skirt--about an inch--was filled with thin cement grout. The tunnel was advanced 18 inches during each 8-hour shift. The work continued around the clock, and the 900-foot river section was completed in only 14 weeks.[4] The entire work was completed almost without incident in just under a year, a remarkable performance for the world's second subaqueous tunnel.

[Ill.u.s.tration: Figure 21.--ENLARGED DETAIL of Brunel's tunneling shield, vertical section. The first two and part of the third of the twelve frames are shown. To the left is the tunnel's completed brick lining and to the right, the individual breasting boards and screws for supporting the face. The propelling screws are seen at top and bottom, bearing against the lining. Three miners worked in each frame, one above the other. MHT model--3/4" scale. (Smithsonian photo 49260-G.)]

[Ill.u.s.tration: Figure 22.--BROADSIDE PUBLISHED AFTER COMMENCEMENT OF WORK on the Thames Tunnel, 1827. (MHT collections.)

OPEN TO THE PUBLIC EVERY DAY (_Sundays excepted_) _from Seven in the Morning, until Eight in the Evening_,

THE THAMES TUNNEL.

Fig. 1 shows a transverse section of the Thames, and beneath it a longitudinal section of the Tunnel, as it will be when completed; with the ascents in the inclinations in which they will be finished.

Fig. 2 shows the two arched entrances of the Tunnel from the shaft.

Fig. 3 is a representation of the iron shield, and shows a workman in each of the compartments.

The Entrance to the Tunnel is near to Rotherhithe Church, and nearly opposite to the London-Docks. The nearest landing place from the river is Church Stairs. The Greenwich and Deptford coaches which go the lower road, start hourly from Charing-cross, and Gracechurch-street, and pa.s.s close by the works at Rotherhithe.

Books relative to the Tunnel may be had at the works.

The Public may view the Tunnel every day (Sundays excepted) from Seven in the morning until Eight in the Evening, upon payment of One Shilling each Person.

The extreme northern end of the Tunnel is for the present secured by a strong wall; but visitors will find a dry, warm, and gravelled promenade, as far as to almost the centre of the river, and brilliantly lighted with oil gas.

The entrance is from Rotherhithe Street, and by a safe, commodious, and easy stair case.

H. Teape & Son, Printers, Tower-hill, London.]

[Ill.u.s.tration: Figure 23.--VERTICAL SECTION THROUGH BRUNEL'S SHIELD.

The long lever, x, supported the wood centering for turning the masonry arches of the lining. (LAW, _A Memoir of the Thames Tunnel._)]

[Ill.u.s.tration: Figure 24.--THAMES TUNNEL. SECTION THROUGH riverbed and tunnel following one of the break-throughs of the river. Inspection of the damage with a diving bell. (BEAMISH, _A Memoir of the Life of Sir Marc Isambard Brunel_.)]

The Tower Subway at first operated with cylindrical cars that nearly filled the 7-foot bore; the cars were drawn by cables powered by small steam engines in the shafts. This mode of power had previously been used in pa.s.senger service only on the Greenwich Street elevated railway in New York. Later the cars were abandoned as unprofitable and the tunnel turned into a footway (fig. 32). This small tunnel, the successful driving due entirely to Greathead's skill, was the forerunner of the modern subaqueous tunnel. In it, two of the three elements essential to such work thereafter were first applied: the one-piece movable shield of circular section, and the segmental cast-iron lining.

The doc.u.mentation of this work is far thinner than for the Thames Tunnel. The most accurate source of technical information is a brief historical account in Copperthwaite's cla.s.sic _Tunnel Shields and the Use of Compressed Air in Subaqueous Works_, published in 1906.

Copperthwaite, a successful tunnel engineer, laments the fact that he was able to turn up no drawing or original data on this first shield of Greathead's, but he presents a sketch of it prepared in the Greathead office in 1895, which is presumably a fair representation (fig. 33). The Tower Subway model was built on the basis of this and several woodcuts of the working area that appeared contemporaneously in the ill.u.s.trated press. In this and the adjacent model of Beach's Broadway Subway, the tunnel axis has been placed on an angle to the viewer, projecting the bore into the case so that the complete circle of the working face is included for a more suggestive effect. This was possible because of the short length of the work included.

Henry S. Drinker, also a tunnel engineer and author of the most comprehensive work on tunneling ever published, treats rock tunneling in exhaustive detail up to 1878. His notice of what he terms "submarine tunneling" is extremely brief. He does, however, draw a most interesting comparison between the first Thames Tunnel, built by Brunel, and the second, built by Greathead 26 years later:

FIRST THAMES TUNNEL SECOND THAMES TUNNEL (TOWER SUBWAY)

Brickwork lining, 38 feet Cast-iron lining of 8 feet wide by 22-1/2 feet high. outside diameter.

120-ton cast-iron shield, 2-1/2-ton, wrought-iron shield, accommodating 36 miners. accommodating at most 3 men.

Workings filled by irruption "Water encountered at almost of river five times. any time could have been gathered in a stable pail."

Eighteen years elapsed between Work completed in about start and finish of work. eleven months.

Cost: $3,000,000. Cost: $100,000.

[Ill.u.s.tration: Figure 25.--TRANSVERSE SECTION THROUGH SHIELD, after inundation. Such disasters, as well as the inconsistency of the riverbed's composition, seriously disturbed the alignment of the shield's individual sections. (LAW, _A Memoir of the Thames Tunnel_.)]

[Ill.u.s.tration: Figure 26.--LONGITUDINAL SECTION THROUGH THAMES TUNNEL after sandbagging to close a break in the riverbed. The tunnel is filled with silt and water. (LAW, _A Memoir of the Thames Tunnel_.)]

[Ill.u.s.tration: Figure 27.--INTERIOR OF THE THAMES TUNNEL shortly after completion in 1843. (_Photo courtesy of New York Public Library Picture Collection._)]

[Ill.u.s.tration: Figure 28.--THAMES TUNNEL in use by London Underground railway. (_Ill.u.s.trated London News_, 1869?)]

[Ill.u.s.tration: Figure 29.--PLACING A segment of cast-iron lining in Greathead's Tower Subway, 1869. To the rear is the shield's diaphragm or bulkhead. MHT model--1-1/2" scale. (Smithsonian photo 49260-B.)]

BEACH'S BROADWAY SUBWAY

Almost simultaneously with the construction of the Tower Subway, the first American shield tunnel was driven by Alfred Ely Beach (1826-1896). Beach, as editor of the _Scientific American_ and inventor of, among other things, a successful typewriter as early as 1856, was well known and respected in technical circles. He was not a civil engineer, but had become concerned with New York's pressing traffic problem (even then) and as a solution, developed plans for a rapid-transit subway to extend the length of Broadway. He invented a shield as an adjunct to this system, solely to permit driving of the tunnel without disturbing the overlying streets.

An active patent attorney as well, Beach must certainly have known of and studied the existing patents for tunneling shields, which were, without exception, British. In certain aspects his shield resembled the one patented by Barlow in 1864, but never built. However, work on the Beach tunnel started in 1869, so close in time to that on the Tower Subway, that it is unlikely that there was any influence from that source. Beach had himself patented a shield, in June 1869, a two-piece, sectional design that bore no resemblance to the one used.

His subway plan had been first introduced at the 1867 fair of the American Inst.i.tute in the form of a short plywood tube through which a small, close-fitting car was blown by a fan. The car carried 12 pa.s.sengers. Sensing opposition to the subway scheme from Tammany, in 1868 Beach obtained a charter to place a small tube beneath Broadway for transporting mail and small packages pneumatically, a plan he advocated independently of the pa.s.senger subway.

[Ill.u.s.tration: Figure 30.--CONTEMPORARY ILl.u.s.tRATIONS of Tower Subway works used as basis of the model in the Museum of History and Technology. (_Ill.u.s.trated London News_, 1869.)

ADVANCING THE SHIELD. FITTING THE CASTINGS.]