348 



NATURE 



[August 9, 1906 



be recommended. The author will find it easier and 

 more profitable to treat each type of motor separately, 

 and then to point out the differences between the 

 various types, than to try and establish diagrams and 

 formula which will meet all cases. 



Val. a. Fynn. 



SUBAQUEOUS TUNNELLING. 

 Tunnel Shields, and the Use of Compressed Air in 

 Subaqueous Works. By W. C. Copperthwaite. 

 Pp. XV + 390. (London : .Archibald Constable and 

 Co., Ltd., 1906.) Price 31X. 6d. net. 



THIS fine quarto volume furnishes a very valuable 

 and comprehensive history of a system of 

 tunnelling, especially under rivers and in water-bear- 

 ing strata, which was inaugurated by Sir Marc 

 Isambard Brunei, as regards the employment of a 

 shield, in the celebrated Thames Tunnel between 

 Rotherhithe and Wapping, commenced in 1825. but, 

 owing to the inrush of the river into the works on 

 two occasions through breaks in the stratum of clay, 

 and financial difficulties, only completed in 1843. 



The second important step in the developinent of 

 the system in a practical form was, curiously enou.gh, 

 taken in constructing a second tunnel under the 

 Thames rather higher up the river, crossing just 

 above the Tower, which was commenced in February, 

 i86g, and completed in November the same year. 

 This Tower Subway, originally proposed by Mr. Peter 

 Barlow, but eventually executed by the late Mr. Great- 

 head, whose name will always be prominently associ- 

 ated with the system of tunnelling under consider- 

 ation, was carried forward through the London Clay 

 luider the shelter of a shield, similar in principle to, 

 though much smaller than, the Thames Tunnel shield. 

 The shield in this instance consisted of a short 

 wrought-iron cylinder laid horizontally, 4J feet long 

 .-aid slightly more than 7 feet internal diameter, 

 stiffened at its front cutting-edge, and provided in- 

 side with a vertical plate diaphragm having a central 

 opening, which could be readily closed, through which 

 tlie men passed for excavating the ground in front 

 preparatory to pushing forward the shield by a series 

 of screws. The novelty consisted in the lining of the 

 tunnel being formed of a series of cast-iron rings, 

 composed of segments bolted together, which were 

 erected under the shelter of the rear part of the 

 cylindrical portion of the shield as it was pushed for- 

 ward ; and as the shield overlapped the lining of the 

 tunnel, and left a slight annular space between the 

 lining and the clay stratum, lime grout was injected 

 through holes provided in the casting, so as to fill 

 up the vacancy left by the shield in its advance. This 

 subway traverses the London Clay throughout, at a 

 minimum depth of 22 feet below the river-bed, no 

 water having been encountered ; and it indicates the 

 general method of constructing tunnels by this system. 

 The shield serves to protect the completed end of the 

 tunnel from the fall of earth at the working face, and 

 acts like timbering in supporting the superincumbent 

 mass and preventing settlement above during con- 

 struction, which is further insured over the completed 

 NO. I919, VOL. 74"] 



tunnel by filling the cavities left by the advancing 

 shield with grout. 



The system, however, as successfully carried out, in 

 the absence of water, in the Tower Subway, was not 

 adapted for passing through water-bearing strata ; 

 and a third step, consisting in the introduction of 

 compressed air, was essential to enable this system 

 to cope effectually with the conditions liable to be 

 encountered in tunnelling under rivers, or at a con- 

 siderable depth below the surface, in loose ground. 

 The completion of this system of tunnelling, by the 

 combined use of a shield, a cast-iron lining put 

 together under shelter of the shield, and compressed 

 air to exclude the water from the works in traversing 

 water-bearing strata, has enabled abandoned tunnels 

 to be completed, and tunnels to be successfully carried 

 out under such unfavourable conditions as would have 

 been considered impracticable by the methods pre- 

 viouslv in use. This combination of shield, cast-iron 

 lining, and compressed air, for carrying a tunnel 

 through water-bearing strata, was resorted to by Mr. 

 Greathead for the first time in 1887, in constructing 

 the City and .South London Railway, the first of the 

 metropolitan tube railways, where it passes through 

 the loose, water-logged gravel of the Thames basin, 

 overlying the London Clay ; and in 1889 it was adopted 

 for continuing the Hudson Tunnel in the silt under- 

 lying the Hudson River separating New York from 

 the mainland, when different systems of carrying for- 

 ward an iron lining by the aid of compressed air, under 

 the shelter of which a brick tunnel was constructed, 

 proved increasingly difficult as the work advanced. 



The shield for the continuation of the two single- 

 line Hudson tunnels was loj feet long and 20 feet 

 outside diameter; whilst the cast-iron lining has an 

 external diameter of 195 feet and 18 feet internal 

 diameter, formed of rings ij feet long, made up of 

 eleven segments and a ke)', put in place by a revolving 

 hydraulic erector. Tliis work was stopped for want 

 of funds in 1891, but was resumed in 1903 and com- 

 pleted last year. Where the silt traversed was very 

 soft, the shield was kept closed and pushed forward 

 by sixteen hydraulic rams ; and to avoid unequal 

 settlement of the tube under the weight of a train, it 

 has been supported at intervals on iron piles driven 

 down to a hard stratum underlying the silt. Com- 

 presseu air had been used successfully for many years 

 in constructing foundations and piers of bridges under 

 water, or in water-bearing strata, before it was 

 applied to subaqueous tunnelling ; but whereas in 

 bottomless, vertical caissons, the compressed air forces 

 out the water uniformly all over the bottom, the 

 pressure of the air at the open end of a horizontal 

 tube meets with less opposition from the water at the 

 top than at the bottom, where the head of water is 

 greater, in proportion to the diameter of the tube. 

 .Accordingly, in large tubes there is a liability in 

 traversing loose soil for the air to escape through 

 the stratum at the top, and for the water to rush in 

 simultaneously at the bottom. To provide for the 

 safety of the men in such a contingency, in addition 

 to two or three platforms at the back of the diaphragm 

 of the shield, with openings at each stage which can 



