Fig. 125.—First Shield Invented by Barlow.
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.
Longitudinal Section.
Cross Section.
Fig. 126.—Second Shield Invented by Barlow.
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.