in small space as soon as the segments are bolted together, and they can be caulked water-tight.
In 1830 Lord Cochrane (afterwards 10th earl of Dundonald) patented the use of compressed air for shaft-sinking and tunnelling in water-bearing strata. Water under any pressure can be kept out of a sub aqueous chamber or tunnel by sufficient air of a greater pressure, and 'men can breathe and work therein—for a time—up to a pressure exceeding four atmospheres. The shield and cast-iron lining invented by Brunel, and the compressed air of Cochrane, have with the aid of later inventors largely removed the difficulties of sub aqueous tunnelling. Cochrane's process was used for the foundation of bridge piers, &c., comparatively early, but neither of these devices was employed for tunnelling until half a century after their invention. Two important sub aqueous tunnels in the construction of which neither of these valuable aids was adopted are the Severn and the Mersey tunnels.
The Severn tunnel (fig. 16), 41⁄3 m. in length for a double line of railway, begun in 1873 and finished in 1886, Hawkshaw, Son, Hayter & Richardson being the engineers and T. A. Walker the contractor, is made almost wholly in the Trias and Coal Measure formations, but for a short distance at its eastern end passes through gravel. At the lowest part the depth is 60 ft. at low water and 100 ft. at high water, and the thickness of sandstone over the brickwork is 45 ft. Under a depression in the bed of the river on the English side there is a cover of only 30 ft. of marl. Much water was met with throughout. In 1879 the works were flooded for months by a land spring on the Welsh side of the river, and on another occasion from a hole in the river bed at the Salmon (Pool. This hole was subsequently filled with clay and the works completed beneath. Two preliminary headings were driven across the river to test the ground. “Break-ups” were made at intervals of two to five chains and the arching was carried on at each of these points. All parts of the excavation were timbered, and the greatest amount excavated in any one week was 6000 cub. yds. The total amount of water raised at all the pumping station sis about 27,000,000 gallons in twenty-four hours.
The length of the Mersey tunnel (fig. 15) between Liverpool and Birkenhead between the pumping shafts on each side of the river is one mile. From each a drainage heading was driven through the sandstone with a rising gradient towards the centre of the river. This heading was partly red out by a Beaumont machine to a diameter of 7 ft. 4 in. and at a rate attaining occasionally 65 lineal yds. per week. All of the tunnel excavation, amounting to 320,000 cub. yds., was got out by hand labour, since heavy blasting would have shaken the rock. The minimum cover between the top of the arch and the bed of the river is 30 ft. Pumping machinery is provided for 27,000,000 gallons per day, which is more than double the usual quantity of water. Messrs Brunlees & Fox were the engineers, and Messrs Waddell the contractors for the works, which were opened in 1886, about six years after the beginning of operations.
In 1869 P. W. Barlow and J. H. Greathead built the Tower foot-way under the Thames, using for the first time a cast-iron lining and a shield which embodied the main features of Brunel's design. Barlow had patented a shield in 1864, and A. E. Beach one in 1868. The latter was used in a short masonry tunnel under Broadway, New York City, at that time. In 1874 Greathead designed and built a shield, to be used in connexion with compressed air, for a proposed Woolwich tunnel under the Thames, but it was never used. Compressed air was first used in tunnel work by Hersent, at Antwerp, in 1879, in a small drift with a cast-iron lining.
In the same year compressed air was used for the first time in any important tunnel by D. C. Haskin in the famous first Hudson River tunnel, New York City. This was to be of two tubes, each having internal dimensions of about 16 ft. wide by 18 ft. high. The excavation as fast as made was lined with thin steel plates, and inside of these with brick. In June 1880 the northerly tube had reached 360 ft. from the Hoboken shaft, but a portion near the latter, not of full size, was being enlarged. Just after a change of shifts the compressed air blew a hole through the soft silt in the roof at this spot, and the water entering drowned the twenty men who were working therein. From time to time money was raised and the work advanced. Between 1888 and 1891 the northerly tunnel was extended 2000 ft. to about three-fourths of the way across, with British capital and largely under the direction of British. engineers—Sir Benjamin Baker and E. W. Moir. Compressed air and a shield were used, and the tunnel walls were made of bolted segments of cast iron. The money being exhausted, the tunnel was allowed to fill with water, and it so remained for ten years; Both tubes were completed in 1908.
The use of compressed air in the Hudson tunnel, and of
annular shields and cast-iron lined tunnel in constructing
the City & South London railway (1886 to 1890) by Greathead,
became widely known and greatly influenced sub aqueous
and soft-ground tunnelling thereafter. The pair of tunnels
for this railway from near the Monument to Stockwell, from
10 ft. 2 in. to 10 ft. 6 in. interior diameter, were constructed
mostly in clay and without the use of compressed air, except
for a comparatively short distance through water-bearing
gravel. In this gravel a timber heading was made, through
which the shield was pushed. The reported total cost was
£840,000. Among the tunnels constructed after the City &
South London work was well advanced, lined with cast-iron
segments, and constructed by means of annular shields and
the use of compressed air, were the St Clair (Joseph Hobson,
engineer) from Sarnia to Port Huron, 1889–1890, through clay,
and for a short distance through water-bearing gravel, 6000 ft.,
18 ft. internal diameter; and the notable Blackwall tunnel
under the Thames (Sir Alexander Binnie, engineer, and S.
Pearson & Sons, contractors), through clay and 400 ft. of water saturated
gravel, 1892–1897, about 3116 ft. long, 24 ft. 3 in.
in internal diameter. The shield, 19 ft. 6 in. long, contained a
bulkhead with movable shutters, as foreshadowed in Baker's proposed
shield (fig. 2).
Fig. 2.—B. Baker's pneumatic shield. Numerous tunnels of small diameter have been similarly constructed under the Thames and Clyde for electric and cable ways, several for sewers in Melbourne, and two under the Seine at Paris for sewer siphons.
The Rotherhithe tunnel, under the Thames, for a roadway, with a length of 4863 ft. between portals, of which about 1400 ft. are directly under the river, has the largest cross-section of any subaqueous tube of this type in the world (see fig. 3). It was begun in 1904 and finished in 1908, Maurice Fitzmaurice being the engineer of design and construction, and Price & Reeves the contractors. It penetrates sandy and shelly clay overlying a seam of limestone beneath which are pebbles and loamy sand. A preliminary tunnel for exploration, 12 ft. in diameter, was driven across the river, the top being within 2 ft. of the following main tunnel. The top of the main tunnel excavation in the middle of the river was only 7 ft. from the bed of the Thames, and a temporary blanket of filled earth, usually allowed in similar cases, was prohibited owing to the close proximity of the docks. The maximum progress in one day was 12.5 ft., and the average in six days 10.4 ft. The air compressors were together capable of supplying 1,000,000 cub. ft. of air per hour.
Some tunnels of marked importance of this type—to be operated solely with electric cars—have been built under the East and Hudson rivers at New York. Two tubes of 15 ft. interior diameter and 4150 ft. long penetrate gneiss and gravel directly under the East River between the Battery and Brooklyn. They were begun in 1902, with Wm. B. Parsons and George S. Rice as engineers, and were finished in December 1907, under the direction of D. L. Hough of the
