Engineering Wonders of the World
Volume I

THE THAMES TUNNEL. 185 the friction between brickwork and ground held the caisson fast, nor could it be moved until 200 tons of bricks had been piled on the top. Early in June the curb reached its final position at the level of the crown of the future tunnel. Brune] now proceeded to remove the curbs, a small part at a time, and continue the brick- work downwards for another 20 feet, leav- ing on the river-side an open- Under= |ng of ^ie sjze of ^he tunnel, secured bv stout timbering. A basin-shaped circular invert at the bottom completed the caisson. Great pub- lic interest was taken in this part of the work, and so many distinguished people visited the scene of operations that at times work was seriously hampered. Considering the difficul- ties of sinking even a firmly-bolted iron cais- son, Brunel deserves the greatest credit for his skilful handling of a great masonry shaft which depended for its cohesion on the tenacity of mortar and the tension of a comparatively few metal rods. Before tunnel-driving could be commenced, a large well had to be sunk under the invert for the pumps to deal with the water drain- ing from the sub-river works. Drainage. This operation proved trouble- some, owing to the treacherous nature of the ground, and occupied a couple of months. It was Brunel’s original intention to drive a small drainage heading from the well under the tunnel, so as to get the water away easily as well as test the ground in advance of the shield. But the directors baulked at what they considered to be an unnecessary extra expense, and requested their engineer to sub- stitute—much against his will—a horizontal cast-iron pipe built into the masonry of the bottom of the tunnel. As often happens when those in authority interfere with the subor- dinate expert, the course laid down by them proved a costly mistake. During the operations described above, the The Great Shield. great shield had been constructed by William Maudslay, of the famous firm of Maudslay, Field, and Company. This ingenious device deserves a somewhat detailed description, as it may be regarded as the father of tunnelling shields, and was certainly the most important item of the plant used. But for it, the scheme would never have materialized. The greatest difficulty in tunnelling through soft and treacherous strata is to support the ground attacked while giving the workmen ac- cess to it for excavation. The most vulnerable part is the roof. If timbering be used, considerable lengths of ground must be exposed while the timber is placed in position ; and, where water lies in ambush, this process becomes extremely risky. Brunel therefore took a lesson from the teredo navalis, or ship’s worm, which bores its way through wood by means of a pair of strong shell plates encircling its head. As this creature protected its mouth, so Brunel decided to hold up the ground immediately in front of the brickwork of the tunnel, as it advanced, by means of a shield covered in on both bides and provided with movable flat top and bottom. The illustration on the next page will help to explain the construction and principle of the device. It consisted of twelve vertical frames, each 3 feet wide and 21J feet high, and divided by two plat- forms into three “ cells.” A frame stood upon two swinging legs, attached by ball joints to a couple of massive flat iron “ shoes.” At the top was the “ head,” carrying longitudinal iron sliders of sufficient length to cover the whole depth of the excavation, except for that part between a slider, when moved forward, and the brickwork behind. Experi- ence caused “ tails ” to be added to overlap the brickwork, just as the corresponding part of a Greathead shield—to be described in a later chapter—overlaps the last ring of an iron tunnel lining.