Tag: Tunnels

By Marc Isambard Brunel

The Minutes of the Proceedings of the Institution of Civil Engineers, 1837

April 11, 1837

Mr. Brunel gave an account of the Thames Tunnel. Having described the nature and difficulties of the undertaking, and the previous attempts which had been made by others to effect a similar work, he explained, by reference to sections, the nature of the strata below the river. He had adopted the rectangular form of the present excavation, because the work would set better than if it had been of any other form, and it also had a better sustaining surface. The necessity of supporting the ground, and of having a sufficient shelter, had led to the adoption of the shield, respecting which so much had been said. The construction of this would be most easily understood, by conceiving twelve books set side by side on their ends. These would represent the parallel frames which, standing side by side, but not in immediate contact, filled up the excavation. Each frame was divided into three boxes or cells, placed one above the other, the adjustment of the floors of which, and other details, were minutely described by Mr. Brunel.

Each frame was furnished with two large slings, by which it might derive support from, or assist in supporting, its neighbours; it had also two legs, and was advanced, as it were, by short steps, having for this purpose an articulation which might be compared to that of the human body. The frame rested on one leg, and then one side was hitched a little forward; then resting on the other leg, the other side was hitched a little, and so on. Hence the shield might be called an ambulating coffer-dam, travelling horizontally.

The brick-work was built in complete rings, and the advantages of this system of building had been fully proved, by the fact of two dreadful irruptions of the river having produced no disruption. Such was the violence of the irruption, that the brick-work had in one part been suddenly reduced in thickness by one-half, and in one place there was a hole, as if pierced by a cannon-ball. At a few feet beneath the tunnel was a bed of quicksand 50 feet deep, and above it were strata of most doubtful consistency, some of which fell to pieces immediately they were disturbed. Still, however, the progress was certain, and only required patience, to allow the ground above to acquire sufficient density. He found gravel, with a mixture of chalk or clay, extremely impervious to water; in some cases he contrived to let out the water from the sand above, and thus obtained ground of sufficient density. The progress had been considerably retarded by land springs, which produced cutaneous eruptions, and destroyed the finger-nails of the workmen.

April 18, 1837

Mr. Brunel continued his description of the works of the Tunnel. He explained how the ground above had suddenly sunk down, owing to the run of at lower stratum of sand. This running sand, which was a great annoyance, consisted of five parts of water to one of sand. Bags of clay and gravel were not so effective, where there were many stones, as the interstices did not become properly filled up; in such cases the coarsest river sand was a better material; the water ran through it at first, but it soon stopped ; a mixture of gravel and clay was nearly impervious to water, but not so impervious as gravel and pounded chalk.

The Ventilation of the Tunnel was provided for by a pipe 15 inches square, passing out under the fire-place of the steam-engine boiler.

Mr. Gibbs stated, that he had found bags filled with clay and tow-waste, exceedingly impervious to water. Being called upon to build a sluice, in a place where piling was impossible, in consequence of the stony nature of the ground, he had formed a coffer- dam, by laying down bags full of clay and tow-waste, in tiers, on the top of each other, up to the surface of the water.

By Richard Beamish, M. Inst. C.E.

The Minutes of the Proceedings of the Institution of Civil Engineers, 1837

April 4, 1837

The paper states that several attempts had been made in former years to effect a communication betwixt the opposite shores of the Thames by means of a tunnel, all of which, however, failed. In 1798, Dodd proposed a tunnel at Gravesend; in 1804, Chapman projected one at Rotherhithe; and in 1807, Vazie commenced the construction of a shaft, II feet diameter, at a distance of 315 feet from the river. With Vazie was associated Trevethick, a man of great practical knowledge as a miner, and by indefatigable labor, a drift-way 5 feet in height, 2 feet 6 inches in breadth at the top, and 3 feet at the bottom, was carried 1046 feet under the river. In the spring of 1808, having first ascended from under a rocky stratum, though with a depth of at least 25 feet betwixt them and the bed of the river, the Thames broke in upon them, and not a single brick having been laid, the work was irretrievably lost.

In 1823 the subject of a tunnel was again agitated, and a company was formed, to carry into execution the plans of Mr. Brunel. The first proceeding was to sink a shaft. Twenty-four piles, with a shoulder on each, were first driven all round the circle intended for the shaft. One side of a wooden platform, or curb, was then laid on this shoulder, whilst the other side rested on an iron curb, having an edge below to which it was attached. Through this curb ascended forty-eight wrought-iron bolts, 2 inches diameter, to the height of 40 feet, the height to which it was proposed to raise the shaft. The regular building of the tower on the curb, with bricks laid in cement, was proceeded with, and yet farther bound together by twenty-six circular hoops of timber, half an inch thick, as the brick-work was brought up. At the top of the tower was placed another curb, and the long iron bolts passing through it, having their ends formed into screws, the whole was screwed solidly into one mass, and completed in three weeks. In a week after it was finished sixteen of the piles having been driven, two by two, opposite each other, the whole structure was sunk half an inch, carrying down with it the remaining eight piles, on which it was brought to a rest uniformly and horizontally, thus permitting the sixteen piles to be abstracted by opening the ground at the back. The whole weight supported by these eight piles was about 910 tons (the weight of the shaft). Having been left for three weeks to dry, and gravel having been heaped under the curb, the remaining eight piles were removed, two by two, till the mass rested on a bed of gravel. The machinery, viz., the thirty-horse high pressure steam engine, with gear for raising the excavated soil was now fixed on the top. The miners were placed inside, and by excavating from around the bottom, the whole descended by its own gravity.

Mr. Beamish then describes the peculiar difficulties which were experienced, previous to the first irruption.

The chasm in the bed of the river, formed by the irruption of 1827, was stopped by bags filled with clay, with hazel rods passed through them, the interstices thus formed being filled with gravel. The irruption of 1828 was met by similar means; but the funds of the company not being then sufficient for proceeding with the work, the shield was blocked up with bricks and cement, and a wall 4 feet in thickness was built within the Tunnel.

The work was then abandoned, and remained untouched for seven years. In 1835 a Treasury loan was granted, subject to the condition, that the most dangerous part of the Tunnel should be executed first. On resuming the works, the first object was to provide a drain for the water from the shield, for which purpose two reservoirs were formed under the middle pier, from which drifts were formed to the bottom of the great excavation and shield. The water was abstracted from the shield at the lowest point, and the pipes of two pumps, worked by the steam engine, being brought into the reservoir, all the difficulty of the drainage was overcome.

The removal of the old and the introduction of the new shield, was a work of no ordinary difficulty. The bricks and cement had, by the strong oxide of iron which the water contained, been converted into a mass harder than most rocks ; and not less than 1646 feet of surface, 342 feet of which constituted the ceiling, had to be supported, on the removal of the brick-work, previous to the introduction of the new shield. The means, however, adopted by Mr. Brunel, and which are described in the paper, were perfectly successful.