A tunnel is simply a hole bored through a hill in order to carry a canal, a road, or a railway, in an advantageous course; and is generally resorted to as a means of lessening the expense which would ensue if an open cutting were made. There are other reasons which may render this mode of proceeding advisable; such as gaining the consent of a landholder, who would object to a cutting through his property, but would have no dislike to a tunnel; or various other local causes: but the general reason for such a mode of proceeding is the consideration of expense.
In order to enter into this inquiry, the first step is to take plenty of borings, for the purpose of examining the geological structure of the proposed hill. If these are considered satisfactory, trial shafts should be sunk quite to the intended bottom, as by these the quantity of water to be expected will be discovered. It should be particularly attended to, as water is one of the greatest annoyances to be encountered in tunnelling. The readiest way is to measure the water into a cask, and ascertain the number of gallons per minute. To do it readily, the shaft should be about six feet in diameter, rather more than less; and no loss will be occasioned by making it large, as, if the situation is properly chosen, it may afterwards be converted into a working shaft.
The necessary power to drain being taken into account, as the geological structure of the ground being ascertained by the borings, we have thus the elements for making an estimate of the relative expense of forming a tunnel or an open cutting. Questions of expediency will also have to be taken into the account; such as, how the spoil from a tunnel can be deposited, and at what expense in the purchase of land; whether earth in large quantities is requisite for an adjacent embankment; and if we lose this by making a tunnel, at what price can a side-cutting be made to compensate the embankment; and various other questions connected with the locality, which can only be determined on the spot. Sometimes the earth for the tunnel can be taken laterally, the working shafts being horizontal; and in case the work is called a gallery, and not a tunnel. This process will in general lessen the expense.
In the present state of civil engineering, a tunnel can be made through any object, from a rock to a quicksand; and the question of making it turns chiefly on the expense, which will vary from L20 a yard in sandstone, which is easy to cut, and which will stand without brick-work, up to L100 and L140 a yard in quicksand and bad ground, requiring twenty-seven inches thickness in the brick-work, and mostly to be laid in concrete. The Thames tunnel has cost about L1200 a yard. Here, however, there have been some difficulties of a peculiar nature to contend with, and it is a double tunnel. In general the rocky strata will be easiest, as no brick-work will be required. Hard shale, and in fact anything which will not cut by a spade, is best treated by blasting, and not by the pickaxe. In some cases tough clays will be found, against which a pickaxe is nearly useless, as it merely indents a hole where it sticks, without loosening anything. Blasting will have no effect on this: pickets will answer very well, and cross-cut saws best of all. The borings should be numerous, so as to give all the data. Chalk, for instance, will often contain large holes filled with gravel, which on being opened during the execution of the work, will pour in on the unsuspecting miner.
After due consideration, having determined on a tunnel the place proposed, the engineer next decides on the size and shape, both being in some measure regulated by what the tunnel is intended to carry. But, as respects the shape, this is also dependent on the nature of the ground, and the degree of weight which the brick-work will have to support. For instance, in a wet quicksand, which approaches the nature of a fluid, with an equal pressure all round, the shape should approximate as nearly as possible to that of a circle; and for any other kind of pressure, the form of the tunnel should be proportionally modified. Curves and rises or falls should by all means be avoided if practicable.
In setting out a tunnel, the engineer should have a transit instrument, placed in a fixed observatory, and standing on a pier insulated from the building; and if there is any road near it, the pier must be insulated from the ground to a depth of six, eight, or ten feet, according to the nature of the traffic on the road, in order to insure stability and a freedom from vibration. A distant mark should then be taken in the line of the tunnel, and a fixed point placed as the adjusting spot for the line of direction; the farther this is off the better. Intermediate marks for the working and ventilating shafts may then be set out correctly; and as these are sunk, plumbmets, suspended from points accurately set out by the transit instruments, may be constantly tried as to the proper direction. These plumbmets should be of iron, and should be let down into buckets of water, to check the vibration; or, what is better, they should hang in cups of mercury. When the shafts are cleared out to the bottom, other transit instruments may be placed in them, the plumb-line and transit being kept as far from each other as possible. The intersection of the vertical hair in the transit with the plumb-line, will then enable the engineer properly to set out the work.
Tunnels have been known to draw water off the wells as far as a mile distant. This may generally be remedied in every case except that of a canal containing salt water, by sinking the wells deeper, which for the most part restores them to nearly their original state. When a salt-water canal is to pass through a tunnel, compensation had better be made to the parties as soon as it is discovered that an injury has been done. In all cases of canal tunnels, care must be taken that they are run deep enough to insure a proper supply of water to allow for waste, lockage, and leakage, where the supply is contingent on a tide-way. It will be very expensive to remedy this afterwards, and will most probably require the erection of a steam-engine to pump up a sufficient quantity.
Some of the old canal tunnels, through an injudicious economy, were constructed without a towing-path. The boats are in these cases pushed through by men lying on their backs on the deck, or on a projecting board placed for the purpose. They then set their feet against the side-walls of the tunnel, and advancing them alternately, foot the boat along, the helmsman steering against them. This barbarous plan, which has occasioned serious loss of life, is now generally abandoned; but no tunnel or canal can be called perfect unless it has a towing-path on both sides. In the tunnels, an iron hand-rail should invariably be set up next the edge of the towing-path; and if there is but one of these, its width should be so increased as readily to admit the horses to pass each other. Some of the old tunnels above described were completed for less than four pounds per yard; and one, the old tunnel on the Grand Trunk Canal at Harcastle, in Staffordshire, constructed by Brindley, only cost L3.10s. 6d. It was ten feet in diameter, and consisted merely of a semicircular brick arch, which sprung from the water-line of the canal. The variation in... size from this may be judged of by the following examples on railways.
| Yards long | Ft. high | Ft. wide | |------------|----------|----------| | Liverpool and Manchester Railway tunnels—Wapping to Edge Hill | 2200 | 16 | 22 | | Lime Street | 1 mile | 19 | 25 | | Primrose Hill, London, and Birmingham Railway | 1250 | 25 | 22 | | Kilshy ditto | 2420 | 27 | 23 | | Canterbury and Whitstable | 880 | 12 | 12 | | Leeds and Selby | 700 | 17 | 22 | | Whitby and Pickering | 130 | 14 | 10 | | Leicester and Swannington | 1750 | 13½ | 10½ |
In the agreement entered into with the contractor, the price of the contract, and a schedule, by which any extra or additional works are to be executed, should be stated; and likewise the mode of payment, and the nature of any retentions. The time for completion, and fines for exceeding this time, should be clearly and distinctly laid down; with the condition that all payments are subject to the engineer's approval of the work. The contractor should find all tools, machinery, labour, and materials of an approved kind, getting out all foundations, excavations, shafts, culverts, drains, roads, &c.; and he should provide all centrigs, coffer-dams, pumping apparatus, scaffolding, fencing, and other requisite materials of every description, according to the specifications, plans, and drawings, and the instructions which he may from time to time receive from the engineer. In the case of a railway, he should lay the permanent way, the materials being in this case found for him; and in a canal he should construct all the locks and gates, if there are any at either end of the tunnel, and form the towing-path. When he does not employ a sufficient number of men on the work, the engineer should have power to engage more, after giving him a week's notice of his intention. These may be retained temporarily or permanently, as may be required, and in all cases using the contractor's materials, the men's pay being deducted from the price of his contract. The engineer should also have the power of ordering the discharge of any foreman or workman acting improperly, or doing his work with carelessness; a power which he should be cautious not to use without strong reasons. The ground over the tunnel should be fenced off previously to commencing the work. The contractor should be restricted from entering on any adjoining land without leave; and if this should be necessary after the leave is obtained, the required land should be immediately fenced in. Temporary roads, for the conveyance of materials from the high roads, when required, should be formed by the contractor; as also those necessary for conveying away the spoil-earth. If the company who are forming the tunnel are called upon by any of the surrounding occupiers, they should have power to compensate them for any damages, and to place the sums thus paid to the contractor's account.
The contractor should not be allowed to sublet any portion of the work without permission in writing from the engineer. He should also be bound to take down or otherwise alter any work not approved of by the engineer, and to remove all unsound materials. All materials, from the moment they are brought on the site of the works, become the property of the persons for whom the tunnel is made, and the contractor must not remove them again without permission; but the company or other persons making the tunnel are not to be answerable for the damages which any materials may sustain. These and similar clauses in the agreement, are of course only provided for cases of emergency, and would seldom require to be acted on. The contractor should make all the alterations and additions which he may be instructed to do by the engineer, and no others. He is to be furnished with copies of the plans, sections, and other drawings, also of the specification, and is himself to see if these are correct; he is also to do all that may be reasonably implied, although not actually expressed, in the drawings and specification. It is usual to deduct ten per cent. from the payments; the whole of which is not returned to the contractor till twelve months after the completion of the work, he being bound to keep it in repair for that length of time. The payments are in general made monthly, including those for extra and additional works, after being approved by the engineer, who is to certify the same. It is also usual in large works of this kind, when the contractor has more than two millions of bricks on the site of the work, to allow him half or two thirds of their cost.
When the tunnel is for a railway, the company deliver the permanent-way materials at the nearest convenient place, and the contractor brings them to the works, defraying the charge of wharfage, if any. In the event of any disputes, the decision of the principal engineer is in all cases to be final.
The drawings should consist of, 1. A general section of the tunnel longitudinally, containing all the measurements to an exact scale; the positions of any ventilating shafts if such are required; their length, breadth, thickness of brickwork, &c.
2. Views of the fronts and wings, with plans and sections of the foundations and wing-walls, showing the curves and battens.
3. Plans and sections of the ventilating shafts, and the iron-work connected with them on the working shafts.
4. Transverse sections of the tunnel, with any other details as to drains, and all other necessary information; and plan and section of fronts.
5. A general plan of the property through which the tunnel passes, showing the width to be fenced in.
6. If for a railway, plans of the rails, blocks, and the mode of laying them. If for a canal, plans of the locks and gates, with all other necessary details.
The specification should define in words the situation and termini of the tunnel, and the number and dimensions of the locks, if for a canal; and should then enumerate the various works, such as fencing off the ground above, one chain in width, till the tunnel is completed; sinking ventilating and working shafts; excavating and building the tunnel; building the fronts; laying the drainage; excavating in open cutting at each end, if required; forming and turfing the slopes, and fencing and ditching them, and the tops of the fronts; depositing all the earth in a given place, so as, when soiled, to be fit for agricultural purposes; ballasting and laying the way, if for a railway; but if for a canal, making all the necessary locks and feeders, forming the towing-path, building lock-houses, deviating roads if required, and keeping the whole in repair for one year.
The temporary fencing should be on each side of the tunnel one chain in width, between which all work and materials are to be comprised. It should be of split oak posts nine feet apart, mortised for the reception of three horizontal oak or larch bars; the posts three and a half feet above ground, and the bars having one intermediate stay or prickpost firmly nailed to each of them. This fencing should be substantially fixed previously to the commencement of any other work, so as to exclude sheep and cattle from the intermediate space, and protect the surrounding land from damage. On the outside of the fencing a ditch should be formed, four feet wide at the top, one foot wide at the bottom, and one and a half foot deep, communicating with any existing water-courses or drains, so as effectively to prevent any of the shafts which may be sunk from injury by the overflow of water during the progress of the work. The strictest attention should be paid to this stip- When ventilating shafts are necessary, they may be built of any required size. In railway tunnels from 2000 to 600 yards long, two shafts sixty feet diameter will be sufficient. They are best built from the top; for instance, excavate ten feet down, and build the shaft for that height, then divide the circumference into parts about ten feet wide, and excavate every alternate one of these, leaving the others to support the ring of brick-work; and when these alternate ones are bricked up to the ring above, the remaining earth is all taken away, and the bricking continued till the second ring is completed and connected with the first, then the same process is repeated till the whole is connected to the required depth.
The thickness of the brick-work is of course in proportion to the size and height. For a shaft sixty feet diameter and 120 feet high, the following would be sufficient in favourable ground. The tunnel, where the shaft intersects it, should be entirely of stone, and on this a course of one foot square, on which the shaft stands; then the next forty-six feet to be three feet thick; the next seventeen feet, two feet eight inches thick; the next seventeen feet, two feet three inches thick; and the last or upper fifteen feet, one foot ten inches thick, coped with stone for the railing. The boundary wall might be one foot one half inch thick, with footings, and an increase in the foundations, if thought necessary.
They should be built in alternate courses of headers and stretchers, each brick being well flushed up; and where the tunnel intersects the shaft it should be faced with stone, each stone to be eighteen inches upon the curve of the shaft, toasting into the brick-work in the soffit of the tunnel by being three and four feet long alternately; the whole neatly fitted in their beds, and neatly hammer-dressed, each being secured to the adjoining one with malleable iron nails run in with lead.
Stone coping should be set upon the shafts a little above the level of the ground, into which iron railing may be fixed. The stones to be secured by lead joggles, three fourths of an inch square, and run one inch into each stone. The iron railing should be about three feet high, of malleable iron, three fourths of an inch square, and five inches distant from centre to centre, set three inches into the coping, and run with lead. A half-round hand-rail should be riveted on each upright, an inch and a half wide, as a finish.
Shafts of the above size should be enclosed with a circular wall seventy-two feet in diameter, twelve feet high, and thirteen inches thick, with a plinth of eighteen inches, and stone coping sixteen inches wide and four inches thick, gathered and throated. In this wall there should be a doorway. The space or path between the wall and the shaft should be paved with bricks, having a gentle slope towards, with holes through the wall about every three feet, to let out the water.
The whole bottom of the shaft should be soundly filled sufficiently deep, with good concrete beaten firmly down; and whenever the excavation is larger than the dimensions of the shaft, the space should be filled perfectly solid with brick-bats, and grouted with Roman cement, with which the shafts are to be built; but the enclosing wall may be built with mortar. Whenever water occurs in sinking any of the shafts, it must be completely excluded from them by lining of puddle, or Roman cement if necessary.
In short tunnels it may sometimes be advisable to drive heading, about four feet wide and five feet high, right through, before excavating it; and the contractor should be allowed to sink what air-shafts he thinks proper, providing they are not in the line of any road, nor within fifty feet of any working shaft. They may be from three to four feet diameter in the clear, supported on iron curbs, carried up ten feet above the surface, and coped with stone.
To determine the number of working shafts, regard must be had to the time allowed for the completion of the work, and the nature of the part to be excavated. The shafts must be just so close together that every face can be raised and bricked half the distance to the next shaft, two months before the given time for completion, leaving these two months for building the fronts, and laying the way if for a railway; and in all cases a larger allowance had better be made, in case of accidents.
The number having been determined on, their places should be shown on the drawings. They should be nine feet in diameter inside, and nine inches thick; the bricks to be all heading, and to be well bedded in Roman cement. The shafts should be kept free from all bulges and imperfections; they should rest on an iron curb-ring, forming a key between the bottom of the shaft and the top of the tunnel; and they should be carried up ten feet above the surface, and coped with stone.
In excavating where the ground is bad, the contractor should not, without permission, advance beyond the completed brick-work more than six feet. Ten feet will in most cases be a convenient length; but in this the engineer must form his judgment from the nature of the ground. The space excavated must be well secured, in the manner to be hereafter stated. The dimensions of these supports are to be approved of by the engineer. The invert, sides, and roof, should be cut out as nearly as possible of the size the tunnel will be when finished. When more than this is taken out, it should be securely made good; in every case one man being employed at each face solely in filling and ramming in, if under the invert, with well-pounded clay, and if on the sides or top, with suitable materials, rammed in with every other course of bricks at the sides, and, as soon as the succeeding excavation will allow it, at the top. But when the ground is very bad, the bars are to be left in, at the pleasure of the engineer. The cills should not in general rest on the brick-work, but on trestles, so as to be independent of the side-walls. If this is not done, the holes where they have rested should be made good, whenever they are withdrawn, with brick and cement.
When any two faces are within fifty yards of each other, a heading should be driven to connect them, and insure the proper direction of the line at the junction. The contractor should also be bound to drive any headings the engineer may direct, either for drainage or otherwise. The engineer should, at the contractor's expense, from time to time give the contractor marks, in the way to be hereafter shown, to let him know how the line of the work is to be regulated.
Twenty-seven inches thick in the sides and top, and eighteen inches in the invert, in Roman cement, will be the most ever required, even in a quicksand; and as the nature of the ground improves, this may be lessened, till we arrive at the point where it will stand of itself. Each brick should be well bedded with a wooden mallet, and every joint thoroughly flushed when mortar is used. When the shape of the tunnel requires it, the bricks must be moulded taper. In the arch they are laid in concentric rings, half a brick thick, taking care that the additional number of bricks requisite for each additional ring is put in, which is easily calculated. In a tunnel twenty-seven feet high, and twenty-three wide, it will be five more for every ring. A course of stone should be laid in at the springing of the side-walls from the invert; the stones about three feet long, well bedded in mortar, on four courses of very sound and hard bricks.
It will generally happen that excavations are to be made at each end after the tunnelling has sufficiently advanced. These excavations should be turfed, if possible, as soon as they are finished; and if turf is not to be had, they must be soiled at least eight inches thick, and sown at the proper season with equal parts of rye and clover seed, in the proportions of three pounds of the mixed seed per acre.
The top of the slopes and the ends of the tunnel should be protected with a ditch, a wood-railing, and a line of quicks. The ditch should be five feet wide at the top, two feet deep, and with slopes that will stand according to the nature of the soil, which, if very bad, will render it necessary to widen the top. The earth from the ditch will form a mound to be neatly turfed, having the best of the vegetable soil in its middle, where a double row of three years old quicks, two years transplanted, should be set in the proportion of twelve quicksets to a lineal yard. The fence of split oak is to be set on the inner edge of the ditch, as before described. The quicks should be weeded and cleaned twice a year, and others planted in the room of those which have not taken root.
A brick drain should be laid the whole length of the tunnel, in Roman cement, with the end-joints open half an inch for the passage in of water from the ballasting; and if the shafts let in water, a conveyance half-round pipe must be fitted to the bottom of them, to lead the water down the side of the tunnel into the drain.
With respect to the spoil, if made into banks, the turf should first be removed; to be afterwards put on the top. Convenient sites should be found, so that the spoil shall be useable for agricultural purposes; and where turf enough to cover it cannot be had, it must be soiled and sown with seed at the proper season.
All damages to the adjoining land must be paid by the contractor; but if the parties making the tunnel are, as is usually the case, a public company, under an act of parliament, they should assist the contractor in getting the necessary land, with the powers of their act. It is understood that the contractor has the full use of the one chain in width along the line of the tunnel, free of all charge, on which to erect any buildings or machinery he may require, laying railways or roads for conveying away spoil and bringing in materials, lifting-engines, pumping-engines, or any other apparatus; and it is also understood, that if any material turns up fit for making bricks, he is to have the advantage of applying it to that purpose.
In case of having any roads to divert, or new ones to make, the ground should be excavated the required width, and one foot deep; when, if it is moist, brushwood should be placed upon the bottom, and then gravel, till the centre, when the whole is well beaten, is two feet above the excavated bottom, with a rounding of six inches on the surface. None of the gravel or broken stones should be larger than will go through a two-inch ring. A ditch, railing, and quicks, as before described, are then to be put up.
The engineer is in all cases to be the judge of the thickness of the brick-work, which should be provided for in the schedule of prices; as also, whether laid in cement or mortar. It should also be clearly defined whether or not the contractor is to be liable for any damage done to the land from the falling of the surface during the execution of the works, or if the regular continuity of the brick-work is destroyed, arising from irregular shrinking or settlement in the arch, imperfection in the centring, or any other cause. He should also be bound to remove all the temporary fencing, and clear the surface after the works are completed.
The materials should all be of the very best kind. The bricks should be sound, hard, and well shaped, being moulded taper wherever the form of the tunnel requires it. The sides may be built in English bond, or alternate heading and stretching; but the arch and the invert must be in concentric half-brick rings. The mortar should be of the best lime and sharp sand, in the proportions of three to one, and passed through a pug-mill; the lime being ground under edge-stones in its dry or unsacked state. The Roman cement should be perfectly fresh burnt, and capable of setting hard; it should be mixed with an equal quantity of sand, and none used which has set. The stone should be of a sound and hard quality, perfectly free from flaws. The cast iron employed in the curbs for the shafts should be of the best No. 2 iron, and the castings perfectly clean and cleared from air-bubbles or pin-holes; and no stopping or packing should be allowed. The whole of the malleable iron bolts, &c., employed, should be made of the best scrap-iron. The bolts for retaining the tunnel fronts should be tested to twenty-five tons, and tested in pairs to try the couplings. When concrete is used, it should be five measures of clean riddled gravel, and one measure of lime, and not mixed till immediately before it is required to be placed in its intended situation.
There should be a specified time for the contractor to begin and finish each portion, such as the working shaft, the ventilating shafts, the excavation, &c.; and he should be bound to execute a given quantity of tunnelling per week, sending the engineer a weekly return of all descriptions of men employed on the works, distinguished into classes.
When the rails, chairs, blocks, and other permanent-way materials are delivered to the contractor, he is then responsible for them. They need not be delivered till the tunnel is complete. Ballasting is to be used of broken stone or gravel for bringing up the blocks to the required height, without any mixture of clay or other substance capable of containing water; it should set quite hard. If broken stones are used, they should be passed through a two-inch ring. The ballasting, which may, if required, be shot down the shafts, an inclined plane at the bottom canting it the right way, should be spread over the invert, and beaten into a firm and solid mass, by means of heavy beaters, worked by at least two men, till it is consolidated at the proper height to receive the blocks or sleepers, which are also to be imbedded as firmly as possible. The contractor will have to drill two holes in each block to receive the treenails, and to cut a proper bed for the chairs on the sleepers. No spiles should be driven into sleepers till proper holes are bored. The sleepers should be imbedded by beaters, and the blocks by lifting them one foot high by a spring lever, and then letting them fall till no difference of level is perceived. The rails must be laid perfectly level and parallel, and the joints even and true, with an allowance for expansion and contraction suitable to the temperature to be furnished by the engineer. The utmost care and attention should be paid to so essential a point as the laying of the permanent way.
In excavating the working or ventilating shafts, it will be the best method to sink with a roll till they are fifty or sixty feet deep; this is worked by hand, and then with a horse-gig. Fig. 13; Plate CCCXCIV. If the ground is hard, the roll should be used longer, as this will be the cheapest way. The roll is worked by two men, and if they have to stand still occasionally for the bucket, or skip as it is termed, to be filled, it will be a less expense than if a horse, horseboy, and bankman (to land the skip), have to wait for the fillers. In working rocks, they will frequently have to wait for the blast.
A working shaft of nine feet diameter will be found sufficient; and in commencing it, place four cills across, as represented in the drawing; fig. 13, about twelve or fourteen inches square. On them place the standards for the witch, as shown; then begin the excavation, going on till the ground discovers signs of weakness; then put in a curb (a), which is a wooden ring the size of the shaft, three inches thick, and as wide as the intended brick-work, for which four inches and a half will in most cases be sufficient. But if the ground is very soft, cut back into it and make the curb wider; then brick the shaft from the curb to the top, and go on excavat- When the side-walls are raised to the springing of the arch, a cill (e), fig. 15, is built in with them; then the centring, which, with its cill (i), is all fixed together, is got up and wedged to the required height. The laggins (kk) are pieces of quartering which go from centre to centre, and on them the bricks are laid; (ee) are the queen-posts, (d) is the straining-piece, (aa) principal beams butting against (b), (mm) are the ribs on which the laggins lie; and if the centring is large, these may be joined over the queen-posts and struts.
In some rare cases the cill (e) is not laid on the side-walls and bricked in, as leaving a hole would, if the weight was great, be dangerous. Whenever this happens, trestles are set up from the bottom to support the cill (e).
Sometimes in quicksands or other bad ground, a slip will happen, in spite of all exertions to the contrary. This occurrence will perhaps take place, if the rakers are not got up, although the crown-bars, side-bars, and cills are up. It arises principally from a pressure on the face, throwing the cill back and letting the bars down. When it does happen, a shaft must be sunk from above, and the ground beaten perfectly firm and solid from bottom to top, and the work then proceeded with as in any other new length. The slip will often extend to the surface, and will then take the appearance of a hollow dome.
In the upper portion of the arch, where the bars cannot be withdrawn, and the vacancy between the brick-work and the excavation filled up and rammed in till the completion of the brick-work, it should invariably be done at the very earliest possible period, which will be when the next length of excavation has proceeded far enough to let the workmen have sufficient space to perform it with somedness; and whenever the material through which the tunnel is making is of so bad a nature as not to admit the withdrawal of the bars and planking, without a risk of disturbing either the form or stability of the brick-work, they must be left in, according to the judgment of the engineer; consequently their value should invariably form an item in the schedule of prices under which the contractor is constructing his work.
Tunnelling under rivers presents considerable difficulty, as has been experienced in that under the Thames. The first tunnel was commenced at Gravesend in the year 1798, but the scheme was speedily abandoned. In 1804 another was commenced from Rotherhithe to Limehouse; but after sinking an eleven-feet shaft forty-two feet deep, the difficulties were such that the work was suspended. The diameter was then reduced to eight feet, and at seventy-six feet deep a drift was carried 923 feet under the river, which was only 150 feet from the opposite shore; the engineer then reported that farther progress was impracticable. Several other plans were devised; till in 1823 Mr Brunel proposed his for a double tunnel between Rotherhithe and Wapping; the soil there being a tenacious blue clay. The Rotherhithe shaft is fifty feet diameter 150 feet distant from the river, being forty-two feet high and three thick, built on the surface and excavated afterwards; the earth and water being drawn out by a steam-engine on the top. It was thus sunk in its place in a body, and by this means passed through a bed of gravel and sand twenty-six feet deep, which had caused great inconvenience in the former attempt. See fig. 11.
The fifty-feet shaft was sunk sixty-five feet; and from the lower level a twenty-five feet shaft was sunk to eighty feet for drainage. At sixty-three feet the tunnel commences dipping two and a quarter feet per 100 feet. The excavation was thirty-eight feet wide and twenty-two and a half high, or a sectional area of 850 feet; and the bottom, at the deepest part of the river, is seventy-six feet below high water. The excavation was effected by a shield composed of twelve frames close together, having each three cells above one another for the miners (fig. 18): 400 feet were bricked. complete in ten and a half months. In May 1827 and January 1828 the river burst in. These accidents were overcome by throwing down bags of clay from above. The work was however suspended for several years, not from any doubt of its success, but from want of funds. It has again been commenced, and has now reached the wharf-wall on the Wapping side.
The shafts will contain easy flights of steps for foot-passengers, and the descents for carriages are intended to be circular, to give them an easy slope. The middle wall was first built solid for greater strength; but openings were then made, so that each tunnel has ready communication with the other. The entire length will be 1300 feet. During the excavation each portion of the shield advances separately; and in front, polling boards are pressed firm against the earth by screws. These boards are removed singly to excavate, and the first is always replaced before a second is taken down. Fig. 17 shows through the tunnel a view of the twelve parts of the shield as worked. They, of course, are all of a length, extending to the outer parts of the brick-work in all parts.
In Plate CCCCXCIV., fig. 14 shows the mode of supporting the shafts in a canal or railway tunnel when finished, by the iron curb, separate drawings of which are given in figs. 6, 7, 8, and 9; fig. 12 shows the mode of drawing the curves for the tunnel exemplified in North Church Tunnel on the London and Birmingham Railway; fig. 5 is the front of Stowhill Tunnel; fig. 1 is the front, fig. 2 is a section of the wing-walls, fig. 3 a section through the centre, and fig. 4 plans at top and bottom of Linslade Tunnel. All the figures are to the same scale, that under fig. 11; except figs. 17 and 18, which are to the scales near them. (n. h.)