TUNNEL. 



TUNNEL. 



angle* with that of the tunnel, but in com* ease* 'obliquely 

 it." Th* eastern end f tin- tunnel it, for a abort diitanot, lined 

 with masonry ; bryond which, for about half a mile, the excava- 

 tion U left without any support from masonry or brickwork, iU 

 extreme width being, in thia part, 80 feet, and its height varying 

 Iran 80 to 40 feet, in order to avoid finishing above in any stratum 

 of doubtful character, which, though sufficiently itrong in the sides, 

 might not be ao trustworthy ai a roof. The aid** of the excavation 

 are cut in the form of an oblong elliptical or nearly a Gothic arch ; 

 but the uppermost bad of oolite, which forms the roof of the tunnel, 

 baa not been completed in that form, but baa, for greater strength, 

 been left flat for a width of two or three feet ; and in order to avoid 

 the risk of exfoliation which would have attended the cutting of the 

 edge* of the strata to a thin wedge shape, had the curved line of tho 

 t .MI. 1 been continued unbroken where it intersects them in the aides 

 of the roof, they are cut in the form of steps, each stratum presenting 

 a thick blunt edge ; ao that, in the words of Paaley's report, " the 

 upper strata of the oolite being supported by those below them, beyond 

 which they each successively project in a moderate degree, like corbels 

 in architecture, the whole enter into that sort of combination which 

 has the strength of an arch without its form ; provided that the vertical 

 fissures do not out through them longitudinally," of which no symp- 

 tom was apparent. The remaining portion of the Box tunnel, passing 

 in the centre through beds of fullers'-earth and clay below the great 

 oolite formation, and towards the western extremity through the infe- 

 rior oolite, U arched over with brickwork, varying in thickness from 

 four to seven concentric half-brick rings, and at the western entrance 

 containing nine such rings. An inverted arch is introduced excepting 

 where the oolite forms the foundation or bottom of the tunnel ; and in 

 some parts a portion of the side walls has been formed of the inferior 

 oolite, unprotected by brickwork, although it has not in any instance 

 been trusted for the roof. The judicious adoption of such changes in 

 the amount of masonry forms an im]>ortant part of the duty of the 

 engineer, and is essential to the construction of tunnels .'it a moderate 

 expense, although it may occasionally lead to failure, as in the case of 

 the Summit tunnel at Littleborough, on the Manchester and Leeds 

 railway, where, in passing through a stratum of blue shale, the invert 

 had been discontinued for a short distance ; but the shale, although 

 apparently hard and firm, was so affected by the atmosphere as to soften 

 and yield under the pressure of the sides, rendering it necessary subse- 

 quently to put in a strong invert of masonry. 



Tunnelling in clay is frequently attended with formidable difficulties 

 which render it very expensive. It is, when tough, a difficult material 

 to remove, blasting being of no use, and spades and pickaxes being 

 almost inapplicable. Lecount states that in such cases hatchets may 

 be used to advantage, but that cross-cut saws answer best. The 

 Primrose Hill tunnel, on the London and North-\Vetern railway, 

 passing through the London clay, is an example of the difficulties of 

 such a material. The engineers, warned by the failure of tho tunnel 

 attempted some years before through the same material at Highgnte, 

 on what is now called the Archway road, which fell in owing to the 

 insufficiency of the brick lining, adopted unusual precautions in the 

 first instance, excavating only nine feet in advance of the brickwork, 

 and supporting the clav by very strong timbering until the arching 

 was complete. Owing however to the extraordinary mobility of the 

 moist clay, the pressure upon the brickwork was so great an to squeeze 

 the mortar from the joint*, and to bring the inner edges of the bricks 

 in contact. The evil was augmented by the form of the bricks, which, 

 according to the custom in and near London, were made with hollow 

 surfaces, and were consequently unfit to bear great pressure, because 

 their edges only could come in contact The result was, as stated in 

 Lecount a ' History of the Railway , connecting London and Birmingham,' 

 pp. 32, 88, that " by degrees the bricks were grinding to dust, and the 

 diiiieniiions of the tunnel insensibly, but irresistibly, contracting." 

 This difficulty was overcome by the use of very hard bricks 

 laid in Roman cement, which, by setting hard before the external 

 preesnre became so great as to force the bricks into actual contact, 

 enabled the whole surface of the brick, instead of its edges only, to 

 imtrt the pleasure. The thickness of the brickwork was also increased, 

 so that in most parts of the tunnel it amounts to twenty-seven inches. 

 The occurrence of a similar material in the line of the Fareham tunnel, 

 oo the Gosport branch railway, occasioned great expense, and produced 

 a slip of the superincumbent earth which carried away about forty 

 yards in length of the brick arching, although it was of the unusual 

 thickness of three feet. 



Tunnels formed through chalk are often impeded by faults or cavities 

 filled with wet gravel or sand, which pour a flood of semifluid matter 

 into the excavation as soon as they are cut into. The irruption of 

 such Voose matrial,as well a of water alone, has in many cases occasioned 

 difficulties almost insurmountable. In the Watfiml tunnel, on the 

 London and North- Western railway, which passes through the \i]i)icr 

 chalk formation, where it is covered with a thick irregular bed of gravel, 

 such breaks occasioned great inconvenience. The chalk bad occasional 

 fissures sometime* as much as one hundred feet deep, filled with dean 

 gravel, '"which," observe* Lecount ('History/ 4c , p. 114), "when 

 worked into, rushed down with such violence, as to plough the wall* 

 of the tunnel as if bulleU had been shot against it" Such an accident, 

 occurring at the foot of one of the working shafts, overwhelmed ten 



mm who were at work in the tunnel, and led to the construction of 

 the large ventilating abaft near the centre of the tunnel, which occupies 

 the site of the cavity. Loose sand is perhaps the most difficult soil 

 that can be met with in tunnelling, but it has been in several instances 

 successfully passed through. In the tunnel on the Leicester and 

 Swannington railway, on* of the earliest railway tunnels, it loose dry 

 running sand was encountered for a distance of five hundred yards, in 

 which it was necessary to make a wooden tunnel to support the soil 

 while the brickwork was executed. When water occurs with the loose 

 soil the difficulty is still greater. This was the case in part of ih,i 

 Wapping tunnel at Liverpool, a portion of which fell in, to a depth of 

 thirty feet from the surface. The Kilsby tunnel, before alluded to, is 

 a more striking example of this kind of difficulty. Mr. Stopbenson, the 

 engineer, conquered this obstacle by sinking shafts, a little beside the 

 line of the tunnel, for pumping out the water, and so draining the 

 sand until it was sufficiently dry for tunnelling. The** shafts wer* 

 sunk through the quicksand by means of wooden tubbing, anil 

 them headings were driven to collect the water and to conduct it to 

 the pumps. Steam-engines were erected to work the pumps, which 

 were used incessantly for nine months before the sand was dry . 

 to allow the work to proceed: during a great part of that t> 

 thousand gallons of- water were removed per minute. When the 

 working of the tunnel was recommenced, headings were driven from 

 the pumping shafts to the bottom of the working shafts, by means of 

 which the tunnel was freed from water. Tho quicksand extends over 

 about four hundred and fifty yards of the length of the tunnel, and its 

 bottom dips to about six feet below the arch. On the occasion of an 

 irruption of water in another part of the tunnel, in which it was 

 desirable to complete the arching of a portion already executed 

 it was possible to get rid of the water, that object was effect 

 floating the men and materials to the spot upon a raft. Water ha* 

 been met with in large quantities in several other tunnels. It 

 so freely from fissures in the freestone rock through which U 

 tunnel U driven, that in November, 1837, the steam-engine etn. 

 in pumping proved insufficient, and the water filled one division 

 tunnel, and rose to the height of fifty -six feet in the shaft, thereby 

 suspending the work until the following July, when the water was over- 

 come by means of a second engine, of fifty-horse power. After another 

 irruption in the same tunnel, the water was pum]>ed out at the rate of 

 thirty-two thousand hogsheads a day. The progress of the Merstham 

 and several other tunnels was stopped for a time in like manner. In 

 no ease however has the irruption of water or the badness of the ground 

 proved so serious a difficulty as in the Thames tunnel. 



Short tunnels are occasionally excavated from the ends only, l>ut 

 those of considerable length are usually formed by sinking \ 

 shafts, about nine feet in diameter, down to the level of the tunnel, 

 and excavating in each direction from the bottom of those shaft*, until 

 the several parties of workmen meet in the intermediate portions. By 

 this means the work can proceed at any required number of | 

 faces, so as to bring the execution of the tunnel, whatever may be it* 

 length, within a moderate period of time. The accurate junei; 

 these detached workings is provided fur in the following manner : 

 In setting out the tunnel, the engineer plants a transit-instrument in 

 an observatory erected in the line of the tunnel, stip|iorting it on a pier 

 insulated from the building to prevent vibration. If a road hap 

 pass near the observatory, the ground should be excavated round the 

 pier to a depth of from six to ten feet, according to the traffic, f<>r the 

 same purpose. A distant mark should then be selected in the lino of 

 the tunnel, and a fixed point placed as an adjusting spot for the line 

 of direction, which point should be at a considerable distance. Inter- 

 mediate marks for the working and ventilating shaft* may then be set 

 out correctly ; and as these shafts are sunk, the points determined by 

 the transit-instrument are carried downwards by carefully suspended 

 plummets, which should be of iron, and let down in buckets of water, 

 or, which U better, in cups of mercury, to check vibration. When the 

 shafts are cleared out at the bottom, other transit-instrumente may be 

 placed in them, the plumb-line and transit being kept as far apart as 

 possible. The intersection of the vertical hairs in the transit with the 

 plumb-line will then enable the engineer properly to set out the work. 

 By these means junctions are effected between the several workings, 

 or */i if tt, with surprising accuracy. In a length of 1620 feet between 

 two shafts of the Box tunnel, which has a slope of 1 in 1'"'. the 

 junction of the two shifts was perfect in point of level, and did not 

 deviate more than an inch and a quarter in any place at the sides. 

 Even in curved tunnels, although the difficulty is increased, great 

 exactness is attainable. In those on the Glasgow and Qreenock r 

 at BUhopton, the deviation from perfect uuneuluecB nowhere exceeded 

 two inches. 



The number of working shafts in a given length of tunnel i 

 mined by the nature of the ground and th. -time allowed for excavation. 

 They Khould be so near together as to allow the tunnel to be exc 

 ami lini-il, if brickwork be requisite. f"r a length equal to <me h.-ilf of 

 the distance between two shafu, at least two months before the time 

 appointed for the completion of tho tunnel ; that time being de 

 to allow for accidents, for building the tunnel front*, ballasting and 

 laying the railway, Ac. The shafts are umially '.< feet in diam. 

 the clear, and lined with brickwork ft inches thiek. laid in cement ; all 

 the bricks being laid as htailcr*, or with their ends towards the centre 



