Fig. 8Fig. 9Fig. 10
Fig. 8Fig. 9
Fig. 8
Fig. 8
Fig. 8
Fig. 9
Fig. 9
Fig. 9
Fig. 10
Fig. 10
Fig. 10
Figs. 8to10.—Typical Sectional Profiles for Tunnel.
—The dimensions to be given to the cross-section of a tunnel depend upon the purpose for which it is to be used. Whatever the purpose of the tunnel, the following three points have to be considered in determining the size of its cross-section: (1) The size of clear opening required; (2) the thickness of lining masonry necessary; and (3) the decrease in the clear opening from the deformation of the lining.
Railway tunnels may be built either to accommodate one or two, three and four tracks. In single-track tunnels a clear space of at least 21â„2ft. on each side should be allowed for between the tunnel wall and the side of the largest standard locomotive or car, and a clear space of at least 3 ft. should be allowed for between the roof and the top of the same locomotive or car. Since the roof of the tunnel is arch-shaped, to secure a clearance of 3 ft. at every point will necessitate making the clearance at the center greater than this amount. In double-track tunnels the same amounts of side and roof clearances have to be provided for, and, in addition, there has to be a clearance of at least 2 ft. between trains. On the three- and four-track tunnels only the width varies while the height remains almost equal to the two track. Referring toFig. 7, and assuming the lineABto represent the level of the tracks, then the ordinary dimensions in feet required for both single- and double-track tunnels are asfollows:—
The dimensions of tunnels built for aqueduct purposes are determined so as to have an area of cross-section equal to the required waterway. In the Croton Aqueduct two different types of cross-sections were used, the circular one for tunnels through rock and the horseshoe section for tunnels through loose materials. In the Catskill aqueduct three different cross-sections have been selected, the circular one for tunnels under pressure and the horseshoe for tunnels at the hydraulic gradient. These, however, through rock have a cross-section formed of a semi-circular arch and vertical side walls, while through earth the semi-circular arch is supported by skewback walls.
In tunnels built for railroad aqueduct sewer and any other purpose the thickness of the masonry lining to be allowed for varies with the material penetrated, as will be explained in asucceeding chapterwhere the dimensions for various ordinary conditions are given in tabular form. The lining masonry is subject to deformation in three ways: by the sinking of the whole masonry structure, by the squeezing together of the side walls by the lateral pressures, and by the settling of the roof-arch. The whole masonry structure never sinks more than three or four inches, and merits little attention. The movement of the side walls towards each other, which may amount to three or four inches for each wall without endangering their stability, has, however, to be allowed for; and similar allowance must be made for the settling of the roof-arch, which may amount to from nine inches to two feet, when the arch is built first as in the Belgian system and rests for some time upon the loose soil.
—Comparatively few of the labor-saving machines employed for breaking up and removing loose soil in ordinary surface excavation are used in tunnel excavation through the same material. Several forms of tunnel excavating machines have been tried at various times, but only a few of them have attained any measure of success, and these have seldom been employed in more than a single work. In the Central London underground railway work through clay a continuous bucket excavator (Fig. 11) was employed with considerable saving in time and labor over hand work. In some recent tunnel work in America the contractors made quite successful use of a modified form of steam shovel. These are the most recent attempts to use excavating machines in soft ground, and they, like all previous attempts, must be classed as experiments rather than as examples of common practice. The Thomson machine,[4]however, can be employedin any tunnel driven through loose soil. It occupies a comparatively small space and may easily work when the timbering is used to support the roof of the tunnel. Steam shovel instead may give efficient result only in the case that the whole section of the tunnel is open at once and there are no timbers to prevent the free swinging of the dipper handle and boom. But in tunnels through loose soils it is almost impossible to open the whole section at once without the necessity of supporting the roof. Consequently the use of steam shovel in loose tunnels is very limited. The shovel, the spade, and the pick, wielded by hand, are the standard tools now, as in the past, for excavating soft-ground tunnels.
[4]The machine was designed by Mr. Thomas Thomson, Engineer for Messrs. Walter Scott & Co.
[4]The machine was designed by Mr. Thomas Thomson, Engineer for Messrs. Walter Scott & Co.
Fig. 11.—Soft Ground Bucket Excavating Machine: Central London Underground Railway.
Fig. 11.—Soft Ground Bucket Excavating Machine: Central London Underground Railway.
—At one period during the work of constructing the Hoosac tunnel considerable attention was devoted to the development of a rock excavating, boring, or tunneling machine. This device was designed to cut a groove around the circumference of the tunnel thirteen inches wide and twenty-four feet in diameter by means of revolving cutters. It proved a failure, as did one of smaller size, eight feet in diameter, tried subsequently. During and before the Hoosac tunnel work a number of boring-machines of similar character were experimented with at the Mont Cenis tunnel and elsewhere in Europe; but, like the American devices, they were finally abandoned as impracticable.
—Briefly described, a drill is a bar of steel having a chisel-shaped end or cutting-edge. The simplest form of hand drill is worked by one man, who holds the drill in one hand, and drives it with a hammer wielded by his other hand. A more efficient method of hand-drill work is, however, where one man holds the drill, and another swings the hammer or sledge. Another form of hand drill, called a churn drill, consists of a long, heavy bar of steel, which is alternately raised and dropped by the workman, thus cutting a hole by repeated impacts.
In drilling by hand the workman holding the drill gives it apartial turn on its axis at every stroke in order to prevent wedging and to offer a fresh surface to the cutting-edge. For the same reason the chips and dust which accumulate in the drill-hole are frequently removed. The instruments used for this purpose are called scrapers or dippers, and are usually very simple in construction. A common form is a strong wire having its end bent at right angles, and flattened so as to make a sort of scoop by which the drillings may be scraped or hoisted out of the hole. It is generally advantageous to pour water into the drill-hole while drilling to keep the drill from heating.
—When the conditions are such that use can be made of them, it is nearly always preferable to use power drills, on account of their greater speed of penetration and greater economy of work. Power drills are worked by direct steam pressure, or by compressed air generated by steam or water power, and stored in receivers from which it is led to the drills through iron pipes. A great variety of forms of power drills are available for tunnel work in rock, but they can nearly all be grouped in one of two classes: (1) Percussion drills, and (2) Rotary drills.
—The first American percussion drill was patented by Mr. J. J. Couch of Philadelphia, Penn., in March, 1849. In May of the same year, Mr. Joseph W. Fowle, who had assisted Mr. Couch in developing his drill, patented a percussion drill of his own invention. The Fowle drill was taken up and improved by Mr. Charles Burleigh, and was first used on the Hoosac tunnel. In Europe Mr. Cavé patented a percussion drill in France in October, 1851. This invention was soon followed by several others; but it was not until Sommeiller’s drill, patented in 1857 and perfected in 1861, was used on the Mont Cenis tunnel, that the problem of the percussion drill was practically solved abroad. Since this time numerous percussion drill patents have been taken out in both America and Europe.
A percussion drill consists of a cylinder, in which works a piston carrying a long piston rod, and which is supported in such a manner that the drill clamped to the end of the piston rod alternately strikes and is withdrawn from the rock as the piston reciprocates back and forth in the cylinder. Means are devised by which the piston rod and drill turn slightly on their axis after each stroke, and also by which the drill is fed forward or advanced as the depth of the drill-hole increases. The drills of this type which are in most common use in America are the Ingersoll-Sergeant and the Rand. There are various other makes in common use, however, which differ from the two named and from each other chiefly in the methods by which the valve is operated. All of these drills work either with direct steam pressure or with compressed air. Workable percussion drills operated by electricity are built, but so far they do not seem to have been able to compete commercially with the older forms. No attempt will be made here to make a selection between the various forms of percussion drills for tunnel work, and for the differences in construction and the merits claimed for each the reader is referred to the makers of these machines. All of the leading makes will give efficient service. It goes almost without saying that a good percussion drill should operate with little waste of pressure, and should be composed of but few parts, which can be easily removed and changed.
—For tunnel work the general European practice is to mount power drills upon a carriage moving on tracks in order that they be easily withdrawn during the firing of blasts. Connection is made with the steam or compressed air pipes by means of flexible hose which can easily be attached or detached as the drill advances or when it is moved for repairs or during blasts. Two, four, and sometimes more drills are mounted and work simultaneously on a single carriage. In America it has been found that column mountings have been more successful for tunnel work than any other form. Thecolumn mounting made by the Ingersoll-Sergeant Drill Co. is shown inFig. 12. In using this form of mounting no tracks or other special apparatus is required; it is not necessary, as is the case with the carriage mounting, to remove the débris before resuming operations, but as soon as the blasting has been finished and the smoke has sufficiently disappeared the column can be set up and drilling resumed.
Fig. 12.—Column Mounting for Percussion Drill: Ingersoll-Sergeant Drill Co.
Fig. 12.—Column Mounting for Percussion Drill: Ingersoll-Sergeant Drill Co.
—Rotary drilling machines, or more simply rotary drills, were first used in 1857 in the Mont Cenis tunnel. The advantages claimed for rotary drills in comparison with percussion drills are: (1) That less power is required to drive the drill, and the power is better utilized; (2) once the machines work easily they do not require continual repairs, and (3) in driving holes of large size the interior nucleus is taken away intact, thus reducing work and increasing the speed of drilling. Rotary drills are extensively used for geological, mining, well-driving, and prospecting purposes; but they are very seldom employed in tunnels in America, although successfully used for this purpose in Europe. The reason they have not gained more favor among American tunnel builders is due to some extent perhaps to prejudice, but chiefly to the great cost of the machine as compared with percussion drills, and to the expense of diamonds for repairs. Those who advocate these machines for tunnel work point out, however, that under ordinary usage the diamonds have a very long life,—borings of 700 lin. ft. being recorded without repairs to the diamonds.
Fig. 13.—Sketch of Diamond Drill Bit.
Fig. 13.—Sketch of Diamond Drill Bit.
The form of rotary drill used chiefly for prospecting purposes is the diamond drill. This machine consists of a hollow cylindrical bit having a cutting-edge of diamonds, which is revolved at the rate of from two hundred to four hundred revolutions per minute by suitable machinery operated by steam or compressed air. The diamonds are set in the cutting-edge of the bit so as to project outward from its annular face and also slightly inside and outside of its cylindrical sides (Fig. 13). When the drill rod with the bit attached is rotated and fed forward the bit cuts an annular hole into the rock; the drillings being removed from the hole by a constant stream of water which is forced down through the hollow drill rod and emerges, carrying the débris with it, up through the narrow space betweenthe outside of the bit and the walls of the hole. There are various makes of diamond drills, but they all operate in essentially the same manner.
The rotary drill principally employed in Europe in tunneling is the Brandt. The cutting-edge of the Brandt drill consists of hardened steel teeth. The bit is pressed against the rock by hydraulic pressure, and usually makes from seven to eight revolutions per minute. Some of the water when freed goes through the hollow bit, keeping it cool, and cleaning the hole of débris. A water pressure of from 300 to 450 lbs. per square inch is required to operate these drills. Rotary rock-drills may be mounted either on carriages or on columns for tunnel work. Several machines have recently been constructed for the purpose of breaking the rock in tunnels without blasting, but they did not meet the approval of tunnel engineers. One of these machines is provided with numerous electric torches, which are applied to the rock at the front. By suddenly chilling the rock with a stream of cold water the stone will crumble away. Another machine was tested, with little success, in the excavation for the New Grand Central Depot in New York. On the face of this machine there is a multitude of chipping drills revolving on four arms and driven by air pressure. They attack the rock and chip it into fragments, which are carried away by an endless band.
When the holes are once drilled, either by hand or power drills, they are charged with explosives. The principal explosives employed in tunneling are gunpowder, nitroglycerine, and dynamite.
—Gunpowder is composed of charcoal, sulphur, and saltpeter in proportions varying according to the quality of the powder. For mining purposes the composition employed is 65% saltpeter, 15% sulphur, and 20% charcoal. It is a black granulated powder having a specific gravity of 1.5; the blackcolor is given by the charcoal; and the grains have an angular form, and vary in size from1â„8in. to3â„8in. Good blasting powder should contain no fine grains, which may be detected by pouring some of the powder upon a sheet of white paper. The force developed by the explosion of gunpowder is not accurately known; it depends upon the space in which it is confined. Different authorities estimate the pressure at from 15,000 lbs. per sq. in. in loose blasts to 200,000 lbs. per sq. in. in gunnery. Authorities also differ in opinion as to the character of the gases developed by the explosion of gunpowder, a matter of vital concern to the tunnel engineer, since they are likely to affect the health and comfort of his workmen. It may be assumed in a general way, however, that the oxygen of the saltpeter converts nearly all of the carbon of the charcoal into carbon dioxide, a portion of which combines with the potash of the saltpeter to form carbonate of potash, the remainder continuing in the form of gas. The sulphur is converted into sulphuric acid, and forms a sulphate of potash, which by reaction is decomposed into hyposulphite and sulphide. The nitrogen of the saltpeter is almost entirely evolved in a free state; and the carbon not having been wholly burnt into carbonic acid, there is a proportion of carbonic oxide.
—Nitroglycerine is one of the modern explosives used as a substitute for gunpowder. It is a fluid produced by mixing glycerine with nitric and sulphuric acids; it freezes at +41° F., and burns very quietly, developing carbonic acid, nitrogen, oxygen, and water. By percussion or by the explosion of some substances, such as capsules of gunpowder or fulminate of mercury, nitroglycerine produces a sudden explosion in which about 1250 volumes of gases are produced. The pressure of these gases has been calculated at 26,000 atmospheres, or 324,000 lbs. per sq. in. Nitroglycerine explodes very easily by percussion in its normal state, but with great difficulty when frozen; hence, in America, at the beginning of its use, it was transported only in a frozen state. Whendirty, nitroglycerine undergoes a spontaneous decomposition accompanied by the development of gases and the evolution of heat, which, reaching 388° F., causes it to explode. Notwithstanding the enormous pressures which nitroglycerine develops, it is very seldom used in its liquid state, but is mixed with a granular absorbent earth composed of the shells of diatoms. The fluid undergoes no chemical change by being absorbed, and explodes, freezes, and burns under the same conditions as in the fluid state.
—The credit of rendering nitroglycerine available for the purposes of the engineer by mixing it with a granular absorbent is due to Albert Nobel of Stockholm, Sweden, who named the new material dynamite. The nitroglycerine in dynamite loses very little of its original explosive power, but is very much less easily exploded by percussion, and can be employed in horizontal as well as vertical holes, which was, of course, not possible in its liquid state. Dynamite must contain at least 50% of nitroglycerine. Some manufacturers, instead of using diatomaceous earth, use other absorbents which develop gases upon explosion and increase the force of the explosion. These mixtures are classed under the general name of false dynamites. A great many varieties of dynamite are manufactured, and each manufacturer usually makes a number of grades to which he gives special names. Dynamite for railway work, tunneling, and mining contains about 50% of nitroglycerine; for quarrying about 35%, and for blasting soft rocks about 30%. It is sold in cylindrical cartridges covered with paper.
—In driving tunnels through rock large quantities of explosives must be used, and it is necessary to have some safe place for storing them. In many States there are special laws governing the transportation and storage of explosives; where there is no regulation by law the engineer should take suitable precautions of his own devising. It is best to build a special house or hut in one of the most concealed portions of the work and away from the tunnel, andprotect it with a lightning-rod and from fire. Strict orders should be given to the watchman in charge not to allow persons inside with lamps or fire in any form, and smoking should be prohibited. The use of hammers for opening the boxes should be prohibited; and dynamite, gunpowder, and fulminate of mercury should not be stored together in the same room. A quantity of dynamite for two or three days’ consumption may be stored near the entrance of the tunnel in a locked box, the keys of which are kept by the foreman of the work. When dynamite has been frozen the engineer should provide some arrangement by which it may be heated to a temperature not exceeding 120° F., and absolutely forbid it being thawed out on a stove or by an open fire.
—When gunpowder is used in tunneling it is ignited by the Blickford match. This match, or fuse as it is more commonly called, consists of a small rope of yarn or cotton having as a core a small continuous thread of fine gunpowder. To protect the outside of the fuse from moisture it is coated with tar or some other impervious substance. These fuses are so well made that they burn very uniformly at the rate of about 1 ft. in 20 seconds, hence the moment of explosion can be pretty accurately fixed beforehand. Blickford matches have the objection for tunnel work of burning with a bad odor, especially when they are coated with tar, and to remedy this many others have been invented. Those of Rzika and Franzl are the best known of these. The former has many advantages, but it burns too quickly, about 3 ft. per second, and is expensive; the latter consists of a small hollow rope filled with dynamite.
Blickford matches cannot be used to explode dynamite, the use of a cartridge being required. These cartridges are small copper cylinders containing fulminate of mercury. They may be attached to the end of the Blickford match, which being ignited the spark travels along its length until it reaches the copper cylinder, where it explodes the fulminate of mercury, which in turn explodes the dynamite. Blasts may also be firedby electricity, which, in fact, is the most common and the preferable method, because several blasts can be fired simultaneously, and because the current is turned on at a great distance, thus affording greater safety to the workmen.
The method of electric firing generally employed in America is known as the connecting series method, and consists in firing several mines simultaneously. The ends of the wires are scraped bare, and the wire of the first hole of the series is twisted together with the wire of the second hole, and so on; finally the two odd wires of the first and last holes are connected to two wires of a single cable or to two separate cables extending to some safe place to which the men can retreat. Here the two cable wires are connected by binding screws to the poles of a battery, or sometimes to a frictional electric machine. The current passes through the wires, making a spark at each break, and so fires the fulminate of mercury, which explodes the dynamite.
Simultaneous firing by electricity by utilizing the united strength of the blasts at the same instant secures about 10% greater efficiency from the explosives. Another advantage of electric firing is that in case of a missfire of any one of the holes there is slight possibility of explosion afterwards, and the place can be approached at once to discover the cause.
—Tamping is the material placed in the hole above the explosive to prevent the gases of explosion from escaping into the air. Tamping generally consists of clay. When gunpowder is used the clay must be well rammed with a wooden tool, and paper, cotton, or some other dry material must be placed between the moist clay and the powder. When dynamite is used it is not necessary to ram the tamping, since the suddenness of the explosion shatters the rock before the clay can be driven from the hole.
A few experienced men should be appointed to fire the blasts. These men should give ample warning previous to the blast in order that all machinery and tools which might be injured by flying fragments may be removed out of danger, and so that theworkmen may seek safety. When all is ready they should fire the blasts, keeping accurate count of the explosions to ensure that no holes have missed fire, and should call the workmen back when all danger is over. In case any hole has missed fire it should be marked by a red lamp or flag.
—When the explosives are ignited a sudden development of gases results, producing a sudden and violent increase of pressure, usually accompanied by a loud report. The energy of the explosion is exerted in all directions in the form of a sphere having its center at the point of explosion, and the waves of energy lose their force as the distance from this central point increases. The energy of the explosion at any point in the sphere of energy is, therefore, inversely proportional to the distance of this point from the center of explosion. In the vicinity of the center of explosion the gases have sufficient power to destroy the force of cohesion and shatter the rock; further on, as they lose strength, they only destroy the elasticity of the material and produce cracks; and still further away they only produce a shock, and do not affect the material. Within the sphere of energy there are, therefore, three other concentric spheres: the first one being where cohesion is destroyed, the second where elasticity is overcome, and the third where the shock is transmitted by elasticity. When the latter sphere comes below the surface, the gases remain inside the rock; but when the surface intersects either of the other two spheres, the gases blow up the rock, forming a cone or crater, whose apex is at the point of explosion, and which is called the blasting-cone. The larger the blasting-cone is, the greater is the amount of rock broken up; and the object of the engineer should, therefore, always be so to regulate the depth of the hole and the quantity of explosive as to secure the largest possible blasting cone in each case. Experiments are required to determine the most efficient depth of hole, and quantity of explosive to be employed, since these differ in different kinds of rock, with the position of the rock strata,etc.; but in ordinary practice, the depths of the holes are made from 2 to 3 ft. in the heading and upper portion of the tunnel, when drilled by hand; and from 6 to 8 ft. when drilled by power drills. In the lower portion of the profile, the holes are made deeper, from 3 ft. to 4 ft. when drilled by hand, and exceeding 6 ft. when drilled by power. The distance of the holes apart should be about equal to the diameter of the blasting-cone; as a general rule it is assumed that the base of the blasting-cone has a diameter equal to twice the depth of the hole. The following table gives the average number of holes required in each part of the excavation for the St. Gothard tunnel in which the heading was excavated by machine drills while the other parts were excavated by hand drills:
[5]The location of the parts numbered is shown byFig. 14,p. 36.
[5]The location of the parts numbered is shown byFig. 14,p. 36.
The quantity of explosives required for blasting depends upon the quality of the rock, since the force of the explosives must overcome the cohesion of the rock, which varies with its nature, and often differs greatly in rocks of the same kind and composition. The quantity of explosives required to secure the greatest efficiency in blasting any particular rock may be determined experimentally, but in practice it is usually deduced by the following rules: (1) The blasting force is directly proportional to the weight of the explosives used, and (2) the bulk of the blasted rock is proportional to the cube of the depth of the holes. It is usually assumed, also, that the explosive should fill at least one-fourth the depth of the hole.
The following table gives the depth of holes and amount of dynamite used at each advance in theFort George Tunnelillustrated onpage 135.
A number of different modes of procedure are followed in excavating tunnels, and each of the more important of these will be considered in a separate chapter. There are, however, certain characteristics common to all of these methods, and these will be noted briefly here.
Fig. 14.—Diagram Showing Sequence of Excavation for St. Gothard Tunnel.Fig. 15.—Diagram Showing Manner of Determining Correspondence of Excavation to Sectional Profile.
Fig. 14.—Diagram Showing Sequence of Excavation for St. Gothard Tunnel.
Fig. 14.—Diagram Showing Sequence of Excavation for St. Gothard Tunnel.
Fig. 14.—Diagram Showing Sequence of Excavation for St. Gothard Tunnel.
Fig. 15.—Diagram Showing Manner of Determining Correspondence of Excavation to Sectional Profile.
Fig. 15.—Diagram Showing Manner of Determining Correspondence of Excavation to Sectional Profile.
Fig. 15.—Diagram Showing Manner of Determining Correspondence of Excavation to Sectional Profile.
—It may be asserted at the outset that the whole area of the tunnel section is not ordinarily excavated at one time, but that it is removed in sections, and as each section is excavated it is thoroughly timbered or strutted. The order in which these different sections are excavated varies with the method of excavation, and it is clearly shown for each method in succeeding chapters. As a single example to illustrate the proposition just made, the division of the section and the sequence of excavation adopted at the St. Gothard tunnel is selected (Fig. 14). The different parts of the section were excavated in the order numbered; the names given to each part, and the number of holes employed in breaking it down, are given by thetableonpage 35. Whatever method is employed, the work always begins by driving a heading, which is the most difficult and expensive part of the excavation. All the other operations required in breaking down the remainderof the tunnel section are usually designated by the general term of enlargement of the profile. The various operations of excavation may, therefore, be classified either as excavation of the heading or enlargement of the profile.
—There is considerable confusion among the different authorities regarding the exact definition of the term “heading†as it is used in tunnel work. Some authorities call a small passage driven at the top of the profile a heading, and a similar passage driven at the bottom of the profile a drift; others call any passage driven parallel to the tunnel axis, whether at the top or at the bottom of the profile, a drift; and still others give the name “heading†to all such passages. For the sake of distinctness of terminology it seems preferable to call the passage a heading when it is located at the top of the profile, and a drift when it is located near the bottom.
Headings and drifts are driven in advance of the general excavation for the following purposes: (1) To fix correctly the axis of the tunnel; (2) to allow the work to go on at different points without the gangs of laborers interfering with each other; (3) to detect the nature of material to be dealt with and to be ready in any contingency to overcome any trouble caused by a change in the soil; and (4) to collect the water. The dimensions of headings in actual practice vary according to the nature of the soil through which they are driven. As a general rule they should not be less than 7 ft. in height, so as to allow the men to work standing, and have room left for the roof strutting. The width should not be less than 6 ft., to allow two men to work at the front, and to give room for the material cars without interfering with the wall strutting. Usually headings are made 8 ft. wide. The length of headings in practice varies according to circumstances. In very long tunnels through hard rock the headings are sometimes excavated from 1000 ft. to 2000 ft. in advance, in order that they may meet as soon as possible and the ranging of the center linebe verified, and so that as great an area of rock as possible may be attacked at the same time in the work of enlarging the profile. In short tunnels, where the ranging of the center line is less liable to error, shorter headings are employed, and in soft soils they are made shorter and shorter as the cohesion of the soil decreases. When the material has too little cohesion to stand alone, the tops and sides of the heading require to be supported by strutting. To prevent caving at the front of the heading, the face of the excavation is made inclined, the inclination following as near as may be the natural slope of the material.
—The enlargement of the profile is accomplished by excavating in succession several small prisms parallel to the heading, and its full length, which are so located that as each one is taken out the cross-section of the original heading is enlarged. The number, location, and sequence of these prisms vary in different methods of excavation, and are explained in succeeding chapters where these methods are described. To direct the excavation so as to keep it always within the boundaries of the adopted profile, the engineer first marks the center line on the roof of the heading by wooden or metal pegs, or by some other suitable means by which a plumb line may be suspended. He next draws to a large scale a profile of the proposed section; and beginning at the top of the vertical axis he draws horizontal lines at regular intervals, as shown byFig. 15, until they intersect the boundary lines of the profile, and designates on each of these lines the distance between the vertical axis and the point where it intersects the profile. It is evident that if the foreman of excavation divides his plumb line in a manner corresponding to the engineer’s drawing, and then measures horizontallyand at right angles to the vertical center plane of the tunnel the distance designated on the horizontal lines of the drawing, he will have located points on the profile of the section, or in other words have established the limits of excavation.
Fig. 16.—Polar Protractor for Determining Profile of Excavated Cross-Section.
Fig. 16.—Polar Protractor for Determining Profile of Excavated Cross-Section.
In the excavation of the Croton Aqueduct for the water supply of New York city, an instrument called a polar protractor was used for determining the location of the sectional profile. It was invented by Mr. Alfred Craven, division engineer of the work. This instrument consists of a circular disk graduated to degrees, and mounted on a tripod in such a manner that it may be leveled up, and also have a vertical motion and a motion about the vertical axis. The construction is shown clearly byFig. 16. In use the device is mounted with its center at the axis of the tunnel. A light wooden measuring-rod tapering to a point, shod with brass and graduated to feet and hundredths of a foot, lies upon the wooden arm or rest, which revolves upon the face of the disk, and slides out toa contact with the surface of the excavation at such points as are to be determined. If the only information desired is whether or not the excavation is sufficient or beyond the established lines, the rod is set to the proper radius, and if it swings clear the fact is determined. If a true copy of the actual cross-section is desired, the rod is brought into contact with the significant points in the cross-section, and the angles and distances are recorded.
The general method of directing the excavation in enlarging the profile by referring all points of the profile to the vertical axis is the one usually employed in tunneling, and gives good results. It is considered better in actual practice to have the excavation exceed the profile somewhat than to have it fall short of it, since the voids can be more easily filled in with riprap than the encroaching rock can be excavated during the building of the masonry. In tunnels where strutting is necessary the excavation must be made enough larger than the finished section to provide the space for it. In soft-ground tunnels it is also usual to enlarge the excavation to allow for the probable slight sinking of the masonry. The proper allowance for strutting is usually left to the judgment of the foreman of excavation, but the allowance for settlement must be fixed by the engineer.
Shafts are vertical walls or passages sunk along the line of the tunnel at one or more points between the entrances, to permit the tunnel excavation to be attacked at several different points at once, thus greatly reducing the time required for excavation. Shafts may be located directly over the center of the tunnel or to one side of it, and, while usually vertical, are sometimes inclined. During the construction of the tunnel the shafts serve the same purpose as the entrances; hence they must afford a passageway for the excavated materials, whichhave to be hoisted out, and also for the construction tools and materials which have to be lowered down them. They must also afford a passageway for workmen, draft animals, and for pipes for ventilation, water, compressed air, etc. The character of this traffic indicates the dimensions required, but these depend also upon the method of hoisting employed. Thus, when a windlass or horse gin is used, and the materials are hoisted in buckets of small dimensions, the dimensions of the shaft may also be small; but when steam elevators are employed, and the material is carried on cars run on to the platform of the elevator, large dimensions must be given to the shaft. Generally the parts of the shaft used for different purposes are separated by partitions. The elevator for workmen and the various pipes are placed in one compartment, while the elevator for hoisting the excavated material and lowering construction material is placed in another.
Shafts may be either temporary or permanent. They are temporary when they are filled in after the tunnel is completed, and permanent when they are left open to supply ventilation to the tunnel. Permanent shafts are usually made circular, and lined with brick, unless excavated in very hard and durable rock. When sunk for temporary use only, shafts are usually made rectangular with the greater dimension transverse to the tunnel. They are strutted with timber. A pump is generally located at the bottom of the shaft to collect the water which seeps in from the sides of the shaft and from the tunnel excavation. The dimensions of this pump will of course vary with the amount of water encountered, as will also the capacity of the pump for forcing it up and out of the shaft, which has always to be kept dry.
The majority of engineers prefer to sink shafts directly over the center line of the tunnel. Side shafts are employed chiefly by French engineers. The chief advantage of the former method is the great facility which it affords for hoisting out the materials, while in favor of the latter method is thenon-interference of the shaft with the operations inside the tunnel. Were it not that the side shaft requires the introduction of a transverse gallery connecting it with the tunnel, it would be on the whole superior to the center shaft; but the side gallery necessitates turning the cars at right angles, and consequently the use of a very sharp curve or a turntable to reach the shaft bottom, which is a disadvantage that may outweigh its advantages in some other respects. It is impossible to state absolutely which of these methods of locating shafts is the best; both present advantages and disadvantages, and the use of one or the other is usually determined more by the local conditions than by any general superiority of either.
When side shafts are employed they are sometimes made inclined instead of vertical. This form is used when the depth of the shaft is small. By it the hauling is greatly simplified, since the cars loaded at the front with excavated material can be hauled directly out of the shaft and to the dumping-place, surmounting the inclined shaft by means of continuous cables. The short galleries connecting the side shafts with the tunnel proper usually have a smaller section than the tunnel, but are excavated in exactly the same manner. Another form of side shaft sometimes used is one reaching to the surface when the tunnel runs close to the side of cliff, as is the case with some of the Alpine railway tunnels.
Tunnels are classified in various ways, but the most logical method would appear to be a grouping according to the quality of the material through which they are driven; and this method will be adopted here. By this method we have first the following general classification: (1) Tunnels in hard rock; (2) tunnels in ordinary loose soil; (3) tunnels in quicksand; (4) open-cut tunnels; and (5) submarine tunnels. It is hardly necessary to say that this classification, like all others, is simplyan arbitrary arrangement adopted for the sake of order and convenience in treating the subject.
—With the numerous labor-saving methods and machines now available, hard rock is perhaps the safest and easiest of all materials through which to drive a tunnel. Tunnels through hard rock may be excavated, either by a drift or by a heading. The difference depends upon whether the advance gallery is located close to the floor or near the soffit of the section.
—In driving tunnels through loose soils many different methods have been devised, which may be grouped as follows: (1) Tunnels excavated at the soffit—Belgian method; (2) tunnels excavated along the perimeter—German method; (3) tunnels excavated in the whole section—English, Austrian and American methods; (4) tunnels excavated in two halves independent of each other—Italian method.
(1) Excavating the tunnel by beginning at the soffit of the section, or by the Belgian method, is the method of tunneling in loose soils most commonly employed in Europe at the present time. It consists in excavating the soffit of the section first; then building the arch, which is supported upon the unexcavated ground; and finally in excavating the lower portion of the section, and building the side walls and invert.
(2) In excavating tunnels along the perimeter an annular excavation is made, following closely the outline of the sectional profile in which the lining masonry is built, after which the center core is excavated. In the German method two drifts are opened at each side of the tunnel near the bottom. Other drifts are excavated, one above the other, on each side to extend or heighten the first two until all the perimeter is open except across the bottom. The masonry lining is then built from the bottom upwards on each side to the crown of the arch, and then the center core is removed and the invert is built.
(3) This method, as its name implies, consists in taking out short lengths of the whole sectional profile before beginning the building of the masonry. In the English method the invert is built first, then the side walls, and finally the arch. The excavators and masons work alternately. The Austrian method differs in two particulars from the English: the length of section opened is made great enough to allow the excavators to continue work ahead of the masons, and the side walls and roof are built before the invert. In the American method the whole section of the tunnel is open at once: excavators and masons work simultaneously, but a very large quantity of timbering is required.
(4) The Italian method is very seldom employed on account of its expensiveness, but it can often be used where the other methods fail. It consists in excavating the lower half of the section, and building the invert and side walls, and then filling the space between the walls in again except for a narrow passageway for the cars; next the upper part of the section is excavated, as in the Belgian method, and the arch is built; and finally the soil in the lower part is permanently removed.
—Tunnels through quicksand are driven by one of the ordinary soft-ground methods after draining away the water, or else as submarine tunnels.
—Open-cut tunnels are those driven at such a small depth under the surface that it is more convenient to excavate an open cut, build the tunnel masonry inside it, and then refill the open spaces, than it is to carry on the work entirely underground. In firm soils the usual mode of operation is to excavate first two parallel trenches for the side walls, then remove the core, and build the arch and the invert. In unstable soils, since the invert must be built first, it is usual to open up a single wide trench. In infrequent cases where a tunnel is desired in a place which is to be filled in, the masonry is built as a surface structure, which in due time is covered.
—The mode of procedure followed in excavating submarine tunnels depends upon whether the material penetrated is pervious or impervious to water. In impervious material any of the ordinary methods of tunneling found suitable may be employed. In pervious material the excavation may be accomplished either by means of compressed air to keep the water out of the excavation, or by means of a shield closing the front of the excavation, or by a combination of these two methods. Tunnels on the river bed are built by means of coffer dams which inclose alternate portions of the work, by sinking a continuous series of pneumatic caissons and opening communication between them, and by sinking the tunnel in sections constructed on land.
The above diagram gives in compact form the classification of tunnels according to materials penetrated and methods of excavation adopted, which have been described more fully in the succeeding paragraphs. It may be noted here again that this is a purely arbitrary classification, and serves mostly as a convenience in discussing the different classes of tunnels without confusion.
The purpose of timbering or strutting in tunnel work is to prevent the caving-in of the roof and side walls of the excavation previous to the construction of the lining. As the strutting has to resist all the pressures developed in the roof and side walls, which may be exceedingly troublesome and of great intensity in loose soils, its design and erection call for particular care. The method of strutting adopted depends upon the method of excavation employed; but in every case the problem is not only to build it strong enough to withstand the pressures developed, but to do this as economically as possible, and with as little hindrance as may be to the work which is going on simultaneously and which will come later. Only the latter general problems of strutting peculiar to all methods of tunnel work will be considered here. For this consideration strutting may be classified according to the material of which it is built, under the heads of timber structures and iron structures.