CHAPTER I. PRELIMINARY CONSIDERATIONS. CHOICE BETWEEN A TUNNEL AND OPEN CUT. GEOLOGICAL SURVEYS. CHOICE BETWEEN A TUNNEL AND AN OPEN CUT.

When a railway line is to be carried across a range of mountains or hills, the first question which arises is whether it is better to construct a tunnel or to make such a dÉtour as will enable the obstruction to be passed with ordinary surface construction. The answer to this question depends upon the comparative cost of construction and maintenance, and upon the relative commercial and structural advantages and disadvantages of the two methods. In favor of the open road there are its smaller cost and the decreased time required in its construction. These mean that less capital will be required, and that the road will sooner be able to earn something for its builders. Against the open road there are: its greater length and consequently its heavier running expenses; the greater amount of rolling-stock required to operate it; the heavy expense of maintaining a mountain road; and the necessity of employing larger locomotives, with the increased expenses which they entail. In favor of the tunnel there are: the shortening of the road, with the consequent decrease in the operating expenses and amount of rolling-stock required; the smaller cost of maintenance, owing to the protection of the track from snow and rain and other natural influences causing deterioration; and the decreased cost of hauling due to the lighter grades. Against the tunnel, there are its enormous cost as compared with an open road and the great length of time required to construct it.

To determine in any particular case whether a tunnel or an open road is best, requires a careful integration of all the factors mentioned. It may be asserted in a general way, however, that the enormous advance made in the art of tunnel building has done much to lessen the strength of the principal objections to tunnels, namely, their great cost and the length of time required for their construction. Where the choice lies between a tunnel or a long dÉtour with heavy grades it is sooner or later almost always decided in favor of a tunnel. When, however, the conditions are such that the choice lies between a tunnel or a heavy open cut with the same grades the problem of deciding between the two solutions is a more difficult one.

It is generally assumed that when the cut required will have a vertical depth exceeding 60 ft. it is less expensive to build a tunnel unless the excavated material is needed for a nearby embankment or fill. This rule is not absolute, but varies according to local conditions. For instance, in materials of rigid and unyielding character, such as rock, the practical limit to the depth of a cut goes far beyond that point at which a tunnel would be more economical according to the above rule. In soils of a yielding character, on the other hand, the very flat slope required for stability adds greatly to the cost of making a cut.

It may be noted in closing that the same rule may be employed in determining the location of the ends of the tunnel, for assuming that it is more convenient to excavate a tunnel than an open cut when the depth exceeds 60 ft., then the open cut approaches should extend into the mountain- or hill-sides only to the points where the surface is 60 ft. above grade, and there the tunnel should begin. If, therefore, we draw on the longitudinal profile of the tunnel a line parallel to the plane of the tracks, and 60 ft. above it, this line will cut the surface at the points where the open-cut approaches should cease and the tunnel begin. This is a rule-of-thumb determination at the best, and requires judgment in its use. Should the ground surface, for example, rise only a few feet above the 60 ft. line for any distance, it is obviously better to continue the open cut than to tunnel.

THE METHOD AND PURPOSE OF GEOLOGICAL SURVEYS.

When it has been decided to build a tunnel, the first duty of the engineer is to make an accurate geological survey of the locality. From this survey the material penetrated, the form of section and kind of strutting to be used, the best form of lining to be adopted, the cost of excavation, and various other facts, are to be deduced. In small tunnels the geological knowledge of the engineer should enable him to construct a geological map of the locality, or this knowledge may be had in many cases by consulting the geological maps issued by the State or general government surveys. When, however, the tunnel is to be of great length, it may be necessary to call in the assistance of a professional geologist in order to reconstruct accurately the interior of the mountain and thereby to ascertain beforehand the different strata and materials to be excavated, thus obtaining the data for calculating both the time and cost of excavating the tunnel.

The geological survey should enable the engineer to determine, (1) the character of the material and its force of cohesion, (2) the inclination of the different strata, and (3) the presence of water.

Character of Material.

—The character of the material through which the proposed tunnel will penetrate is best ascertained by means of diamond rock-drills. These machines bore an annular hole, and take away a core for the whole depth of the boring, thus giving a perfect geological section showing the character, succession, and exact thickness of the strata. By making such borings at different points along the center line of the projected tunnel, and comparing the relative sequence and thickness of the different strata shown by the cores, the geological formation of the mountain may be determined quite exactly. Where it is difficult or impracticable to make diamond drill borings on account of the depth of the mountain above the tunnel, or because of its inaccessibility, the engineer must resort to other methods of observation.

The present forms of mountains or hills are due to weathering, or the action of the destructive atmospheric influences upon the original material. From the manner in which the mountain or hill has resisted weathering, therefore, may be deduced in a general way both the nature and consistency of the materials of which it is composed. Thus we shall generally find mountains or hills of rounded outlines to consist of soft rocks or loose soils, while under very steep and crested mountains hard rock usually exists. To the general knowledge of the nature of its interior thus afforded by the exterior form of the mountain, the engineer must add such information as the surface outcroppings and other local evidences permit.

For the purposes of the tunnel builder we may first classify all materials as either, (1) hard rock, (2) soft rock, or (3) soft soil.

Hard rocks are those having sufficient cohesion to stand vertically when cut to any depth. Many of the primary rocks, like granite, gneiss, feldspar, and basalt, belong to this class, but others of the same group are affected by the atmosphere, moisture, and frost, which gradually disintegrate them. They are also often found interspersed with pyrites, whose well-known tendency to disintegrate upon exposure to air introduces another destructive agency. For these reasons we may divide hard rocks into two sub-classes; viz., hard rocks unaffected by the atmosphere, and those affected by it. This distinction is chiefly important in tunneling as determining whether or not a lining will be required.

Soft rocks, as the term implies, are those in which the force of cohesion is less than in hard rocks, and which in consequence offer less resistance to attacks tending to break down their original structure. They are always affected by the atmosphere. Sandstones, laminated clay shales, mica-schists, and all schistose stones, chalk and some volcanic rocks, can be classified in this group. Soft rocks require to be supported by timbering during excavation, and need to be protected by a strong lining to exclude the air, and to support the vertical pressures, and prevent the fall of fragments.

Soft soils are composed of detrital materials, having so little cohesion that they may be excavated without the use of explosives. Tunnels excavated through these soils must be strongly timbered during excavation to support the vertical pressure and prevent caving; and they also always require a strong lining. Gravel, sand, shale, clay, quicksand, and peat are the soft soils generally encountered in the excavation of tunnels. Gravels and dry sand are the strongest and firmest; shales are very firm, but they possess the great defect of being liable to swell in the presence of water or merely by exposure to the air, to such an extent that they have been known to crush the timbering built to support them. Quicksand and peat are proverbially treacherous materials. Clays are sometimes firm and tenacious, but when laminated and in the presence of water are among the most treacherous soils. Laminated clays may be described as ordinary clays altered by chemical and mechanical agencies, and several modifications of the same structure are often found in the same locality. They are composed of laminÆ of lenticular form separated by smooth surfaces and easily detached from each other. Laminated clays generally have a dark color, red, ocher or greenish blue, and are very often found alternating with strata of stiatites or calcareous material. For purposes of construction they have been divided into three varieties.

Laminated clays of the first variety are those which alternate with calcareous strata and are not so greatly altered as to lose their original stratification. Laminated clays of the second variety are those in which the calcareous strata are broken and reduced to small pieces, but in which the former structure is not completely destroyed; the clay is not reduced to a humid state. Laminated clays of the third variety are those in which the clay by the force of continued disturbance, and in the presence of water, has become plastic. Laminated clays are very treacherous soils; quicksand and peat may be classed, as regards their treacherous nature, among the laminated clays of the third variety.

Inclination of Strata.

—Knowing the inclination of the strata, or the angle which they make with the horizon, it is easy to determine where they intersect the vertical plane of the tunnel passing through the center line, thus giving to a certain extent a knowledge of the different strata which will be met in the excavation. On the inclination of the strata depend: (1) The cost of the excavation; the blasting, for instance, will be more efficient if the rocks are attacked perpendicular to the stratification; (2) The character of the timbering or strutting; the tendency of the rock to fall is greater if the strata are horizontal than if they are vertical; (3) The character and thickness of the lining; horizontal strata are in the weakest position to resist the vertical pressure from the load above when deprived of the supporting rock below, while vertical strata, when penetrated, act as a sort of arch to support the pressure of the load above. The foregoing remarks apply only to hard or soft rock materials.

In detrital formations the inclination of the strata is an important consideration, because of the unsymmetrical pressures developed. In excavating a tunnel through soft soil whose strata are inclined at 30° to the horizon, for instance, the tunnel will cut these strata at an angle of 30°. By the excavation the natural equilibrium of the soil is disturbed, and while the earth tends to fall and settle on both sides at an angle depending upon the friction and cohesion of the material, this angle will be much greater on one side than on the other because of the inclination of the strata; and hence the prism of falling earth on one side is greater than on the other, and consequently the pressures are different, or in other words, they are unsymmetrical. These unsymmetrical pressures are usually easily taken care of as far as the lining is concerned, but they may cause serious cave-ins and badly distort the strutting. Caving-in during excavation may be prevented by cutting the materials according to their natural slope; but the distortion of the strutting is a more serious problem to handle, and one which oftentimes requires the utmost vigilance and care to prevent serious trouble.

Presence of Water.

—An idea of the likelihood of finding water in the tunnel may be obtained by studying the hydrographic basin of the locality. From it the source and direction of the springs, creeks, ravines, etc., can be traced, and from the geological map it can be seen where the strata bearing these waters meet the center line. Not only ought the surface water to be attentively studied, but underground springs, which are frequently encountered in the excavation of tunnels, require careful attention. Both the surface and underground waters follow the pervious strata, and are diverted by impervious strata. Rocks generally may be classed as impervious; but they contain crevices and faults, which often allow water to pass through them; and it is, therefore, not uncommon to encounter large quantities of water in excavating tunnels through rock. As a rule, water will be found under high mountains, which comes from the melted ice and snow percolating through the rock crevices.

Some detrital soils, like gravel and sand, are pervious, and others, like clay and shale, are impervious. Detrital soils lying above clay are almost certain to carry water just above the clay stratum. In tunnel work, therefore, when the excavation keeps well within the clay stratum, little trouble is likely to be had from water; should, however, the excavation cut the clay surface and enter the pervious material above, water is quite certain to be encountered. The quantity of water encountered in any case depends upon the presence of high mountains near by, and upon other circumstances which will attract the attention of the engineer.

A knowledge of the pressure of the water is desirable. This may be obtained by observing closely its source and the character of the strata through which it passes. Water coming to the excavation through rock crevices will lose little of its pressure by friction, while that which has passed some distance through sand will have lost a great deal of its pressure by friction. Water bearing sand, and, in fact, any water bearing detrital material, has its fluidity increased by water pressure; and when this reaches the point where flow results, trouble ensues. The streams of water met in the construction of the St. Gothard tunnel had sufficient pressure to carry away timber and materials.

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