  Admiral Davis’s Report—Table of the Tunnels of the different Isthmean Routes—Altitude of Ridge at Darien—Comparative Cost of Canals with and without Tunnels—Lift Locks and Thorough Cut—Tide in the Atlantic and Pacific—Moderate Lockage can not Obstruct the Navigation—Gisborne on Thorough Cut—His Error as to Velocity of Water—Objections to Strait—Tabular Statement of the Cost of Tunnels, English, French, German, and American—Tunnel of Mont Cenis—Hoosac Tunnel—Profiles of Mont Cenis and Hoosac Tunnels—Dimensions of Ship Tunnel—Cost of Open Canal—General Michler’s Report—Guard Locks Necessary—Cost of System of Lift Locks—Conclusions Supported by Garella and Michel Chevalier. In compliance with a resolution of the Senate, dated March 19, 1866, we have an admirable report from Admiral Davis. In this report the relative merit of different lines is exhibited; carefully prepared tables, showing the amount of freight which would pass the Isthmus; a list of ninety publications and fourteen maps, are appended. Ten of these maps, based on recent surveys, supply much valuable information. “It is to the Isthmus of Darien,” says Admiral Davis, “that we must look for a solution of the question of an interoceanic ship canal.” And he quotes from Airian, “who has made a careful study of this subject,” the assertion that, “with regard to the Cordillera, in proportion as it advances, proceeding from the base of the Isthmus, it descends a good deal, and is only, so to speak, a range of hills or isolated peaks, the bases of which are intersected by ravines, which point out to engineers the true route of the canal. The Indians in the neighborhood of Caledonia Bay make use of these passages. One of them is elevated fifty metres (164 feet), and is covered with a luxuriant growth of mahogany, palm, ebony, and other trees.” “This description,” Admiral Davis remarks, “is not based on actual measurement, but from probabilities deduced from M. Garella’s survey of another part of the Isthmus, and from data, equally conjectural, drawn from the published statements of Messrs. Cullen and Gisborne.” A thorough exploration may justify this conjecture, but no data exists for fixing the absolute altitude at 164 feet. The value of the statements of Messrs. Cullen and Gisborne may be contested. It will be seen from the altitude given in the table below, that however correct in point of fact these opinions may be, they are not sustained by the figures taken from the maps accompanying the Admiral’s report: Table showing the length of Railroads and Canals, length of Tunnels, altitudes of Summits, estimated cost of some of the lines proposed for uniting the two Oceans, from actual surveys:
At the Isthmus of Darien altitudes of from one to two thousand feet are found. Cullen’s pass of 150 feet proved to be estimated at one-ninth of its true height. The least elevation of the divide is that given by M. Bourdial. This engineer did not cross the Isthmus, and his statement is so vague, the reader is left in doubt whether he actually reached the summit. Notwithstanding this uncertainty, there still exists a faint hope that “it is to the Isthmus of Darien we must first look for a solution of the question of an interoceanic canal.” From another statement in this very valuable report, we feel reluctantly compelled to dissent. By imposing unnecessary conditions in the statement of the problem, its solution may be indefinitely postponed. “The interoceanic canal,” it is affirmed, “in width, depth, in supply of water, in good anchorage and secure harbors at both ends, and in absolute freedom from obstruction by lifting-locks, or otherwise, must possess, as nearly as possible, the character of a strait.” To insist that the canal must possess the character of a strait, may give rise to the necessity for a thorough-cut of such extreme depth, or a tunnel of so great length, as to render the work practically impossible. A line suitable for a thorough-cut may possibly be found, but so important a project should not be endangered by limiting its practicability to a communication of that nature. If, by the employment of “lift-locks,” the cost of the canal can be materially reduced, the question to be considered is, to what extent such structures would obstruct navigation? This question depends upon the amount of trade drawn to the Isthmus by the canal. The relative cost of the two methods for piercing the Isthmus can be best determined by a comparison of the cost of a canal in an open country with one by means of tunnels. These considerations, since they afford criteria for judging of the merits of different routes, may be considered more minutely. Let us assume the trade passing over the Isthmus—were the canal now completed—to increase one hundred per cent. in ten years; there would then be 2,066 tons in transitu daily, requiring seven ships of about 300 tons burthen each. Locks of four hundred feet long by ninety feet wide can be filled or emptied in twenty minutes; and this time can be reduced for smaller vessels by additional lock-gates, and for larger vessels by an increase in the size and number of filling valves. The entire trade likely to seek this route, increased four hundred per cent. of its present amount, could be passed through one lock in about four hours and forty minutes. As the vessels come from opposite directions, one-half of the number would be waiting for lockage at the same point, which would reduce the time required for this purpose to two hours and twenty minutes. Eight locks, having an average lift of twelve and one-half feet, would delay the increased commerce eighteen hours and forty minutes, and would raise the level of the canal fifty feet; while to raise the level one hundred feet the delay would not exceed two days. As a summit level may be a necessary part of any Isthmean canal, it is manifest that the resulting lockage can not seriously obstruct navigation. The design of an artificial strait may therefore be reasonably abandoned, if, by so doing, the extraordinary cost of tunneling is excluded by the employment of a small number of lift-locks. On account of the rise of the tide on the Pacific coast guard locks, not much less costly than lift-locks, must be an essential part of any canal from ocean to ocean. The mean tide of the two oceans is about the same.
Mr. Lloyd found a difference of 27.44 feet between high and low water at Panama. The Red Sea is 3 inches higher than the Mediterranean. The Atlantic at Brest is 3½ feet higher than the Mediterranean at Marseilles. The small variation in the mean tide at Panama of the two oceans is probably due to the action of winds and the Gulf Stream. At Panama the highest flood tide rises about ten and one-half feet above the level of the mean tide of the Atlantic, and the extreme ebb falls about the same number of feet below it. The alternate currents through the new strait, caused by the rise and fall of the tide, would prove a serious inconvenience to navigation. The Pacific tide, piling up at the head of the new cut, and entering the strait with considerable violence, would be propelled toward the Gulf in a manner analogous to the progression of the tidal wave in a river. Upon the ebb of the tide a reverse current would prevail. Navigation would not only be obstructed by these alternate currents, but the channel would be choked by drifting timber washed into the canal during the rainy season. Silt and sand would be deposited in bars at the outlet of the canal, or swept inward to form shoals where the current could no longer transport it. Mr. Gisborne, in his report, devotes some space to speculations on these results. “There can be no doubt,” he remarks, “that at high water there will be a current from the Pacific to the Atlantic, and that during the ebb tide there will be a current in the opposite direction. The extent of these currents, and the place of their greatest effect, depends on the comparative sectional area of different portions; and if the cross-section is uniform throughout, will be some time after high tide in the Pacific and at the Atlantic end of the canal. The phase of the tide wave (or the appreciable effect of the tide) will take one and one-half hours to reach from one end to the other, and presuming the current to be uniform in the whole length”——“the question may be examined as a maximum, i. e., what will be the surface velocity of Employing Du Buat’s formula, with the following quantities:
he deduces a maximum surface velocity of three miles per hour. The assumed average fall per mile is strictly a variable function, and at its maximum would give a result greatly in excess of that deduced by Mr. Gisborne. There is no reason for this assumption of a fall of 0.33 of a foot per mile. It directly involves the question to be determined, since the velocity depends upon the inclination of the surface. The value deduced by the formula is not the maximum but the minimum velocity attained in the canal upon the assumed fall per mile. There is another error in Mr. Gisborne’s statement. “The tide,” he remarks, “would take one and one-half hours to reach from one end to the other, presuming the current to be uniform; what,” he asks, “will be the surface velocity in a canal thirty miles long?” This statement contradicts his calculations, and involves also the question at issue. If the tide travels to the end of a canal thirty miles long in “one and one-half hours,” it is evident that it must move at the rate of twenty miles per hour, a velocity which renders Mr. Gisborne’s strait impracticable for navigation. In fact, neither assumption is tenable. The problem is very complex, or, rather, with the data given, indeterminate. It is well known that the tide is propagated up the channel of a river in a succession of long waves, or swells, and that when the tidal wave is entering the mouth of the river, the waves which have reached the head are returning. The same movement is observed, on an exaggerated scale, in the successive breakers which roll in to meet the one which is returning, after it has expended its force upon the beach. In the case of the Isthmean Canal, the rising tide, after having passed the mean, will have a downward slope into the canal. In rivers, notwithstanding the local rise of the water, the slope is never reversed, but is simply reduced in its angle of inclination. The problem involves the inclination of the surface, or the determination of the limits of tidal action at successive stages of While these objections are valid against a thorough-cut canal without locks, they do not apply to a strait of a quarter of a mile in width. As the cost of a canal is the chief difficulty in the way of its construction, it is necessary to abandon the idea of a strait, and to adopt that of a thorough-cut with guard-locks, as the only known means of protecting the canal from the injurious effects of the tide. In order to form a correct opinion of the cost of canals with and without tunnels, attention is called to the expense incurred in the execution of this kind of work. Dimensions and Cost of some English Tunnels.
Canal tunnels are rarely larger than 16½ feet by 18 feet high. Supposing the same dimensions to obtain in French tunnels, the cost per lineal yard of the following named tunnels will furnish a basis for comparison:
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The cost of the Thames tunnel was greatly increased by a shield, designed by Brunel, to keep out the water. Omitting this tunnel from comparison the English works exceed the French, or Continental, in cost of construction. The boldest work of the kind yet undertaken is the Mt. Cenis tunnel, to connect France and Italy by a continuous railway. In length it is seven miles, with a width of 26' 6 and a height of 20' 8. Its completion is anticipated in April, 1871. The monthly advance by hand-labor was twenty-two and a-half yards. The progress is doubled by machinery, and during the past year has averaged 330 feet per month. Air, compressed by water power, is conveyed inside to give motion to chisels, which form cavities for blasting by gunpowder. The average progress per day in 1865, with the machinery, was about 9 feet. The estimated cost was $550 per running foot, but the rate was increased to $640; the entire cost of the tunnel being estimated at $9,200,000. The use of machinery at Mt. Cenis was found to expedite the work, but at an increase of expense. The trial of machinery at the Hoosac tunnel, upon the Troy and Greenfield Railroad, has not been favorable to its employment. This tunnel will be four and three-quarter miles long. Originally projected with a width of 24 feet, and a height of 20 feet, it has been contracted to 14 feet wide, and 18 feet high. The estimated cost was $2,696,229. The rate first assumed was $137 per running foot. The rate per cubic yard varies from $5 to $22, and $30, for the excavation of shafts. The contract prices for the Hoosac tunnel, in 1869, were as follows:
Although more than two hundred railroad tunnels have been constructed in the United States, and an unknown number of canal tunnels, facts in regard to them are difficult of access. Recent bids for tunnel work upon United States railroads have been offered at $5.40 per cubic yard for excavations. Canal tunnels, of the ordinary dimensions of 297 square feet area, would cost $113.20 per running foot. The uncertainty of the nature of tunnel excavation, the unexpected difficulties to be overcome, baffle all anticipatory estimate. The variable rates in the preceding tables establish this fact. The average cost per running yard upon French canals is about $152, which sum probably includes arching. Rates of labor in the United States would increase the cost about four times this amount. Comparing the contract price of American tunnels, as given above, with the table of English tunnels, and bearing in mind that the cost of arching is included in the latter, we find in Nos. 3, 6, and 9, the cost of English tunnels is in excess; number 3 being nearly double, and number 9 one-tenth more, while, in every other case, the cost at American rates is greater, varying from one-third to five and one-half times more. The shale, schist, and trachyte of the Isthmean ridge is of variable consistence. Many places exhibit friable, seamy strata, disintegrating upon exposure to the atmosphere. A tunnel of the dimensions to admit the passage of ships, when carried through rock of this character, will require a lining of masonry to prevent falling material from obstructing the way. To pass ships with the topmast struck, the intrados of the arch should be 100 feet above the surface of the water. A semi-ellipse with semi-transverse, and conjugate diameters of 100 feet, added to the canal prism of thirty feet in depth, will give an area of tunnel equal to 10,104 superficial feet, or to 1,976,263 cubic yards per mile. [Click image to enlarge.] This amount, taken from the careful and elaborate estimates contained in General Michler’s report, may be assumed as a basis of comparison of the two proposed methods of intermarine communication, viz.: by uniting the two oceans upon one level by a tunnel, or by means of a moderate number of “lift-locks.” Eight locks, four at each end of the canal, or sixteen locks, eight at each end of the canal, will raise the summit fifty feet above tide in the first case, and one hundred in the second, and will cost eight millions, and sixteen millions respectively. Since two guard locks will be requisite for either method of communication (i. e. by “strait,” or canal with lift-locks), their cost should be excluded from the above sums, which are thereby reduced to six millions, and fourteen millions of dollars. These sums are fixed as the probable limits of the cost of a system of lift-locks sufficient to overcome the divide of the Isthmus, and also to supply the reader with a standard, by which he may judge of the merits of different routes. The construction of a ship tunnel is, as has been said, “a herculean task,” and it is not apparent that “the prejudice against it will be removed by the operations at Mt. Cenis.” A moderate number of lift-locks seems preferable to a tunnel of one mile in length, which, in turn, would be more economical than an excessive number of locks. A greater number than we have mentioned may be deemed excessive. A thorough-cut upon the level of the ocean would be a desirable method of canalization, but it seems like hampering the important design of an intermarine highway for the commerce of the world, with an impracticable condition, to insist that it should possess “absolute freedom from obstruction by lifting locks,” or that it should possess, in any degree, the “character of a strait.” In this statement I find I have the support of M. Garella and Michel Chavalier. The opposition to the system of lift-locks appears to have originated in the objection expressed in Mr. Wheaton’s letter to Mr. Buchanan, to the large number of these structures, recommended in M. Moro’s plan for the canalization of the Isthmus of Tehuantepec. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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