The Panama Canal and its Makers Part 3

It will be seen from the map that the dredged sea channel by which our vessel will reach deep water on the Pacific pa.s.ses to the west of the Isle of Naos instead of to the east, as was proposed in the earlier plans.

Returning now to the Gatun locks. The mitre sill of the top lock is 37 feet above mean sea-level, _i.e._, 48 feet below the surface of the lake, which is 85 feet above mean sea-level. But the bottom of the lake here is only about 5 feet above sea-level, the total depth of water immediately above the locks and dam being 80 feet. It follows that, in the extreme case of both gates of one of the top locks (as well as the roller gate) being wrecked, the level of the water in the lake can only fall to the level of +37, which would leave a depth of 32 feet immediately above the dam. Ships of large draft could therefore lie there without being stranded. Moreover, the lake is so large that the outflow through the broken locks would only lower the level 2 feet _per diem_, so that more than three weeks would elapse before the water sank to the level of the mitre sill.

Again, the channel provided by the broken lock would be so small that in the Ca.n.a.l below the calculated current which would result from the outflow would have a velocity of only 3-1/2 miles per hour.

Above the Pedro Miguel and Milaflores locks there is not the same surplus depth of water, so that vessels might be grounded if the locks were broken. Moreover, as there is no wide-spreading lake above Pedro Miguel, the outflow of water would generate a somewhat swift current above the lock, which might be a source of danger to ships.

This circ.u.mstance serves to enforce the apparent paradox that the great area of Lake Gatun is in several respects an element of safety, not, as the layman might suppose, of danger. The hydrostatic pressure upon the dam depends, of course, solely upon the depth of water, not upon the area of the lake, while the greater the contents of the reservoir the more nearly stagnant are its waters.

As there is to be no lock at La Boca, the dams shown there on the plan and profile will not have to be constructed, so that it is not necessary to deal with the questions to which they formerly gave rise.

In the vicinity of the locks at Pedro Miguel and Milaflores, however, dams have to be constructed to hold up the water. At both places the dams will be short, and will be founded upon hard rock,[16] and in each case the head of water to be held up will only be about 40 feet, instead of 80, as at Gatun. The construction of the dams at Pedro Miguel and Milaflores is not, therefore, regarded with anxiety.

[16] "Ca.n.a.l Zone Pilot," pp. 316-317.

The great Gatun dam remains the one important experiment in the whole scheme of the high-level Ca.n.a.l, and much attention is being devoted to the planning of this work. The alluvial foundation is a disadvantage shared by the Bohio site formerly chosen, and all other sites in the lower Chagres valley; so that, having decided upon the Panama route, and a high-level ca.n.a.l, there appears to be no alternative to the construction of a dam upon this kind of bottom. The details of the proposed structure, as elaborated in April, 1908, were as follows:--

The length of the great earthen dam at Gatun is 7,700 feet, its breadth no less than 2,060 feet. The weight of the dam per linear foot is more than 60 times the horizontal pressure of the water in the lake, so that the pressure could not move the whole ma.s.s; and the weight of the dam is spread over such a great width that it is not thought that the ground will sink beneath it. The form of the plan and section is shown on the map, and an idea of the topography may be obtained from the photographs, which I took in April, 1908. The south-eastern end of the dam abuts on the hill of hard, fine-grained, argillaceous sandstone in which the lock-site is being excavated.

The dam, according to these plans, is not to be merely superposed upon the surface, as originally proposed in 1905. Embedded in its earthy ma.s.s there is to be a puddled core, and a trench will be excavated to a level of 40 feet below the sea (-40 feet) for the lower part of this core. Nor is this all that is to be done to check seepage beneath the earthen dam.

From the bottom of the trench excavated for the puddled core, sheet piling, made of 4-inch timbers, is to be driven down for another 40 feet, so that sheet piling and puddled core together will form an impervious barrier to -80 feet; that is to say, 80 feet below the surface-level of the sea, or about 85 feet below the lowest natural surface of the ground. The puddled core is carried up through the earthen dam to the level of +90, that is to say, 5 feet above the level of the lake, which is to be 85 feet above sea. The crest of the dam will be +135 feet, _i.e._, 50 feet above the level of the lake; this excess of height being to provide top weight for increased stability of the whole structure, and also for the purpose of compacting the underlying material. The underwater slopes of the earthy materials have been reduced from the 1:3 of 1905 to 1:5. On the other hand, it has been decided that the width of 2,625 feet given in 1905 was in excess of utility, and that a reduction of between 500 and 600 feet can be made without loss of strength or efficiency.

[Ill.u.s.tration: EXCAVATION IN THE CUT.]

[Ill.u.s.tration: PIPE FOR DIVERSION OF A RIVER, NEAR EMPIRE.]

About half way across the valley occurs a low hill, on which a house is shown in the photograph. This hill is on the crest-line of the dam, and is useful as giving support to the sides of the regulating channel which will be excavated in it. The material of the hill, however, is not the hard argillaceous sandstone of the lock site, but merely alluvial. The regulating works themselves will be built of concrete: a solid ma.s.s built up to +69 feet, and on this piers will be constructed 8 feet in thickness, between which will be the sluice-gates. By their means the level of the lake will be prevented from rising unduly in flood time.

The capability of the dam to maintain the waters of the lake at a sufficient level in the dry season depends upon their not finding a ready way either through the dam itself or below it. The construction of the dam is believed to guarantee its own practical impermeability. Not only is there a puddled core, but the mud, sand, and rocks of which the princ.i.p.al ma.s.s will be composed will be laid down in the manner best calculated to secure compactness. With regard to underground flow, there is an underlying bed of indurated clay which is regarded as sufficiently impervious, and wherever the puddled core and piling are imbedded in that clay it may, I think, be a.s.sumed with some confidence that the leakage will be unimportant. On referring to the section (map), however, it will be seen that there are in the valley two old river gorges, which to a depth of 200 and 260 feet are filled only with gravel, sand; sand, sh.e.l.ls, and wood; clayey sand, and so forth. These gorges, measured on the section shown in the figure, have widths of about 1,200 and 500 feet respectively at the depth to which the sheet piling goes, and extend about 120 and 180 feet below. How much water may escape by these gorges it is difficult to say. This leads us to the next division of our subject.

_On the Supply of Water Available for the Needs of the High-level Ca.n.a.l._

The construction of the Suez Ca.n.a.l was a work of excavation pure and simple. The construction of any kind of ca.n.a.l across the Isthmus of Panama involves another task, second only in importance to the primary work of excavation, viz., that of regulating the rivers.

In the case of a sea-level ca.n.a.l the problem would have been how to get rid of their waters, particularly in the rainy season.

In the actual case of an 85-foot-level ca.n.a.l, the regulation of the rivers, particularly of the Chagres, presents two aspects, viz.:--

(1) In the wet season, disposing of the surplus waters.

(2) In the dry season, conserving water supplied by the rains so as to meet the waste caused (_a_) by locking, (_b_) by evaporation, (_c_) by percolation.

The arrangements for taming the torrents of the Chagres and its tributaries have already been described. They are, briefly, the construction of the Gatun dam and its spillway.

Turning to the other aspect of the problem, I have to answer the question, What is the guarantee that there will be sufficient water in the dry season?

[Ill.u.s.tration: IN THE CUT, WIDTH 500 FEET.]

[Ill.u.s.tration: IN THE CUT, LOOKING SOUTH TOWARDS CULEBRA.]

Probably there is no problem of the Panama Ca.n.a.l which has received more prolonged and careful study than this. From the outset the French engineers commenced collecting data relating to the hydrology of the Isthmus, and when funds grew low, and the proposed level of the ca.n.a.l began to rise, the matter received ever-increasing attention. The _Comite Technique_ of the New Panama Ca.n.a.l Company commenced in 1894 elaborate investigations to determine the catchment area, the amount of rainfall, and the discharge of rivers. Brigadier-General Henry L. Abbot (late Corps of Engineers, U.S.A.), whose investigations upon the Mississippi are known the world over, was a member of this Committee of the New Panama Company until the work was taken over by the Government of the United States, for whom he continued to act; and he was a member of the Board of Consulting Engineers, signing the minority report in favour of an 85-foot-level ca.n.a.l in January, 1906. A continuous study for seven years is an advantage enjoyed by few of the American engineers, and the book on "Problems of the Panama Ca.n.a.l" published by General Abbot in 1905 (new edition 1907) deals very fully and ably with the hydrology and meteorology of the Isthmus. The observations were continued under the direction of Don Ricardo M. Arango, who has also a long experience on the Isthmus. I shall not attempt to summarise the ma.s.s of data upon which the authorities rely in their calculation that there is a sufficient water supply for the needs of the Ca.n.a.l during the dry season, contenting myself with showing, as above, that in this department of study, which more than all others connected with the Ca.n.a.l demands long experience, this requisite has in fact been secured. Yet whatever depends upon climate is liable to unexpected accidents, and personally I regard as an important safeguard the fact that at Alhajuela, on the Chagres, 9 or 10 miles above Obispo, there is an excellent site for a dam, which would form a reservoir where some of the surplus water of the wet season could be stored, and supplied to the Ca.n.a.l as required. The details for such a dam were elaborated in connection with one of the earlier plans of the Ca.n.a.l, so that the necessary data would be immediately available in case its construction should become necessary in the future.

_Harbours and Fortifications._

There are no storms in the Bay of Panama, and but little additional protection from weather is needed there for shipping. The entrance to the Ca.n.a.l being at La Boca, a new city will grow up there. This will be the second westward migration of the terminal port, the present city of Panama lying between Old Panama and La Boca.

Colon is exposed to northers, and protection against the heavy sea which then rolls in will have to be provided. Whether this will be done by breakwaters or by forming an interior basin is not yet decided, and the cost of this part of the Ca.n.a.l works is therefore not yet known.

The Ca.n.a.l, as already stated, is to be fortified; but I made no inquiries as to the location or character of the proposed fortifications, a matter which I regarded as outside my province. The cost of fortifications is included in the provision made by Congress for the Ca.n.a.l.

CHAPTER III

ON THE PRESENT CONDITION OF THE CULEBRA CUT, AND ON THE METHODS EMPLOYED FOR EXCAVATION AND DISPOSAL OF THE SPOIL

REFERENCE once more to the plan and profile on the map will show at a glance the length and position of the rocky divide, the whole of which is termed the Culebra Cut, from the name of the town near the highest point. The proposed form and dimensions of this cut, throughout the 5 miles of the greatest height, is also shown (the section adopted at the commencement of 1906), and the stage reached in April, 1908, is shown by the photographs. The line drawn across the above section at a level of 120 feet above bottom (160 feet above sea), shows the general level of the bottom of the workings at Culebra itself at the time the photographs were taken. A narrow pilot cut, only, was then 20 feet lower.

[Ill.u.s.tration: ROCK DRILL.]

[Ill.u.s.tration: ROCK DRILLS AT WORK IN THE CUT.]

All that part of the section below this line (+160) remained to be excavated.

Most of the rock above this line has been removed, but not all, for the final width is not, of course, reached at any level until the central portion has been excavated below that level.

The level of the original rock line shown in this section was +275, _i.e._, 235 above ca.n.a.l bottom, so that the photographs show excavation of 115 feet of rock. There was, however, soil above the hard basaltic rock, of varying thickness--removed to the slope 1:2 as shown on the section. The highest original surface of the soil on the centre line of the Ca.n.a.l (between Golden Hill and Silver Hill at Culebra) was +312 feet,[17] so that the photographs in which Golden Hill appears show a total excavation of 152 feet along the centre line. As this line pa.s.sed along a saddle between the two hills, the original surface at the sides was considerably higher, so that the total height shown in the photographs from the bottom of the cut to the highest berm, or ledge, on Golden Hill is considerably more than 152 feet.

[17] The profile at end of volume shows the stage of excavation when the height here had been reduced to +210.

The bottom of the Ca.n.a.l will be 272 feet below the original saddle, and its depth below this berm, which is seen on the photograph, is considerably more. Thus will the gorge appear when the excavation is finished and before the water is allowed to flow in. When full, the surface of the water will be 227 feet below the original saddle, and the pa.s.senger on a vessel will gaze upon the scarped banks of a somewhat greater height than this.

For a tide-level ca.n.a.l, not only would the depth be 85 feet greater, but, as the slope could not be made steeper, the width of the whole cutting would be correspondingly increased.

With reference to the slope of the sides, it is important to note that it has not been found practicable to adhere always to the proposed section, which has to be made flatter, thus considerably increasing the amount of excavation required. The behaviour of living rock is not susceptible of the precise specification which can be applied to quarried stone on the one hand or loose gravel on the other.

Mechanically it is complex, both on account of its structure and of the _role_ which water plays in its economy. In the case of the Culebra rock, the volcanic d.y.k.es by which it is traversed have altered the nature of the rock in their vicinity, and the part played by water is considerable, owing to the wetness of the climate. Moreover, the rock does not remain wholly unchanged when exposed to air, but deteriorates by "weathering," a chemical and physical process which proceeds much faster in an equatorial climate than in the temperate zones. The climate, however, has a compensating action, in so far as the rapid growth of vegetation soon clothes and protects the scarped slopes, thus acting as a "revetment."

Alighting at Culebra station on the Panama Railway, and proceeding to the western side of the cut, one obtains the most impressive view of the Ca.n.a.l works, and this is the spot usually visited by travellers and tourists. I first stood there in January, 1907, and returned in April, 1908. The impressions obtained were very different on these two occasions. In January, 1907, after two and a half years of American occupation, what struck me most was the enormous ma.s.s of material which had been removed by the French companies, and the comparatively insignificant appearance of the American excavations, which could readily be distinguished from the older work, already coated with vegetation. It was then that I began to appreciate the heroic labours of the French engineers, whose achievements under circ.u.mstances of great difficulty are being daily more and more appreciated and praised by their successors. Turning to study the progress of work, I watched with delight the operations of the 100-ton steam shovels, which at a distance, when the human hands are not seen, appear endowed with volition, and remind the spectator of elephants at work. The cars were loaded with surprising celerity, and the dirt-train was hauled off to the distant dump by an old Belgian locomotive, part of the machinery taken over from the New Company. But then the hitch came--there were no cars to take the place of those already filled, and the steam shovel was idle. Looking round, I found that many other steam shovels and their crews were idle from the same cause, the machinery for transportation not having been provided in proper proportion to the machinery of excavation. That the time required for the completion of the rock-cut was limited by the possible rate of transportation of spoil, and not by that of excavation, had long been known, and the report of the Board of Consulting Engineers contains elaborate diagrams of s.p.a.ce available for shovels and for tracks. It was apparent, therefore, that the organisation of the work was not yet perfected. In like manner, as far as I could judge during my first short visit, the West Indian labour was not yielding the best results, owing to white foremen and coloured labourers not being in perfect harmony.

[Ill.u.s.tration: THE CUT, LOOKING NORTH FROM CULEBRA.]