CHAPTER XI.

THE PROBLEM OF CONSTRUCTION.

We may now begin to consider the canal itself, the problems which its designers had to solve, the methods of construction, and the features of the completed work. As we have seen, the first two and a half years were a time mainly of preparation for the titanic enterprise of excavation and construction. In fact, it might have been better if the work during that period had been entirely restricted to scavenging, sewering, and so forth. The labourers were hurried a little too fast to the isthmus, before the isthmus was properly cleaned up to receive them. Hence the yellow fever panic and difficulties which might have been avoided. The people of the United States were responsible for this over-haste at the start. The great thing, they cried, is to "make the dirt fly." They wanted evidence that the steam-navvies were actually at work in the bed of the canal and that the task was well in hand. In fact, the public at home took an interest in the canal operations which was sometimes embarrassing. Some newspaper man at the isthmus would report an accident or unforeseen difficulty, probably with a good deal of exaggeration, an anxious excitement sprang up among the people, and special commissions had to go to the isthmus in order to investigate the true state of affairs and if possible restore confidence at home.

As the reader knows, the Americans had no clean slate on which to write at Panama. They succeeded two French companies which had been at work for twenty years. True, the New Panama Canal Company which succeeded the Lesseps Company had not greatly perspired over the undertaking. It had kept a certain amount of work going, chiefly in order to maintain its concession. All the same, the French had ploughed a pretty deep furrow between Colon and Panama, and much of the work they had done was fortunately available whichever type of canal should be adopted, high-level or tide-level. They had carried out a good deal of dredging for the channel through the tidal flats at either end of the canal, and they had made a very visible impression on the "continental divide" at what is known as the Culebra Cut. Altogether the French companies excavated 81,548,000 cubic yards. The Americans inherited from their predecessors a large amount of machinery and tools, in addition to a great deal of work well done. Much of the machinery, even of the Lesseps Company, was found to be in serviceable condition, and operations could be continued with it, though the extent and efficiency of the plant was, of course, as time went on, greatly increased.

The main problem which the American engineers had to solve was how to deal with the Chagres River. On the tide-level scheme, that violent and capricious stream, which in the rainy season was navigable for half its length of 100 miles, would have had to be diverted into another channel or ponded back in its upper waters by a high dam at Gamboa, some of the overflow of which might perhaps have been permitted to pass into the canal. But, as we have seen, the Chagres would have to be utilized and at the same time controlled if the high-level plan was adopted. A river which is capable of rising 35-1/2 feet in twenty-four hours needed a great deal of regulation and discipline before it could be used as the feeder of the upper reaches of a lock canal. The only way to do this was to diffuse its waters over a vast artificial lake which it would keep full, but in which its floods and current would be effectually tamed. This could only be done by a huge dam intercepting the course of the river in its lower reaches, at some point before it entered the Caribbean Sea. When the New Panama Canal Company changed its plans and decided for an elevated waterway, it was intended to construct such a barrier at Bohio, a point much higher up stream than Gatun, the site ultimately chosen by the American engineers. The Isthmian Canal Commission which reported in 1901, also arranged for a dam at Bohio to control the Chagres River. On this plan the river would have been intercepted much higher up, and the artificial lake would have been much smaller. But when the Americans finally decided on the high-level type in 1906, the site of the proposed dam was shifted from Bohio to Gatun, nearer the river's mouth, which involved the inundation of a much vaster area of country. This position for the dam was first suggested by a French engineer, Godin de Lepinay, who, in a paper read before the congress of engineers in Paris in 1879, advocated a lock canal with a dam controlling the Chagres River at Gatun. This, then, was the biggest problem peculiar to the high-level scheme, for the cutting through the "continental divide," though an even more titanic labour, would have had to be accomplished whatever type of canal had been adopted.

No feature of the construction has been subject to so much criticism and anxious solicitude as this Gatun dyke. On it depends the maintenance of Gatun Lake and the supply of water for the canal. If the dam fails, everything fails. The real cause of the difficulty was the foundation upon which this big artificial hill had to be laid. The great dam at Assouan in Egypt is based upon the eternal granite, upon which masonry of natural stone is built. It is, therefore, part and parcel of the solid framework of our planet, and will probably last as long. The Gatun dam is, however, founded upon the alluvial deposits of the Chagres River. This alluvium consists of gravel firmly cemented with mud and clay, and is unquestionably water-tight. These deposits go down in places to a depth of 280 feet before the solid rock is reached. The dam had, therefore, to be laid down on the top of them.

Now this foundation, though water-tight, is soft. It would have been impossible to place upon it a massive structure of rock or concrete. The deposits would have given way under its weight. The only plan was to dump down in the valley an earthen dam, making it very broad so as to distribute the weight over as large a space as possible of the alluvium underneath. A steep slope would have been impossible, for the weight of the central portion would have pushed the clay and gravel outwards, and the whole mass would have subsided. The earth-dam was to block the valley through which the Chagres had hitherto flowed uninterruptedly to the sea. This valley is a mile and a half wide, and this is, therefore, the length of the dam. Its base is 2,100 feet wide. It is 398 feet through at the surface of the water, 100 feet wide at the top, and was to be 115 feet above sea-level. The last figure has, it seems, been brought down to 104 feet, which will be an advantage, as the weight upon the foundations will be proportionately less.

In the middle of the dam the level of the lake is controlled by a channel called the "spillway," with walls and floor of concrete, by which the surplus waters will be sluiced off into the old bed of the Chagres River and so passed on to the sea. The entrance to this channel is closed with falling gates or doors. This safety-valve will no doubt be capable of dealing with the biggest and quickest rise of the lake-level that is ever likely to take place. It can pass off 137,000 cubic feet of water a second, the water issuing at a speed of 35 feet a second. But, to complete the security, the big culverts of the mighty Gatun locks close by can be turned open, and 170,000 cubic feet a second carried off there. Indeed, as regards the Gatun Lake the anxiety, if there be any, is that the water-supply will be insufficient rather than dangerously excessive.

The level of the lake is to be kept at 85 feet above mean sea-level--that is, the dam, or a considerable length of it, will be exposed to what is called a "head" of water of 85 feet. The lake itself will be 164 square miles in extent. There have been many rational anxieties on the sufficiency of the dam. A certain American senator, however, who visited the works during the construction, worried himself rather unnecessarily on this last figure. Colonel Goethals was showing a congressional delegation round the works, and in the course of the survey they came to the dam with the broad expanse of water behind it. "Colonel," he said, "how is it that so small a body of earth as the Gatun dam can hold in check such a tremendous body of water as the Gatun Lake?" The chief engineer explained that the pressure of a body of water is determined by its height and not by its volume. The inquirer seems not to have been satisfied with the statement of this hydrostatic law. Senator Knox, afterwards Secretary of State, then came to his aid. "Senator," he said, "if your theory were true, how could the dykes of Holland hold in check the Atlantic Ocean?" This was a clincher, and the sceptic joined in the laugh at his own expense.

All the same, the Gatun dam has two extremely responsible and heavy duties to perform. It has to withstand the horizontal thrust of a head of 85 feet of water so as not to be carried bodily down the Chagres bed into the Atlantic. And it has to block up the valley so effectually that the water of the lake shall not percolate through at any point. There is every reason to believe that, in spite of all alarums and excursions during its construction, it will fulfil both these requirements. Its composition and construction may be briefly described. Two bulwarks of big rocky fragments were built up on either outer line or "toe" of the structure. This rough material was obtained from the lock site, or Mindi, or the Culebra Cut twenty-six miles away. The area between these piles is filled with silt, and water pumped into it by hydraulic dredges from the Chagres valley. The surplus water is carried off through pipes. The sodden silt remains and is packed down and consolidated by atmospheric pressure. Such a "hydraulic fill" is impervious to water, the thrust or "head" of which is very quickly lost in the minute interstices or pores of the material. It will be seen how such a structure differs from a dam of concrete or stone masonry. It is porous, while at the same time impervious to water.

The future traveller through the Panama Canal will probably never guess the immensity of the labour that has gone to the building of the Gatun dam. Already, indeed, it looks so much like a part of the natural landscape that it might well escape special observation altogether. Yet nothing less than 21,145,931 cubic yards of material were laid down--enough to make a wall of earth three feet high and three feet thick reaching nearly halfway round the world. The spillway itself contains 225,485 cubic yards of concrete.

It will be noticed that in the dam proper there is no core of masonry or puddled chalk or clay whatever. It was at one time intended that there should be. I have alluded to the alarmist rumours that were raised again and again at Panama and created much uneasiness in the United States. These were especially concerned with the great dam, and that word must have frequently been on the lips of the engineers in more than one significance. Every possible test was applied to determine the exact character of the underlying materials, to ascertain whether there was any connection between the swamp areas to the north and south through the deposits in the gorges which the earthwork was to bridge, to prove the ability of the material below to support the structure, and to find out whether suitable material for the dam could be found in its neighbourhood. "As the result of all these investigations," wrote Colonel Goethals,[15] "it may be briefly stated that the underlying material is impervious to water; that it possesses ample strength to uphold the structure that will be placed upon it, and, the subsoil being impervious, that there is no connection between the swamps above and the sea below."

In order to make assurance doubly sure, Colonel Goethals planned the dam so as to include triple interlocking steel sheet-piling across the valley, driven down to bedrock, and decided to carry the dam to a height of 135 feet. Even so, the news of a collapse was wired home, and this so impressed President Roosevelt that he sent a commission of engineers to the isthmus accompanied by President-elect Taft. The investigations had a different result from what had been expected. Instead of being dissatisfied with the size and strength of the dam, the engineers declared that it was being built too high and that the steel piling was unnecessary. It must be admitted, therefore, that the efficiency of the Gatun dam has been subjected to the most rigorous tests, and that no further anxiety on the subject need be felt.

With the blocking of the Chagres outlet at Gatun, the waters of the lake have gradually accumulated until they cover an area of 164 square miles. Not only the Chagres itself but its tributaries, the Trinidad and others, are thus ponded back. The reservoir extends up a number of long and winding arms, and is thus very irregular in shape. The bed of the channel itself was cleared of brushwood and trees, but the rest of the valley was thickly overgrown. As the waters rose, therefore, and gradually submerged this primeval forest, a rather dismal spectacle was presented of decay and destruction. The lake has, indeed, completely altered the aspect of the country. Villages and even small towns, whose names had come down from the days of the old navigators, lie buried for ever beneath the waters of Lake Gatun. Even now the great expanse of water with its wooded islands looks like a natural feature of the landscape rather than yesterday's creation of engineering enterprise. The vessels in transit will, of course, keep to the dredged and buoyed channel, but the channel will itself be invisible, and the traveller, after tossing on the restless Caribbean Sea, will enjoy the full sensation of a cruise over a landlocked fjord or lake. Lake Gatun is indeed twice the size of Lago Maggiore and four-fifths the size of Lake Geneva. The journey from Gatun to Bas Obispo, where the waterway again assumes the appearance of a canal and enters the Culebra gorge, is 22 miles, but the same 85-foot level is maintained right to the locks at Pedro Miguel, where the waters of Lake Gatun are again retained by a dam connecting the walls of the lock with a hill to the west. The rest of the lake is held in by the natural configuration of the country, the only outlets being at the Gatun spillway and, of course, through the locks.

But we must not overlook the main purpose of the lake, which is to supply the water for the canal and the lockages. For this purpose everything, of course, depends on the rainfall at the isthmus, and the question arises whether this may be relied upon to replenish the canal with the needful water-supply. Colonel Goethals estimates that in an average dry season 58 "lockages," or transits of the canal, per day would be possible, which is a greater number than the twenty-four hours of the day would permit, allowing vessels to follow each other at intervals of one hour. Happily, a resource is still left if the supply of water should show signs of proving insufficient. At Alhajuela, on the Chagres River, some nine or ten miles above Obispo, there is an excellent site for a dam, forming a reservoir where some of the surplus water of the rainy season could be stored and supplied to the canal as required in the dry months. Details of the construction of such a dam were prepared in connection with a former canal-scheme, and would be available in case of need.

FOOTNOTES:

[15] _The National Geographic Magazine_, February 1911.