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.