It was first raised to a height of fifteen feet, so as to cause it to
enter the sand by virtue of its own gravity. It thus penetrated to a
depth of about twenty inches. After this the workmen were ordered to
man the windlasses and hoist out some of the sand. This caused the
tubbing to descend about eight inches more, when it came to a
standstill. It was now loaded with 17,000 pounds of pig iron, but in
vain, for it refused to budge. Mr. Chavatte therefore had recourse to
a dredge with vertical axis, constructed as follows:
Upon a square axis, A B (Pl. 2, Figs. 1, 2, and 3), provided with
double cross braces, C D, and strengthened by diagonals, were riveted,
by their upper extremities, two cheeks, G H, whose lower extremities
held the steel plates, I J I' J', which, in turn, were fastened to the
axis, A B, by their other extremities. These plates were so inclined
as to scrape the surface of the ground over which they were moved.
They each carried two bags made of coarse canvas and strengthened by
five strong leather straps (Figs. 2 and 4). To the steel plates were
riveted two plates of iron containing numerous apertures, through
which passed leather straps designed for fastening thereto the lower
part of the mouth of the bags. That portion of the mouth of the latter
that was to remain open was fastened in the same way to two other
plates, X Y, X¹ Y¹ (Fig. 1), held between the lower cross-braces.
When the apparatus was revolved, the plates scraped the earth to be
removed, and descended in measure as the latter entered the bags.
These bags, when full, were hooked, by means of the five rings which
they carried, to the device shown in Fig. 8 (Pl. 2), and raised to the
surface and emptied into cars.
The dredge was set in motion by four oak levers (Figs. 5 and 6). Two
of these were manned by workmen stationed upon the surface flooring,
and the other two by workmen upon the flooring in the tubbing. The
axis was elongated, in measure as the apparatus descended, by rods of
the same dimensions fastened together by cast iron sleeves and bolts
(Fig. 7).
The steel plates were not capable of acting alone, even in cases where
they operated in pure moving sand containing no pebbles, for the sand
was too compact to be easily scraped up by the steel, and so it had to
be previously divided. For this purpose Mr. Chavatte used rakes which
were in form exactly like those of the extirpators, U and V, of Figs.
1, 2, and 3, of Pl. 2, except that the dividers carried teeth that
were not so strong as those of the extirpators, and that were set
closer together. These rakes were let down and drawn up at will. They
were maneuvered as follows:
The dredge descended with the extirpators pointing upward. When their
heads reached the level of the upper floor, the tools were removed.
Then the dredge was raised again. In this way the extirpators lay upon
the floor, and, if the lifting was continued, they placed themselves
in their working position, in which they were fixed by the bolts A" B"
C" (Fig. 1). After this, the apparatus was let down and revolved. The
rakes divided the earth, the scrapers collected it, and the bags
pocketed it.
The great difficulty was to cause the tubbing to descend vertically,
and also to overcome the enormous lateral pressure exerted upon it by
the earth that was being traversed. Water put into the shaft helped
somewhat, but the great stress to be exerted had to be effected by
means of powerful jack screws. These were placed directly upon the
tubbing, and bore against strong beams whose extremities were inserted
into the masonry.
As a usual thing it is not easy to use more than four or six such
jacks, since the number of beams that can be employed is limited,
owing to the danger of obstructing the mouth of the shaft. Yet twelve
were used by Mr. Chavatte, and this number might have been doubled had
it been necessary. As we have seen, the frame, K K (Pl. 1, Fig. 3),
was provided with an oak circle traversed by 32 bolts. The length of
these latter was two inches and a quarter longer than they needed to
have been, or they were provided with wooden collars of that
thickness. Later on, these collars were replaced with iron bars that
held the wood against which the jacks bore in order to press the
tubbing downward (Pl. 1, Figs. 10, 11, 12, and 13).
Mr. Chavatte's great anxiety was to know whether he should succeed in
causing the first section of tubbing to traverse the four feet of
gravel; for in case it did not pass, he would be obliged to employ a
second section of smaller diameter, thus increasing the expense. He
was persuaded that the coarse gravel remaining in the side of the
shaft would greatly retard the descent of the tubbing. So he had
decided to remove such obstructions at the proper moment through
divers or a diving bell. Then an idea occurred to him that dispensed
with all that trouble, and allowed him to continue with the first
section. This was to place upon the dredge two claw-bars, T (Pl. 2,
Fig. 3), which effected the operation of widening with wonderful ease.
To do this it was only necessary to turn up the bags, and revolve the
apparatus during its descent. The claw at the extremity of the bar
pulled out everything within its reach, and thus made an absolutely
free passage for the tubbing.
The sands and gravels were passed by means of a single section of
tubbing 31 feet in length, which was not stopped until it had
penetrated a stratum of white chalk to a depth of two yards. This
chalk had no consistency, although it contained thin plates of quite
large dimensions. These were cut, as if with a punch, by means of the
teeth of the extirpator.
It now remains to say a few words concerning the sinking of the shaft,
which, after the operation of the dredge, was continued by the process
called "natural level" The work was not easy until a depth of 111 feet
had been reached. Up to this point it had been necessary to proceed
with great prudence, and retain the shifting earth by means of four
iron plate tubes weighing 54 tons. Before finding a means of widening
the work already done by the dredge, Mr. Chavatte was certain that he
would have to use two sections of tubbing, and so had given the first
section a diameter of 16½ feet. He could then greatly reduce the
diameter, and bring it to 15¾ feet as soon as the ground auger was
used.
After two yards of soil had been removed from beneath the edge of the
tubbing, the earth began to give way. Seeing this, Mr. Chavatte let
down a tube 13 feet in length and 15.4 in diameter. The exterior of
this was provided with 12 oak guides, which sliding over the surface
of the tubbing had the effect of causing the tube to descend
vertically. And this was necessary, because this tube had to be driven
down every time an excavation of half a yard had been made.
Afterward, a diameter of 15.35 feet was proceeded with, and the small
central shaft of 4¼ feet diameter was begun. This latter had not as
yet been sunk, for fear of causing a fall of the earth.
Next, the earth was excavated to a depth of 8.2 feet, and a tube 16.4
feet in length was inserted; then a further excavation of 8.2 feet was
made, and the tube driven home.
After this an excavation of 26¼ feet was made, and a tube of the same
length and 14½ feet in diameter was driven down. Finally, the shifting
soil was finished with a fourth tube 19½ feet in length and 14 feet in
diameter.
A depth of 111 feet had now been reached, and the material encountered
was solid white chalk. From this point the work proceeded with a
diameter of 13.9 feet to a depth of 450 feet. The small shaft had been
sunk directly to a depth of 475 feet. At 450 feet the diameter was
diminished by three inches. Then an advance of a foot was made, and
the diameter reduced by one and a half inch.
The reason for this reduction in the diameter and change in the mode
of deepening was as follows:
The Chaudron moss-box, when it chances to reach its seat intact, and
can consequently operate well, undoubtedly makes a good wedging. But
how many times does it not happen that it gets injured before reaching
its destination? Besides, as it often rests upon earth that has caved
in upon its seat during the descent of the tubbing, it gets askew, and
later on has to be raised on one side by means of jacks or other
apparatus. Under such circumstances, Mr. Chavatte considered this
moss-box as more detrimental than useful, and not at all
indispensable, and so substituted beton for it, as had previously been
done by Mr. Bourg, director of the Bois-du-Luc coal mines.
[Illustration: FIGS. 1, 2, 3, 6 AND 4.--Details of dredge. FIGS. 5 6
AND 6.--Details of maneuvering lever. FIG. 7.--Mode of lengthening the
axis of the dredge. FIG. 8.--Hooks for lifting the dredge bags. FIG.
9.--Arrangement of valves in the beton box. FIG. 10.--Device for
centering the tubbing.