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.
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.
This engineer likewise suppressed the balancing column, which is often a source of trouble in the descent of the tubbing, and forced his tubbing to center itself with the shaft through a guide with four branches riveted under the false bottom that entered the small shaft (Pl. 2, Fig. 10). Mr. Bourg so managed that there remained an empty space of ten inches to fill in with beton. Mr. Chavatte had at first intended to proceed in the same way, but the two last tubbings, upon which he had not counted, forced him to reduce the space to 5¾ inches. Under such circumstances it was not prudent to employ the same means for guiding the base of the tubbing, because, if the central shaft had not exactly the same center as the large one, there would have been danger of throwing the tubbing sideways and causing it to leak. Seeing which, Mr. Chavatte strengthened the lower part of the base ring and placed it upon another ring tapering downward, and 27½ inches in height (Pl. 1, Fig. 5). The object of this lower ring was to force the tubbing to remain concentric with the shaft, to form a tight joint with its upper conical portion, and to form a joint upon the seat with its lower flange, so as to prevent the beton from flowing into the small shaft.
After the shaft was pumped out, digging by hand was begun with a diameter of 12 feet. After descending 20 inches an 8×10 inch curb was laid, in order to consolidate the earth and prevent any movement of the tubbing. Then the excavating was continued to a depth of 31½ inches, and with a diameter of 9¾ feet. At this point another curb was put in for consolidating the earth. Finally, the bottom was widened out as shown in Fig. 7, so that three basal wedged curbs could be put in. This done, the false tubbing was put in place; and finally, when proceeding upward, the last ring composed of twelve pieces was reached, the earth was excavated and at once replaced with a collar composed of twelve pieces of oak tightened up by oak wedges. Each of these pieces was cemented separately and in measure as they were assembled.
Through motive of economy no masonry was placed under the base of the three wedged curbs. In fact, by replacing this with a wedged curb of wood traversed by six bolts designed to fix the cast iron curb immediately above, Mr. Chavatte obtained a third curb that he would have had to have made of cast iron.