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.
* * * * *