Girders “B."—­Eighteen of these girders were required, each weighing about 6,000 lb.  The timber bents supporting these girders, shown on Fig. 2, were of long-leaf yellow pine.

The total weight, including the elevated railway structure, surface railway structure, pipes, etc., supported during the work, amounted to about 5,000 tons.

Details of the Work.—­The method in general is shown on Figs. 4 and 5.  At first the east side of the avenue was closed and excavated down to rock, the earth was mined out under alternate yokes of the surface railway structure, and temporary posts were placed under the yokes to support the structure while the remainder of the earth was being removed.  Then needle-beams and posts were placed under each yoke.  The concrete forming the track structure was then enclosed with planking to prevent it from cracking and falling.  I-beams were then placed under the needle-beams carrying the structures, and these were carried on posts; they were changed alternately until the excavation had been taken out to a depth of about 16 ft. below the surface.  In placing these I-beams, heavier blocking was used in the center of the span than at the ends where the bents would come, to prevent the subsidence of the track owing to the sag in the I-beams.  As much excavation, to a depth of about 20 ft., was taken out adjoining the elevated railway foundations as could be done with safety.  Fig. 2, Plate XLVII, shows this condition of the work.  The 48-in. brick sewer was broken, and the sewage was pumped across the excavation.

The overhead girders “B” were then put in place, and two of the girders “C” were used as temporary shoring girders at each column.  These, as shown by Fig. 3, Plate XLVII, were placed parallel to the elevated railway, with blocking between them and the girders “B.”  Double bents, independent of each other, were placed under the ends of these temporary shoring girders, and these were braced securely to prevent possible dislodgment during the removal of the rock.  The weight of the structure was then taken by jacking up the girders near the bents until the column was lifted off the old foundation; blocking was put in between the girders and the bents during the jacking, so that when the jacks were released the base of the column was still clear of the old foundation.  One 80-ton jack was used for this purpose, and the general method is shown by Fig. 1, Plate LII.

[Illustration:  FIG. 4. (Full page image)

METHOD OF EXCAVATING NINTH AVENUE PLAN AND ELEVATION SHOWING VARIOUS STAGES
OF THE WORK]

[Illustration:  FIG. 5. (Full page image)

METHOD OF EXCAVATING NINTH AVENUE SECTIONS SHOWING VARIOUS STAGES OF WORK

No. 1

Condition Prior to Commencement of Work

No. 2

East side of Avenue cut down about 20 ft.  Beams with Supporting Posts placed under Surface Railway Tracks.  Girders B and Temporary Shoring Girders C for supporting Elevated Ry. in place.