[Illustration: 
  Fig. 8. 
  SKETCH SHOWING FORMS AND BRACING FOR NINTH AVENUE WALL]

The sections of concrete built in trench varied in height from 13 to 59 ft. from the base to the top of the back wall.  With the exception of the Seventh Avenue wall, 50 ft. in height, and the Ninth Avenue wall, 62 ft. in height, none of those sections constructed by the bench method was more than 14 ft.  The forms and bracing for these walls were substantially the same, except that the low walls were built in lengths of approximately 50 ft., while the forms for the Seventh and Ninth Avenue walls were only 20 ft. long.

The forms and bracing for the Ninth Avenue walls are shown on Fig. 8.  These forms were built in one piece and moved ahead from section to section, and they were firmly braced from the bottom with raker braces to a point 36 ft. above the base, the upper part being held in place by 3/4-in. bolts passed through the forms and anchored by cables to bolts grouted into the rock behind.

After the forms had been set and braced, an 8-in. brick wall was laid up the face of the rock, containing a vertical line of three-cell hollow tile block every 5 ft. of length, and laid to conform as nearly as possible to the face of the rock, all voids being filled with broken stone.  Water-proofing, similar to that described for the walls in the trench, was then applied to the brick and tile wall for the full height, and firmly braced to the front forms, the braces being removed as the concrete reached them.  The concrete was mixed at the street level and deposited through chutes, as described previously.

Tables 1, 2, and 3 show the quantity of cement used in each section of retaining wall, and give figures by which the quantities of other materials may be determined.

Pit Excavation.—­The pit excavation during the horse-and-truck period was largely preparatory work done to get the excavation in good shape for handling spoil trains after Pier No. 72 and the trestle approach were finished.  This required an open cut from Ninth to Seventh Avenues at a sufficient depth below the sewers and other substructures in the avenues to clear a locomotive, and wide enough for both running and loading tracks, also the building of the cast-iron sewer in Eighth Avenue across the entire excavation, with enough of the temporary bridging to support it.  The building of the trestle in Eighth Avenue was essentially a part of the pit excavation, as the progress of one depended greatly on that of the other.

Excavation was commenced on July 12th, 1904, for the crossing under Ninth Avenue, and in the pit east of Ninth Avenue along 32d Street.  The line chosen for the opening cut was down the center of the pit, as it was not safe to excavate near the bounding streets until after the completion of the enclosing retaining wall.  The excavation was started by hand, but three 70-ton Bucyrus steam shovels were put to work