Concrete Retaining and Face Walls.—­It was essential to have the greatest space possible at the bottom of the excavation, and, inasmuch as the yard was to be left open, it was necessary to provide some facing for the rock on the sides in order to prevent disintegration, due to exposure, and give a finished appearance to the work.  Above the rock surface a retaining wall of gravity section was designed, the top being slightly higher than the yards of the adjoining properties.  The face wall was designed to be as thin as possible, in order to allow the maximum space for tracks.

The excavation, therefore, was laid out so that the back of the retaining wall would not encroach on the adjoining property, but would practically coincide with the property line at positions of maximum depth.

The batter on the face of the wall was 2 in. per ft., and a bridge seat 3-1/2 ft. wide was formed at an elevation of 22 ft., minimum clearance, above the top of the rail.  This bridge seat was made level.  The maximum height of the south wall is 49 ft., and of the north wall 65 ft.

The face walls were classed as “Upper Face Walls,” extending from the base of the retaining wall to the bridge seat, and as “Lower Face Walls,” extending from the bridge seat to the base of the wall.  The general design is shown on Fig. 8.

In considering the design of the face wall it was felt that, the wall being so thin, ample provision should be made to prevent any accumulation of water and consequent pressure back of the wall; therefore, no attempt was made to water-proof it, but provision was made to carry off any water which might appear in the rock.  Box drains, 2 ft. wide and 6 ft. from center to center, were placed against the rock, so that, there being but 4 ft. between the drains, and the wall having a minimum thickness of 2 ft., any water in the rock would not have to go more than 2 ft. to reach a drain, and would probably pass along the face of the rock to a drain rather than through 2 ft. of concrete.  These drains were connected with pipes leading through the wall at its base.

[Illustration:  FIG. 8. (Full page image)

RETAINING AND FACE WALLS NORTH SIDE]

These box drains occurred so frequently, and decreased the section of the wall so materially, that it was thought desirable to tie the wall to the rock.  This was done by drilling into the rock holes from 6 to 15 ft. in depth, and grouting into each hole a 1-1/2-in. rod having a split end and a steel wedge.  The outer end of each rod was fitted with a 12 by 12 by 1/2-in. plate and a nut, and extended into the wall, thus tying the concrete securely to the rock.  The drains being 6 ft. from center to center, the tie-rods were placed midway between them, and 6 ft., from center to center, vertically and horizontally.  Fig. 8 shows the arrangement of these rods and drains.  Around the Express Building site, just west of Ninth Avenue, on the south side of the work, the bridge seat was omitted, and the face wall was designed 2 ft. thick from top to bottom.  The batter on the 31st Street wall was made variable, the top and bottom being constant distances from the center line and on different grades.