Upper Face and Retaining Wall.—­In cases where the top of the retaining wall was at a higher elevation than the mixer, it was necessary to raise the concrete in a bucket with a derrick, and dump it into cars on the trestle above the top of the coping.  Concrete was deposited through chutes, as in the lower face wall, continuously from the bottom of the face wall to the top of the retaining wall.  At the commencement of each section of the retaining wall a layer of mortar was put on the rock.  A 1:2:3 mixture of concrete was used in the face wall, and a 1:3:6 mixture in the retaining wall.

As the face walls were so thin, the number of batches of concrete per hour was reduced, for the form filled so rapidly that the concrete, before it set, exerted an excessive pressure against the form, and this tended to make it bulge.  The proper rate at which to place the concrete behind a form 50 ft. long, with a wall 2 ft. thick, was found to be about fifteen 1/2-yd. batches per hour.

Cracks in Walls and Longitudinal Reinforcement.—­Before the concrete walls were started, the contractor suggested using forms 100 ft. long and building the walls in sections of that length; it was decided, however, to limit the length to 50 ft.

The south walls, in sections approximately 50 ft. long, were built first, starting at Tenth Avenue and extending for about 500 ft.  Soon after the forms were removed, irregular cracks appeared in the walls between the joints in practically every section.  It was thought that these cracks might be due to the wall being very thin and being held at the back by the tie-rods; there was also quite a material change in the section of the wall at each drainage box.  Although it was admitted that these cracks would have no effect on the stability of the wall, it was thought that, for appearance sake, it would be desirable to prevent or control them, if possible.  The first method suggested was to shorten the sections to 25 ft., which would give an expansion and contraction joint every 25 ft., it being thought that sections of this length would not crack between the joints.  This, however, was not considered desirable.  An effort was then made to prevent cracks in a section of wall, about 46 ft. long, on the south side, by using longitudinal reinforcement.  In the lower and upper face walls, 3/4-in. square twisted steel rods were placed longitudinally about 4 in. in from the face and about 1 ft. 4 in. apart vertically.  The sections of these walls were finished on April 10th, and May 5th, 1909, respectively.  At present there are no indications of cracks in these sections, and they are practically the only ones in the south walls which do not show irregular cracks.