being (165 × 60/100) + (62.5 × 40/100) = 124 lb. per cu. ft.

Various angles of repose were used for this material in the
investigation, and it was finally decided that 30° was the greatest
angle that could be expected, whereas the worst condition that could be
anticipated was that the sand and water would act separately and give a
pressure as follows:

Hydraulic pressure from liquid weighing 62.5 lb. per cu. ft. plus
pressure from sand with angle of repose at 30° and weight as follows:

Weight of 1 cu. ft. in air = 165 × 60/100 = 99 lb.

Weight of water displaced by 1 cu. ft. = 60/100 × 62.5 lb. = 37.5 lb.

Weight in water, therefore = 61.5 lb. per cu. ft.

These combined weights, of course, are equal to the weight of the
combined material in the previous assumption.

_Sixth._--The usual requirement that the resultant of both horizontal
and vertical forces should, at all points, fall within the middle
third of the wall, or, in other words, that there should be no
tension in the concrete.

[Illustration:
Plate LIV.
Diagram Showing Widths of Base of Retaining Wall Required
for Different Batters and Pressures, Pennsylvania Station]