98 ANNUAL REPORT 
Top Soil 
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SECTION THROUGH PIERS 
SECTION THROUGH CROWN OF ARCH 
Fig. 41.—Cross Section of the Filtration Chambers, Philadelphia, Pa. 
34 of an acre. These basins are built into the ground and covered over with 
sand, gravel and earth, and then sodded. The floors and arched roofs 
of concrete are about 6 inches thick. The side walls vary from about 4 
feet, 2 inches thick at the base, to 12 inches thick at the spring line of 
the roof. The roof consists of groined arches resting upon the walls and 
a series of concrete piers 22 inches square at the top and 34 inches square 
at the base. The arches have a 14 foot span, 3 foot rise and are 6 inches 
thick at the crown and 21 inches thick over the piers. Over the arches 
are spread 4 inches of gravel to act as a draining medium to carry all 
collected storm water to small inlets situated over each pier, allowing the 
