Fig. 7.
The average mechanical analyses of the three grades of gravel are shown by Fig. 8. Their effective sizes were 23, 8, and 3 mm. respectively, and for convenience they are designated by these numbers. The average uniformity coefficient for each grade was about 1.8.
The 23-mm. gravel entirely surrounded the 6-inch pipe-drains, and was carried slightly above their tops. In some cases it was used to cover nearly the whole of the floor, but this was not insisted upon.
The 8-mm. gravel was obtained in larger quantity than the other sizes, and was used to fill all spaces up to a plane 21⁄2 inches below the finished surface of the gravel, this layer being about 2 inches thick over the tops of the drains, and somewhat thicker elsewhere.
The 3-mm. gravel was then applied in a layer 21⁄2 inches deep, and the surface levelled.
The preliminary estimates of cost were based upon the use of filter-sand from a bank near the filter-site. Further examination showed that this sand contained a considerable quantity of lime, and it was found by experiment with a small filter constructed for that purpose that the use of this sand would harden the water by about 2 parts in 100,000, and the amount of lime contained in the sand, namely, about 7 per cent, was sufficient to continue this hardening action for a considerable number of years. This was regarded as a serious objection to its use, and the specifications were drawn limiting the amount of lime in the sand. This excluded all of the local bank sands. The river-sands which were used were nearly free from lime, and in the end the sand as secured was probably not only free from lime, but more satisfactory in other ways, and also cheaper than the bank-sand would have been.
Diameters in Millimeters
MECHANICAL COMPOSITION OF FILTER SAND AND GRAVELS.
(ARROWS SHOW REQUIREMENT OF SPECIFICATION)
Fig. 8.
The specifications of the filter-sand require that “The filter-sand shall be clean river-, beach-, or bank-sand, with either sharp or rounded grains. It shall be entirely free from clay, dust, or organic impurities, and shall, if necessary, be washed to remove such materials from it. The grains shall, all of them, be of hard material which will not disintegrate, and shall be of the following diameters: Not more than 1 per cent, by weight, less than 0.13 mm., nor more than 10 per cent less than 0.27 mm.; at least 10 per cent, by weight, shall be less than 0.36 mm., and at least 70 per cent, by weight, shall be less than 1 mm., and no particles shall be more than 5 mm. in diameter. The diameters of the sand-grains will be computed as the diameters of spheres of equal volume. The sand shall not contain more than 2 per cent, by weight, of lime and magnesia taken together and calculated as carbonates.”
