RATE OF FILTRATION.–The rate of filtration through different soils was experimented with by forming a dike in the tank previously mentioned, as shown in [Fig. 22].
The dike was made full 8 ft. high, 7 ft. wide on top, with a slope on the up-stream side of 2 on 1, and on the down-stream side 4 on 1. This gave a base width of 55 ft. Immediately over the top of the dike there was placed 3 ft. of soil to slightly consolidate the top of the bank and permit the filling of the tank to the top without overflowing the dike. The water pressure in different parts of the dike was determined by placing horizontal pipes through the soil crosswise of the tank. These pipes were perforated and covered with wire gauze, being connected to vertical glass tubes at their ends. The end of the slope on the down-stream side terminated in a box having perforated sides and filled with gravel, thus enabling the water to percolate and filter out of the bank without carrying the soil with it.
When the soil was shoveled loosely into the tank, without consolidation of any kind, it settled on becoming saturated and became quite compact. It took five days for the water to appear in the sixth gauge pipe near the lower end of the tank. After the pressure, which was maintained constant, had been on for several weeks, the seepage amounted to one gallon in 22 minutes. When the soil was deposited by shoveling into the water, the seepage amounted to one gallon in 34 minutes.
The relative filtering capacities of soils and sands were thought to be better determined by the use of galvanized iron cylinders of known areas.
[Fig. 23] shows one of the cylinders. These latter experiments confirmed those previously made at Lawrence, by Mr. Allen Hazen, for the Massachusetts State Board of Health. They showed that the loss of head was directly proportioned to the quantity of water filtered and that the quantity filtered will vary as the square of the diameter of the effective size of the grains of the filtering material.[4]
The material classed as “permeable” at the North Dike of the Wachusett Reservoir has an effective diameter of about 0.20 mm. A few results are given in the following table:
Amount of Filtration in Gallons per Day, Through an Area of 10,000 Sq. Ft., With a Loss of Head or Slope of 1 ft. in 10 ft.
| Material. | Unit ratios. | U. S. gallons. | |
| (1) | Soil | 1 | 510 |
| (2) | Very fine sand | 14 | 7,200 |
| (3) | Fine sand | 176 | 90,000 |
| (4) | Medium sand | 784 | 400,000 |
| (5) | Coarse sand | 4,353 | 2,200,000 |
To be sure that the accumulation of air in the small interstices of the soil was not the cause of the greatly reduced filtration through it, another series of experiments was conducted in the wooden tank, as shown in [Fig. 24].
A pair of screens was placed near each end of the tank, filled with porous material, sand and gravel, and the 50-ft. space between filled with soil. The soil was rammed in 3-in. layers, and special care taken to prevent water from following along the sides and bottom of the tank. One end was filled with water to near the top, while the other end gave a free outlet.