. Bulletin of the Department of Agriculture. Agriculture; Agriculture. DEAIISrAGE OF lEEIGATED LAND. 21 by a stratum of water-bearing material in which the water is generally under pressure. The hardpan is very difficult to penetrate with a trench, and the underlying material makes a poor bedding for tile. Moreover, the hardpan is practically impervious to water and to plant roots, so there is no need for deep drainage. The drain is laid on top of the hardpan and relief weUs are bored through at frequent inter- vals, as shown in figure 14. The pressure causes an artesian flow which is car
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. DEAIISrAGE OF lEEIGATED LAND. 21 by a stratum of water-bearing material in which the water is generally under pressure. The hardpan is very difficult to penetrate with a trench, and the underlying material makes a poor bedding for tile. Moreover, the hardpan is practically impervious to water and to plant roots, so there is no need for deep drainage. The drain is laid on top of the hardpan and relief weUs are bored through at frequent inter- vals, as shown in figure 14. The pressure causes an artesian flow which is carried away by the drain. Should there be seepage on top of the hardpan it may be intercepted at the upper edge of the tract. DRAINAGE OF GRAVEL POCKETS. Rehef wells are especially useful in the drainage of soils underlain by gravel beds or pockets, particularly at the foot of benches, where the bottom land contains so much quicksand that it is difficult to lay a drain at the proper depth. Figure 15 shows the method of. â .:â â ::S an of :â :â .'â /.'â â ' ' â â " â¢â â â¢â _.â¢â¢â¢â¢â¢â â¢â¢â¢â¢.⢠⢠⢠⢠⢠â¢Â«=⢠â¢j"GH,de'l. Fig. 15.âSection illustrating relief-well method applied to drainage of soils fed from gravel deposits. application. The relief well should be sunk into the gravel. Enor- mous quantities of water are developed in this way; there is a case on record where a single well drained 100 acres of very wet land. DRAINAGE OF SHALE KNOLLS. Another application of the relief-well system is shown in figure 16 in which case the source of seepage is a buried shale knoll. The seep- age is carried between the shale layers and is under pressure which is relieved by means of wells, these being connected to outlet drains. APPLICATION OF THE UNIFORM SYSTEM. There arc some conditions where tln^ uMiloriu method of arranging the drains as uscfl in the humid s(u-tion is aj)])li('al)le. Among these is the caH(; of
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