DRAINAGE OF IRRIGATED 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 od top 
of the hardpan and relief wells 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. 
Relief 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 
Figure 15 shows the method of 
lay a drain at the proper depth. 
Fig. 15.— Section 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 are some conditions where the uniform method of arranging 
the drains as used in the humid section is applicable. Among these 
is the case of a tract lying nearly level, having a fairly homogeneous 
soil or perhaps a substratum of sand at moderate depth, and receiv- 
ing but little water from outside sources, the excess water being that 
due to the irrigation of the tract itself. The gridiron system, as 
shown in figure 17, is a most efficient plan under such conditions, the 
drains being placed from 200 to 450 feet apart. The latter figure 
