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 



■.:■ ■::S an of :■:■.'■/.' ■ 









■ ' ' ■ ■ " •■■•■_.•••••■••••.• • • • • •«=• •j"GH,de'l. 



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 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 u 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 wat(M- Ixung that 

 due to the irrigation of the tract itself. The gridiron system, as 

 shown in figure 17, is a most eflicient plan uiuh^r such conditions, tho 

 drains being placed from 200 to 4r>0 U'v.i, ii])iiil-. The. Iiittcr figuie 



