ROCK DRAINS 



161 



The drains must be large enough to insure the prompt runoff of water. 

 Under exceptional circumstances the French have managed to dry ground 

 to a great depth, sometimes up to 10 to 13 feet, but the expense is much 

 greater. First-class drains are generally built where the slope is steep, in 

 order to assure rapid runoff. They are less Hkely to be dislocated by soil 

 movements. At the head of drains little walls are constructed as props. 



Fig. 12. — Paved drains at Bastan (Hautes-Pyrenees). 

 graph.) 



(French official photo- 



When the drainage water is abundant it is often united in a paved trench 

 built in the valley bottom to carry the water under the dams. Where the 

 soil movements are due to a leaky canal it is often better to stop the cause 

 of the infiltration rather than to take up the expensive work of drainage. 

 To prevent running water from eroding the soil it is often advisable to 

 collect it in Httle canals, about 20 inches in depth, which follow the slope 

 and which are filled with little stones or fascines. These canals can be 

 replaced by paved drains if the earth is in movement. Trees are planted 



