LAND DRAmAGE 643 



obstruction by breaking up into such a mass of fine root- 

 lets that the tile is finally closed. Any kind of tree or 

 plant may cause this difiSculty if permitted to de^'elop 

 under certain conditions. Trouble from roots occurs onlv 

 where the tile carries water from a spring or borae other 

 continuous source, so that in dry periods the water may 

 leak out at the joints into the adjacent dry soil. This 

 leads the roots in the direction of the tile. In the ab- 

 sence of such a spring, plant roots do not appear to inter- 

 fere with drains. Where a drain carrying water con- 

 tinuously comes near a tree, especially if the adjacent 

 soil is likely to become drv, the joints of tile should be 

 closed by cement. 



544. Protections of joints on curves. — Special care 

 may be needed in order to protect the joints on turns 

 where the outer side may be too open. The larger the 

 size of the tile, the longer will be the opening on a given 

 curve. Short turns should not be made. Stones are 

 usually unsafe material to place around the joints of a 

 tile under such conditions, especially in soil that is likely 

 to erode easily. If so used, special care should be em- 

 ployed to protect the joints with caps. 



545. Foundation for tile. — Tiles should have a firm 

 foundation, and if the bottom of the ditch is soft it msi^y 

 be advisable to bed them in sand or cinders or lay them 

 on a board. Soft muck and quicksand make this most 

 necessary. Ordinarily the bottom of the trench is finished 

 on the undisturbed earth, which affords a firm setting. 



546. Arrangement of drainage systems. — The ar- 

 rangement of a system of underdrains should be deter- 

 mined by the slope of the land and the structure of the 

 soil No fixed rule can be laid down. The aim must be 

 to place the drains in the line of movement of water in 



