CONSTRUCTION METHODS 183 



and for a depth of 8 feet the top width would be 20 feet 

 or more. 



"In the installation of a drainage system it should be 

 borne in mind that the improvement is permanent, and 

 that after the tile is once covered up it is more expensive 

 to uncover and relay it with larger tile than to install a 

 new drain, so it is false economy to cut down on the size 

 of tile. It is much better to err on the side of too great 

 capacity than too small. 



4 * Construction Methods. In many instances owing to 

 lack of humus the soils of the arid region are very fluxible 

 when wet and the construction of drainage systems is very 

 difficult and requires painstaking care and ingenuity. 

 Special methods and devices have to be employed, and 

 special 'machinery has been developed. 



" Drain lines must be laid out carefully and grade stakes 

 set. The completed drain must be true to grade and as 

 straight as possible. For hand trenching it is advisable 

 to stretch a cord on the ground along one edge of the 

 proposed trench, to obtain good alignment. To insure 

 accurate grade at all points, grade plants should be set 

 up at each station at a uniform height above the grade of 

 the drain. A stout cord then may be stretched over the 

 middle line of the trench from plank to plank and every 

 point on this cord will be the given height above grade. 

 Grade may be established at one end of each tile with a 

 grade pole having a length equal to the distance from the 

 cord to the proper location for the tile. This may be 

 accomplished by keeping the cord taut by suspending a 

 tile or other weight at each end and measuring down 

 from the cord at the desired points. 



"Construction work always should start at the outlet 

 of each line and proceed up the slope, so that the water 

 developed will drain away. 



