PRELIMINARY OPERATIONS 155 



too great to permit this. In the loosest soils the slope 

 should not be greater than 1 in 13, but it may be 

 increased on tougher and more rocky ground. It may 

 also be possible to carry these drains down a steep slope 

 by terracing the fall, i.e. by cutting it into steps which will 



Fig. 48. 



check the rush of the water. In this case it may become 

 necessary to put in masonry aprons to prevent scour. 



The collecting drains should be constructed in such 

 a way as to collect as much water as possible while 

 allowing the water so collected to How off without 

 check. They should therefore have a gentle slope, i.e. 

 obliquely to the general slope of the ground. 



