MASONRY WALLS AND STEPS 57 



in order to ensure stable equilibrium and then, after say 

 a month, remove the surplus material. As the good soil 

 sinks into the soft material in which the lake is being con- 

 structed an upheaval of course takes place, which necessi- 

 tates further excavation. 



In order to provide a solid bottom for the lake, 1-inch 

 boards are laid loosely on scantling and covered with 6 

 inches of good subsoil. 



MASONRY WALLS AND STEPS 



Retaining Walls. It is essential that masonry walls, 

 whether of stone or concrete, be kept dry. In order to 

 secure this, proper provision must be made both for the 

 collection and for the disposal of water. 



If a retaining wall is backed by a grassy slope, a grass 

 gutter should be provided to intercept surface water before 

 it reaches the wall. Catch-basins or inlets are placed in 

 this gutter at proper intervals, usually about 125 feet, and 

 the water entering them is carried off through a storm- 

 water sewer. (See Fig. 19, page 55.) 



Ground water must also be taken care of. To do this 

 it is well to place broken stone behind the wall to act as a 

 reservoir and passageway for the water, which can be 

 drawn off through the wall by means of " weep holes " 

 spaced from 15 to 50 feet apart. A square hole in the wall 

 about 4 inches by 4 inches in cross-section answers the pur- 

 pose. Or a 3- or 4-inch iron pipe of the kind commonly 

 called " seconds " may be used instead. The first method 

 is frequently followed in stone retaining walls and the 

 second in those made of concrete. The weep hole may 

 discharge directly upon the surface, or the water may be 

 carried off below the surface. The first method will suffice 

 if no path or walk is adjacent, since the quantity of water 



