FILLING OF CAVITIES 113 



amount of water incorporated in the mixture. A 

 rich mix is one in which one part (by loose bulk) 

 of cement is mixed with two parts of sand and 

 three of gravel. A medium mixture runs about 

 1 :3 :6, and a lean mixture, 1 14:8. Arranged ac- 

 cording to wetness, a very wet mixture is one 

 which is mushy enough to run off the shovel, a 

 medium mixture is jelly-like and will quake on 

 tamping, while a dry mixture closely resembles 

 damp earth. 



For filling trees a rich mixture should ordinarily 

 be employed, because the actual amount of cement 

 used is small and because it is highly desirable that 

 the filling set quickly and become very hard. In 

 positions where there is but little strain on the fill- 

 ing, as beneath or near the surface of the soil, or 

 in a large cavity with a small opening, a mixture 

 of medium richness can be used with entire safety. 



The degree of wetness is not so easily disposed 

 of. For the most part a rather dry mixture has 

 been used for filling trees. The difficulty of mak- 

 ing a form into which to deposit the concrete al- 

 most compels the workman to use a mixture so 

 dry that he can build it up without a form and 

 without danger of its slumping down through the 

 opening of the cavity. This is the easiest way to 

 put in a concrete filling, and it has been almost 

 the universal way. It is constantly being made 

 clearer and clearer, however, that the dry system 



