FOUNDATIONS AND RETAINING WALLS 213 



level with the top, the thickness b of the base of the wall in terms 

 of its total height h should be as follows : * 



For wall of cut stone or large ranged rubUc f in mortar, 



b = .35 h. 

 For wall of good common scabbled mortar rubble, or brick, 



b = .40 h. 

 For wall of well scabbled dry rubble, 



b - .50 h. 



These empirical rules may be regarded as representative of the best 

 American practice, and may be used to give a first approximation in 

 making a tentative design. 



By inclining the wall backward the angle between the earth thrust 

 P 1 and the wall is decreased, and consequently the resultant It, is 

 made to approach more nearly the center of the base. This allows 

 the thickness of the base to be decreased and thus lessens the 

 amount of material in the wall, although it slightly increases its 

 depth. However, there is a restriction upon the amount of inclina- 

 tion which is permissible, for the inclination also has the effect of 

 increasing the tendency to slide on the base or joints. In practice 

 these considerations are balanced by inclining the back of the wall 

 at a small angle, say 5 or 10, to the vertical (i.e. a = 80 or 85), 

 and at the same time cutting the footing into steps perpendicular 

 to the line of action of the resultant It, thus securing economy of 

 material without sacrificing stability. 



The thickness of the top of the wall is determined by the necessity 

 of providing for the lateral pressure of the earth, due to the action of 

 frost. Since the action of frost is greatest near the top of the wall 

 where the material is most exposed, it is likely to push the top over 

 if the wall is made only thick enough to resist the pressure due 

 to the weight of the earth. This consideration, therefore, limits the 

 least thickness of the wall at the top to about two feet for masonry, 

 or somewhat less than this amount for concrete, since the latter has 

 no joints and therefore offers a greater moment of resistance. 



* Engineer's Pocket-Book, 1002, p. 603. 



t Masonry composed of rough, undressed stones is called rubble ; scabbled rubble has 

 the roughest irregularities knocked off with a hammer. 



