340 RETAINING WALLS 



be safe from overturning if the point w where the line of action 

 of R meets the base came anywhere inside the base of the 

 wall; and, theoretically, the pressure P could be increased 

 until n coincided with a that is, until the line of action of the 

 resultant pressure R passed through the toe a. But practical 

 considerations require that, under ordinary conditions, the 

 point n should fall within the middle third of the base of the 

 wall. It must be stated, however, that the distance a n may 

 safely be reduced somewhat from one-third to even one-fifth the 

 width of the base, if the foundation is perfectly rigid and the 

 masonry of the best. This will give a maximum intensity of 

 pressure on the foundation at a 3$ times the intensity there 

 would be if the center of pressure were at the center of the base. 



Stability Against Sliding. The total pressure R on the base 

 may be resolved into a vertical component oj ( = W+ Y) and a 

 horizontal thrust jr ( = X) the latter tending to produce sliding 

 on the base. This thrust must not exceed the product of the 

 normal pressure oj and the coefficient of friction between the 

 wall and its foundation; otherwise expressed, the angle roj 

 must not exceed the angle of friction between the wall and its 

 foundation unless some external means, such as earth placed in 

 front of the wall at the base, is employed to strengthen the 

 wall against sliding. 



Ordinarily, the friction of the back filling is disregarded in 

 determining the resistance to sliding. The neglect of this factor 

 of stability against sliding is warranted in the majority of cases, 

 because, the thickness of wall required for stability against 

 overturning gives ample weight to resist sliding, and the added 

 help of the filling in front of the foundation, required on account 

 of frost and other surface influences, is generally sufficient to 

 make up for the neglected friction of the filli-ng. It is, however, 

 sometimes advisable to take it into account, for though latent 

 when there is no motion of the wall, the instant that the wall 

 begins to move, or is about to do so, whether by overturning or 

 by sliding, the filling begins to slide or is ready to do so 

 down the back of the wall, and brings the friction into action. 





