290 PRACTICAL STRUCTURAL DESIGN 



thrust has a larger factor of safety than one designed to resist 

 a thrust at an angle. The author designs retaining walls on the 

 assumption of a horizontal thrust. 



The resultant of the weight of the wall and the thrust must 

 pass through the base at such a point as will keep the toe pres- 

 sure within the allowable soil pressure per square foot. It is 

 considered best to keep the resultant within the middle third 

 of the wall. In the figure the thrust is shown as meeting a ver- 

 tical line through the center of gravity at a point one-third the 

 height above the base. This makes the thrust line strike the wall 

 a trifle above the one-third point. With a horizontal thrust the 

 application is exactly one-third above the base. 



The diagram here given is independent of the shape of the 

 wall. In fact a single line representing the back of the wall could 

 have been drawn just as well. The diagram merely gives the 

 amount of thrust, its direction and the point of application. A 

 separate diagram may be drawn from the wall if the work is to 

 be graphical, and only the thrust line from this figure will be 

 required. 



For a well-built concrete wall not reinforced the width of the 

 base can be one-third the height for ordinary earth. For a brick 

 or well-built cut stone wall with cement mortar joints the bottom 

 width can be one-third the height + 1. For an ordinary stone 

 or brick wall the thickness of the base should be at least one-half 

 the height. Such empirical rules make it very easy to draw plans 

 for walls. With reinforced concrete walls it is necessary to know 

 the pressure and overturning moment so the wall may be designed 

 to resist definite forces. 



A reinforced concrete retaining wall is built in the shape of a 

 capital L. The weight of the earth on the outstanding rear leg is 

 counted as part of the weight of the wall, the back of the wall 

 being vertical and extending upward from the rear end of the 



1 "~ S^fl & 



slab. The coefficient of friction is k = T -. 7 > and the angle 



1 +sin < 



corresponding to this is used in the graphical analysis instead of 

 the angle C. 



The thrust acting at one-third the height tends to overturn 

 the wall. The vertical front face of the wall must, therefore, be 

 designed as a cantilever beam to resist this moment, anchoring 

 into the base. The small toe in front is extended to widen the 



