As the resiiltant cuts the base at the edge of the middle third,- the factor 

 of safety against overturning is 3 and the entire structure will be in com- 

 pression. 



3960 To determine the factor of safety against sliding, multiply the 

 effective downward force acting on the structure by a coefficient of static 

 friction and divide by the horizontal thrust of the wave force. If the 

 foundation is not dressed smooth, a friction coefficient of 0.5 or 0.6 is 

 generally considered adequate. The factor of safety should not be less 

 than 2.0. 



wt X 0.5 ^ 165,000 X 0.5 = 2.87 (11^6) 



Rg 28,750 



which exceeds two and is satisfactory, 



397 o As the structure is symmetrical, the resultant of the vertical 

 loads cuts the center of the base of the structure at a distance of l5 feet 

 from the inner edge. The distance Z from the inner edge to the point where 

 the resultant cuts the base, assuming uplift pressures in a triangular 

 distribution, is given by 



165,000 - 1^90 X 20 = ^^^~2 



Z = 10.3 feet. 



(1U7) 



398c The maximum pressure against the foundation would be at the 

 inside edge with maximum wave conditions « If Z is the distance between 

 the inside edge of the base and the point where all the resultant forces 

 intersect the base, V is the effective downward force per unit length 

 of wall, G is the pressure on one square unit of foundation, and A is 

 the width of the breakwater, then the ground pressure at the shoreward 

 edge of the breakwater is: 



G = 2V (2 - 3Z) (li^g) 



G - 2 (165,000) ( 2 ~ 3 (I0o3) ^ ^ 10,700 pounds, 

 30 30 



This pressure would be large for most foundation conditions. Should this 

 ground pressure be too great for the bearing power of the bed at the site, 

 the base width of the structure must be increased until the pressure is 

 within the allowable load. 



193 



