elevation of the ifall base above still water level, (Mote h ruay be 

 negative. ) 



219, From the pressure diagram of Fig-ure 80, the dynaiixLc thrust R, 

 is given by: 



R, . = P (h - h ) if h is positive, (35) 



R, ■■= P (h ) if h is negative 

 cl— m c w 



The total tlir.ust is then gi'^'en by 



K = R^ '^ Ps^~"T^^ ^3°) 



The overturning moment abou.t the ground line at the seaward face of the 



structure isj ^, , s ,, , v2 

 (h - a ) (h ~ h ) 



M =/R. ,"\ -—-^ - P^^ -^-7~ i ii h„ is positive (37) 





2 



^c II ^\ "^ P P 



(tt- '^ h ) + P 7 if h is negative. 



220» Effect of Face Slope on Wave "^'ressui^es . - The preceding forinulas 

 may be used for all cases of structures T-zith essentially vertical faces. 

 If the face is sloped bacla'fards at sn angle with the horizontal (Fig-ure 81a) 

 the horizontal component of the dynaiaic pressui-e due to waves breaking 

 either on or seax^ard of the Xirall should be reduced to 



P ' = P cos^ 9 (38) 



m m ^ 



The vertical component of this wave force may be neglected in stability 

 computations s Forces on stepped face structures (Figure 8lc) may, for 

 design calculations^ be coiiputed as if the face were vertical, since it 

 is probable that dynainic pressm-es of the sarae order as those computed for 

 vertical walls wouJ.d eristv Forces on cux'ved non-re-entrant face structures 

 (FiguT'e 8lb) may be calculated by using a line from the top to the bottom, 

 of the face to determine an average slope. Re-entrant curved face walls 

 may be considered as ^^ertical. 



221, Wav e Forces on Rubble Kound Structures . - Until recently, the 

 design of rubble mound structures was largely based on e:xperience and general 

 Icnowledge of a particular site's conditions. Efforts to rationalize the 

 design of ttese structures have been made » These entail the observation of 

 failujT'es and the determination of constants to be applied to var-ious para- 

 meters in an attenpt to explain the failures » Because of the empirical 

 nature of the formulas developed, they are generally e^qsressed in -terms of 

 the size stone required to Tiiithstand design -urave condi'&ion«. These formulas 

 have been partially substantiated in model studies* However, they still are 

 only a guide and cannot be blindly substituted for experience, 



222 o In 1938, Iribarren presen'bed formulas for the design of rubble 

 mound st-ructures. These formulas permitted calculation of sea side slopes 



112 



