distance of 200 feet to withstand heavy waves and prevent scour or damage 

 to the timber sections. The stone was transported to the site by barges 

 and lighters from quarries at Rockport, Massachusetts. VJhen the work was 

 finished the beach betvxeen high and low water had s moderate slope and 

 extended about 320 feet seaward of the original shore line. The experience 

 with this beach and the groins was satisfactory ever a period of 30 

 years, during v^hich several hurricanes occurred. 



ii57. Forces Acting on 'groins . - The forces acting on a groin include 

 earth pressure and wave action. Earth pressures are determined from the 

 C'ifferential ground elevations as developed in Part I. Wave pressures are 

 determined from the application of the design vjave to the structure. 

 Determination of the characteristics of the design wave is discussed in 

 the section on i-;ave heights. 



STRUCTURAL DESIiiM 



ii.58. Vertical Sheet Pile Groins . - This type of groin may be con- 

 structed of tiiTiberj concrete; or steel, depending on life expectancy, cost., 

 availability of materials, etc. The sample design included here in is for 

 a steel struct'jre. Similar iiisthods would be used for either timber groins 

 or concrete sheet pile groins. The conditions assumed represent about 

 the maximum that a groin xrould be required to withstand. 



kS9 • Loading Conditio ns. - The uiost severe loading conditions wouj..d 

 occur when the water level v;as at extreme low, snd under the assuinption 

 that the fill on the updrift side of the groin was saturated with sea 

 water (see Figure lU7)= With a 9-foot differential fill at tlie groin j, 

 the loading diagram against the piling will be as shown in Figure lU7 B. 

 On the updrift side the forces vShown are active pressures. From the top 

 of the piling to extreme low water I ground line-', this active pressure 

 increases uniformly at a rate of 



P = wh^ tan2^l/2)(90 - 0^ (l52) 



where for h = 1 foot 

 w = 110 



» 25^ (Beach sand) 

 tan2 ^/2 (90 - 0) = O.Ul 

 p a 110 X Oohl 2 U5 pounds o 



Assuming the voids in the sand saturated with sea water or Oeii "' tU s 26 

 pounds, the weight of the beach fill per cubic foot would be kS ^ 26 or 

 71 pounds. 



Il60o From extrem.e low water (ground 3.ine) to the bottom of the 

 piling, the further increase in pressure p-' - foot of depth is 



p - 6U + (no - 0,6 X 6I4) (O.I4I) 

 p = 93 pounds 



219 



