369, Distance of Deadjnan From Wall ^ - The de adman should be located 

 outside the possible failure planes of the various layers of earth fill 

 aiid also outside the intersection of its own failure plane to the surface 

 with the line of possible failure of the sarth fill. Since in failing, 

 the wall would rotate seaward about the point of zero loading, the mimiraiun 

 distance is determined - as in Figure II6 - hj dividing the heights of 

 the various fill components by the tangents of the various internal friction 

 angles (failure angles) and summing the result j i.e. for the wall to fail 

 the, corrponent fill failure planes must coincide, 



370» For the case being considered, the minimum distance of the dead- 

 man from the wall is given by 



'an' 0K tan 0) tan (?_ tan jJp tan 0- tan a 



where 



tan a = yl + tan C^ - tan -, " 0»58 



therefore L = .^^^^ "^ tSTia* "^ tSTT^"* " tST^ "" tiFlU» "^ ot^: 

 = ii.08 + ?,2 + 6,Iiii + 6,hh + 6,9h + 11.70 

 = liO,5 feet from the wall. 



OFFoHOHE BlgAKT'/ATSRS 



GEMJRAL CONSIDERATIONS 



371* Because of their high cost, offshore breakwaters have been 

 little used solely for shore protection* However, because they have 

 use as a protective structure, because they may be required to provide a 

 sand trap for by-passing material, and because they may be used to retain 

 the toe of a beach fill, design data are included for tj'pical exaiiiples of 

 the most frequently constructed types. Other types mas'" be designed by 

 using the methods illustrated for the design of seawalls, 



372 » Types and Description , - Several conventional types of permanent 

 offshore breakirraters are now in general use, varjang as to material of 

 which constructed and as to cross section* The principal construction 

 materials are stone, concrete, steel and timber. In addition fascine 

 mattresses, asphalt, and bitiunen have occasionally been used (Kuiper 19$l)» 

 The selection of a material or combination of riiatei-ials for breakwater 

 construction is based on cost and adaptability as pertains to conditions 



180 



