I ro NOTES ON DOCKS AND DOCK CONSTRUCTION 



The following remarks, bearing specially on dock walls, arc 

 taken verbatim from Sir Benjamin Baker's paper on " The 

 Lateral Pressure of Earthwork." 1 



" A dock wall is subject to far greater contingencies than an 

 ordinary retaining-wall, and the required strength will be in- 

 cluded only within correspondingly large limits. 



" Hydrostatic pressure alone may more than double or 

 halve the factor of safety in a given wall. Thus, with a well- 

 puddled dock bottom the subsoil water in the ground at the 

 back of the walls will frequently stand far below the level of 

 the water in the dock, and the hydrostatic pressure may thus 

 wholly neutralize the lateral thrust of the earth, or even 

 reverse it. 



" On the other hand, with a porous subsoil at a lock entrance, 

 the back of the walls may be subject, on a receding tide, to the 

 full hydrostatic pressure due to the range of tide plus the lateral 

 pressure of the filling. 



" Again, the water may stand at the same level on both sides 

 of the wall, but may or may not get underneath it. If the wall 

 is founded on rock or clay, there is no more reason why the 

 water should get under than that it should creep through any 

 stratum of a well-constructed masonry or puddle dam ; and, 

 under these conditions, the pressure of the water will increase 

 the stability by diminishing the lateral thrust of the filling. 



" With rubble filling, assuming the weight of the solid stone 

 to be 155 pounds per cubic foot, and the voids to be 35 per cent., 

 the weight of the filling would be 100 pounds per cubic foot in 

 air, and 59 pounds in water, whilst the lateral thrust will be that 

 due to the latter weight. 



" If, however, as is perhaps more frequently the case, the wall 

 is founded on a porous stratum, the full hydrostatic pressure will 

 act on the base of the wall, and reduce its stability practically 

 by about one half. Thus, the 30-ton concrete-block walls on 

 rubble mounds at Marseilles and elsewhere have the stability 

 due to a weight of say 130 pounds per cubic foot in the air, and 

 66 pounds per cubic foot in sea-water; but the rubble filling at 

 the back of the wall, being similarly immersed, is also reduced 

 in weight, and consequently thrust to a corresponding extent, so 

 the factor of safety is unaffected. 



" In walls with offsets at the back and water on both sides, 



ol. Ixv. p. 180. 



