126 SECTIONAL ADDRESSES. 



vertical sides with the heavy seas beating thereon, nearly at right angles — 

 there seems little chance of disturbance of relatively small stones. 



It should be understood, however, in this connection that thought 

 must be given to possible currents arising from any cause whatever. It is 

 well known that stones of considerable dimensions are carried along our 

 London sewers if a velocity of a few feet per second arises. 



Only one small movement of the 60-ton blocks in Valparaiso break- 

 water occurred in an exceptionally heavy storm, and that, I think, was 

 attributable to an interesting and temporary modification of the top 

 courses to assist construction. The movement was very small and, it adds 

 to the interest to note, it was not on the sea side but on the sheltered side 

 of the breakwater. It was attributable, I think, to the fact that a tem- 

 porary longitudinal depression was left in the centre of the structure for 

 construction purposes. The pier top has a total width of 45 ft. 6 in. , and only 

 the outer rows of blocks in the top course were placed. Thus, a longitu- 

 dinal channel protected from the sea in which to transmit the blocks, for 

 construction purposes, to the crane at the outer end of the work was pro- 

 vided. This depression formed a canal when a heavy swell existed and 

 was filled by the sea. None of the joints in the breakwater blocks are 

 cemented or filled with grout, therefore voids existed between them, into 

 which columns of water flowed. Since water is incompressible, the falling 

 masses of green sea on the surface of this filled canal acted like a hydraulic 

 ram which transmitted vertical blows from the falling masses of fluid into 

 horizontal pressure between the blocks. The blows moved the top courses 

 of blocks towards the inside by a few inches but, fortunately, not sufficiently 

 to require extensive demolition and replacement to put matters right. Had 

 the storm continued for several days it is possible that these voids would 

 have caused the partial destruction of the top courses of the breakwater. 

 This canal was intended to be filled with mass concrete, and this has now 

 been done, and no further trouble can arise from this cause. 



Another and similar case occurred in the old Admiralty Pier at Dover, 

 which is faced on both sides with granite blocks. A number of the blocks 

 have been forced out at right angles to the axis of the breakwater by what 

 was clearly equivalent action. In other words, the stone jointing was not 

 full of cement mortar and voids were left. Water accumulated behind the 

 inner ends of these poorly jointed blocks, and the force of the hammer 

 blows reacting from the almost vertical wall created what amounted to a 

 hydraulic ram, and so forced the granite blocks out of the face of the 

 breakwater. 



The exact action of the sea on structures is waiting the solution of the 

 scientist to determine what forces exist and are exerted by moving masses 

 of water in great storms. The knowledge of the effect of waves of any 

 definite length or height is not determinable with sufficient exactitude by 

 any scientific data that I know of. 



I remember while the harbour works were under construction at Dover 

 three concrete blocks piled one on top of the other, each averaging about 

 thirty-five tons, were standing on the unfinished pier. They rested on 

 some timber packings a few inches thick at a level of about 8 ft. above 

 high-water mark. During a south-westerly storm in the Channel, they 

 were carried across the pier many feet and into the sea on the lee side. 



I 



