considerable, and rooks up to 3 tons and mora have been moved* 



In the United States an attempt "was made to consolidate the 

 stones, for the first time in 1936 by filling the voids with 

 asphalt mortar. This method was used in the Netherlands for the 

 first time in 1933 by the Engineer in Chief of Delfland, A. C. 

 Kolff, on some beach groins near Scheveningen, Since 1945 it has 

 been increasingly used. 



a. Groin at the mouth of Columbia river 1936. - The groin 

 is situated at the Pacific Ocean in an area of heavy wave-action 

 and reaches from 18 m below to 7«5 ra above low water. It lias a 

 top width of 12 m and is constructed of stones from I to 12 tons. 

 The asphalt grouting contained 15-18$ asphalt mortar and was 

 applied only to the upper part. 



b. Galveston Jetty 1936. - The asphalt-grouting of this 

 structure proved satisfactory. 



Co Delfland Groins. - The voids of the groins were filled 

 with an asphalt mort.gr containing 20% asphaltic bitumen [50/6O 

 (6%), 60 /SO {!%) and 80/lOO (756 j, 10% filler and 10% dune sand. 

 Three groins were treated from 1938 to 194-0 and seventeen more from 

 194-6 to 1948 . On each ^roin 300 tons of the mortar were used, 

 or an average of 0.5 tons per m 2 . This reinforcing method of 

 the beach groin proved entirely satisfactory. When a floating 

 mine exploded against one of these groins it was found that the 

 voids were filled to a dep bh of 1 m below the surface » 



d. South Jetty of the outer shipping channel at Ymuidan, 

 1938. - This jetty which was often badly damaged during heavy 

 gales , was provided with asphalt mortar as described under c. 

 Between the dumped stones 0.8 tons of asphalt mortar per vt? 

 and between the set stones 0,5 tons per m 2 were used. Since 

 then no more damage has occurred. 



e. North- and South piers at Hook of Holland 1946-50, 

 (Figure 20). - The piers are 2 km in length, with the top, 



4 m wide at mean sea level. The heavy stone protection (stones 

 of 0.1 to 3 tons) of the outer section could not resist the 

 heavy ground swell so that it was often damaged, and the normal 

 gauge railroad which t"'as laid on oak girders on top of the 

 jetty required abnormally high expenditures for maintenance. 

 In view of this, it has been decided to convert the railroad 

 into a single lane road for trucks and to grout the set stone 

 and dumped stone with asphalt-mortar where the attack is 

 heaviest. 



The outer 1 km section of the North pier and the so-called 

 connecting jetty, forming the connection between the South 

 jetty and the low jetty which was constructed at a later date, 

 were thus reconstructed (Figure 21-26) from 1946 to the middle 



55 



