Figure 22 where the cells failed in a 1984 storm. Because of the intensely 

 strong signal return from the vertical sheet-pile walls, minor damage to the 

 walls, such as gaps a few inches wide, may be masked or not resolved. The 

 faint lines and texture on the cell images are diffraction hyperbolae and 

 noise. Diffraction hyperbolae often occur from strong circular reflectors of 

 acoustic energy (like steel pipes and circular sheet-pile cells) due to 

 reflections of energy from the horizontal side lobes. Another unusual feature 

 shown in Figure 22 is the ghost image caused by cross talk from the other 

 channel. Cross talk occurs when the reflected sound energy is so strong that 

 it causes the transducer on the opposite side of the towfish to vibrate. 



43. The Burns Harbor breakwater, constructed between 1967 and 1970, is 

 a multi-layer rubble-mound design using 10- to 16-ton limestone blocks on the 

 exposed surfaces (Figure 23). Soil borings prior to construction indicated 

 that approximately 10 ft of soft clay were underlain by much harder clay. 

 Before construction, the soft clay was to have been excavated and replaced 

 with a sand mat which was to extend at least 30 ft beyond the toe of the 

 structure. The excavated clay was disposed onto the lake bed about 150 to 

 300 ft from the toe of the structure in 50-ft depths. 



HARBOR SIDE 



CONSTRUCTION DESIGN 

 SCALE 



1 10 20 30 40 50 FT 



Figure 23. Cross section of Burns Harbor breakwater 



44. The lakeside survey conducted in 1985 was not able to provide con- 

 clusive evidence of a sand mat extending beyond the base of the structure 

 (Figure 24); also, no conclusive evidence of a sand mat was found on the 

 harbor side. Areas of sand waves occur at distances greater than 30 ft from 

 the base of the structure, but these are believed to be veneers of sand moved 

 by waves and currents over a predominantly clay lake bottom. It is possible 

 that wave action could have redeposited soft clay sediments on top of the sand 



37 



