condition c was terminated when all visible movement stopped (Figs. 49 

 and 50). As experience during tests I, II, and III had shown that wave 

 conditions a and c produced the most significant damage, structure IV 

 was not subjected to wave condition d. The sand bed beyond the toes of 

 the structure was not surveyed in test IV, but when the tank was drained 

 at the end of the test, only slight changes in the sand bed were observed. 



IV. ANALYSIS 



1. Bag Problems . 



The method of placing bags successfully produced the planned structure 

 when the correct underwater dimensions of the bags were known and the bag- 

 dropping equipment was positioned according to a placement diagram; however, 

 handling the bags during placement was a problem. The crane clam was 

 equipped with a bag grip made of two short sections of steel pipe bolted 

 to the jaws (Fig. 51). Torn bags found in structures I and II were sus- 

 pected of having been ripped on the ends of the pipe sections (Fig. 52). 

 The sections were extended and pipe elbows were welded to the ends to 

 protect and give additional support to the bags (Figs. 53 and 54). In cold 

 weather the stockpiled, sand- filled bags froze solid at night, making the 

 bags difficult to grip and more susceptible to tearing. A tent of black 

 plastic placed over the stockpile at night and warmed by a small kerosene 

 heater succeeded in keeping the bags pliable enough to be gripped without 

 further damage by the modified clam jaws. 



Both damaged and undamaged bags displayed forms of instability under 

 wave action. Sand was "sucked out" of torn bags until the bags were nearly 

 empty. Evidently, this loss of sand contributed to the settlement during 

 tests I and II. Damaged bags were found in every structure, usually lying 

 in the installed position, implying that the bags were torn during place- 

 ment. Undamaged bags included even those bags that were displaced suddenly 

 from the crests of the structures to the sand bed. Most changes in the 

 structures were due to movement of undamaged bags. The nylon material was 

 slippery and trapped air, which added buoyancy to the bags, formed cush- 

 ioning bubbles on the tops and side, and prevented adequate interlocking 

 despite the carefully overlapped layers. 



2. Structure Stability . 



a. Configuration Changes . In general, all four of the structures lost 

 crest elevation during the tests (Figs. 12, 24, 33, and 48). The overall 

 changes in configuration differed among the structures. Structure I was 

 symmetric until damaged during the midtest draining of the tank, then the 

 back slope flattened and the front slope steepened during wave conditions 

 c and d. Although the crest of structure II broadened as bags were dis- 

 placed during wave condition a, the structure was symmetric until wave 

 condition c, then the back slope flattened more than the front slope. 

 The crest of structure III lost bags and broadened during wave conditions 

 a, b, and c, steepening the back slope in the process. The front slope 



55 



