to carry part of the load. The balance o£ the load is distributed over 

 the sea bottom by the floats. 



The main truss would be required to operate from five different 

 positions along the longitudinal axis of the breakwater during placement 

 of each unit. The screed box, while depositing and leveling stone, and 

 traveling across the structure axis, would be positioned and moved with 

 the aid of counterweights resting on the bottom beneath the extreme ends 

 of the main truss. The counterweights would be suspended from the main 

 truss and relocated for each position. 



Once the bed is prepared for the next unit, the screed box would be 

 run to one end of the main truss; then, the screed box and the counter- 

 weights would be raised free of the bottom, and the main truss fully 

 retracted on the gantry to a position adjacent to the set unit. At this 

 point, the screed box and counterweights are hanging far enough out from 

 the side to clear the inplace units supporting the gantry. 



With the main truss in a fully retracted position, the pipe legs and 

 floats are jacked free of the bottom with the buoyant force of the floats 

 still partly supporting the unloaded gantry. At this stage the entire 

 assembly can be run back on its track onto the next-to-last previously 

 set unit, leaving the newly screeded area free of obstructions for setting 

 the next unit. 



With the new unit in place and secured, gantry track will be laid, 

 and the gantry moved ahead for laying and screeding the bed for the next 

 imit. This will not interfere with filling of the unit with sand and 

 stone, which should be a continuous operation once the unit is on the 

 bottom. 



Once the gantry is operating from permanently set units, the first 

 two temporary units can be refloated and placed in their final position. 



c. Alternate Screeding Procedure . In an alternate screeding proce- 

 dure, the operation may be carried on from specially rigged barges, from 

 which the stone is placed and leveled, as shown in Figure 20. Because 

 of the need for good weather conditions, the use of floating equipment 

 should be avoided. However, it may be necessary under certain conditions, 

 e.g., bottom preparation for the first units placed. 



After each precast unit has been placed, preparations are made for 

 screeding the bed of the next unit. Where necessary for placing addi- 

 tional units, screed barge anchors are picked up and relocated. The 

 screed barge is moved into position and anchor lines connected to anchor- 

 line buoys. The anchor lines are tensioned and the screed barge is 

 positioned for the screeding operation. A screed-pulling anchor is then 

 planted; wire ropes for the counterweights are laid out and the counter- 

 weights set on the bottom. Ballast tanks in the barge are flooded and 

 the hull submerged to the proper depth below water level by tensioning 

 the counterweight wire ropes. In a semisubmerged position, the barge 



37 



