high water may pass over the top, but drainage of water off the marsh will 

 cease when the water reaches a prescribed level. In the past, unreinforced 

 bank plugs have been formed merely of compacted marsh soil. Due to subsidence, 

 water and animal movement, and the lack of stabilizing vegetation, this sort 

 of plug has often failed. Simple reinforcement methods such as pilings, metal 

 sheets, or concrete-capped shell provide better plug security, but they are 

 not used universally. 



Thus, it is desirable to build permanent canal plugs or wiers. When a 

 site is chosen, a dragline is used to place a number of small pilings into the 

 soil. The pilings are placed across the ditch, and there may be several rows 

 of them in places. Heavy boards, corrugated iron, or even concrete slabs may 

 be fastened to the outside rows of piles to form two parallel walls. The 

 outside piles may be braced, and then the interior section between the walls 

 is filled with spoil. If cattle are to use the plug as a walkway, it is 

 elevated slightly and covered with shell to give it a hard surface. Otherwise, 

 fill or, in some cases, a concrete cap, is placed across the top of the plug. 

 If a wier is being constructed, ironwork may be placed so that movable slats 

 can be used to control the water level. While these improved bulkheads, 

 plugs, and wiers are desirable, they are not industry standards, and there 

 have been many instances where not even earthen plugs were installed. 



In moderately firm marsh soils, cathodic protection is sometimes used. 

 However, when the soils become too moist or when the pipe is actually laid in 

 water, protection of pipe is more dependent upon complete sealing to exclude 

 water and to insulate the pipe. Here layers of mastic - asphalt derivatives - 

 and concrete are used to seal the pipe. 



Pipeline construction - unstable marsh soils and shallow water bodies. 

 In some areas, the marsh soil is so unstable that it cannot support the weight 

 of a dredge. In other instances, requirements of the pipeline do not allow 

 the push method to be used. Instead, the "flotation" technique is used. The 

 flotation technique utilizes a canal wide enough for the pipeline barge and 

 other vessels to float through. The pipe is laid from a pipeline barge that 

 traverses the length of the canal. 



Hydraulic or bucket dredges are used to excavate the flotation canal. 

 The methods used are the same as those used for dredging well site canals and 

 slips. Typically, the canal is 12 to 15 m (40 to 50 ft) wide and 1.8 to 2.4 m 

 (6 to 8 ft) deep for vessel passage. A narrow deeper ditch is dredged within 

 the wide canal for actual pipeline placement. Spoil is placed alongside the 

 canal or pumped away from the side. On occasion, other pipelines may cross 

 the canal. Hydraulic dredges are usually used to clear a path under these 

 pipes (Petroleum Extension Service, 1972). 



It is rare that dredged material is backfilled into a flotation canal, 

 though Willingham et al. (1975) reported the backfilling of a 1.2-km (2-mi) 



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