will have been repaired. A means of burying these slack sections of cable, 

 whether it be the primary burial means or a subsystem, must be provided. 



d. Additional specific requirements as developed in Reference 1 are 

 shown in Table 1 . 



STATE-OF-THE-ART 



Cables and pipelines have been buried on land and under waterways 

 for many -years and lately in ocean bottoms. This section discusses the 

 variety of techniques used to accomplish burial tasks, examines their 

 advantages and disadvantages, and, where applicable, references burial 

 systems using these techniques underwater. 



Trenching /Excavat ing 



Trenching and excavating are the most common methods used for install- 

 ing buried cables and pipelines. Typical equipment used includes backhoes, 

 chain /bucket trenchers, and excavating wheels. In the mid-1960s, a Belgian 

 firm modified an excavating wheel trencher to bury 600 feet of power cable 

 at depths to 40 feet in the river Scheldt [5]. The trencher was capable 

 of excavating a 5-foot-deep, 20-inch-wide trench at a rate of 150 ft/hr. 

 About the same time, a conventional backhoe was modified for underwater 

 use that could excavate 200 feet of 4-foot-deep trench, 18 inches wide, 

 in a day. Recently, a commercial cutter wheel trencher was modified and 

 used in 120 feet of water to trench cable in sandstone. CEL recently used 

 a similar trencher in coral (Figure 2). 



Trenchers such as these are attractive in that they can be used in 

 material as hard as granite. Wheel and chain/bucket trenchers can be 

 equipped with cable feed mechanisms that allow placing the cable while the 

 trench is being excavated. Backhoes require a three-step operation - trench- 

 ing, placing the cable, and backfilling. In soft or sandy materials, a 

 means to keep the trench from slumping in must be provided until the cable 

 is installed. The major drawback for the trenching /excavating technique 

 is that it is an inherently slow process. Supply power, usually in the 

 50-to-150-hp range, is well within the range considered feasible for deep- 

 ocean cable burial. 



Plowing 



Plowing in cables and small -diameter pipelines was developed princi- 

 pally to increase the efficiency of installation. The cable can be in- 

 stalled through a feedshoe that immediately follows the plowshare. Little 

 or no surface restoration is required since very little earth is forced 

 out of the slot. Plowing cables has been proven feasible for deep -ocean 

 cable burial by the Sea Plow (discussed in the Introduction) and by two 

 Japanese firms that have developed plows. Repeater handling has been accom- 

 modated by lowering auxiliary plowshares to widen the ditch, or by plowing 

 a repeater-sized ditch over the entire cable route. The hardware required 

 for plowing cables is relatively simple, a particularly attractive feature 

 for deep-ocean application. 



