Dredging 



Dredging, which combines a rotating cutting head and suction pump 

 for spoil removal, is a very effective means of removing soil. Two similar 

 devices, the Mole [27] and the Gopher [28], are pipeline machines that 

 straddle the pipe and have mechanical cutters on each side of the pipe 

 angled towards each other. The cutters dislodge the soil which is then 

 removed by a suction dredge pump. The Gopher also has water jets and air- 

 lift pipes to help remove the spoil. The most technically advanced dredging 

 system for burying pipelines is currently under development by Tecnomare 

 (Italy) [29] . It is a tracked crawler machine with two dredge cutters 

 mounted on articulated arms. The system may be programmed to dredge a 

 prearranged path, or it may be manually controlled from the surface. Dredge 

 spoil is pumped to the rear of the machine to bury the pipeline after 

 it has settled in the trench. The system can be made neutrally buoyant 

 and is supplied from the surface with 1,300 hp. Dredging is a proven under- 

 water excavation technique, but generally requires large amounts of 

 power, excavates more soil than necessary for burying a cable, is slow, 

 and does not lend itself well to backfilling. 



Fluidizing 



Fluidizing is a technique where water is pumped into the soil at 

 such a rate that as it flows out of the soil, the individual soil particles 

 are buoyed up by the water. The soil /water mixture achieves a fluid 

 or ''quick'' condition which will not support applied shear forces. 



Shell Laboratories (The Netherlands) has developed a fluidizing system 

 for burying pipelines [30] . The soil is fluidized under a predetermined sag 

 length, and the weight of the pipe and fluidizing device causes the 

 pipeline to "sink" into the fluidized soil. This technique works in 

 sandy (noncohesive) soil, but to date it has been stymied by cohesive 

 (clay) soils as the intergranular forces cannot be overcome and the 

 soil will not fluidize. 



Related Techniques 



Other techniques which have been used or proposed for burying cables 

 and pipelines include cavitation cutting, high-pressure water jetting, 

 directional drilling, and piercing tools. 



Cavitation cutting is basically a forced erosion process that depends 

 on the formation and violent collapse of bubbles in a fluid. The cavitation 

 erosion is caused by the shock wave produced when the bubble collapses, 

 and the energy density is sufficient to erode materials such as rock 

 and metal. The intensity of the cavitation, and, therefore, the penetration 

 rate, increases with hydrostatic pressures [31]. Cavitation cutting 

 development is still in its Infancy, and acoustic transducers powerful 

 enough to produce the necessary threshold energy levels for high ambient 

 pressures have not been developed. This technique produces localized 

 energy densities effective for drilling through rock. 



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