Analysis of plowing forces for a finite-width blade in dense, ocean bottom sand

Burying cable in dense ocean bottom sands requires huge plowing forces. This is because of the “pore water” effect. When compact sand shears, the grains must roll over one another. In so doing the spaces (pores) between the grains expand. Water pressure in these pores must decrease in order to draw in water to keep the pores filled. The drag of this permeating pore water squeezes the grains together, increasing the frictional force that shears the sand. Hence the drawbar is increased. The faster and deeper the plow tool operates, the greater the decrease in pore water pressure, and hence the greater the drawbar-up to a limiting speed at which either cavitation (vacuum) or grain crushing occurs. The cavitation limit is a function of ocean depth. The grain crushing limit is a function of the sand properties. This paper analyzes ocean plowing forces for a finite-width plow blade in ocean bottom sand. Equations are derived for the plowing force as a function of plow speed, ocean depth, blade depth, blade width and various soil parameters including friction angle, weight density, permeability, unconfined volumetric strain and critical confining stress. The analytical results are shown to be in reasonably good agreement with experimental results obtained by the Naval Facilities Engineering Service Center