Chemical Overlays for Seafloor Sediments

Abstract

Various materials and methods have been investigated to control the turbidity caused by the disturbance of seafloor sediments. The method selected as the most promising consists of the formation of plastic film over the sediments by casting from a compatible solvent system. A formulation has been developed, and equipment to dispense it has been designed, fabricated, and is being evaluated. INTRODUCTION Poor visibility is one of many problems which man encounters when he carries out undersea operations. It has been reported that under ideal conditions, and at shallow depths, divers have been able to see clearly for 200 feet in all directions1 but generally the limit of visibility off the Southern California coast is fifty feet. This limit is drastically reduced if seafloor sediments are disturbed so that an undersea "dust cloud" is created. Material of this type can remain suspended for hours with the result that work must be greatly curtailed or suspended. In salvage operations, any prolonged delay could result in partial or complete failure of the mission. As man begins to place permanent structures on the sea floor, sediment stabilization is a problem which will have to be solved during construction and operation of these installations. CHEMICAL METHODS Three methods were considered as approaches toward solving the sediment stabilizing problem:use of flocculating agents,stabilization of the soil mass,formation of an overlay to cover the sediment. Flocculating agents are effective in increasing the settling rate of suspended material. However, if the settled material is disturbed, an additional waiting period is required for it to settle again. Stabilization of the soil mass would require that rather large quantities of binding materials be injected into the sediment with a minimum of disturbance. This technique should be considered only when a permanent or semi-permanent foundation is required. Formation of an overlay to cover the sediment was selected as the best of the three methods to solve the present problem. Three methods were considered as possible solutions to the production of an overlay:the formation of a polymer gel by chemical reaction at the site,the formation of a gel by increasing the viscosity of a liquid because of low ambient temperature at the site,the formation of a plastic film at the site by casting from a compatible solvent system. Roe and Williams2 and Epstein and Widman3 have reported on one or more of these methods, and the information can be summarized as follows:Formation of a polymer gel. Acrylamide can be polymerized in a soil formation4 and a proprietary product is sold for this purpose. Oceanographers have used this chemical grout in high concentrations in taking seafloor core samples.5 Polyacrylamide forms a tough gel under water, but the monomer is slightly neurotoxic and would not be suitable for diver use.