Breakout of objects from underconsolidated sediments

Guha, Shib Nath (1979) Breakout of objects from underconsolidated sediments. Masters thesis, Memorial University of Newfoundland.

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Abstract

There is a growing worldwide interest in the exploration, production and utilization programs for offshore oil, natural gas and other minerals. As a part of these efforts considerable interest has been directed in the last decade towards marine geotechnology. Adhesive resistance of ocean sediments is one of the important properties of marine soils which finds an application in ship salvage, refloating of exploratory platforms and relocation of marine installations such as pipelines, oil rigs, well-head manifolds, well linings and ice-soil interaction studies. -- There is considerable research work reported in the literature about the adhesion and skin friction between terrestrial soils and construction materials. However, ocean floor soils differ from most terrestrial soils in their properties. Surficial ocean sediments are generally cohesive and highly underconsolidated, having very low shear strength with high liquidity index. Such soils have negligible angle of shear resistance. In the studies of interaction of such soils with embedded objects, only adhesion is of primary importance. Very little information is available at present about the adhesive resistance of these underconsolidated sediments. A closely related property is the base suction at the interface of the object base and soil, which is important in the breakout problem. -- An attempt has been made in this investigation to study the adhesion and base suction during pull out tests in an artificially sedimented soil having properties comparable to those of ocean sediments. Laboratory tests were carried out to determine (a) the adhesion between the soil and objects of different surface roughness, (b) the suction at the base of prismatic objects of different base configurations during pull out, and (c) the total breakout force for fully embedded objects. -- Only one type of soil was used in all the tests and the sample was prepared by mixing a pre-weighed dry soil in a known volume of water. -- The adhesive resistance was found to depend on the surface roughness of the embedded objects as well as on the soil cohesion. Surface unevenness and physio-chemical action were two additional factors influencing adhesion, as observed in the tests on plexiglas and glass plate respectively. -- Base suction for prismatic objects was found to be a function of the depth to breadth ration (D/B) of the object. It is higher for higher D/B values. Base configurations do not appear to have any significant influence on the base suction values. -- A fairly good correlation was found between the breakout force determined from the experiments and that estimated analytically. -- Pull out tests conducted with fresh water ice sheets indicate that the adhesion - cohesion ratio is very nearly constant in the range of soil strengths tested.

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