A Study of Finite Strain Consolidation Aspects of Soft Soils

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This proposal address a specific problem within a group of problems of considerable interest in geotechnical engineering. The group of problems all relate to efforts to develop a general stress-strain- time theory for soils, e.g., to develop a constitutive model. Such a model is required for solution of any problem in which the state of stress in soil is changed and we need to estimate deformations, e.g., when we construct buildings, bridges, dams, etc. The specific problem to be addressed by this research is the analysis of what happens in soils around driven pile foundations as a function of time. Piles are long thin structural members that are driven into the ground and used to provide support for structures. Driving of the piles causes large strains in the soils in the vicinity of the piles and results in large changes in soil properties which, vary radially outwards from the pile. Further, excess pressures develop in the water filling the non-solid parts of the soil, and this water then flows outwards in a time-dependent fashion. As the water flows, the soils densify and become stronger, thus causing an increase in the load capacity of the piles. Engineers count on this increased capacity when they design the foundation, but if the build up in capacity is too slow, or the construction too fast, it is possible to have failures in the foundations. The problem is particularly severe with offshore oil production structures where piles may be up to ten feet in diameter and more than 2000 feet long, and be designed to carry loads of as much as 5000 tons per pile. No load testing has ever been economically possible for such large piles so the design is based on tests on small piles augmented by theory. This proposal is concerned mainly with the theoretical part of this problem. The theoretical part of the study involves an attempt to develop a general model to describe the time-dependent response of the soil near deep foundations, to the changed conditions caused by the installation and subsequent loading of the foundation. There is also an experimental part in which the principal investigators will perform tests with small penetrometers to simulate the action of a pile. Analyses to find the soil properties that must have existed in order to explain penetrometer performance is another aspect of this research.