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Pore Pressures in Soft Ground under Surface Loading; Theoretical Considerations.

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Final rept. Jan-Dec 75,

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Most soft clays in their natural state exhibit a small degree of overconsolidation resulting from changes in ground water level, delayed consolidation, or other causes. The overconsolidation ratio is commonly in the range of 1.0 to 2.5. Under surface loading pore pressures in such a deposit will develop as for an elastic material until the effective stresses reach a yield condition or failure condition. In the latter case the soil can continue to carry additional total stresses within confined zones and pore pressures in these zones will continue to increase. In the former case the soil will continue to deform plastically until it reaches failure, showing, in general, different pore pressure responses in these two phases. Thus pore pressure response at any point in a soft clay deposit under increasing surface loading may show two or three distinct phases, although in some cases the plastic and failure responses may be almost indistinguishable. In this report evidence is presented for the yield condition from a study of published laboratory test results. Expressions are then developed for the pore pressures to be expected in the field for each of the three phases -- elastic, plastic, and contained failure -- as a function of the vertical total stress increment at any point in the soil mass. Author

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  • Soil Mechanics

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