Advantages of Midheight Pore Pressure Measurements in Undrained Triaxial Testing

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Courtesy of ASTM International

Most soil mechanics laboratories still prefer the use of rough end platens during triaxial testing despite an awareness that nonuniform stress conditions occur within the soil specimen. This practice may to some extent be due to a lack of awareness of the magnitude of inaccuracies which may result. This paper describes results from unconsolidated undrained triaxial compression and extension tests in which pore pressures were monitored at the specimen base and at midheight using a circumferentially mounted miniature transducer. The rate of imposed displacement was varied by two orders of magnitude. The significance of displacement rate and specimen end condition is illustrated by presentation of resulting stress ratios and stress paths. Base measured pore pressures were found to be up to 50% higher than the midheight values, depending on the testing rate used. The results indicated that currently accepted criteria for the determination of acceptable displacement rates are adequate.

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