Elimination of membrane compliance in undrained triaxial testing.II. Mitigation by injection compensation

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Abstract
A computer-controlled injection-correction system has been developed to continuously and completely mitigate the adverse effects of membrane compliance for undrained testing of granular soils. The system was developed with the explicit idea that it could be quickly and easily adapted to most any up to date triaxial testing apparatus. A relationship between effective confining stress and volumetric compliance can be predetermined, as volumetric-compliance errors have been shown to be a direct and repeatable function of effective confining stresses for a given material. The predetermination of volumetric-compliance errors represents a suitable basis for control of injection–removal compensation for membrane compliance during undrained testing. A closed-loop computer-controlled system continuously monitors changes in sample effective confining stresses such that precalculated volumetric-compliance errors can be continuously offset by injecting or removing volumes of water equal to those errors throughout the duration of each test. The validity of the injection system was verified by comparison of the compensated tests to results of large-scale (300 mm diameter) tests of the same materials, as compliance effects for large-scale tests of this material were negligible. Key words : membrane, penetration, compliance, undrained testing, triaxial, compensation, liquefaction.

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