ASTM International

Automated Triaxial Testing of Soft Clays: An Upgraded Commercial System

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The paper evaluates the ability of a widely used automated commercial testing system to perform Ko consolidated-undrained triaxial compression and extension (CKoUC/E) tests on normally consolidated clay. The commercial system uses control software on a microcomputer to operate three digital pressure-volume controllers that control and measure the pore water pressure and volume, the radial stress, and the axial stress and strain for a specimen within a Bishop and Wesley Hydraulic Triaxial Apparatus. Because the commercial system calculates axial stress and strain by relying on indirect measurements assuming idealized behavior of the axial loading assembly, the system was modified to directly measure axial stress and strain. The system was also enhanced to enable shear in triaxial extension and to improve certain other testing procedures. Comparison of results from CKoUC/E tests obtained by the resulting reference system with prior data on resedimented Boston Blue Clay shows good to excellent agreement in 1-D compressibility and undrained stress-strain-strength characteristics. In contrast, stresses recorded by the commercial system resulted in significant overestimates of preconsolidation pressure, undrained strength, and the effective stress envelope, primarily caused by unquantifiable behavior of the axial loading assembly. Based on these results and other experience, the paper suggests features warranting special attention when selecting a commercially available automated triaxial system in order to upgrade the experimental capabilities of a laboratory.

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