STP883: In-Situ Undrained Shear Strengths and Permeabilities Derived from Piezometer Measurements

    Bennett, RH
    Head, research oceanographer, and marine geotechnical branch head, Seafloor Geosciences Division, Naval Ocean Research and Development Activity, NSTL, MS

    Li, H
    Head, research oceanographer, and marine geotechnical branch head, Seafloor Geosciences Division, Naval Ocean Research and Development Activity, NSTL, MS

    Valent, PJ
    Head, research oceanographer, and marine geotechnical branch head, Seafloor Geosciences Division, Naval Ocean Research and Development Activity, NSTL, MS

    Lipkin, J
    Technical program coordinator, Seabed Programs Division, Sandia National Laboratories, Albuquerque, NM

    Esrig, MI
    Vice-president, Woodward Clyde Consultants, Wayne, NJ

    Pages: 18    Published: Jan 1985


    Abstract

    Existing theories and models describing stress changes and consolidation-time effects around a pile were used to derive in-situ permeabilities and undrained shear strengths from piezometer probe measurements in smectite- and illite-rich soils. Permeabilities derived from piezometer measurements are in reasonable agreement with laboratory measurements, and calculated undrained shear strengths agree well with strength measurements using standard field and laboratory techniques.

    Undrained shear strengths Su were estimated using insertion pressures Ui, determined from both the 10.2- and 0.8-cm-diameter probes, and the relationship, Ui = 6 × Su. A strength measurement determined with the small diameter probe inserted in the disturbed zone of a previously emplaced 2.5-cm-diameter cylinder showed a significant strength reduction equal to half the value determined for the soil (strength) in the zone unaffected by the implanted cylinder. Differences in decay rates were significant, indicating severe soil disturbance in close proximity to the cylinder.

    Multisensor piezometer probes (2), 10.2 cm in diameter, were deployed in shallow-water fine-grained smectite-rich soils of the Mississippi delta. Pore-water pressures were measured at subbottom depths of 6.5, 12.6, and 15.6 m. Insertion pressures, time-dependent pore pressure decay, and ambient excess pore pressures were determined.

    Single sensor piezometers (2), 0.8 cm in diameter, were developed for deep-ocean investigations. Before high pressure testing (55 MPa), probes were inserted in reconstituted illitic marine soil to depths of 16.9 and 26.4 cm below the soil-water interface. Insertion pressures and their decay characteristics were monitored.

    Significant differences were observed in the pore-pressure decay rates produced by the small and large diameter probes. Decay times for the induced pressures to reach t100 values were on the order of tens of hours for the large diameter probes, whereas the t100 values of the small diameter probes were on the order of minutes. These differences in decay times were a function of the differences in probe diameters (radii) and soil permeabilities.

    Keywords:

    piezometers, pore pressures, shear strength, permeability, heat transfer, marine geology, piezometer probes, consolidation, mass physical properties


    Paper ID: STP36331S

    Committee/Subcommittee: D18.05

    DOI: 10.1520/STP36331S


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