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doi:10.2204/iodp.proc.308.207.2009

Results

Table T1 lists symbols commonly used in the text. Morgan and Ask (2004) presents the background of these values in more detail. The preconsolidation phase was longer than normal, 39 h, because an external pore water leak was found. The source of the leak could not be identified, and the preconsolidation phase was terminated once it was clear that remaining parameters were stable. The first test results from the preconsolidation phase are listed in Table T2, and the full results can be found in Microsoft Excel format as PRECONSOL.XLS in STRESS in “Supplementary material.”

The reconsolidation phase was run for >645 h (Fig. F3). The test unexpectedly shut down four times, but test conditions were reestablished reasonably well (at 100 h, there was an increase in cross-sectional area; at 100 and 140 h, small shifts in axial strain).

Sample 1320-31-1 was loaded to a total effective vertical stress, σ_v′, of 24 MPa, corresponding to effective horizontal stress, σ_h′, of >16 MPa and vertical strain, ε_v, of 16% (Fig. F3). Initial inspection of samples and analyses of test results proposed that Sample 1320-31-1 was uncemented and undisturbed. However, posttest computer tomography scans and paleomagnetic studies (anisotropy of magnetic susceptibility) reveal that the sample indeed is disturbed and comprises three drill biscuits (Fig. F1). Because of the short spacing between the three drill biscuits and because the sample was collected in a basin in which sediments are assumed to be transversely isotropic, the results may still be of some interest, especially at greater stresses, at least for stress-stress plots.

Table T3 shows the results from the first hours of testing during the K₀ reconsolidation phase, and Figure F4 shows how the stress ratio and vertical strain vary with increasing stress during the entire test. The complete data set from the test results is available in Microsoft Excel format as RECONSOL.XLS in STRESS in “Supplementary material.” The plot of σ_v′ versus ε_v best shows the initial nonlinear deformation when microcracks are closed and the transition from elastic to plastic deformation. In comparison, the stress-stress plots reveal more subtle changes. The preconsolidation stress is inferred to be 2.03 MPa based on the change in slope in the stress-strain curve shown in Figure F4C. Based on shipboard data, the in situ effective vertical stress for hydrostatic water pressure, σ_vh′, is calculated to be 2.27 MPa, which results in a maximum pore fluid pressure in excess of hydrostatic water pressure, P*^max, of 0.24 MPa. This corresponds to a λ* of 0.11. The small difference between σ_vh′ and σ_y′ is probably insignificant, especially when comparing the results with studies that use other methods to determine σ_y′ (or preconsolidation pressure). The results propose that the sample is normally consolidated. There is a subtle change in stress ratio across σ_y′, and the stress ratio, K₀, changes from 0.70 before yield to 0.69 after yield for the entire data set. A more detailed inspection of the data reveal minor shifts in the stress magnitudes when the test unexpectedly shut down four times: postyield K₀ values range from 0.65 to 0.68 within the continuous test sequences (Fig. F4A). The overall trend reveals that the stress ratio is constant up to an effective vertical stress of nearly 25 MPa.

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