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doi:10.2204/iodp.proc.314315316.213.2011

Results

Sample identification and a summary of CRSC test results are shown in Table T2. A complete CRSC data sheet can be found for each test in CRSC in “Supplementary material.” Figures in CRSC in “Supplementary material” show time series data of vertical effective stress (σ′_v) and basal excess pore pressure (Δu) for all the tests, the consolidation curves in e – log(σ′_v) and ε–log(σ′_v) formats, excess pore pressure ratio, coefficient of consolidation (C_v), SED, and hydraulic conductivity (K) for each CRSC test.

The graphical determination of a P′_c value is not reliable unless the inflection point on the e – log(P) curve is clearly defined. The effect of sample disturbance on test quality was evaluated following the criteria of Lunne et al. (1997). This designation considers the overconsolidation ratio (OCR), where OCR is equal to P′_c/σ′_vh, plus the change of void ratio during consolidation tests. The Δe value is equal to the difference between the initial pretest void ratio (e_i) and the void ratio at the intersection point of P′_c on the consolidation curve using the SED method. The rating is based on Δe/e_i. Sample quality is poor to very poor for most specimens (Table T3). Disturbance is particularly severe for the specimens from Site C0006.

P′_c values are similar using the Casagrande and SED approaches (Table T2). All specimens from Sites C0002, C0006, and C0007 exhibit significant amounts of apparent overconsolidation (Fig. F5). In other words, the test-derived values of P′_c are greater than the calculated values of in situ hydrostatic vertical effective stress (σ′_vh) at the sampling depths. Differences between values of P′_c and σ′_vh range from 0.31 to 9.14 MPa. The overconsolidation ratio based on SED-determined values of P′_c varies from 1.69 to 8.58.

The compression index (C_c) refers to the slope of the virgin portion of the e – log(P) curve (Fig. F4). The index is calculated using the following equation:

Values of C_c range from 0.390 to 0.780 (average = 0.584) at Site C0002. Values of C_c at Sites C0006 and C0007 range from 0.236 to 0.418 (average = 0.302) in the trench-wedge and upper Shikoku Basin facies (Table T2).

Calculated values of intrinsic permeability decrease during CRSC tests as vertical effective stress increases and porosity is progressively lost (Fig. F6). Permeability values for strata from Site C0002 show log-linear decreases with porosity. Values decrease from a range of 6.53 × 10^–17 to 9.83 × 10^–16 m² at porosities of 52% to 54% (σ′_v = 1–2 MPa) to a range of 1.50 × 10^–19 to 3.72 × 10^–19 m² at porosities of 16% to 22% (σ′_v = 40 MPa). A plot of permeability versus porosity for Sites C0006 and C0007 also exhibits a log-linear trend. Values decrease from a range of 4.11 × 10^–18 to 2.00 × 10^–15 m² at porosities of 43% to 46% (σ′_v = 1–2 MPa) to a range of 3.28 × 10^–19 to 1.59 × 10^–17 m² at porosities of 12% to 19% (σ′_v = 20 MPa).

We calculated the in situ intrinsic permeability of each sample from the in situ void ratio and the porosity values shown in Table T4. We obtained those values by extrapolating the linear portion of the n – log(k) relation to the porosity and equivalent void ratio to match a desired value of P′_c or σ′_vh on the SED curves (Fig. F6) (e.g., Long et al., 2008). There is no consistent trend of intrinsic permeability changing with depth at Site C0002; values range from 2.67 × 10^–17 to 2.56 × 10^–18 m² (Fig. F7). In situ intrinsic permeability deceases systematically with depth at Sites C0006 and C0007 from values of 1.85 × 10^–16 m² at 33 m CSF to values of 5.61 × 10^–17 m² at 400 m CSF (Fig. F7).

We also calculated in situ hydraulic conductivity from values of in situ intrinsic permeability by taking into account changes in temperature, fluid density, and fluid viscosity with depth (Table T4). Temperature-dependent changes in permeant properties assume a pore water salinity of 35‰ and follow the equations of El-Dessouky and Ettouny (2002) and Fofonoff (1985).

The statistics used to evaluate microfabric are summarized in Table T5. Figure F8 shows all ESEM images for the horizontal and vertical sections of the samples. Figure F9 shows the orientations of the particles (apparent long axes) as rose diagrams, together with values of standard deviation and index of orientation. Cumulative curves are shown in Figure F10. Average values for index of orientation are ~0.33 (Sites C0006 and C0007) and ~0.38 (Site C0002). This index is expected to increase as depth of burial increases. Vertical sections generally show greater indexes of orientation than the horizontal sections, which should be expected as mudstones develop a weak bedding-parallel fissility. Exceptions to this pattern include several samples from Sites C0006 and C0007 where bedding dips are >40° (Table T5; Fig. F9). Samples from Site C0002 and the accreted upper Shikoku Basin facies at C0007 yielded the highest values of orientation index (>0.40), indicating better preferred alignment of platy grains. Plots of the index of orientation versus C_c and P′_c show weak positive correlations (Fig. F11), which indicates that the mudstone microfabric has very little influence on the state of consolidation over the range of burial depths covered by this study.

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