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doi:10.2204/iodp.proc.322.209.2014

Results

Summaries of CRSC test results are shown in Table T2. A complete CRSC data sheet for each test can be found in Microsoft Excel format in CRSC in “Supplementary material.” Also included are figures showing time series for effective axial stress and basal excess pore pressure, consolidation curves in e versus log(σ′_v) and ε versus log(σ′_v) formats, excess pore pressure ratio, coefficient of consolidation, SED, and hydraulic conductivity.

Compression index

Figures F4, F5, and F6 show all the virgin curves from consolidation tests. Values of C_c for samples from Sites C0011 and C0012 range from 0.31 to 2.70 (average = 0.99) (Table T2). The overall pattern is that C_c values decrease with increasing depth (Fig. F7). Samples of hemipelagic-pyroclastic facies from the upper Shikoku Basin show higher indexes (>1.0) and more scatter, whereas deeper stratigraphic units are less compressible with average values of ~0.5. The values of compression index at Site C0018 show no systematic changes with depth or stratigraphic unit (Fig. F7), ranging from 0.42 to 0.60 (average = 0.53) (Table T2).

Permeability

Values of hydraulic conductivity and intrinsic permeability during laboratory tests generally decrease in log-linear trends as the vertical effective stress is progressively increased and specimens compact to lower porosity. Test results from Sites C0011 and C0012 are grouped according to lithostratigraphic unit (Fig. F8). Samples from the hemipelagic-pyroclastic facies (Unit I) yield bands of permeability values that decrease from a high of 7.6 × 10^–16 m² at n = 70% (σ′_v = 1 MPa) to a low of 7.3 × 10^–19 m² at n = 41% (σ′_v = 5.6 MPa). Samples from the deeper volcanic turbidite facies (Unit II) decrease in permeability from a maximum of 2.2 × 10^–17 m² at n = 65% (σ′_v = 1 MPa) to a minimum of 1.5 × 10^–19 m² at n = 31% (σ′_v = 23 MPa). For older samples (Unit III and deeper), values decrease from 2.3 × 10^–18 m² at n = 49% (σ′_v = 1.0 MPa) to 3.8 × 10^–20 m² at n = 21% (σ′_v = 32 MPa).

Values of in situ intrinsic permeability calculated for Site C0012 sampling depths range from 3.0 × 10^–15 m² to 3.6 × 10^–19 m² (Table T2), consistent with the results of Screaton et al. (2013), Dugan and Zhao (2013), and Song et al. (in press). The condensed section at Site C0012 shows an erratic depth-dependent pattern with several excursions from the typical trend of progressive burial compaction (Fig. F9). Scatter is more extreme in the hemipelagic facies (e.g., at ~263 meters below seafloor [mbsf]), although some of the higher values could be due to microcracks induced by drilling. Comparable results from the expanded section at Site C0011 reveal a more consistent trend in which permeability progressively decreases with depth (Fig. F9); those k_i values range from 1.2 × 10^–16 m² to 1.6 × 10^–19 m² (Table T2). Data from the volcanic turbidite facies display more scatter in comparison to overlying and underlying units.

For Site C0018, laboratory permeability values for the slope-basin deposits decrease from 6.4 × 10^–16 m² at n = 60% (σ′_v = 0.2 MPa) to 1.3 × 10^–18 m² at n = 25% (σ′_v = 14 MPa) (Fig. F10). The calculated in situ values define a smooth trend of steadily decreasing permeability with depth, with no obvious differences between the mass transport deposits and hemipelagic deposits (Fig. F9). The values range from a high of 3.3 × 10^–16 m² at ~59 mbsf to a low of 1.5 × 10^–17 m² at ~311 mbsf (Table T2).

Maximum past effective normal stress

All of the P′_c values for samples from Site C0011 are greater than the calculated values of in situ hydrostatic vertical effective stress at the equivalent sampling depths (Fig. F11); such OCR values are consistent with moderate levels of apparent overconsolidation. The individual values of P′_c range from 0.76 to 11.37 MPa (Casagrande method) and 0.72 to 11.08 MPa (SED method). Differences between the values of P′_c and σ′_vh range from 0.43 to 6.01 MPa. We see no systematic changes in OCR with increasing depth and no obvious shifts in OCR across lithostratigraphic boundaries (Fig. F11).

Samples from Site C0012 yield P′_c values that range from 0.50 to 7.33 MPa (Casagrande method) and 0.47 to 8.25 MPa (SED method). Differences between those values of P′_c and σ′_vh range from 0.26 to 4.75 MPa, and the OCR ranges from 2.14 to 4.08. The magnitude of apparent overconsolidation at Site C0012 is consistently higher than for equivalent facies at Site C0011 (Fig. F11). On the other hand, changes in OCR are erratic with depth. We find unusually high OCR values within the hemipelagic/pyroclastic facies (Unit I), which has been affected by slumping within the upper ~100 m (Expedition 333 Scientists, 2012b). Another excursion to higher OCR occurs near the top of the hemipelagic facies (Unit III); those strata were likewise affected by slumping and development of an unconformity (Expedition 322 Scientists, 2010b).

Results from Site C0018 are more straightforward. The values of P′_c range from 0.54 to 2.60 MPa (Casagrande method) and 0.43 to 2.37 MPa (SED method). Differences between the values of P′_c and σ′_vh range from 0.11 to 0.70 MPa. OCR values vary from 0.76 (slightly underconsolidated) to 1.82 (modestly overconsolidated). OCR values show no obvious trend with depth, facies change, or depositional process (Fig. F11).

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