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doi:10.2204/iodp.proc.314315316.213.2011

Methods

Sample handling and preparation

All notations and abbreviations for this report are summarized in Table T1. Shipboard coring techniques on the D/V Chikyu include the hydraulic piston coring system (HPCS), rotary core barrel (RCB), and extended shoe coring system (ESCS). The coring system for each sample is shown in its identification code, with H for HPCS, R for RCB, and X for ESCS. Figures F2 and F3 provide the stratigraphic context for each WR sample interval. Sample intervals range from 32 m core depth below seafloor (CSF) to 920 m CSF. Samples from Site C0002 are from the lower forearc basin facies (five samples from Unit II) and the basal starved-basin facies (five samples from Unit III). Samples from Sites C0006 and C0007 are limited to accreted strata within the hanging wall of the frontal thrust; this suite includes trench-wedge deposits (six samples) and the upper Shikoku Basin facies (one sample).

WR core samples typically measure 10–20 cm in length. They are usually cut within several hours of recovery, capped and taped, sealed with wet sponges in aluminum vacuum bags, and maintained at a temperature of ~4°C during shipment and storage to prevent moisture loss prior to the tests. From each WR core, we subsampled homogeneous and intact mud/mudstone after examination of X-ray computed tomography (CT) scans. Immediately prior to testing in the shore-based laboratories, samples are extruded from the core liners and a trimming jig is used to shape the cylinders. Sample diameter for the PSU constant rate of strain (CRS) system is either 50 or 36.6 mm, depending on the required stress levels. The sample diameter for MU tests is fixed at 41 mm. Initial height of a sample in the fixed-wall consolidation ring is ~20 mm for most PSU tests and ~24 mm for MU tests (Table T2).

Samples for ESEM imaging of sediment fabric were selected from “clusters” of specimens taken immediately adjacent to WR sample intervals. We cut faces both parallel and perpendicular to the core axis. The orientation of bedding (dip angle with respect to horizontal) for each sample interval is shown on Figures F2 and F3 (see the “Expedition 315 Site C0002” [Expedition 315 Scientists, 2009], “Expedition 316 Site C0006” [Expedition 316 Scientists, 2009b], and “Expedition 316 Site C0007” [Expedition 316 Scientists, 2009c] chapters). These bedding dips are important for relating microfabric anisotropy to in situ orientation of planar features within the deposits.

Index properties

After trimming each WR sample for a CRSC test, we retained two or three pieces of trimmings for water content measurements. Water content is measured by oven drying the samples to constant mass at 105°C (ASTM, 2006). Water content is calculated by taking the difference in the weight of the sample before and after oven drying and dividing the difference by the oven-dried weight (Blum, 1997; see the “Expedition 316 methods” chapter [Expedition 316 Scientists, 2009a]). The calculated values of initial void ratio include a correction for salt in the pore water and are reported in Table T2, along with the values of void ratio (e) from the closest sampling interval of a shipboard measurement of moisture and density (MAD). Water content measured in shore-based laboratories can be compared to the shipboard MAD to assess loss of moisture during shipment and storage. In some instances, shore-based e values are higher than shipboard e values; these higher values can be attributed to differences in composition, texture, or disturbance between the nearby sampling intervals.

Constant-rate-of-strain consolidation tests

We conducted CRSC tests in an oedometer system following the general protocols and test configuration specified by the American Society for Testing and Materials Standard D4186-06 (ASTM, 2006). Samples were trimmed and then placed in a stainless steel specimen ring used to maintain a condition of zero lateral strain. The consolidation cell and lines were then evacuated of air using a vacuum pump, and the specimen was backpressured to values of u_b = ~0.2–0.4 MPa (MU) or u_b = ~0.3–0.4 MPa (PSU) using de-aired synthetic seawater (1.75 g NaCl in 500 mL distilled water) for 24 h to ensure saturation and dissolve any air remaining in the system lines. During backpressuring, the axial load was specified to maintain a vertical effective stress (σ′_v) of <0.025 MPa in the PSU tests, and the axial load actuator maintained a height with a strain of 0.2% at MU.

Specimens are laterally confined during tests with a fixed-wall consolidation ring. Constant-rate-of-strain loading is applied using a computer-controlled load frame, with the sample base undrained and the sample top open to the backpressure. We continuously monitor the sample height (H, in millimeters), the applied vertical total stress (σ_v, in kilopascals), and the basal pore pressure (u, in kilopascals). The maximum axial load with the MU loading frame is 44 kN; for a specimen with a diameter of 4.14 cm, this corresponds to a maximum vertical total stress of 33 MPa. For pore water backpressure ranging from 0.2 to 0.4 MPa, the maximum vertical effective stress is 32.6 to 32.8 MPa. The maximum axial load for the PSU device can reach 50 kN. Fixed-wall consolidation rings allow for specimen diameters of 25, 36.6, and 50 mm, which correspond to maximum total stresses of ~100, ~50, and ~20 MPa, respectively.

Tests are extended to peak axial stresses of ~20 MPa at MU and to either 20 MPa or 40 MPa at PSU (Fig. F5). For each test, we specify the rate of displacement (or strain rate) in order to maintain an anticipated ratio <0.10 for basal excess pore pressure (Δu) to the total axial stress. Strain rates for the tests are shown in Table T2. Unloading was recorded for a subset of the specimens, and these results are shown as part of the stress-strain and stress-void ratio curves in CRSC in “Supplementary material.”

We use the following equations to compute the axial strain (ε), base excess pore pressure (Δu), vertical effective stress (σ′_v; namely, average effective axial stress within the specimen), hydraulic conductivity (K), intrinsic permeability (k), coefficient of volume compressibility (m_v), and coefficient of consolidation (C_v) (ASTM, 2006; Long et al., 2008):

and

Displacements are measured by a linear variable differential transformer (LVDT) mounted at the top of the consolidation cell and are corrected to account for the compliance of the testing system. Equation 1 defines the natural axial strain of the specimen and is used for subsequent calculations; we report the strain as a percentage in all supplementary tables and figures in CRSC in “Supplementary material.” In CRSC tests, permeability values are generally only considered to be reliable once the strain distribution within the sample reaches steady state (ASTM, 2006). To calculate intrinsic permeability from hydraulic conductivity under laboratory testing conditions, we used a value of fluid viscosity (v) equal to 0.001 Pa·s for water at 20°C (the temperature at which all tests were conducted) and a value of fluid density (ρ) equal to 1027 kg/m³.

As a routine outcome of consolidation tests, comparisons are made between each test-derived value of the maximum pretest consolidation stress (P′_c) and a calculated value of in situ hydrostatic vertical effective stress (σ′_vh) at the equivalent burial depth, assuming conditions of monotonic and uniaxial loading (e.g., Holtz and Kovacs, 1981). We employed two methods to obtain the value of P′_c for each test: the Casagrande (1936) method, which uses the e – log(P) curve, and the work (strain energy density) method (Becker et al., 1987), which uses the plot of strain energy versus effective stress (Fig. F4). In the case of one-dimensional consolidation, the strain energy density (SED) is calculated by

SED is the work per unit volume done by the effective stress (Becker et al., 1987; Germaine and Germaine, 2009).

The depth gradient for hydrostatic vertical effective stress at each site is calculated by subtracting the values of hydrostatic pore pressure from the overburden stress (total normal stress). The overburden stress is constrained by integrating the shipboard bulk density and depth data (see the “Expedition 315 Site C0002” [Expedition 315 Scientists, 2009], “Expedition 316 Site C0006” [Expedition 316 Scientists, 2009b], and “Expedition 316 Site C0007” [Expedition 316 Scientists, 2009c] chapters). For a normally consolidated specimen, the test-derived and calculated values of P′_c and σ′_vh are equal. Overconsolidation refers to a case in which the value of P′_c exceeds the calculated value of σ′_vh at the depth from which the specimen was cored, and underconsolidation refers to a situation in which P′_c is less than σ′_vh. We used a graphical solution to pick the associated values of void ratio at the points where P′_c and σ′_vh intersect the consolidation curves.

Microfabric analysis

Instrument settings for the FEI Quanta 600 FEG scanning electron microscope follow the procedures described by Yue et al. (submitted). The images are taken from wet, uncoated specimens with an imaging resolution of ~4 nm, and the dimensions of the field of view are ~145 by 130 µm with 2000× magnification. The approach used for quantification of the specimen’s microfabric follows the graphic standard deviation (sorting) statistics of Folk and Ward (1957), as described in more detail by Yue et al. (submitted). After processing each digital SEM image, we construct cumulative frequency curves to show the angle of apparent long axes for particles oriented from 0° to 180° across imaging surfaces, with those surfaces cut parallel and perpendicular to the core axis. The standard deviation of particle orientation (d) equals

where Φ₈₄, Φ₁₆, Φ₉₅, and Φ₅ represent the graphic picks for the angles of particle orientation at the eighty-fourth, sixteenth, ninety-fifth, and fifth percentiles, respectively, on the cumulative frequency curve. This graphic statistic avoids the laborious calculations required by moment statistics (Chiou et al., 1991). Numerically, the maximum value of d is equal to 72.3° (i.e., a case in which Φ₈₄ = 180, Φ₁₆ = 0, Φ₉₅ = 180, and Φ₅ = 0). Each d value is normalized to this maximum value by calculating the index of orientation (i), as defined by the following formula:

We also display fabric results using rose diagrams to illustrate the number of grains (apparent long axes) aligned within each 10° orientation bin from 0° to 180° across the imaging surface. If the fabric of a sedimentary deposit shows strong preferred grain orientation relative to the core axis, then the standard deviation of orientation will be smaller, the slope of cumulative frequency curve will be steeper near the mode, the curve will be more nonlinear, and the index of orientation will be closer to 1. Interpretations of these results (e.g., differences between horizontal and vertical faces) need to take the dip of bedding into account, as shown in Figures F2 and F3.

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