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doi:10.2204/iodp.proc.314315316.218.2011

Experimental methods

Sample handling and preparation

During the drilling expeditions, whole-round samples of ~10–20 cm length were selected for postexpedition geotechnical tests using X-ray computed tomography (CT) scans to identify minimally disturbed intervals of hemipelagic mud/mudstone. Whole-round samples were sealed in plastic core liners using plastic caps and electrical tape, sealed within aluminum vacuum bags with wet sponges to minimize desiccation, and stored in a refrigerator until the end of the expedition. At that time, samples were shipped with cold packs by courier to the PSU and MU laboratories, where they were kept refrigerated until they were trimmed and used for CRS tests.

After removing whole-round samples from the core liner, we carefully trimmed cylindrical specimens to fit in the fixed steel ring of an oedometer system (Fig. F3) using a sharp stainless steel cutting shoe (to cut the specimen to the exact ring diameter) and a wire saw and trimming jig to attain flat top and bottom specimen faces. The specimen diameter for PSU tests was either 50 mm or 36.6 mm, depending on the test, and the specimen diameter for MU tests was 41 mm (Table T3). The initial specimen height was ~20 mm for PSU tests in most cases and ~24 mm for MU tests. During specimen preparation, we took trimmings for measurement of water content and initial void ratio (ASTM International, 2006; Long et al., 2008). We determined specimen water content and void ratio from wet mass and dry mass after oven drying at 105°C for 24 h, following the IODP method used for moisture and density (MAD) measurements that accounts for salt mass in seawater (Blum, 1997; Expedition 315 Scientists, 2009a). These values are reported in Table T3. In addition, we obtained a second measurement of index properties after completion of the CRS test by oven drying the test specimen in the steel ring and using the same method.

After trimming, we placed specimens in the fixed ring of the oedometer system and deaired the system and pore fluid lines using a vacuum pump. We then backpressured the specimens to values of ub = ~200 kPa (MU) or ub = ~300–400 kPa (PSU) using deaired synthetic seawater for 24 h to ensure saturation and to dissolve any air remaining in the lines. To maintain contact between the loading ram and specimen and to prevent swelling during backpressuring, we held the axial load at a value ~10–20 kPa higher than the backpressure to maintain an effective axial stress (σa) of <25 kPa for the PSU tests and specified the axial stress to maintain a condition of 0.2% axial strain for the MU tests.

Consolidation test procedure and computations

We conducted CRS tests at the two laboratories, following the protocols and test configuration specified by the American Society for Testing and Materials (ASTM) (ASTM International, 2006). In our CRS tests, we deformed the specimen at a constant rate of strain, controlled via a specified displacement rate boundary condition applied by a computer-controlled load frame (Fig. F3). The sample is confined in a steel ring to maintain a condition of zero lateral deformation (i.e., uniaxial strain), is undrained at its base, and is open to backpressure (ub) at its top. During each experiment, we continuously monitored specimen height (h), total axial stress (σa), and basal pore pressure (u). Tests were run to peak axial stresses of ~20 MPa at MU and either ~20 or ~40 MPa at PSU. For each test, we chose the displacement rate (or strain rate) to maintain an anticipated ratio of basal excess pore pressure to a total axial stress (Δua) less than ~0.10. Strain rates for the tests are shown in Table T3. We recorded the loading phase for all tests and the unloading phase for a subset of the experiments; these results are shown in the stress-strain and stress-void ratio curves in Figures F4, F5, F6, F7, F8, F9, F10, F11, F12, F13, F14, F15, F16, F17, F18, F19, F20, F21, F22, F23, F24, F25, F26, F27, F28, F29, F30, F31, F32, F33, F34, F35, F36, F37, F38, F39, F40, F41, and F42.

We computed the axial strain (ε), base excess pressure (Δu), average effective axial stress within the specimen (σa), hydraulic conductivity (K), intrinsic permeability (k), coefficient of volume compressibility (mv), and coefficient of consolidation (Cv) as follows (ASTM International, 2006; Long et al., 2008):

ε = Δh/h0, (1)

where displacements are measured by a linear voltage differential transformer (LVDT) mounted at the top of the consolidation cell (Fig. F3) and are corrected to account for the compliance of the testing system. Equation 1 defines the natural axial strain (Δh/h0) of the specimen and is used for subsequent calculations; in Figures F4–F42 and in all supplementary tables we report the strain as a percentage.

Δu = uub. (2)

σa = [σa – (2/3)Δu] – ub. (3)

K = ( × h × h0 × γw)/2Δu. (4)

k = Kvw. (5)

mv = Δε/Δσa. (6)

Cv = K/(mv × γw). (7)

We computed the compression index (Cc) from the change in specimen void ratio as a function of effective axial stress; this relationship is also shown graphically as the slope of the virgin compression curve (Fig. F4):

Cc = (Δe)/Δlog(σa), (8)

where e is specimen void ratio (Table T2). As part of our data analysis, we determined the maximum preconsolidation stress (Pc), which is taken to represent the in situ effective stress under conditions of monotonic and uniaxial vertical loading (e.g., Holtz and Kovacs, 1981). We conducted this analysis for all specimens obtained from slope sediments but do not report Pc values for sediments sampled from the interior of the thrust wedge at Site C0001 (i.e., the accretionary prism) because the assumption of uniaxial vertical loading is most likely violated there. To estimate Pc, we employed the Casagrande method (Fig. F4A) (Casagrande, 1936) and the work (strain energy density) method (Fig. F4B) (Becker et al., 1987). In the case of one-dimensional consolidation, the strain energy density (SED) is given by (Becker et al., 1987; Germaine and Germaine, 2009)

SED = int(σa × δε). (9)

We compare these values to the in situ vertical effective stress expected under conditions of uniaxial vertical burial and hydrostatic pore fluid pressure (Po), computed by integrating bulk density values obtained from shipboard measurements from the seafloor downward. The overconsolidation ratio (OCR) is defined by Pc/Po; OCR = 1 indicates normal consolidation, whereas OCR < 1 indicates underconsolidation and OCR > 1 indicates overconsolidation. It should be noted that estimates of Pc are subject to considerable uncertainty because of sample disturbance, the effects of drastically differing timescales between natural sedimentation-driven compaction rates and those achieved in laboratory experiments, the effects of cementation on specimen yield stress (e.g., Holtz and Kovacs, 1981), and the effects of tectonic consolidation under stress regimes in which the maximum effective stress is not vertical.

ESEM fabric analyses

Samples for ESEM imaging of sediment fabric were taken shipboard from locations immediately adjacent to each whole-round core sample. We prepared samples for ESEM imaging following the general procedures described in Yue et al. (submitted), and scanned two mutually perpendicular faces for each sample, one horizontal and the other vertical. ESEM images for each of the two faces are provided in ESEM in “Supplementary material.” The approach used for fabric quantification follows the graphic standard deviation (sorting) statistics of Folk and Ward (1957), as described by Yue et al. (submitted). After processing the digital scanning electron microscope image, we constructed cumulative frequency curves to show the orientation of apparent long axes for clasts oriented from 0° to 180° across imaging surfaces cut parallel and perpendicular to the core axis. The degree of clast orientation (d) equals

d = [(ϕ84 – ϕ16)/4] + [(ϕ95 – ϕ5)/6.6], (10)

where ϕ84, ϕ16, ϕ95, and ϕ5 represent graphical picks for the angles of clast orientation at the 84th, 16th, 95th, and 5th percentiles, respectively, on the cumulative frequency curve. This graphical statistic avoids the laborious calculations required by moment statistics (Chiou et al., 1991). In theory, the maximum value of d is equal to 72.3° (i.e., a case in which at least 16% particles are oriented at an angle of 0° and infinitely close to 180°, respectively). We normalized each d value to this maximum by calculating the index of orientation (i), as defined by the following formula:

i = 1 – (d/72.3). (11)

We also report 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. Interpretations of these results (e.g., differences between horizontal and vertical faces) need to take the dip of bedding into account, as shown in Figure F2. If the fabric of a sedimentary deposit shows strong preferred grain orientation relative to the core axis, then the degree of orientation (d) will be smaller, the slope of cumulative frequency curve will be steeper near the mid-point and more nonlinear, and the index of orientation (i) will be larger and closer to 1.