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doi:10.2204/iodp.proc.322.205.2013

Results

Atterberg limits

Atterberg limits results on the Nankai silty claystone–silica mixtures are shown in Table T5 and Figure F3. The LL, PL, and PI of the Nankai silty claystone are 68%, 29%, and 39%, respectively. Because LL > 50%, the Nankai silty claystone is classified as a high-plasticity soil. With decreasing clay fraction, LL, PL, and PI decrease (Table T5; Fig. F4). We performed two measurements for the Nankai silty claystone: one on air-dried material and one on oven-dried material. LL is significantly affected by oven-drying; however, PL is not. LL measured on the air-dried bulk material is 8% higher than LL measured on the oven-dried material. This discrepancy most likely results from the loss of interlayer water from smectite during oven-drying, which perturbs values of water content and should be accounted for in any Atterberg limits correlations.

Particle size analysis

The clay-size fractions (<2 µm) by mass are 56%, 50%, 48%, 41%, 36%, and 32% for mixtures with 100%, 88%, 76%, 64%, 52%, and 40% Nankai silty claystone and silica as the remainder (Table T6; Fig. F5). Thus, the specimens fall into the categories silty clay and clayey silt. Data sheets and curves similar to Figure F5 for each sample are available in PARTSIZE in “Supplementary material.” Sand, silt, and clay percentages are given in Table T6.

Resedimentation

All six compression curves during resedimentation experiments are shown in Figure F6 (Table T7). Each data point represents the void ratio and vertical effective stress at the end of a stress increment. The first digitally measured void ratio (at 2.6 kPa) systematically decreases from 2.50 to 1.57 for specimens with 100% to 40% Nankai silty claystone. The compression index (C_c) decreases from 0.63 to 0.37 for specimens with 100% to 40% Nankai silty claystone, showing that the samples become stiffer the more silica they contain.

Consolidation testing

We conducted CRS tests on the Nankai silty claystone–silica mixtures. Table T3 summarizes details of each CRS test. Figures F7, F8, F9, F10, F11, and F12 show the consolidation curves in both ε-log(σ′_v) and e-log(σ′_v), normalized excess pore pressure (Δu/σ_v), hydraulic conductivity (K), coefficient of consolidation (C_v), and intrinsic permeability (k) for each CRS test. The CRS data sheets are in CRS in “Supplementary material.”

Because all six CRS tests were performed on homogeneous samples prepared in the laboratory under controlled conditions and contain only Nankai silty claystone and silt-size silica, we can compare the results in order to analyze the compositional influence on the compression and flow behavior. The initial void ratio (e_i) decreases from 1.63 to 0.98 for samples with 100% to 40% Nankai silty claystone. The compression index (C_c), which refers to the slope of the normally consolidated portion of the compression curve in e-log(σ′_v) space, is determined over the vertical effective stress range between 0.2 and 5 MPa and ranges from 0.64 to 0.26 for samples with 100% to 40% Nankai silty claystone (Table T3). C_c decreases with increasing vertical effective stress, particularly for the clay-rich samples. For a vertical effective stress range between 5 and 20 MPa, C_c varies from 0.36 to 0.24 for samples with 100% to 40% Nankai silty claystone (Table T3). The expansion index (C_e), which refers to the slope of the unloading portion of the compression curve in e-log(σ′_v) space, is determined over the vertical effective stress range between 5 and 20 MPa and ranges from 0.056 to 0.017 for samples with 100% to 40% Nankai silty claystone.

Vertical intrinsic permeability (k) varies log-linearly with porosity (log[k] = γn + log[k₀]) and increases with decreasing clay fraction. At a given porosity of 40%, the difference in permeability between mixtures with 100% and 40% Nankai silty claystone is two orders of magnitude. Values of γ and log(k₀) are listed in Table T3.

The coefficient of consolidation (Equation 9) ranges from 6.1 × 10^–9 to 3.2 × 10^–7 m²/s for samples with 100% to 40% Nankai silty claystone. In case of tests CRS088 and CRS089, C_v does not approach a constant value; instead, C_v continues to decrease with increasing vertical effective stress. Thus, in those cases we report C_v at maximum vertical effective stress of 21 MPa.

SEM imaging

One BSE image at a magnification of 14,000× for each sediment mixture after consolidation to a vertical effective stress of 21 MPa is shown in Figure F13. Additional BSE and SE images at vertical effective stresses of 21 MPa as well as 100 kPa are in SEM in “Supplementary material.” BSE images show a strong decrease in clay particle alignment with decreasing clay fraction. Pores in the Nankai silty claystone are either elongated pores between similarly oriented clay sheets or triangular to crescent-shaped pores in folded clays, particularly in between smectites (Fig. F13A, F13B). In coarser samples large jagged pores are preserved where silt grains touch each other and form silt bridges (Fig. F13I–F13L). A gradual increase in mean pore size with decreasing clay fraction occurs.

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