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doi:10.2204/iodp.proc.343343T.201.2015

Experimental results

Uniaxial CRS results

Results from each CRS experiment are shown in Figures F5, F6, F7, F8, F9, and F10; Tables T3 and T4; and TESTDATA in “Supplementary material.” Both permeability and porosity decrease with increasing effective axial and mean stress (σ′_A and σ_m′), and permeability decreases log-linearly as a function of porosity below 37% porosity. Permeability of the prism sample (Section 343-C0019E-5R-1) decreases from 7.3 × 10^–18 m² at 20 MPa effective axial stress (σ_m′ = 14.6 MPa assuming K₀ = 0.6) to 3.5 × 10^–19 m² at 85 MPa (σ_m′ = 62.3 MPa), and porosity decreases from 46% at 1 MPa (σ_m′ = 0.73 MPa) to 27% at 85 MPa (σ_m′ = 62.3 MPa). The underthrust sample (Section 20R-1) did not achieve a sufficient pore pressure gradient to determine the permeability (Fig. F8). Porosity decreased from 49% at 1 MPa (σ_m′ = 0.73 MPa) to 37% at 95 MPa (σ_m′ = 69.6 MPa) (Fig. F9).

Using the relationship between effective stress and porosity for each sample, as shown in Figures F8 and F11, we estimated in situ porosity (ϕ_{in situ}) based on an in situ effective vertical stress of 5.51 MPa (σ_m′ = 4.04 MPa) for the prism sample (Section 343-C0019E-5R-1) and 7.31 MPa (σ_m′ = 5.36 MPa) for the underthrust sediment sample (Section 20R-1) (Table T4). In situ effective vertical stresses were calculated under the assumption of hydrostatic pore pressure. Estimated in situ porosity values are in agreement with the lower range of porosity values reported from shipboard measurements, 44% and 47% for the prism and underthrust samples, respectively. Shipboard porosities exhibit considerable scatter (from 40%–68%) and may be overestimated because the measurements include the effects of elastic rebound. This makes direct comparison between laboratory and shipboard measurements difficult.

Using the permeability-porosity relationship for the prism sample (Fig. F7), we estimated in situ permeability (k_{in situ}) from the in situ porosity values (e.g., Dugan and Daigle, 2011; Guo et al., 2011). Estimated in situ permeability for the prism sample is 4.3 × 10^–18 m² and corresponds to an in situ hydraulic conductivity (K_{in situ}) of 2.7 × 10^–11 m/s (Table T4). In situ hydraulic conductivity is estimated using a fluid viscosity based on the in situ temperature (~4°C) measured in Hole C0019E (Expedition 343/343T Scientists, 2013).

The coefficient of consolidation (C_v) decreases with increasing effective axial stress to a constant value of 4.2 × 10^–7 m²/s and 9.0 × 10^–6 m²/s for the prism and underthrust sediment samples, respectively (Figs. F6, F9). The compression index (C_c) is 0.70 and 0.57 for the prism and underthrust sediment samples, respectively (Table T3). Preconsolidation stress and OCR for both samples indicate significant overconsolidation. P_c′ values for the prism and underthrust samples are 17.0 and 20.5 MPa (Fig. F4), respectively, corresponding to OCR values of 3.1 for the prism sample and 2.8 for the underthrust sample (Table T3).

Isostatic loading path results

As in the uniaxial tests, both porosity and permeability decrease with increasing effective mean stress, and permeability decreases with decreasing porosity (Figs. F12, F13, F14; see also TESTDATA in “Supplementary material”). Permeability of the prism sample (Section 343-C0019E-5R-1) decreases from 5.4 × 10^–18 m² at 3.6 MPa effective mean stress to 2.9 × 10^–19 m² at 54.6 MPa, and porosity decreases from 46% at 3.6 MPa to 29% at 64.6 MPa. Permeability of the underthrust sample (Section 20R-1) decreases from 4.0 × 10^–18 m² at 3.6 MPa to 3.2 × 10^–19 m² at 86.6 MPa, and porosity decreases from 48% to 36% over this stress range (see TESTDATA in “Supplementary material”). Estimated in situ porosities (ϕ_{in situ}), defined as the porosity corresponding to the overburden assuming hydrostatic pore pressure, are 45% at a σ_vh′ of 5.5 MPa (σ_m′ = 4.04 MPa) and 47% at 7.3 MPa (σ_m′ = 5.36 MPa) for the prism and underthrust samples, respectively. These values are consistent with shipboard porosity measurements (Fig. F11) (Expedition 343/343T Scientists, 2013). Estimated values of in situ permeability (k_{in situ}) for the prism and underthrust samples are 4.9 × 10^–18 and 3.6 × 10^–18 m², respectively, and correspond to an in situ hydraulic conductivity of 3.1 × 10^–11 and 2.3 × 10^–11 m/s, respectively. In situ permeability and hydraulic conductivity values are also in good agreement with those estimated from CRS tests (Fig. F15; Table T4).

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