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doi:10.2204/iodp.proc.314315316.208.2011

Laboratory testing methodology

Whole-round core samples from Sites C0004 and C0006–C0008 (Table T1) were capped and sealed after collection and stored at 4˚C to help maintain the natural water content. Samples were taken out of the sealed core liner to conduct CRS consolidation experiments and grain-size analysis following American Society for Testing and Materials (ASTM) International standards (ASTM International 2003; ASTM International, 2006). CRS consolidation experiments are performed at room temperature (20˚C) in a rigid confining ring to maintain uniaxial strain. Each specimen was trimmed using a trimming jig, a wire saw, and a sharp-edged spatula to minimize disturbance during preparation and to provide a specimen diameter that was the exact diameter of the rigid confining ring. Once the specimen was in the confining ring, a wire saw, a sharp-edged spatula, and a recess tool were used to shape the specimen into a right cylinder of a fixed height. The use of the trimming jig, confining ring, and recess tool facilitate making specimens of identical diameter and height. Each specimen had an initial height (H_o) of 2.41 cm and an initial diameter of 5.09 cm. A constant, controlled cell pressure (P_c; 386 kPa) is applied to each specimen to ensure saturation. After an initial saturation period of at least 8 h, each specimen is axially deformed at a constant rate of strain. The strain rate () was adjusted (ranged from 0.3%–2.0%/h) for each specimen to ensure a pore pressure ratio <0.10 (ASTM International, 2006). The pore pressure ratio depends on the strain rate and the permeability of the specimen. Total axial stress (σ_a), instantaneous sample height (H), and basal pore pressure (P_p) are recorded throughout the experiment. Each experiment is completed at a consolidation stress exceeding the hydrostatic effective vertical stress (σ_vh′) for the specimen; σ_vh′ is total vertical stress less hydrostatic fluid pressure. Total vertical stress is determined from bulk density (ρ_b) data (see Expedition 316 Scientists, 2009a, 2009b, 2009c, 2009d). Hydrostatic fluid pressure is calculated assuming a constant seawater density (ρ_w = 1024 kg/m³).

CRS consolidation experiments provide data to constrain hydraulic conductivity (K) for laboratory conditions and compressibility and to estimate the overconsolidation ratio (Tables T2, T3). Hydraulic conductivity was calculated based on the strain rate and base excess pressure (ASTM International, 2006)

K = HHoγw/2Δu,

(1)

where

• = strain rate,
• H_o = initial specimen height,
• H = instantaneous specimen height,
• Δu = base excess pressure, and
• γ_w = unit weight of water.

Base excess pore pressure is defined as difference between the basal pore pressure (P_p) and the cell pressure (P_c) (Δu = P_p – P_c). A smoothed-base excess pore pressure, based on a three-point moving average, is used to calculate hydraulic conductivity. A six-point moving average is used to smooth the strain rate.

We use hydraulic conductivity-void ratio (K-e) data during normal consolidation to define a log-linear relation between K and void ratio for each specimen (Figs. F1, F2) (Lambe and Whitman, 1969). Each specimen-specific model (e.g., Fig. F2) is used to estimate the hydraulic conductivity at the in situ void ratio. We assume that the void ratio at the laboratory-determined preconsolidation stress (e_pc′) of each specimen represents the in situ void ratio. Initial specimen void ratio was determined in our laboratory from mass and density measurements following the approach presented by Blum (1997). Void ratio during consolidation was determined using the strain data. Hydraulic conductivity is converted to permeability (k = Kµ/ρ_wg) using standard seawater density (ρ_w = 1024 kg/m³), constant dynamic viscosity (µ = 0.001 Pa·s), and the acceleration due to gravity (g = 9.81 m/s²), which are the laboratory conditions. From the K-e relationship, in situ void ratio, and relation between hydraulic conductivity and permeability, we define the in situ permeability at the in situ void ratio (k_epc′ ) for each specimen (Table T3).

The coefficient of consolidation (c_v), describing coupled deformation and fluid flow, is calculated from the hydraulic conductivity and deformation data using the coefficient of volume compressibility (m_v) (ASTM International, 2006; Craig, 1992),

Stress-strain data during normal consolidation are used to define the compression index (c_c) (Craig, 1992),

The compression index quantifies the relationship between void ratio and vertical effective stress during normal consolidation (Fig. F1; Table T3).

The preconsolidation stress (σ_pc′) for each vertically oriented specimen is estimated using the work-stress method (Becker et al., 1987). The preconsolidation stress represents an estimate of the maximum effective stress a specimen has experienced. To determine σ_pc′, we extrapolate the linear portions of the preyield and postyield behavior (Fig. F3). The intersection of the extrapolations defines σ_pc′ (Becker et al., 1987). We use the preconsolidation stress and the hydrostatic effective vertical stress to define the overconsolidation ratio (OCR) for each vertically oriented specimen (OCR = σ_pc′/σ_vh′) (Table T3). We use the void ratio at the preconsolidation stress to estimate the in situ void ratio (e_pc′) (Table T3). An OCR <1 suggests in situ overpressure, an OCR = 1 suggests the sample is at its maximum past effective stress with hydrostatic fluid pressure, and an OCR >1 suggests unloading. Sample disturbance can produce consolidation data with poorly defined σ_pc′ (Santagata and Germaine, 2002). Saffer (2003) and Dugan and Germaine (2008) present methods to estimate error on interpreted preconsolidation stresses.

To complement the geotechnical data, we also characterized the grain-size distribution for the vertically oriented specimens (Tables T1, T3). All grain-size analyses followed the ASTM standard for particle-size analysis (ASTM International, 2003). No particles were retained by the 2 mm sieve, so the distribution of particles was determined by settling analysis using a hydrometer. To determine the distributions, air-dried specimens are mixed with distilled water and sodium hexametaphosphate, a dispersing agent. The solution is mechanically stirred to create a dispersed suspension, which is transferred into a glass sedimentation cylinder. Regular hydrometer and water temperature readings of the solution are made as the particles settle from suspension. At the end of sedimentation analysis, the contents of the sedimentation cylinder are dried and total particle mass is determined. All data are processed following the ASTM standard and corrected for temperature effects (ASTM International, 2003). The settling analyses are used to define the D₅₀ (median particle diameter), D₂₅ (particle diameter at which 25% of particles are smaller by mass), and D₇₅ (particle diameter at which 75% of particles are smaller by mass) particle diameters and the percentages of sand (>62.5 µm), silt (2–62.5 µm), and clay (<2 µm) (Table T3).

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