Proc. IODP, 341, Data report: permeability, consolidation properties, and grain size of sediments from Sites U1420 and U1421, offshore southern Alaska

doi:10.2204/iodp.proc.341.201.2016

Methods

Whole-round samples were sealed and stored at 4°C to preserve pore fluids prior to testing. Shipboard selection of whole-round samples was conducted by the R/V JOIDES Resolution staff based on visual inspection to identify areas free of cracks and voids with minimal coring disturbance. Grain size measurements were performed on samples taken immediately adjacent to those used for CRS consolidation tests.

CRS consolidation tests

We performed CRS consolidation tests at room temperature (22°C) following American Society for Testing and Materials (ASTM) International standards (ASTM International, 2006). Consolidation was performed in the vertical direction (parallel to the long axis of the core). The sample was first extruded from the core liner and trimmed into a fixed ring using a wire saw, spatula, and recess tool. After trimming, the initial sample dimensions were 2.41 cm in height and 5.09 cm in diameter. The ring and sample were then loaded into the sample chamber (Fig. F2). Alundum porous stones and woven polymer filter paper were placed on the top and base of the sample to allow drainage of pore fluid during consolidation. The sample chamber was sealed, filled with tap water, and placed in the consolidation frame. We then pressurized the sample chamber to 386 kPa and left it for at least 8 h to ensure complete saturation. During the saturation stage, the stress on the piston actuator of the load frame was controlled to ensure zero axial strain.

After the saturation stage, the drain valve at the base of the sample was locked and the consolidation stage started. During consolidation, the axial strain rate $(\dot{ε})$ was held constant, and the fixed ring ensured zero radial strain. We monitored the pore pressure ratio (defined as [P_p – P_c]/P_c, where P_p is the pore pressure at the base of the sample and P_c is the pressure in the consolidation chamber) and adjusted between 0.5% and 1%/h to maintain a pore pressure ratio between 0.01 and 0.15. During the test, the total axial stress (σ_a), instantaneous sample height (H), and P_p were recorded. We performed unload-reload cycles upon reaching 15% axial strain to assess the elastic consolidation properties of each sample. The unload cycle proceeded until reaching 10% of the value of σ_a at the start of the unload cycle, followed by 18 h of creep. After the creep step, the sample was reloaded and consolidation proceeded until a final axial strain of 25% or σ_a = 22 MPa, whichever was reached first.

We used the data from the CRS consolidation tests to determine compression index (C_c), swelling index (C_s), and permeability (k) (all nomenclature is provided in Table T1). C_c describes the pore volume change during virgin consolidation (Fig. F3) (i.e., elasto-plastic strain that occurs at effective stresses greater than the maximum effective stress to which the sample has been subjected) (Craig, 1992) and was computed from the virgin consolidation portion of the test as

C_c = (eσ_a′ – eσ_a′₊Δσ_a′)/[log(σ_a′ – Δσ_a′/σ_a′)],

(1)

where

C_c = compression index,
e = void ratio, and
σ_a′ = axial effective stress (Pa).

C_s describes the pore volume change during elastic reconsolidation (Fig. F3) (i.e., recoverable stain that occurs at effective stresses less than the maximum effective stress to which the sample has been subjected) (Craig, 1992) and was computed from the reload portion of the test using Equation 1. Void ratio was determined from the initial void ratio of the sample and strain data recorded during the test; initial void ratio was determined from mass and density measurements following the method of Blum (1997). It is important to note that the value of σ_a′ that separates elastic reconsolidation from virgin consolidation is affected by in situ stresses the sediment has experienced as well as stresses imparted during core recovery. The in situ stresses experienced by sediments at Sites U1420 and U1421 are further complicated by a history of glaciation extending over these locations at various times in the past (Manley and Kaufman, 2002). The results of the consolidation experiments must therefore be interpreted with care.

Permeability during the test was computed as

k = ( $\dot{ε}$ HH₀µ)/(2Δu),

(2)

where

k = permeability (m²),
$\dot{ε}$ = strain rate (1/s),
H = instantaneous specimen height (m),
H₀ = initial specimen height (m),
µ = dynamic viscosity of pore fluid (Pa·s), and
Δu = base excess pressure (Pa).

We assumed µ = 0.001 Pa·s. We computed Δu as the difference between the pore pressure at the base of the sample and the fluid pressure in the consolidation cell (Δu = P_p – P_c). In the permeability computation, Δu was smoothed using a six-point moving average, and the resulting permeabilities were extrapolated to the initial permeability (k₀) at the initial porosity of the sample by assuming a log-linear relationship between permeability and porosity during virgin consolidation (e.g., Neuzil, 1994) (Fig. F4).

We used permeability and effective stress data to determine the coefficient of consolidation (c_v) during the test (ASTM International, 2006; Craig, 1992):

c_v = k/µm_v,

(3)

where

c_v = coefficient of consolidation (m²/s),
k = permeability (m²),
µ = dynamic viscosity of pore fluid (Pa·s), and
m_v = coefficient of volume compressibility (1/Pa).

The coefficient of volume compressibility is defined as the change in axial strain per unit increase in axial effective stress (Craig, 1992) and was computed as

m_v = (εσ_a′ + Δσ_a′ – εσ_a′)/[(σ_a′ + Δσ_a′) – σ_a′],

(4)

where ε = axial strain.

Grain size measurements

We conducted grain size measurements following the ASTM standard for particle-size analysis (ASTM International, 2007). Trimmings from the CRS consolidation sample were oven-dried at 105°C for at least 24 h and powdered using a ceramic mortar and pestle. Following this, the powdered samples were passed through a 2 mm sieve. A total of 50 g of the portion of the sample that passed through the 2 mm sieve was then mixed with deionized water and 5 g of sodium hexametaphosphate deflocculant and left to soak for at least 16 h. After soaking, the samples were further dispersed using a milkshake mixer, poured into a glass settling column, and diluted with deionized water to a total volume of 1 L. After dilution, the column was agitated for 60 s and then left to settle. During settling, the bulk density of the solution was measured periodically using ASTM Hydrometer 151H. The mass fraction of particles remaining in suspension (m_p) at the time of hydrometer measurement is given by

m_p = [ρ_sV(ρ – ρ_f)]/[(ρ_s – 1000)m_s],

(5)

where

m_p = mass fraction of particles remaining in suspension,
ρ_s = specimen grain density,
V = volume of solution (m³),
m_s = dry mass of specimen (kg),
ρ = hydrometer reading (kg/m³), and
ρ_f = density of solution fluid without sediment (kg/m³).

The specimen grain density was determined by taking the average of the grain density values determined by shipboard moisture and density (MAD) measurements at each site (2800 kg/m³ at Site U1420 and 2890 kg/m³ at Site U1421). The maximum grain diameter (D) of the particles still in suspension at the time of each hydrometer measurement is given by

(6)

where

D = equivalent particle diameter (m),
µ = solution fluid viscosity (Pa·s),
L = effective depth from solution surface to center of hydrometer bulb (m),
t = time of hydrometer measurement (s), and
g = acceleration due to gravity (m/s²).

L was determined for Hydrometer 151H from table 2 of the ASTM standard (ASTM International, 2007).

Because all the hydrometer analyses indicated that a significant fraction of the particles settled out prior to the first hydrometer reading (15 s after the end of the agitation, corresponding to D = 0.081 mm from Equation 6), we performed sieve analysis on separate aliquots of the bulk powdered samples using a set of sieves with mesh openings of 12.7, 9.52, 6.35, 4.75, 2.38, 2.00, 1.70, 1.40, 1.18, 1.00, 0.850, 0.600, and 0.425 mm. The results of the sieve analyses were merged with the hydrometer results to yield the complete grain size distribution. The median grain diameter was determined from this complete grain size distribution. Relative mass fractions of sand, silt, and clay were determined using grain diameter cutoffs of 0.0625 mm for sand–silt and 0.002 mm for silt–clay. We quantified the degree of sorting by determining the standard deviation (σ) of the distributions in ϕ units following the method of Folk and Ward (1957).

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