-
B =
differential pressure/ cell pressure
During ODP Leg 190, whole-round samples were collected by the Shipboard Scientific Party of the JOIDES Resolution. The samples were cut from the cores after routine testing for physical properties but before splitting. They were then encased and wax-sealed while still in the core liner for transportation and subsequent storage. The whole-round cores were maintained under continuous refrigeration at 5°C until testing. The four samples tested (190-1173A-13H-4, 86–101 cm, 18H-6, 20–40 cm, 190-1174B-27R-3, 120–137 cm, and 33R-4, 20–35 cm) are primarily hemipelagic muds, with ash laminations in Sample 190-1173A-18H-6, 20–40 cm (Table T1). These four samples were selected to represent undeformed sediments at the two sites.
The flow-pump technique has a number of advantages over the more traditional falling-head and constant-head techniques for the measurement of permeabilities of fine-grained and marine sediments (i.e., Olsen et al., 1985; Morin and Olsen, 1987; Aiban and Znidarcic, 1989). Advantages include increased accuracy due to the electronically controlled flow rate and automated data recovery, a constant-head difference at steady-state conditions that minimizes damage to samples caused by excessive gradients, and reduction of consolidation induced by seepage. Also, the absence of a fluid/air interface minimizes air in the system and back pressure helps to ensure dissolution of small air bubbles and saturation.
Prior to testing, each sample was cut to a 38 mm x 76 mm cylinder. The trimmed sample was encased within a latex sleeve, capped at both ends with filter paper and a porous disk, and then saturated with deaired water. The samples were confined by pressurized deionized water within a triaxial cell. An infusion flow pump and digital hydraulic actuator, linked by a differential pressure transducer, controlled input and output of permeant to the sample within the triaxial cell. All aspects of the system were computer linked, and measurements and data logs were automated. Deionized, deaired water was used as permeant to prevent corrosion. A permeant with a chemistry designed to simulate natural porewater may help minimize the possibility of clay swelling and shrinkage (Stover et al., 2001), but the low rates of flow used here mean that the sample was exposed to little permeant other than its natural pore fluid.
The sample was saturated by the application of back pressure and the degree of saturation was assessed by employing the Skempton B-test. This involved systematically isolating the sample fluid while altering the confining pressures and monitoring the associated change in differential pressure. When a B-value of ~0.95 for the samples was obtained,
where
differential pressure/ cell pressure
(Skempton, 1954), the sample was judged to be sufficiently saturated.
Bolton et al. (2000) noted significant variations in permeability at effective pressures <100 kPa and relatively little change at values >100 kPa (Kemerer and Screaton, 2001). Preliminary investigations during the present work suggest that differing rates of flow may also affect permeability; therefore, the present tests employed a range of effective pressures and flow rates. The effective pressures were increased incrementally; samples were tested only after reaching equilibrium at each increment, until there was no significant decrease in permeability. This procedure also helps ensure a reduction in the effects of rebound and other changes in the sample due to coring procedures and sample preparation. Back pressure was maintained at 350 kPa, and tests were carried out at a series of confining fluid pressures of 375 to 800 kPa. Effective pressures therefore ranged from 25 to 450 kPa. Effective pressures in situ would be much greater than effective pressures generated by the triaxial cell; therefore, maximum effective pressure values should be used.
Tests were conducted at various rates of flow for each confining fluid pressure, but the rate of flow in each individual test was kept constant. Samples were not deformed during testing. Because differences between hydraulic head at the points of permeant entry and exit can cause loss of energy and permeability fluctuations, head gradients were monitored for the presence of steady-state conditions to ensure equilibrium of pressure throughout the sample. Darcy's law was used to obtain hydraulic conductivity (K) using the equation
h/l), (1)
where
h = head difference (m) (calculated from pressure difference across sample), and
l = sample length (m),
and intrinsic permeability figures were derived from hydraulic conductivity values using the equation
g/µ), (2)
where
= water density (1000 kg/m3), and
The viscosity value for water at 20°C, the temperature at which all the tests were conducted, was used.
