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doi:10.2204/iodp.proc.308.207.2009

Methods and materials

Samples

Whole-round Sample 308-U1320A-31X-1, 120–150 cm, was collected from 276.4–276.7 meters below seafloor (mbsf), from the base of Brazos-Trinity Basin IV where sediments were assumed to have been normally consolidated (i.e., the internal pore pressure has been dissipated so that hydrostatic pressure has been maintained during deposition). Cylindrical Sample 1320-31-1 was cored with a water-lubricated rotary coring tube from the whole-round core sample. The sample had a diameter of 21.23 mm and a height of 55.27 mm (i.e., ~2.5 times the diameter). Initial bulk density was calculated from measurements of the wet volume and wet weight of the sample, and porosity was derived from this bulk density value and shipboard values of grain and water densities. The shore-based values of bulk density and porosity were 1.93 g/cm³ and 48%, respectively. Hence, shore-based bulk density is slightly lower and porosity is slightly higher than those of adjacent shipboard values (2.02 g/cm³ and 43%, respectively).

Sample 1320-31-1 is a gray clay with darker vaguely subhorizontal and probably bioturbated layers. It was collected from Unit V, which is dominated by hemipelagic generally bioturbated clay with rare silt lamina often containing fragments of foraminifers (Flemings et al., 2005). Coarser grains may be derived from river plumes and/or very low density turbidity currents.

Visual inspection of the sample revealed no drilling disturbance, so it was assumed that the sample was undisturbed. However, posttest investigation of the remaining whole-round sample revealed the presence of drill biscuits (Fig. F1).

Testing setup and procedure

The experiment was carried out with the test equipment of Karig (1996). This laboratory was previously housed at Cornell University, Ithaca (USA), but it is now located at Luleå University of Technology.

The equipment consists of a triaxial cell mounted in a computer-controlled servo-hydraulic INSTRON 1324 load frame (cf. figure 6 of Morgan and Ask, 2004). Figure F2 shows the instrumentation of the test sample within the triaxial cell. Horizontal strains are measured by an array of eight linear variable differential transformers (LVDTs) across four diameters at the midheight of the sample. One pair of LVDTs measures the vertical strain over the middle half of the sample, and one LVDT mounted outside the triaxial cell measures the external vertical strain. A latex jacket isolates the sample from the silicon oil confining fluid. Testing was conducted under drained conditions, with the pore fluid allowed to drain in and out through both ends of the sample. The slow loading rate and the double drainage are assumed to result in fully drained conditions within the sample. A total of 11 digitized channels monitored loading and dimensional data, which were saved every 15 or 30 min during testing, providing detailed information about sample deformation and strength.

The testing sequence included a preconsolidation phase, which lasted for 39 h, at constant vertical and horizontal stresses of ~1.7 MPa and a back-pore pressure of 1.0 MPa. Each sample was first brought to a uniform isotropic stress state to ensure that all remaining gases in the system were in solution during the test phase. The reconsolidation phase began immediately after the preconsolidation phase and followed a K₀ reconsolidation computer-controlled stress path: vertical stress was increased at a constant rate (11.5 Pa/s) while the horizontal stress (confining pressure) was adjusted by computer control to maintain a constant cross-sectional area of the sample. At the outset of the K₀ reconsolidation test, the sudden change in stress state from initially isotropic stresses to uniaxial strain led to boundary effects. System compliance effects and closure of microcracks also affected the response during the initial phase of the K₀ reconsolidation tests.

K₀ stress ratio, yield stress, and pore fluid pressure

The stress ratio, K₀, is defined as the ratio between the effective horizontal and vertical stresses (σ_h′ and σ_v′, respectively) (Table T1), which maintain the condition of uniaxial strain. K₀ of elastic and plastic conditions are obtained by linearly fitting σ_v′ and σ_h′ data pre- and postyield stress. K₀ may also be calculated from plots of σ_m′ versus Δσ (Table T1).

The effective vertical yield stress, σ_y′, marks the transition from elastic to plastic elastic deformation along the K₀ reconsolidation stress path. For uncemented sediments, the yield stress corresponds to the preconsolidation pressure. There are several methods for determining the preconsolidation pressure (e.g., Casagrande, 1936; Becker et al., 1987; Wang and Frost, 2004). Because it is difficult to obtain the true preconsolidation pressure, it is hard to evaluate the relative merits of the different methods. In this paper, I have adopted the method of Karig (1993): I have used various relationships among the collected data (e.g., σ_h′ versus σ_v′, Δσ versus σ_m′, and σ_v′ versus ε_v) and picked the yield stress at the point where the rate of deviation from the elastic slope began to change rapidly.

The effective vertical yield stress, σ_y′, may be compared with the calculated in situ effective vertical stress for hydrostatic water pressure, σ_vh′ (e.g., Karig, 1996). The difference between the two values is a measure of the maximum pore fluid pressure in excess of hydrostatic water pressure, P*^max (Table T1). The magnitude of the importance of the overpressure is often shown by the overpressure ratio, λ* (e.g., Long et al.). The ratio between P*^max and σ_vh′ gives a maximum value of λ* (Table T1).

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