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doi:10.2204/iodp.proc.314315316.218.2011

Results

CRS test results

Results for each experiment are shown in Figures F5–F42 and in Table T3. In general, compression behavior (values of C_c and P_c′) and hydraulic conductivity and permeability (K and k) are similar for sediments from the three drill sites, and the results are consistent between the two laboratories. Values of C_c range from 0.535 to 1.058 for samples from Site C0001, with most values ranging between ~0.6 and 0.8 (Table T3). Values of C_c for specimens from Sites C0004 and C0008 range from 0.624 to 0.768 and from 0.419 to 0.638, respectively.

Experimental results also show that permeability varies strongly with effective stress and porosity. Permeability varies log-linearly with porosity and decreases from 1 × 10^–17 to 3 × 10^–16 m² at porosities of ~50%–55% (σ_a′ of ~1–2 MPa) to 1.5 × 10^–19 to ~6 × 10^–19 m² at porosities of 23%–30% (σ_a′ of 40 MPa) (Fig. F43). Trends of decreasing permeability with increasing effective stress (Fig. F43A) and with progressive compaction (Fig. F43B) are similar for the slope apron and slope basin samples we tested from all three drill sites and exhibit no systematic variation with sample depth or site. The permeabilities of samples from the accretionary prism at Site C0001 are also similar to those of the slope sediments. Using the relationship between permeability and porosity for each specimen as shown in Figure F43B, we estimated the in situ hydraulic conductivity of each core sample from the in situ void ratio values shown in Table T3 (Fig. F44) (e.g., Long et al., 2008). Results indicate that in situ hydraulic conductivity deceases systematically with depth and follows a similar trend at all three sites, from values of 3.3 × 10^–10 to 1.0 × 10^–9 m/s at ~50 mbsf to 3.3 × 10^–11 m/s at 440 mbsf.

The Casagrande and SED approaches yield similar values of P_c′ for our experiments (Table T3) and indicate normal consolidation (conditions of hydrostatic pore pressure and uniaxial strain) at Site C0001 to at least ~140 mbsf and at Site C0008 to at least ~210 mbsf (Fig. F45). This result is consistent with drilling results that did not document substantial evidence for significant fluid overpressure or fluid flow (Screaton et al., 2009b). In contrast, specimens from the footwall of the megasplay at Site C0004 (underthrust slope apron) exhibit significant overconsolidation (OCR values of ~2.3–3.3).

We also semiquantitatively assess sample quality and disturbance, which may be caused by drilling, sampling, and specimen handling. Disturbance generally results in a decrease in effective stress relative to the in situ effective stress and destructuring of the sediment fabric such that laboratory stress-strain response may not be representative of in situ behavior (e.g., Lunne et al., 1997; Germaine and Germaine, 2009). For each test, we report a Specimen Quality Designation (SQD) following Lunne et al. (1997), based on a measure of Δe/e_i and OCR, where Δe is the change in void ratio from the initial void ratio e_i to the estimated in situ void ratio measured along a uniaxial consolidation path (Table T3). SQD generally ranges from B (good to fair) to C (poor) for shallow samples from Site C0001 (<200 mbsf). Deeper samples from Site C0001 and those from Sites C0004 and C0008 are generally rated D (very poor). Despite the fact that the relationships between void ratio and effective stress from CRS tests generally show characteristics of higher quality samples (e.g., well defined yield stress), the potential consequences of sample disturbance in interpretation of these results should be appreciated.

Fabric results

Table T1 summarizes the results of our ESEM fabric analysis. Figures F46, F47, and F48 show rose diagrams of the grain orientations (apparent long axes) imaged across horizontal and vertical faces relative to the core axis. The corresponding cumulative orientation curves are shown in Figures F49, F50, and F51. Average values for the orientation index of samples from the three sites range from ~0.30–0.32, and we see a general trend of fabric development increasing with depth at Sites C0004 and C0008, particularly in vertical sections (Table T1; Figs. F46, F47, F48). In comparison, samples from the accretionary prism at Site C0001 and slope sediments in the footwall of the megasplay fault at Site C0004 exhibit higher values for the orientation index (>0.40) (Table T1; Figs. F49, F50), indicating a higher alignment of grains.

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