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Laboratory testing results

Thirty-six of 41 uniaxial consolidation tests were successfully performed on samples from Sites U1351–U1354. For five of the consolidation tests, the inflection point on the e-log could not be clearly defined in the graphical determination. The results are summarized in Table T2. The first section of the table gives sample name and sample depth information. The second section gives information about area of specimen, diameter of specimen, initial height of specimen, the initial pretest void ratio measured on the specimen (ei), and the applied maximum load (σ′vm). The third section gives the water content, bulk density (ρb), weight per unit volume (ρb – 1), and dry bulk density (ρd) derived from oedometer tests and shipboard MAD measurements located closest to the tested sample. The final section gives the void ratio at the intersection point (epc), the vertical effective stress, the maximum past effective overburden stress determined by the graphical method of Casagrande (1936) (σpc), the compression index, the constrained modulus at the point of first maximum loading, and the calculated OCR. The results are graphically presented as semilogarithmic plots of void ratio versus vertical effective stress and plots of constrained modulus versus vertical effective stress (Figs. F3–F38).

The maximum past effective overburden stress versus depth is graphically presented for Sites U1351–U1354 in Figure F39. As expected, the maximum past effective overburden stress for each site increases with depth. For the uppermost 25 m, the maximum past effective overburden stress correlates well among each site. Below 25 mbsf, the maximum past effective overburden stress tends toward higher values for the shelf sites (U1353 and U1354) than for the slope site (U1352). The outermost shelf site (U1351) shows comparable values to the slope site (U1352).

The compression behavior (values from Cc) is similar for sediments from all four sites (Table T2). Cc ranges from 0.15 to 0.27 for samples from Site U1351. Cc ranges from 0.15 to 0.32 for samples from Site U1353. Cc ranges from 0.17 to 0.30 for samples from Site U1354. Cc for samples from the slope site (U1352) show the widest range from 0.11 to 0.39.

In general, the soil resistance against deformation (Es) is similar for sediments from all four sites and increases with depth (from 19.30 to 510.36 MN/m2). Es ranges from 12.17 to 83.66 MN/m2 for the upper 90 m, with most values ranging between 20 and 60 MN/m2. For depths below 90 mbsf, Es ranges from 89.53 to 510.36 MN/m2, with most values >140 MN/m2.

OCR shows that sediments are strongly overconsolidated in the uppermost 80 m of all sites, with ratios ranging from 0.15 to 3.05 for the shelf edge proximal Sites U1351 and U1352 and ratios ranging from 1.34 to 3.86 for the shelf edge more distal Sites U1353 and U1354. Below 90 m, the sediment seems to be normal to underconsolidated (values ranging from 0.15 to 1). The two uppermost samples from Site U1352 (at 2.89 and 10.09 mbsf) show noncredible high OCR values of 9.73 and 5.41, respectively.

We used the criteria of Lunne et al. (1997) to evaluate the effect of drilling disturbance on test quality. The results are summarized in Table T3. Δe was calculated as the difference between the initial void ratio and the void ratio at the intersection point on the consolidation curve. The degree of sample disturbance is rated on the volume change Δe/ei, where ratios <0.04 are designated as very good to excellent, ratios from 0.04 to 0.07 are good to fair, ratios from 0.07 to 0.14 are poor, and ratios >0.14 are very poor. The tested samples range from fair to very poor (0.06–1.0), whereas deeper samples are generally rated very poor. The semiquantitatively evaluated results with poor to very poor rates contradict the visual observation, which describes the samples as undisturbed and not obviously deformed. Disturbance generally results in a decrease in effective stress relative to the in situ effective stress. Saffer et al. (2012) suggested that possible restructuring of the sediment fabric because of disturbance has an impact on the laboratory stress-strain response, which may not be representative of in situ behavior. The consequences of drilling disturbance while drilling on a shelf of interpretation of these results should be appreciated.