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Triaxial tests

So far, four samples have been analyzed from Site U1324. Sedimentological description (see the “Site U1324” chapter) for the three samples of normally deposited material indicated slightly different settings: normally deposited hemipelagic sediments, levee turbidites, and distal turbidites. The samples were taken from depths between 154.4 and 406.7 meters below seafloor (mbsf). The results of the triaxial testing are given in Figures F2, F3, F4, and F5 and are tabulated in Table T1. Occasional initial offsets in the stress-strain diagrams (Figs. F3, F5) are attributed to sitting effects during the initial phase of compression. The offset of one of the stress paths from zero deviatoric stress in Figure F2 (experiment B at 1200 kPa cell pressure) is due to a minor axial load applied accidentally at the start of the experiment. Both types of deviations are minor, and we have chosen to display original experimental data and not to apply corrections here. Peak shear stress and Young’s modulus were determined from the stress-strain data and changes in pore pressure during the course of the tests. Stress paths were recorded to build a base to derive friction coefficients, angles of friction, and cohesion. Supplementary measurements provided data on water content and grain density. All measured peak shear stresses are very small and lie between 45.3 and 140.7 kPa. The observed range of Young’s modulus (from initial slope M0 in the stress-strain curve) is between ~2 and 6 kPa in the samples, except for the sample from 153.4 mbsf, which has a distinctly higher Young’s modulus (range: 13.6 to 17.4 kPa) and is by far the stiffest of all samples analyzed. As this is the only core catcher (CC denomination) sample that we have tested, we suspect that the high cohesion, peak shear strength, and derived Young’s modulus have their origin in fabric changes that occurred at the bottom end of the core during the coring process when the hydraulic piston corer was advanced into the sediment. If this is so, then due care should be taken in the interpretation of geotechnical data from core catcher samples.

Permeability is in the range of 10–16 to 10–17 m2, and hydraulic conductivity determined during the consolidation stage is around 10–9 to 10–10 ms–1. Both types of values correspond well with determinations made by N.T.T. Binh et al. (unpubl. data) and Long et al. Where the data could be determined reliably, grain density of the tested samples is ~2.7 g/cm3, and water content ranges from 18.3% to 30.7%. During testing, pore water pressure generally rose before reaching the yield point of the samples in all tests, within the first 5%–10% axial shortening. This probably relates to collapse of pore space when fractures or through-going shear zones are being formed in the samples. In the later part of the deformation history, pore pressure fell back to starting value or slightly below. When considering the stress-strain plots in detail, it is evident that all samples except for the hemipelagite from 153.4 mbsf at Site U1324 do not weaken or strengthen during progressive deformation. The mentioned sample, however, shows cyclic changes in strength on the order of ~5%–10%.

One sample from the base of a major MTD at Site U1322 has been analyzed so far. Here, tests showed that the material is weaker than the normally sedimented material: peak deviatoric stress ranges from ~27 to 42 kPa, but Young’s modulus is similar to the weak normally sedimented samples (5.7–7.6 kPa). Figure F6 shows essential test data from this sample graphically, and the data are given in Table T1.

Stress paths from all five samples indicate that the material behaved in a somewhat overconsolidated fashion during the tests, but this effect is least notable in those samples that were consolidated and sheared at ~1.7 MPa confining pressure. This is an indication that the in situ effective stresses in the depth range investigated at Site U1324 may be close to this value. This interpretation is supported by the preconsolidation pressures of ~1.5 to ~3 MPa for the depth range between 300 and 500 mbsf, as determined for samples from Site U1324 by Long et al., and Dugan and Germaine. The almost exact similarity of the stress paths from two identical tests carried out on two aliquots of the whole-round sample from Core 308-U1324C-7H (Fig. F4) shows that tests are fully reproducible on similar materials.

Inferences that could be made about static coefficients of friction from stress path data in the consolidated-undrained tests are at a very preliminary stage, but inferences from Mohr envelopes would yield surprisingly high values (0.8 or more) for the normally sedimented samples at the range of mean effective stress considered (20–140 kPa). For the MTD sample, the friction coefficient estimate is definitely lower (~0.44 at a mean effective stress range of 20–40 kPa). Inferred cohesion would be estimated in the range of 10–20 kPa, underlining the possible very weak nature of Ursa Basin sediments located at the base of MTDs.

Ring shear tests

Four samples, two each from Sites U1322 and U1324 have been analyzed so far, and the results are shown graphically in Figures F7, F8, F9, and F10 and are tabulated in Table T2. As experiments were under drained conditions, comparison with the results to those from the undrained triaxial tests is not straightforward, but some similarities are evident. Shear strength recorded at ~1 MPa normal stress (8 kg axial load) is very low at 100–300 kPa, rising more or less linearly to values between 3 and 4.5 MPa at ~16 MPa normal stress (128 kg axial load).

Friction coefficients from all samples on the basis of these data are in the range of 0.13–0.31, with internal angles of friction of ~7.4°–17.2°. These are values not unusual for smectite-rich clays and muds, like those from the Ursa Basin. There is no obvious difference between the frictional behavior of the three samples from normally sedimented sections and the one from a MTD, except for the fact that the MTD material (sample from Core 308-U1322B-26H; see Fig. F9) is the weakest and shows the least sensitivity of frictional coefficients to changes in shearing rate and axial load. As samples subjected to ring shearing are remolded with no remaining primary microfabric, this is tentatively related to composition and/or the mode of clay flocculation and charging effects. Further investigations will show whether this is a phenomenon inherently related to mass transport deposits. More analyses of samples from both groups of sediments are needed, however, to further explore this question.

Regarding the sensitivity of strength with respect to changes in the velocity of shearing, some observations can be made in the uppermost diagrams of Figures F7, F8, F9, and F10. At high overburden pressure (15.237 MPa), the reaction of the material to an increase in shearing velocity is generally strengthening in all samples, although in one case (Core 308-U1322D-3H; Fig. F6) there is some weakening immediately after changing to a higher shearing rate. At intermediate overburden pressure (7.624 MPa), the MTD sample (Core 308-U1322D-26H; Fig. F10) shows velocity softening when shearing is accelerated from 0.18 to 1.8 mm/min. Velocity softening is also observed in this sample at 3.817 MPa overburden pressure. At intermediate overburden pressure, samples from Cores 308-U1322D-3H (Fig. F6) and 308-U1322B-4H (Fig. F8) show velocity softening, whereas the levee turbidite sample (Core 308-U1324B-50H; Fig. F9) tends to harden over a wide range of overburden pressures when subjected to increases in shearing rate.

Overall, it can be shown (Fig. F1) that the peak shear strengths observed in the triaxial tests and cohesions computed from the ring shear test data (taking measured shear stress and friction coefficients at 0.962 and 1.914 MPa into account) are mostly within the range of shear strengths determined shipboard during Expedition 308. Exceptions are the two samples from shallow depths at Site U1324 that show higher values in the experiments than in the shipboard data. At least for the sample tested triaxially, this is tentatively ascribed to the fact that a core catcher section was tested. We suspect that this could be an effect of fabric changes induced during the hydraulic piston coring.