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doi:10.2204/iodp.proc.308.210.2009

Laboratory testing methodology

All laboratory tests (Table T1) were conducted in accordance with the American Society for Testing and Materials (ASTM) standards. For tests where ASTM standards do not exist, the procedures followed the established Massachusetts Institute of Technology (MIT) geotechnical laboratory protocols.

Radiography

Radiography allows the selection of the best quality material for advanced geotechnical testing. All whole-round samples were X-rayed following a procedure similar to ASTM standard D4452 (ASTM International, 2003). X-ray images were used to assess sample quality, presence of inclusions, general soil type, and variation in soil layering. Radiographs are available in Nelson et al.

Specimen index properties

Water content is measured by taking the difference in the weight of a soil before and after oven drying and then dividing this difference by the oven-dried weight. For each experiment we provide the water content of the test specimen (w_n) (Tables T2, T3). We also provide the initial void ratio (e_i) of each specimen (Tables T2, T3). Void ratio is defined as the volume of voids divided by the volume of solids. From the void ratio and water content we calculate the initial saturation (S_i = w_nG_s/e_i) for each specimen (Tables T2, T3). We assume a constant specific gravity of the solid grains (G_s = 2.78) for these clay-rich samples. All variables are defined in Table T4.

Undrained strength testing

Consolidation and strength properties were measured from the results of K_o-consolidated undrained (CK_oU) triaxial tests on specimens from Sites U1322 and U1324. The MIT geotechnical laboratory has developed a standard method for performing CK_oU tests. In addition, ASTM standard D4767 (ASTM International, 2004) was used as a reference for the triaxial testing. Undrained strength testing can be divided into four stages. The first stage of the test involves sample preparation by trimming the specimen in a trimming jig using a wire saw. After the sample is trimmed (~1.75 cm radius; ~8 cm height), it is placed on the triaxial base with a nylon filter fabric and a porous stone placed on each end. Side drains were not used. Two thin, impermeable membranes are rolled over each specimen and sealed with three O-rings each at the top cap and bottom base of the triaxial chamber. The triaxial cell is then filled with silicon oil and tightly sealed. Distilled water was used as the fluid in the drainage system, which is connected to the top and base of the specimen.

Backpressure saturation is the second stage of the test. This phase ensures full saturation of the specimen. To do this, a modest pressure is applied to dissolve any air bubbles in the specimen. Next, a small, isotropic effective stress is applied to the specimen such that there is minimal to no axial strain. This effective stress is applied to seat the specimen in the triaxial cell. For the specimens from the Ursa region, the applied effective stress ranged from 16 to 76 kPa (Table T2). This isotropic effective stress is maintained while the axial stress and cell pressure are increased incrementally by the same value. To test for specimen saturation, the drainage lines are closed and the axial stress and cell pressure are increased incrementally and the B value (ASTM International, 2004) is measured. A B value of 0.98–1.00 is desired; however, it was not achieved in all experiments (Table T2). After the B value is measured, the drainage lines are opened for the consolidation phase of the test.

The third test stage is K_o consolidation. During K_o consolidation the specimen is consolidated one-dimensionally in the axial direction (i.e., no radial strain) following the SHANSHEP testing technique (Ladd, 1986). K_o consolidation allows vertical strain on the specimen but maintains the radius of the specimen. This simulates burial of the sediment in a confined basin where sediments deform vertically but are confined laterally and do not strain in the lateral direction. K_o is the ratio of the radial effective stress to the vertical effective stress required to maintain no radial strain (K_o = σ_r′/σ_a′). We define K_o at the maximum vertical consolidation stress (σ_vc′) as the consolidation lateral stress ratio (K_c) (Table T2). K_o consolidation rates are provided in Table T2. After reaching the desired consolidation stress, total vertical stress, cell pressure, and pore pressure were held constant for a set time (t_s) to allow excess pore pressure to dissipate and to allow some secondary compression (Table T2). For all specimens the maximum vertical consolidation stress exceeds the in situ effective vertical stress to ensure the specimen is on the primary consolidation path.

The final stage of the test is undrained shearing. Prior to starting the shear, a leak check is performed by closing the drainage valves for 30 min. During this time, the backpressure should remain constant. After the leak check, the specimen is sheared with the drainage lines closed. Shearing with the drainage lines closed prevents any fluid drainage during shear, maintains a constant volume of the specimen, and allows us to define the undrained strength parameters for each specimen. The shear rates are provided in Table T3. Positive shear strain data indicate a compression test, whereas negative shear strain data indicate an extension test.

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