LABORATORY TESTING METHODOLOGY

All laboratory tests were conducted in accordance with the American Society for Testing and Materials (ASTM) standard for each test. In the cases where ASTM standards do not exist, the procedures followed were according to established MIT geotechnical laboratory protocols.

Radiography and Logging

All samples were X-rayed at MIT's radiography facility in order to assess the sample quality, presence of inclusions, general soil type, and variation in soil fabric. The X-ray procedure followed is similar to ASTM standard D4452 (ASTM International, 2003f). Radiography allows selection of the best quality material for testing. The tube X-rays are found in "Appendix A." In addition to the X-rays, a tube log was provided for each tube ("Appendix B.). These logs contain the various tests performed on samples from each tube, as well as their location relative to the tube length.

Remolded and Resedimented Sample Preparation

Because of the limited amount of intact, good-quality soil available for testing, a number of tests were conducted on laboratory reconstituted specimens. Remolded specimens were prepared by first mixing together trimmings left over from sample preparation and highly disturbed soil that could not be used for intact sample testing. The soil was then allowed to air dry until the water content was ~40%. For consolidation tests, the soil was pressed into the consolidation ring, making sure that the soil was tightly packed into the ring. For strength tests, the soil was placed in a mold that had the same dimensions of a triaxial test specimen. The soil was slowly packed, making sure that it was placed tightly in the mold and there were no voids.

Test results showed that preparing the soil using this method does not produce results similar to those of tests on intact specimens, especially in undrained shearing. Because of this, a more complicated method for preparing remolded specimens with a higher water content was employed. This method, called resedimentation, is similar to the process used to prepare resedimented Boston blue clay (Germaine, 1982). The soil for resedimentation is prepared by mixing trimmings from previous tests and soil deemed unsuitable for intact sample testing in a blender with distilled water to produce a slurry. The slurry is thickened by placing it in a 100C oven and removing it every hour for ~5 min to stir it and let it cool. This process of stirring and cooling the soil ensures the soil is only thickened and not fully dried. Once the slurry has thickened, it is ready for bench consolidation. For consolidation tests, the slurry is placed in the consolidation ring and tested immediately. For strength tests, the slurry is first scooped into a tall oedometer ring. The slurry must be slowly scooped into the ring to prevent the formation of air pockets and voids. The slurry is incrementally loaded until the vertical effective stress reaches ~50 kPa. A load:increment ratio of 1 was used, with each load being maintained for at least 24 hr to ensure the completion of primary consolidation. Once 50 kPa is achieved, the load is reduced until an overconsolidation ratio (OCR) of 4 is reached. The sample is then extruded and trimmed as an undisturbed sample. The results show that resedimented specimens exhibit similar behavior to intact specimens, especially in undrained shearing.

Index Tests

Only one Atterberg limit test was performed on each tube. Hence, in order to obtain an average liquid and plastic limit for each tube, small amounts of soil from various locations within the tube were mixed together and used for each Atterberg limit test. The liquid limit was obtained by placing the soil in a casagrande cup, grooving the soil with an ASTM groove tool, and counting the number of blows necessary to close the groove by 0.5 in. The water content at 25 blows is the liquid limit. The plastic limit is the water content of a soil when rolled until crumbling occurs at a diameter of 1/8 in. The Atterberg limits tests were conducted in accordance with ASTM D4318 (ASTM International, 2003b).

Water content is measured by taking the difference in the weight of a soil before and after oven drying and dividing this difference by the oven-dried weight. In the consolidation and strength tests, two water contents are measured: wc and wn. Wc refers to the water content measured from the leftover trimmings during sample preparation. Wn refers to the water content of the test specimen itself.

Loss on ignition is performed by placing a small amount (~5 g) of oven-dried sample in a muffle furnace at 440C for 24 hr. The change in weight divided by the original weight gives the amount of loss on ignition. The test was performed in accordance with ASTM D2974 (ASTM International, 2003e).

Particle size analysis is used to determine the distribution of particle size for the soil. For the fine-grained particles, the particle size distribution is determined by performing the hydrometer test described in ASTM D422 (ASTM International, 2003d).

Mineralogy

Soil mineralogy can be identified using X-ray diffraction (XRD). The MIT XRD facility uses a Rigaku Rotaflex 180-mm diffractometer with a graphite-diffracted beam monochromator using CuK radiation ( = 1.5418 ). The diffractometer is normally rotated between 4 and 56.

Initial random powder XRD tests showed the presence of calcium carbonate. In order to eliminate the effect of calcium carbonate on the results of the tests, calcite-treated random powder samples were also prepared. The calcite was removed by HCl digestion in accordance with ASTM D4373 (ASTM International, 2004b).

XRD results show that the samples contain a significant amount of nonclay particles. As such, random powder XRD was performed on the clay-size fraction only. The clay-size fraction was separated by sedimentation in a volumetric flask. Sedimentation was performed by mixing the soil into a slurry and placing it in a flask filled with water at pH 9. The soil was then allowed to sediment in the flask for 48 hr. The sedimentation time was determined from the settlement time of the clay-size particles during the particle size analysis test.

Consolidation Testing

The MIT geotechnical laboratory has developed a standard method for performing CRSC tests. In addition, ASTM D4186 (ASTM International, 2003c) was used as a guideline in conducting CRSC tests.

The CRSC test can be divided into three stages. The first stage of the test involves sample preparation. This is performed by placing the sample in a trimming jig that lowers the consolidation ring into the soil. Excess soil is slowly trimmed from around the perimeter, and the ring is pushed into the soil in small increments. After the sample is trimmed into the CRSC ring, it is carefully placed in the CRSC cell. The CRSC cell is then filled with salt water and tightly sealed with the piston locked in place.

The second stage of the test is the backpressure saturation stage. The purpose of backpressure saturation is to ensure all the air bubbles go into solution. In this stage, a small effective stress is applied such that there is minimal to no change in axial strain. For the Hydrate Ridge soil, the applied effective stress ranges from 0.05 to 0.4 kgf/cm2. Then, while maintaining the same effective stress, the axial stress and cell pressure are increased in increments of 1 kgf/cm2 until the cell pressure reaches 4 kgf/cm2.

The third stage of the test is the consolidation itself. All of the tests were run at a strain rate of 0.5%/hr. The strain rate was selected such that the maximum value of the pore pressure ratio does not exceed 4%. In addition, an unload–reload cycle to an OCR of 10 was introduced in all tests. For the standard- and small-diameter samples, the maximum vertical effective stress applied ranged from 20 to 25 and 80 kgf/cm2, respectively. Prior to the unload–reload cycle and after the maximum vertical effective stress was reached, the applied stress was held constant to allow the excess pore pressure to dissipate and some secondary compression to set in. The hold stress portion was held for 6 hr for the standard-diameter sample and at least 12 hr for the small-diameter sample.

Undrained Strength Testing

The MIT geotechnical laboratory has developed a standard method for performing CKoU tests. Furthermore, ASTM D4767 (ASTM International, 2003a) was used as a reference for the triaxial testing. This test 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 to the size of a triaxial specimen, it is placed on the triaxial base with a nylon filter fabric and porous stone placed on both ends. No side drains were used during the tests. Two thin impermeable membranes are rolled over the soil and sealed with three O-rings each at the top cap and bottom base. The cell is then filled with silicon oil and tightly sealed. Distilled water was used as the fluid in the drainage system.

The second stage of the test is the backpressure saturation stage. The purpose of backpressure saturation is to ensure the soil is fully saturated by applying enough pressure to dissolve all the remaining air bubbles in the soil. In this stage, a small effective stress is applied such that there is minimal to no change in axial strain. For the Hydrate Ridge soil, the applied effective stress ranges from 0.1 to 0.2 kgf/cm2. Then, while maintaining the same effective stress, the axial stress and cell pressure are increased by an increment of 0.5 kgf/cm2. The axial stress and cell pressure are increased incrementally until the measured B-value is 1.00 + 0.02, which indicates complete saturation, or until the backpressure reaches 3 kgf/cm2.

The third stage of the test is the Ko-consolidation stage. In Ko-consolidation, the sample is consolidated one-dimensionally (i.e., no radial strain). The MIT geotechnical laboratory employs the SHANSEP testing technique (Ladd, 1991). After consolidation, the applied stress was held by keeping the vertical, cell, and pore pressures constant for 24 hr. The hold stress portion is necessary to allow the excess pore pressure to dissipate and allow some secondary compression to set in.

The final stage of the test is the undrained shearing stage. Once the specimen has undergone 24 hr of hold stress, 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. The specimen is sheared at 0.5%/hr until a distinct failure plane has developed or 10% strain has been reached.

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