Application of X-ray Computed Tomography in Marine Clays

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1 Proceedings of the Twentieth (2010) International Offshore and Polar Engineering onference Beijing, hina, June 20 25, 2010 opyright 2010 by The International Society of Offshore and Polar Engineers (ISOPE) ISBN (Set); ISSN (Set); Application of X-ray omputed Tomography in Marine lays S.L.Yang, K. Schjetne and T. Kvalstad Norwegian Geotechnical Institute, Oslo, Norway ABSTRAT Application of X-ray computed tomography (T) in offshore soil investigations is discussed in this study. Soil sample quality, different soil units and change of structures in the clay samples may be obtained and the study showed that x-ray T is very useful for planning of advanced laboratory tests for offshore soil investigation. It is also a tool for identification of different soil units, and is suggested to be used together with interpretation of the borehole logs and classification tests. Gas bubbles can be formed in marine clays due to gas hydrate dissociation and also by oversaturation of gassy pore water due to pressure release during sampling, and the cracks formed can be investigated by x-ray T technique. KEY WORDS: x-ray T; lab test planning; soil units; crack; gas bubbles. INTRODUTION X-ray computed tomography (T) is a non-destructive technique that allows visualization of the internal structure of objects, determined mainly by variations in density and atomic composition. The T scanning has been widely used for years as a medical diagnosis technique. To date, this technique has been increasingly used in the field of nondestructive material testing. Recently, the study of geomaterials (including granulates, soils, rocks and concrete) has become one of the more active and challenging fields for the application of high-resolution T. T has been used in many aspects of geotechnical engineering. Most of previous studies were carried out on the application of T in fracture and localized deformation of rock, sandstone and limestones (Louis et al., 2006; Renard, et al., 2006). Some studies were performed for evaluation of failure structures, minifabrics, localized deformation and cavity generation in soils (Ashibli et al., 2000, Mukunoki, et al., 2006, heng and Ngan-Tillard, 2006, Takahashi, 2006). In this study, some applications of x-ray T in offshore soil investigations are given. Normally, offshore soil investigations are very expensive, and it is necessary to use the samples efficiently. Selection of the good quality samples for further laboratory tests is important. High resolution X-ray T can be used for planning of the advanced laboratory test programmes. At the same time, T technique can give images which can show the change in density, structure and granular shapes, and show if the sample contains shell fragments or iron material. From the above information, the boundary between different soil layers are possible to be determined. The identification of different soil layers from lab tests can be combined with images from T scanning. A more accurate depth for each layer is obtained since a continuous profile can be obtained by T scanning. Finally in this paper, T technique is used in characterizing cracks, which were formed by of gas bubbles expansion in a marine clay. Gas hydrate dissociation is often cited as a triggering mechanism for submarine slope failure (Sultan, et al., 2004; Jung and Vogt 2004; Mienert et al., 2005; Nixon and Grozic, 2007), however, very little direct evidence of these effects on slope stability exists. When gas hydrate is melting, water and gas will be released causing considerable volume expansion, and gas bubbles will be formed in the marine sediments. There is not much data available on the influence of gas bubbles on the behavior of marine clays. X-ray T scan technology is very helpful to visualize the structure of the soil samples under a certain load or after a change in pressure. The image processing can even quantify the voids in the samples. In this study, the T scan was used to see if there is any crack or low density area in the soil after pressure reduction. PLANNING OF ADVANED SOIL TESTING FOR OFFSHORE SOIL SAMPLES When samples are obtained from an offshore soil investigation, one task is to plan the following laboratory tests. Undisturbed samples should be used for the advanced tests like oedometer, direct simple shear (DSS) and triaxial tests. X-ray T scanning is a very useful tool for visualization of the quality of the samples and for selecting the depths where high quality specimens can be prepared. Fig. 1 shows a lot of cracks in a sample. The sample is disturbed and should not be used for advanced tests. While the image in Fig. 2 shows a good quality sample, the sample in Fig.3 is partly disturbed, and the disturbed part 493

2 should be avoided for advanced tests. The cross sections shown in these figures were taken every 10 cm of the sample. It should be noted that white color represents low density, dark color represents high density in these images. This applies for all the images in this study. Fig.3 Partly disturbed samples IDENTIFIATION OF DIFFERENT SOIL UNITS Fig. 1 Disturbed sample Fig. 4 shows a profile from an offshore borehole. Data for water content, unit weight and measured undrained shear strength from fall cone, triaxial tests and DSS tests, and also interpreted undrained shear strength from T-bar tests are included. Based on the data from the borehole logs, three units were identified. A change happens at around 3 m and 6 m from water content, unit weight, undrained shear strength profile. However, the exact depth for the different units cannot be determined. If we look at the images both from cross sections and longitudinal sections from the T scanner, a change happens at depth 2.9 m (Fig. 5), and an obvious change of the image occurs at depth 6.7 m (Fig. 6). The exact depth for the different layers (Unit I, Unit II and Unit III) can be obtained from the T images. Another example is given in Figures 7 and 8, and the exact depth for the layers Unit I and Unit II can be determined as 2.95 m based on the T images. The two soil units were identified by combining all available geological and geotechnical engineering information. The undrained shear strength in the borehole logs were interpreted from PT data. The cross sections were taken every 10 cm of the sample. As can be seen in this study, x-ray T images are very useful in characterizing different soil units. This technique is a supplementary tool for determining the depth to the different soil layers. Fig.2 Undisturbed samples 494

3 0 Soil description Unit I LAY, very soft, dark greyish brown X-ray image 2.74 m Water content, % Unit weight, Undrained shear strength, kn/m 3 s u, kpa E AUE AU DSS Fall cone 2.9 m Depth below seabed, m 5 Unit II LAY, very soft, very dark grey, shell fragments, mica crystals 6.7 m 3.14 m 6.44 m Interpreted from T-bar test Unit III 10 LAY, calcareous to carbonate, slightly sandy, soft, dark grey to black 7.04 m E Fig. 4 Borehole logs, lab test results and T images for borehole 1 (Note: Zero seabed of the T-bar testing was not accurate) ROSS SETIONS 2.24 m Fig. 5 T images for the samples from borehole 1 at a depth range from 2.24 m to3.24 m hange at 2.9 m 3.24 m Depth from 2.24 to 3.24 m 495

4 ROSS SETIONS 6.24 m Fig. 6 T images for the samples from borehole 1 at a depth range from 6.24 m to7.24 m hange at 6.7 m m Depth from 6.24 m to 7.24 m 0 Soil description Unit I X-ray image 2.61 m Water content, % Unit weight, Undrained shear strength, kn/m 3 s u, kpa AU AUE E LAY, very soft, very dark grey Depth below seabed, m 5 Unit II 2.95 m LAY, soft, with traces of organic matter and shell gragments very dark grey Interpreted from PT test 3.31 m Fig. 7 Borehole logs, lab test results and T images for borehole 2 496

5 ROSS SETIONS 2.41 m Fig.8 T images for the samples from borehole 2 at a depth range from 2.41 to 3.41m hange at 2.95 m m Depth from 2.41 m to 3.41 m RAKS IN MARINE LAY An oedometer cell modified from a triaxial cell to allow application of backpressure was used in this experiment (Fig.9). Dissolved O 2 was diffused to the intact marine clay sample (diameter 50 mm, length 50 mm) under a back pressure of 10 bar (981 kpa) in this study. The diffusion process lasted for more than one month. Then a vertical consolidated stress of 100 kpa was applied to the sample and the deformation was recorded. After the consolidation was finished, the sample was sent to the T scanner (Fig.10). The equipment was modified to be movable while keeping the high back pressure and the vertical consolidation stress. Fig.9 ell used in the experiment First, the sample was scanned at about 981 kpa back pressure and 100 kpa vertical consolidation stress. Then the back pressure was reduced in steps while keeping the vertical consolidation stress constant. The sample was scanned at several pressure steps and O 2 bubbles were observed escaping from the sample. The deformation of the sample during the test was measured. First, the sample was scanned at about 981 kpa back pressure and 100 kpa vertical consolidation stress. Then the back pressure was reduced in steps while keeping the vertical consolidation stress constant. The sample was scanned at several pressure steps and O 2 bubbles were observed escaping from the sample. The deformation of the sample during the test was measured. 497

6 O 2 gas bubbles were observed in the cell at a back pressure level of 200 kpa. Fig. 11 shows examples of cross sections of T scan samples before (A) and after (B) the back pressure was reduced. racks were observed in the processed images for the sample after pressure was reduced. The cross sections in figure 11 were taken every 1 mm of the sample, and were taken from middle part of the sample.. Final-1 Final-2 Final-3 Final-4 Fig.10 ell in the x-ray Original-1 Original-3 Original-2 Original-4 B Selected final cross sections of the samples after the back pressure was reduced to 0 kpa Fig.11 ross sections from the T scanning ONLUSIONS This study showed that x-ray computed tomography is a useful tool for planning of advanced laboratory test programmes. When the results from laboratory classification tests and undrained shear strength tests are available, the x-ray computed tomography technique can be used as a supplementary method for identification of different soil units and sample quality evaluation. A more accurate depth for different soil layers is also possible to obtain in this way. Gas bubbles can exist in marine clays due to the dissociation of gas hydrate and evolution of free gas from the pore water caused by pressure reduction during sampling. With increasing expansion of gas bubbles, fractures may develop and interconnect creating continuous cracks in the clay matrix. X-ray computed tomography can be used to find the possible cracks in the marine clay, and the shape and structures of the cracks can be characterized. AKNOWLEDGEMENTS A Selected original cross sections of the samples after consolidation and diffusion of O 2 at a back pressure of 981 kpa The help from Knut Dalen, the operator for the T scanner at Norwegian University of Life Sciences, is greatly appreciated. This paper is partly a contribution to the Norwegian Research ouncil PETROMAKS project "Gas hydrates on the Norway-Barents Sea - Svalbard margin", (GANS, Norwegian Research ouncil project No /S30). 498

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