DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS SUCHIT D. GUMASTE, KANNAN R. IYER, SUSMITA SHARMA and D.N. SINGH about the authors Suchit D. Gumaste Indian Institute of Technology Bombay, Department of Civil Engineering Powai, Mumbai-400076, India E-mail: suchit.gumaste@gmail.com Kannan R. Iyer Indian Institute of Technology Bombay, Department of Civil Engineering Powai, Mumbai-400076, India E-mail: kannankr20@gmail.com Susmita Sharma Indian Institute of Technology Bombay, Department of Civil Engineering Powai, Mumbai-400076, India E-mail: susmita.sharma4@gmail.com corresponding author Devendra Narain Singh Indian Institute of Technology Bombay, Department of Civil Engineering Powai, Mumbai-400076, India E-mail: dns@civil.iitb.ac.in Abstract This paper presents details of investigations that were conducted to determine the fabric (i.e., the arrangement of soil grains and pores) of undisturbed marine clay samples that were retrieved from 5 m to 65 m below the seabed. Impedance Spectroscopy (IS), which is a non-destructive and non-invasive technique, was employed to determine the electrical conductivities of the marine clay samples in their longitudinal and transverse planes of sedimentation. These results were employed to define the extent of the fabric anisotropy in terms of an anisotropy coefficient, Ae, as a function of depth. In addition, Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) were employed to study the fabric and pore-size distribution of these samples, respectively. Based on these investigations it has been observed that Ae increases with sampling depth, which is indicative of the alteration from flocculated fabric, at shallower depths, to the dispersed fabric, at deeper depths. The study highlights the importance and usefulness of the anisotropy coefficient, Ae, for determining the alteration in the fabric of marine clays, due to self-weight consolidation. Keywords Marine clays, anisotropy, laboratory tests NOTATIONS adc n G e Yd,Yt w w adcU Odd OAC A Ae ds d dm dd IS l L/S MIP P SEM V. Hg V HgI Z' DC conductivity of the sample porosity of the sample specific gravity voids ratio dry and total unit weight of the sample, respectively gravimetric water content frequency of the AC DC conductivity in the transverse and longitudinal directions, respectively AC conductivity area of the electrodes anisotropy coefficient sampling depth pore diameter mean pore diameter dominant pore diameter impedance spectroscopy distance between the electrodes liquid to solid mercury intrusion porosimetry pressure scanning electron microscopy cumulative volume of mercury intruded in the sample incremental volume of mercury intruded in the sample real part of the impedance ACTA GEOTECHNICA SLOVENICA, 201^/2 21. S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS 1 INTRODUCTION Marine clays are formed through a process of the sedimentation of suspended particles in seawater. During this process, clay and silt particles flocculate and settle as flocs on the seabed due to the cations present in the seawater [1]. When more and more flocs accumulate at the top, the bottom layers consolidate due to the weight of the sediments lying above. This process, called self-weight consolidation, was previously studied by other researchers [2-6]. Marine clays are soft in consistency and hence exhibit a poor shear strength and a high compressibility [7], and hence due to such adverse engineering behaviour, these soils pose great challenges to the civil-engineering profession [8]. An overview of the studies conducted by previous researchers [9-18] reveals that factors like degree of consolidation, extent of agglomeration, swelling and shrinkage characteristics of sediments, and the chemical constituents of seawater, are responsible for the postsedimentation fabric (i.e., the spatial arrangement of the particles and the pores) of marine clays. Therefore, a close interrelation between the fabric, and the origin and degree of consolidation of marine clays is expected. Also, as soil fabric is one of the major factors influencing the strength and deformation characteristics of clays [17, 19-34], investigations into the fabric of marine clays that have undergone varying degrees of self-weight consolidation need to be conducted. In this context, efforts made by some of the previous researchers are worth mentioning. The fabric of marine clays, corresponding to the same depths below the seabed, from the eastern and western coastal cities of India, has been studied with the help of Scanning Electron Microscopy (SEM) [17], and it has been reported that these soils exhibit a mainly flocculated or dispersed fabric, depending upon the characteristics of the depo-sitional environment (e.g., the chemical constituents of seawater). Incidentally, researchers [35] have proposed various models to explain the flocculated, random and dispersed fabric. Efforts have also been made by researchers [36, 37] to study the interrelation between the microstructure of clays and their physical properties using the XRD and SEM techniques. In this context, the mineralogical and micro-fabric analyses of marine clays from Hong Kong are worth mentioning [38, 39]. However, not many efforts have been made to establish a correlation between the fabric of the marine clays with the conditions prevailing during their formation and the degree of compactness, in a quantitative manner. It is believed that such relations would be helpful in understanding the overall effect of the process of self-weight consolidation on the fabric changes in these clays. It should be noted that changes in the fabric also influence the degree of anisotropy of the permeability, shear strength, and other physical properties, as reported by previous researchers [40], and hence quantification of the fabric changes in terms of the fabric anisotropy becomes essential. Keeping in view the above-mentioned aspects, investigations were carried out on undisturbed marine clays, retrieved from different depths below the seabed. The main objective of these investigations is to quantify the change in the fabric anisotropy with respect to the sampling depth and thereby to elaborate how the fabric of marine clays alters when they are subjected to self-weight consolidation. Details of the investigations performed to achieve this objective are presented in the following. 2 LABORATORY INVESTIGATIONS 2.1 DETAILS OF THE SAMPLES Undisturbed marine clay samples (Table 1), collected from the south-eastern coast of India, were used in this study. These samples were retrieved from the same borehole with the help of Shelby tubes of internal diameter 76mm, pushed from the seabed and then transferred to PVC tubes (200 mm long, 90 mm internal diameter and 2 mm wall thickness). Due care was taken to prevent the migration of moisture from the sample, and any physical damage occurring to it, during its transportation from the site to the laboratory, by wrapping it with a cellophane membrane and a bubbled polythene sheet. Both sides of these tubes were covered with PVC caps and sealed with wax. The depth of these samples ranged from 5 to 65 m below the seabed. The specific gravity G of these samples was determined with the help of a helium gas pycnometer (Quantachrome, USA) [41,42]. The total unit weight yt , water content w, dry density y voids ratio e, and porosity q, of these samples were also determined (Table 1). 2.2 PHYSICAL CHARACTERIZATION A certain portion of each sample was oven dried at 105±5 °C, pulverized, and used for establishing the particle size distribution characteristics [43] and the consistency limits [44]. The results obtained from this exercise are presented in Table 2. Based on the Unified Soil Classification System (USCS) [45] classification, as 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS Table 1. Physical properties of the samples. Sample designation Depth (m) water content, w (%) Total unit weight, yt (kN/m3) Dry unit weight, yd (kN/m3) Specific gravity, G void ratio, e Porosity, n (%) S1 5.50 50.6 16.8 11.2 2.56 1.29 56 S2 8.45 35.7 17.9 13.2 2.73 1.07 52 S3 9.45 38.2 20.5 14.8 2.91 0.96 49 S4 12.50 56.0 18.9 12.1 2.51 1.07 52 S5 15.10 27.6 19.0 14.9 2.64 0.77 44 S6 18.55 35.3 19.9 14.7 2.68 0.82 45 S7 21.50 47.6 19.8 13.4 2.48 0.85 46 S8 27.15 32.2 19.7 14.9 2.77 0.86 46 S9 33.50 36.7 19.1 14.0 2.53 0.81 45 S10 36.45 36.9 22.6 16.5 2.73 0.65 40 S11 38.55 28.3 20.6 16.1 2.67 0.66 40 S12 40.55 25.8 21.2 16.9 2.82 0.67 40 S13 47.50 29.4 20.9 16.2 2.49 0.54 35 S14 50.60 56.0 25.5 16.3 2.57 0.57 36 S15 64.40 17.6 21.5 18.3 2.68 0.47 32 Table 2. Physical properties of the samples: Particle size distribution characteristics and classification of the samples. Sample % Liquid limit Plastic limit Plasticity Index USCS* designation Clay fraction Silt fraction Sand fraction (%) (%) (%) S1 60 39 1 65 27 38 MH S2 53 43 4 71 29 42 CH S3 66 30 4 72 36 36 MH S4 54 44 1 73 32 41 CH S5 57 40 3 78 40 38 MH S6 60 39 1 77 39 38 MH S7 59 40 1 78 31 47 CH S8 64 30 6 80 34 46 CH S9 65 34 1 78 33 45 CH S10 63 37 0 72 31 41 CH S11 60 38 2 78 39 39 MH S12 59 40 1 76 33 43 CH S13 78 20 2 82 34 48 CH S14 73 27 0 97 37 60 CH S15 62 34 4 78 31 47 CH * Unified soil classification system (as per ASTM D 2487-06e1) depicted in Fig. 1, it can be inferred that these samples marine sediments are relatively uniform up to the depth belong to the MH and CH groups. This indicates that the of interest. 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS 80 70 60 50 40 30 20 10 0 | ■ Experimental data CH ■ , CL m/ ■ /* ' /w . : / MH/OH . ym-io\. A _ S dct (1) s dd 20 40 60 80 100 LL (%) Figure 1. Location of different samples on the USCS plasticity chart. 2.3 CHEMICAL CHARACTERIZATION The range of chemical composition (by % weight) of the sample, in the form of major oxides, was determined by using an X-ray Fluorescence setup (Phillips 1410, Netherlands). It was observed that these samples contain SiO2 as a major oxide (58 to 66 %) along with 14 to 17 % of Al2O3 and 2 to 13 % of Fe2O3. The chloride and sulphate contents of the sample were determined on an extract of 2:1 water-to-soil ratio (designated as L/S, by weight), with the help of an Indion Easy test kit (supplied by Ion Exchange India Ltd., Mumbai, India). It was observed that these samples have a sulphate content varying between 5 to 30 ppm for an L/S equal to 10 and 20. However, the chloride content of these soils was found to vary from 700 to 1000 ppm and 300 to 550 ppm, for an L/S equal to 10 and 20, respectively. A water-quality analyser (Model PE 136, Elico Ltd., India), fitted with a glass calomel electrode, was employed for measuring the pH of the soil solution with different liquid-to-solid ratios (L/S=10 and 20). The pH values of these soil solutions fall in the range 8.1 to 8.9. 2.4 INVESTIGATIONS ON THE FABRIC ANISOTROPY In order to establish the variation of the fabric with the depth of the sampling of the specimen, the anisotropy coefficient, Ae , which is defined by Eq. 1 [46-48], was employed: where a^ct and 0^ are the DC conductivities in the horizontal (i.e., transverse to the plane of sedimentation) and vertical (i.e., longitudinal to the plane of sedimentation) directions, respectively. The methodology based on Impedance Spectroscopy (IS) [49-51], developed by previous researchers [48], was employed to measure 0^ct and 0c and hence to compute Ae. In addition, Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) were also conducted on the specimens of the samples in order to observe the particle-to-particle interaction and the pore size distribution, respectively, as discussed in the following. 2.5 ELECTRICAL CONDUCTIVITY MEASUREMENTS With the help of a stainless-steel sample extruder (exhibiting an area ratio of less than 10%) and a piston attached to it, samples (38 mm diameter and 30 mm long) were retrieved from the PVC tubes. To avoid any distortion during the sample collection and extraction, the extruder was lubricated with silicon grease. The DC conductivities, 0^c , of these samples were determined for the longitudinal direction and the transverse direction by employing an LCR (Inductance, Capacitance and Resistance) meter (Agilent 4284A). The methodology for sample preparation, the detailed information regarding the test setup, the method of calibration and the procedure adopted for determining 0^c , reported by previous researchers [48, 52], are discussed in the following. The test setup, depicted in Fig. 2, was employed for measuring the electrical conductivity, 0^c , of the sample. This setup consists of two acrylic plates (100 mm x 100 mm x 10 mm) and at the centre of each of these plates a stainless-steel electrode (50 mm x 50 mm x 2 mm) is fitted. These electrodes are mirror finished, passivated and are connected to brass bolts, across which an electrical potential can be applied. The sample can be fitted between these plates by adjusting their distance with the help of a steel rod and screws arrangement, as depicted in Fig. 2(d). The calibration of the test setup was made by applying open- and short-circuit corrections, which help in eliminating the unwanted impedance generated due to connecting the cables of the LCR meter and the stray capacitance [48, 52]. After the calibration of the test setup, the 0^c of the sample (38 mm in diameter and 30 mm in height) was determined, as explained in the 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS Figure 2. (a) Schematic of the sample holder, (b) Longitudinal (A-A) view of the sample, (c) transverse (B-B) view of the sample, the photographs of (d) the sample holder and (e) the test setup. following. As depicted in Fig. 2(b and c), the top and bottom surfaces, and the about 10-mm-thick curved surface on the opposite sides of the sample, were painted with a conducting silver paint. Next, the AC conductivity, aac , of the sample was determined by employing a LCR meter, which works in the frequency range 20 Hz to 1 MHz and by applying an electric field of 1 V. As reported by previous researchers [52], the real part of the impedance, Z, which gets displayed on the LCR meter corresponding to a different frequency of AC (designated as w), was recorded and by employing Eq. 2 the aac was computed. l s, Z A (2) where l and A are the distance between the electrode plates and area of the electrodes, respectively. Subsequently, aAc was plotted with respect to w (on x-axis) and the intersection of the linear portion of the plot on the aAc axis (w=0) was determined, which yields the of the sample [52]. 2.6 DETERMINATION OF SOIL FABRIC The fabric of the sample was determined using SEM and MIP. To achieve this, approximately 5-mm-sized cubic specimens were extruded from the middle one-third portion of the samples, after the electrical conductivity was recorded. The pore-fluid present in the specimen was removed by employing an air-drying technique [48]. As the soil specimens are non-conducting in nature, there will be a charge accumulation on their surface, which results in blurring and poor quality of the images during the SEM studies. In order to minimize this, the specimens were placed on a sample holder, with a conducting carbon base, and were coated with an about 15-nm-thick gold-palladium (a conducting material) layer by employing a coating device (JCF 1600, JEOL). The SEM images (corresponding to 500x to 8000x magnification and depicting the view perpendicular to the plane of deposition) of the specimen were obtained by employing Quanta 200 ESEM instrument. Although the ESEM allows different modes (i.e., the high-vacuum mode and the ESEM mode) for image capturing, which 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS also facilitates an analysis of wet specimens, the high-vacuum mode, which necessitates the use of air-dry specimens, was employed in the present study to obtain a better resolution of the images. At lower magnifications, general fabric features were studied, while at higher magnifications the particle assemblage, the type of the contacts and the pore space morphology were inspected. The pore-size distribution characteristics of the specimen were obtained with the help of a MIP (Quantachrome, USA). The MIP works on the principle that the applied pressure required for the intruding mercury (a nonwetting liquid) into the pores is inversely proportional to their size [53]. The test is performed by increasing the pressure, P (up to 225 MPa, which corresponds to the pore diameter, 0.0064 ^m), on the mercury filling the cell containing the sample. The pore-size obtained from the measured pressure, by assuming a contact angle and a surface tension of the mercury as 140° and 0.480 N/m, respectively [54], when plotted against the cumulative volume of the mercury intruded into the sample, V^g , facilitates a determination of the pore-size distribution characteristics of different specimens (see Fig. 9). 3 RESULTS AND DISCUSSION The magnitude of odc for the soil samples measured in two directions, designated as odct and Oc , is presented in Table 3. It is clear from the data presented in the table that odct is higher than o^ for all the samples. This indicates that the electrical conductivity measurements are dependent on the direction along which they are measured and this directional dependency (i.e., electrical anisotropy) can be quantified by employing Eq. 1. Although Odc depends strongly upon the voids ratio, particle arrangement and the pore-fluid conductivity, Ae would be a function of the anisotropy of the particle arrangement or the fabric anisotropy [48, 55]. With Table 3. DC Conductivities and anisotropy coefficient of different samples. Sample Depth odd oda A designation (m) (S/m) (S/m) e 51 5.50 2.14 5.53 1.61 52 8.45 2.84 8.65 1.75 53 9.45 2.86 9.06 1.78 54 12.50 2.52 6.92 1.66 55 15.10 3.10 9.40 1.74 56 18.55 2.79 9.14 1.81 57 21.50 1.95 6.53 1.82 58 27.15 2.34 9.13 1.97 59 33.50 1.53 4.96 1.79 510 36.45 1.58 6.15 1.97 511 38.55 2.29 10.10 2.10 512 40.55 2.12 9.61 2.13 513 47.50 0.81 3.68 2.14 514 50.60 2.45 11.86 2.20 515 64.40 0.26 1.36 2.27 Odd = DC conductivity in the transverse direction; Odd = DC conductivity in the longitudinal direction; Ae = anisotropy coefficient yd(kN/nrO (a) (b) Figure 3. Variation of (a) Ae and (b) Yd with the depth of different samples. 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS this in view, Ae for different samples was computed (see Table 3) and its variation with respect to the sampling depth, ds , is depicted in Fig. 3(a). Also, the variation of Yd , with respect to ds is depicted in Fig. 3(b). It is clear from the trends depicted in Fig. 3 that with an increase in ds , Ae and Yd increase linearly. These trends can be attributed to the variation in the spatial arrangement of the soil particles and the pores, along the depth of the sediment deposition. This can be confirmed from Table 4, where the macro pores reduce nearly linearly with depth (i.e., from 42.04 % for S1, at 5.5 m depth, to 6.61% for S15, at 64.4 m depth). Table 4. Summary of the MIP results for different samples. Sample Designation Macropores (%) Meso- pores (%) Micropores (%) *dd (|im) *d um (|im) S1 42.04 52.59 5.37 0.04-0.3 0.043 S4 30.22 65.00 4.78 0.03-0.1 0.036 S7 23.43 68.84 7.73 0.02-0.06 0.029 S11 12.80 84.13 3.07 0.02-0.06 0.026 S14 9.55 84.73 5.72 0.008-0.02 0.023 S15 6.61 78.77 14.62 0.007-0.03 0.018 *dd = Dominant pore diameter; *dm = Mean pore diameter Furthermore, up to 15 m depth, comparatively smaller values of Ae (=1.7) are observed. In this context, researchers [48] have reported that Ae for undisturbed samples is higher than that of the remoulded samples and undisturbed soil samples, with dispersed fabric structure, exhibit Ae=2.0. It has also been stated by these researchers that the transition from the flocculated to the dispersed structure, for the remoulded samples, occurs at Ae=1.6. Hence, Ae=1.7 reflects a random arrangement, or a small degree of preferred orientation of the particles, in the natural soil deposits. This type of particle arrangement, referred to as a 'Flocculated' fabric, would normally be associated with comparatively younger sediments in the seabed. In this fabric, particles get attracted to one another in a very loose, haphazard manner, resulting in higher values of the voids ratio, e. However, a further decrease in e, with a corresponding increase in ds , clearly indicates that sediments develop denser fabrics. This phenomenon can also be attributed to a subsequent increase in the overburden pressure, which the initial fabric would not be able to sustain as a result of the continuous accumulation of sediments. In other words, self-weight consolidation [2-6] would cause the rearrangement, or reorientation, of the particles in the preferred direction, progressively, which is reflected in an increase in Ae [>2.0 for ds>35 m, see Fig. 3(a)]. As such, higher values of Ae clearly indicate an increasing degree of preferred orientation of the particles in the horizontal plane. In other words, the fabric appears to be more anisotropic with the increase in depth, which is a typical characteristic of the dispersed fabric. Hence, it can be stated that with an increase in the depth of marine clays, the initially developed flocculated fabric is changed to a dispersed fabric and this alteration could be captured, quantitatively, in terms of Ae , relatively easily and quickly. In order to verify this hypothesis the results obtained from MIP and SEM were further analysed, as discussed in the following. Samples S1, S4, S7, S11, S14 and S15 were analysed by employing the SEM and MIP techniques. Based on the data presented in Table 1, the samples used in the present study can be considered to represent sediments ranging from shallow depths (up to 35 m) to greater depths (>35 m). The SEM images of the specimens of these samples are depicted in Figures 4 to 8. For instance, the SEM images of specimen S1 (taken in the horizontal plane of the sample, as depicted in Figure 4), which is a younger sediment than its counterparts, reveal that the fabric exhibits mainly 'edge-to-edge' and/ or 'edge-to-face' contacts between the clay particles that are randomly orientated [23]. From the figure it can also be observed that it is a typical case of a honeycomb fabric [56] or a cellularity, by which sediments have been deposited in the form of continuous chains, as depicted in Fig. 4(a). This phenomenon can be attributed to the electrolyte-rich conditions of the marine environment, which is mainly due to the presence of a high chloride content. And due to this, the sediments are deposited in the form of flocs or agglomeration, and hence as a result a highly porous fabric is observed. Incidentally, Fig. 4(b) does not exhibit any sign of stratification (parallel orientation of clay platelets). Thus, it is a typical fabric of the marine clay that has been subjected to a negligible overburden pressure, due to the shallow depth of the deposition. On the other hand, for specimen S4 the SEM images are depicted in Figure 5. These micrographs reveal the presence of a typical matrix fabric [56], wherein larger gaps between the silt particles are found to be occupied by agglomerated clay platelets. From the results presented in Table 2, a comparatively higher percentage of silt fraction can be confirmed for sample S4. Such a type of fabric is relatively common among poorly and moderately consolidated marine clays (i.e., those deposited in an estuary), with a dry density not higher than 16.5 kN/m3 [57]. 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS Figure 4. SEM images of S1 along horizontal (transverse) plane. Figure S. SEM images of S4 along horizontal (transverse) plane. Figure 6. SEM images of S11 along horizontal (transverse) plane. Figure 7. SEM images of S14 along horizontal (transverse) plane. 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS Figure 8. SEM images of S15 along horizontal (transverse) plane. However, with a further increase in depth, the fabric acquires a dense structure with a specific orientation (face-to-face orientation) of particles with rough or smooth surface features and the presence of small pores), as depicted in Figures 6, 7 and 8, for the specimens S11, S14 and S15, respectively. This again can be attributed to the self-weight consolidation at deeper depths. Hence, the distinctive orientation of the clay platelets is observed to be in the transverse plane that is perpendicular to the direction of self-weight consolidation and which ultimately results in a 'face-to-face' contact between them [23]. These platelets increasingly become oriented parallel to each other, thereby making the fabric highly anisotropic. Thus, it can be concluded that with an increase in the depth, the particles and pores rearrange or reorient themselves into a new fabric that can be referred to as a 'dispersed' fabric. In order to verify the above-mentioned findings, which are based on SEM, MIP studies were also conducted on the same specimens. The variation of the percentage cumulative volume of mercury intruded in the specimen, V^g , with respect to the pore diameter d was plotted as depicted in Fig. 9, from which the percentages of micro-pores (d<0.01^m), meso-pores (0.01^m1.5 ^m, increases. This can be attributed to the reorganization and reorientation of clay platelets with an increase in depth (i.e., due to self-weight consolidation), which is responsible for the alteration of the randomly oriented fabric to the dispersed fabric. Hence, it can be inferred that the change in the fabric anisotropy, which in turn depends on alterations in the fabric structure, is related to a change in the size of the pores. Furthermore, in order to highlight the importance of the study, the results obtained from the IS, SEM and MIP were superimposed in Fig. 10. It is clear from this figure, and Tables 1, 3 and 4, that the specimens S1, S4 and S7 exhibit a value of Ae between 1.61 and 1.82 and 0.05 0.04 E 0.03 2. 0.02 0.01 Fabric zones A-Honeycomb B-Matrix C-Dense (rough surface features) D-Dense (smooth surface - features) Computed valued Flocculate' 1.0 1.3 1.5 1.8 2.0 2.3 2.5 Figure 10. Variation in the fabric structure for Marine clay. dm values between 0.029 to 0.043 ^m. The lower values of Ae and the relatively higher values of dm indicate a honeycomb-type or matrix-type flocculated structure [60]. The analysis of the SEM images of these specimens (see Figs. 4 and 5), as described earlier, also qualitatively confirms the flocculated structure. In addition, it can be seen that for specimens S11, S14 and S15, Ae lies between 1.97 and 2.27 and the dm values are observed to be between 0.018 to 0.026 ^m. Hence, it can be inferred that the higher Ae (> 2.0) and lower dm would be representative of the dispersed structure. These fabric arrangements could be qualitatively observed in their SEM images (see Figs. 6 to 8). Hence, it can be hypothesized that Fig. 10 presents guidelines for determining the fabric (i.e., its type and the corresponding dm) of the marine clays, if their Ae is known. It must be appreciated that a determination of Ae is relatively easy and less time consuming as compared to the SEM and MIP techniques. However, for a generalization of these findings, exhaustive investigations should be conducted on marine clays obtained from different locations and that too in the depth ranges beyond those covered in the present study. Incidentally, it is worth mentioning here that the marine clay samples, which were retrieved from the offshore environment and transported to the laboratory, might have undergone changes from their undisturbed in-situ state due to various factors, such as sampling disturbance, relief of stresses from the sample during and after soil sampling, and smearing effects [61]. Also, the extraction of the sample from deeper locations would cause stress relaxation due to the removal of overburden and confining stresses and might result in a swelling of the samples [62, 63]. Furthermore, the soil below the drilled borehole might be subjected to higher vertical stresses, which might lead to localized compaction and disturbance of the soil layer beneath. Also, the mechanical disturbance caused by driving the sampler causes shear distortion and subsequent compression or remoulding of the soil (i.e., smearing) very close to the inner face of the sampler [63, 64]. These effects of stress relaxation, smearing and local disturbances could be different depending on the sampling depth and the overall characteristics of the soil (i.e., physical, chemical and mineralogical properties). The authors would like to reiterate that though the above mentioned factors might result in changes in the pore and fabric structure of the soils, which is responsible for changes in their anisot-ropy, the sampling procedure adopted in this study (i.e., the use of a sampler with an area ratio less than 10% for undisturbed soil sampling and careful extrusion of specimens for MIP, IS and SEM studies) ensures that these effects are minimal. 22. ACTA GeOTeCHNICA SLOVeNICA, 2014/2 S. D. GUMASTE ET AL.: DETERMINATION OF THE FABRIC ALTERATION OF MARINE CLAYS 4 CONCLUDING REMARKS In this study efforts were made to investigate the variation of the fabric of marine clays that were obtained from different depths below the seabed, in their undisturbed state. To achieve this, the anisotropy coefficient, Ae , was determined with the help of Impedance Spectroscopy and the same was found to vary from 1.5 to 2.25, for samples obtained from depths of 5 m to 65 m. This variation can be attributed to the self-weight consolidation, which alters the fabric of these samples and renders them increasingly anisotropic, with an increase in the depth. This hypothesis was substantiated by observing the scanning electron micrographs of some selected samples from different depths and by determining their pore size distribution characteristics with the help of Mercury Intrusion Porosimetery. 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