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Preparation and identification of clay samples with controlled fabric
Engineering Geology, 9(1975) 13--38 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
PREPARATION AND IDENTIFICATION OF CLAY SAMPLES WITH CONTROLLED FABRIC
RAYMOND J. KRIZEK, TUNCER B. EDIL and I. KUTAY OZAYDIN
The Technological Institute, Department of Civil Engineering, Northwestern University, Evanston, Ill. (U.S.A.) Department of Civil Engineering, University of Wisconsin, Madison, Wisc. (U.S.A.) Dames and Moore, Park Ridge, Ill. (U.S.A.) (Received December 5, 1973; revised and accepted July 23, 1974)
ABSTRACT Krizek, R. J., Edil, T. B. and Ozaydin, I. K., 1975. Preparation and identification of clay samples with controlled fabric. Eng. Geol., 9: 13--38. Techniques have been developed to prepare reasonably homogeneous, reproducible bulk samples of a kaolinite clay (Hydrite 10) with predetermined microfabrics and to reliably identify these microfabrics both quantitatively and qualitatively. Eight samples with quite diverse histories were produced in the laboratory by controlling the chemstry of the clay--water system, the consolidation stress path (either isotropic or anisotropic), and the magnitude of the consolidation stresses. The fabrics of these samples are identified and quantified by the combined use of scanning electron microscopy,optical microscopy, and X-ray diffractometry, and reasonably comprehensive appraisals of particle associations and orientation are obtained. Anisotropic consolidation was found to induce a preferred particle orientation, whereas isotropic consolidation tended to provide basically random samples. The anisotropically consolidated samples from dispersed slurries exhibited somewhat greater particle orientation than those from flocculated slurries, and, although considerable particle orientation occurred at low values of the consolidation stress, increases in the major principal consolidation stress did accentuate the particle orientation. The presence of domains or small groups of particles is suggested in certain samples, especially in those consolidated isotropically.
INTRODUCTION
The influence of microstructure on the macroscopic behavior of clays has long been recognized, and the problems associated with understanding this complex relationship have challenged researchers for several decades. Much of the difficulty involved in developing such an understanding has stemmed from the inability to systematically and quantitatively isolate and evaluate the various phenomena that contribute to the overall response. More specifically, the problem of (a) preparing reproducible bulk samples (from which specimens for various engineering tests can be trimmed) with a predetermined microfabric, and (b) reliably identifying both quantitatively and
14 qualitatively the fabric of these samples has proven to be an awesome task; these two aspects will be addressed in this work. FABRIC AND STRUCTURE As used herein, fabric is defined as the geometrical aspects of particle arrangements, and structure is the combination of fabric and the associated interparticle forces between adjacent particles. Since structure is extremely difficult to measure and quantify whereas fabric is more amenable to measurement and quantification, and since much can be inferred about structure from its associated fabric, the study of clay fabric constitutes a most important step in understanding the behavior of clays. However, soil fabric is not concerned with only the spatial arrangement of simple, discrete particles, but also with the arrangement of c o m p o u n d particles; consequently, the levels and units of fabric organization become important. Herein~ fabric is viewed at two general levels, namely, macrofabric and microfabric. Macrofabric refers basically to the fabric units which are observable by the unaided eye or by a low magnification microscope; terms like peds, crumbs, and aggregates have been used to describe these macroscopic units. For example, a p e d is defined as a soil aggregate consisting of a cluster of particles and separated from each other by surfaces of weakness, such as fissures, voids, or skins of different composition material (Brewer, 1964). Microfabric consists of units that are observable only under either an optical or an electron microscope. In this study emphasis is placed on this level of fabric, and the term "fabric" will be used to denote primarily microfabric. At this level units may range from single particles acting independently to domains which consist of two or more particles acting as a unit. In general, the levels of fabric are not sharply separated, but there may exist a rather continuous hierarchy of units ranging from single particles to domains to various size peds (primary, secondary, and tertiary). In microfabric analysis we are concerned with determining the geometrical aspects associated with particle arrangements; these are the orientation characteristics, particle shape and size distribution, porosity (or void ratio), and the distribution of pore sizes. The first of these parameters is a directional parameter, whereas the rest are scalar quantities. Particle orientation is defined by the angular relationship between the linear or planar elements (clay particles) and a set of chosen reference axes, and in this study fabric may further be considered essentially s y n o n y m o u s with orientation, since the other parameters, with the exception of porosity, were maintained nearly constant. The terms oriented and random are very important in describing the orientation characteristics of a clay. When there is parallel orientation between linear or planar elements, they are said to possess preferred orientation or to be oriented. The term random implies that the probability of particle orientation is equal in all directions and that a preferred direction of orientation does not exist. Most clays have fabrics which range between perfectly oriented and perfectly random.
15 CLAY INVESTIGATED Since the purpose of this work is to prepare reasonably homogeneous bulk samples with varying microfabrics and to identify the resulting fabrics, the factors controlling the selection of theclay to be studied are primarily (a) its suitability to induce various fabrics, and (b) the availability of procedures to identify and measure the resulting fabric. There has been developed within the last few years a considerable background of experience and technique for controlling and identifying the fabric of kaolinite; accordingly, kaolinite was selected as the clay mineral to be used in this investigation. However, the commercially available kaolinites, even though they are mineralogically the same, differ from each other in certain properties which are important in controlling fabric; some of these properties are grain-size distribution, grain morphology, nature of exchangeable ions, and presence of cementing agents. Three Georgia kaolinites (Hydrite R, Hydrite 10, and Hydrite UF) were investigated for possible use in this work. Hydrite R contains a b o u t 20% particles that are larger than 2 microns; these often form units consisting of stacks of platelets {also called crystallites) which exhibit a generally cubical shape (see Fig.lb), as opposed to the platelet shape of individual kaolinite particles (see Fig.la), and they are not amenable to inducing orientation. Furthermore, the presence of silt-size grains causes local disturbances in the overall fabric. As a consequence of these considerations and after some preliminary testing, Hydrite R was discarded as unsuitable for this investigation. Hydrite UF {ultra-fine) is available in very small particle sizes, but it is difficult to induce orientation because of such small sizes with relatively small differences in dimensions. Furthermore, Hydrite UF behaves much differently from most other kaolinites owing to its high specific surface and associated increased colloidal effects. Therefore, Hydrite 10 was selected as possessing the most desirable characteristics from the fabric point of view; this clay has the following physical properties: liquid limit, 62%; plastic limit, 34%; specific gravity, 2.64; silt fraction, 4%; and clay fraction (< 2 microns), 96%. SAMPLE PREPARATION Although the methods of sample preparation described herein are not intended to model or duplicate the soil-making processes of nature, they do produce a range of fabrics which includes the fabric of most naturally occurring sedimentary clays. Attention is given to the conditions which exist during the formation of sedimentary deposits, since the environmental conditions during deposition and the post-depositional load history play an important role in controlling the resultant fabric of sedimentary clays (Gillott, 1968). The fabric of clay deposited from an aqueous suspension is not only affected by the movements of the flowing medium, but also by the composition of the fluid phase, which determines the flocculation state of the clay minerals; also, the nature of the settling units, whether single particles or flocs, is important. As the overburden load increases, the water
16
5~
(a) H y d r i t e 10
2~ J
I
(b) H y d r i t e R Fig.1. Particle morphology of two hydrite kaolinites
17 content and the void ratio decrease, and this process of compression is accompanied by structural, physical, chemical, and possibly mineralogical changes in the soil skeleton and its included pore fluid. Accordingly, these two important factors (namely, the pore-fluid chemistry of the slurry and the stress path followed during the slurry consolidation) are used to produce samples with desired fabrics.
Dispersion and flocculation characteristics The dispersion of a kaolinite suspension is enhanced by a low cation concentration in the pore fluid, monovalent ions, a high pH to prevent positive charges on the edges of kaolinite particles, and a high water content to increase interparticle distances. When any of these conditions is n o t satisfied, the repulsive force regime is altered, the attraction becomes stronger between negative and positive double layers, which are associated with the faces and the edges of particles, respectively, and flocculation occurs. For the kaolinite used in this work, it was found that a critical pH less than approximately 4 was conducive to flocculation. Since the magnitude and nature of particle associations in the slurry state, whether in the field or in the laboratory, depend primarily on the clay mineralogy and the species and concentrations of ions present in the pore fluid, a measure of the relative effectiveness of different electrolytes with respect to floccultion or dispersion of a certain suspension was obtained from a series of sedimentation tests which were" c o n d u c t e d in test tubes on suspensions of the actual material used for sample preparation. Various amounts of an electrolyte were added to a specified a m o u n t of the clay, and the contents were mixed by shaking; then, the tubes were allowed to stand for approximately one week during which time the contents were observed visually and the thicknesses of the accumulated sediments were measured. In this way, the o p t i m u m flocculation or dispersion concentrations were b o u n d e d b e t w e e n upper and lower limits. This range was further narrowed in a second series of tests in which the concentrations were varied in smaller increments between the previously established limits. Although the results of these tube sedimentation tests are considered to have only relative significance, they allow a direct comparison of concentrations of different electrolytes to obtain the o p t i m u m flocculation or dispersion characteristics of a particular clay suspension with a specified clay concentration. In the dispersion tests the time to flocculation is a measure of the strength of the dispersion, that is, the magnitude of the repulsive force regime; longer times to flocculation are indicative of stronger repulsive forces. The appropriate concentration of dispersant is the one which maximizes the face-to-face particle associations. In the flocculation tests the volume of the sediment is used as a measure of effectiveness, since random particle associations yield an open structure with associated large volumes. Table I gives the dispersion characteristics of both Hydrite 10 and Hydrite R with NaOH solution; results are given in terms of the height ratio of the
18 TABLE I Dispersion characteristics of Hydrite 10 and Hydrite R Concentration of NaOH (moles/l) in solution
0 0.005 0.010 0.020 0.050
Ratio of height of supernatant to height of total suspension (1.5 g clay per 9.0 g solution) Hydrite 10
Hydrite R
1 day
2 days
0.142 0.185 0 0 0
0.175 0.164 0.040 0.062 0.059
1 week 0.148 0.246 0.097 0.031 0.110
1 day
2 days
1 week
0.091 0.055 0 0 0
0.127 0.154 0.076 0.060 0.029
0.058 0.698 0.092 0.061 0.156
supernatant solution to the total suspension at different times. In the case of both kaolinites the following observations can be made. T he 0.005 M NaOH suspension settled at the end of one day with a clear supernatant solution and sediment; however, the o t h e r three suspensions form ed very slight amounts of clear supernatant solutions and remained in suspension with density increasing with depth w i t h o u t a clearly defined sediment surface, even at the end of a week. Fr om these observations it can be concluded that the 0.005 M NaOH solution does n o t yield satisfactory dispersion, probably due to the insufficient supply o f OH ions to offset the attractive force regime, Although there is some difference bet w een the results for the ot her three concentrations, these differences are somewhat irregular and n o t considered significant. The observed t e n d e n c y for a decrease in the dispersion characteristics after one week with an increase in concent rat i on b e y o n d some o p t i m u m is pr oba bl y due to the fact t h at flocculating effects of the Na cation begin to overcome the dispersing effects of the OH group. Previous experience showed t hat high concentrations of dispersants tended to yield a high degree o f sensitivity in the consolidated blocks. With this fact in mind, the lowest o f the three concentrations t hat caused sufficient dispersion (0.01 M NaOH) was selected as the o p t i m u m dispersion c o n c e n t r a t i o n for b o th kaolinites and used in the preparation of dispersed slurries for slurry consolidation. This c o n c e n t r a t i o n corresponds to a pH o f 6 (which is above the critical pH o f 4) at the water c o n t e n t of 250%. Figure 2 illustrates the flocculation characteristics of the two types of clays with NaC1 and CaC12 ; results are expressed in terms of the height ratio o f the sediment to the total suspension as a function o f time for a period of one week. These data are summarized in Fig.3, where the same ratio is p l o tted versus the salt c o n c e n t r a t i o n at the end o f one week, when sedimentation practically reached equilibrium. An examination of these results indicates th at the 0.0001 M CaCl2 and the 0.01 M NaC1 concentrations for Hydrite 10 and the 0.001 M CaC12 and the 0.001 M NaC1 concent rat i ons for H y d r i t e R result in the largest normalized volume of sediment. T he decrease in sediment volume with high salt-concentration (2 M NaC1 or 2 M CaC12 ), as co mp ar e d with the sediment volume at lower concentrations, is
19
& 2M NoCI v O.IM NoCl
= O.OIM NoCl=
1.0
I Hydrite I0
l~L e-
Distilled° O.O01MwaterNaCI
L I t' 2M CoCle O.OIM CoCIz =O,O001MCoCl= I O.IM CoCI= ~ QOOIMCoCII • Distilled Water I Hydrite I0
0.9
E ~ D
0.8
O
~-
0.7
-
-
0
(a) 0"60 1.0
2
4
/
o
/
6 I
B
(c)
IOxlO 3 0 1.0 \
.,dr.e.
(b)
2
4
I0 x I0 z
I
~\
Hydrite R
I
0.6
r 0
<
2
4
r 6
-8
Oe ~ IOxlO 3
0
2
4
6
Elapsed Time (minutes) Fig.2. Flocculation characteristics of two hydrite kaolinites. 1.0
t
"5§
r----
+
o~ = 8 09 o 8 -I - -
•"r 0 E ~ ~ ~
(d)
~
->~.__
06
!0 NaCI Hydrite I0 [3 CoCl= Hydrite I0 - --V NaCI Hydrite R ~ CaCI2 Hydrite R
j
L
0,4' 10-4
i0-~
L iO-Z
i¸
!
I0 -I
I0 o
IO
Concentration of Solution (moles/liter)
Fig.3. Effect of salt concentration on equilibrium sediment volume of two hydrite kaolinites.
I0 x 103
20 believed to be due to the face-to-face aggregation of particles, which yields a " b o o k - h o u s e " particle arrangement wherein several particles aggregate faceto-face, but the aggregates exhibit a random association with each other. Suspensions with distilled water flocculate and give a height ratio lower than the height ratio in the case of the optimum concentration suspensions of Hydrite 10 with NaC1 and CaC12. However, in the case of Hydrite R, a distilled-water suspension had a height ratio higher than those of all saltflocculated suspensions with both NaC1 and CaC12. Flocculation of suspensions prepared with distilled water is basically due to the low pH. The pH of the Hydrite 10--distilled-water suspension at a b o u t 250 percent water content is 3.5 (the critical pH is 4). Figure 4 shows scanning electron micrographs of the final sediments of the Hydrite 10 suspensions prepared at the o p t i m u m dispersion and optimum flocculation concentrations, as well as that prepared with distilled water. The dominant face-to-face association of particles is very clearly illustrated in Fig.4a, which shows the sediment obtained from the dispersed slurry. However, due to the nature of the forces acting during the placement and drying processes, all particles do not necessarily orient in a certain direction. In the case of the sediments from the flocculated suspensions (due either to the low salt concentration or the low pH), the particles do not exhibit the dominant face-to-face associations illustrated in Fig.4a, but rather they exhibit random associations, as indicated in Figs.4b and 4c.
Effects of stress history One of the most important factors causing post
5~
I
I
5~
I
(b) From Salt-Flocculated Slurry
Fig.4. Scanning electron micrographs of Hydrite 10 sediments.
(a) From Dispersed Slurry
I~
5~
I
t~
(c) From Distilled Water Slurry
I
22
Techniques to control stress conditions during slurry consolidation Two techniques were developed to control the stress conditions during the process o f slurry consolidation. The anisotropic slurry consolidometers (Sheeran and Krizek, 1971) consist basically of rigid cylindrical chambers (20 cm in diameter and 50 cm in height) in which vertical stresses were applied by means of a piston, and vertical drainage was provided by porous stones located in the piston and base; at the end of anisotropic consolidation, a relativly h o m o g e n e o u s block of soil 20 cm in diameter and 10--15 cm in height was obtained. The isotropic slurry consolidometers (Edil, 1973) involve the use of double flexible rubber membranes filled with slurry to form a sphere (about 25 cm in diameter) which was floated in a liquid of slightly higher density within a pressure chamber, loaded hydrostatically, and drained at two diametrically opposite points; at the end of isotropic consolidation, a sphere of soil a p p r o x i m a t e l y 15--20 cm in diameter was obtained. In both systems, a back-pressure of 7 kg/cm 2 was applied to the pore fluid of the slurry to ensure saturation. Slurries were stored for a b o u t a m o n t h after mixing the kaolinite pow der with the appropriate fluid in order to achieve equilibrium in the distribution of the fluid and cations t h r o u g h o u t the slurry. Subsequently, the slurry was de-aired under vacuum for a b o u t 24 h before it was placed into the consolidometers. The stresses were applied incrementally, starting with 0.07 kg/cm 2 and doubling each previous load. In the case of anisotropically consolidated samples each increment was maintained until the com pl e t i on of primary consolidation or until the rate of vertical d e f o r m a t i o n became very slow (less than 0.25 cm/day); for the isotropically consolidated samples, either a comparable time was allowed or the pore fluid out-flow rate was m o n i t o r e d to establish the completion of primary consolidation. After the consolidation of the slurry was complete in both cases, the sample was allowed access to its respective pore fluid as the load was removed incrementally, following the loading path in reverse and equilibrating after each increment. R ebound with access to the emitted pore fluid prevents the d e v e l o p m e n t of excessive soil suctions and the associated disruptive influence on the fabric of the sample. All specimens had a degree o f saturation (ratio of the volume of water to the volume of voids in the soil) on the order of 98--100%. Although other investigators have p r o d u c e d small-size specimens with controlled fabrics, the techniques utilized herein were successful in producing extreme, as well as intermediate, fabric samples in sufficiently large sizes so that a num ber of conventionalsize compression or consolidation specimens could be trimmed w i t h o u t e n co u n ter in g b o u n d a r y disturbances. METHODS OF FABRIC ANALYSES Various techniques, bot h direct and indirect, have been developed to allow the qualitative and quantitative investigation of clay fabric; those e m p l o y e d in this work are scanning electron microscopy, polarized light
23 microscopy, and X-ray diffractometry. Since each of these techniques has its advantages and disadvantages, the combined use of all three seems essential to obtain a reasonably comprehensive appraisal of the clay fabric.
Scanning electron microscopy The scanning electron microscope provides a three
Optical microscopy Polarized-light microscopy is an indirect m e t h o d in which fabric interpretations are based on the properties of the transmitted polarized light that passes through a clay section. It gives the average effect of a group of particles in a certain area, and, while suffering from some of the disadvantages of electron microscopy, it has the advantage of revealing the variabilities that occur above the submicroscopic level; moreover, the optical m e t h o d provides a fast and easy determination of fabric homogeneity in a sample. Although there have been attempts to quantify the results of this procedure by measuring the light intensity with a photomultiplier tube, the extension of precise single-crystal optics to a fabric consisting of many crystals requires some assumptions regarding the superposition of the individual particle effects, and these assumptions are not readily verifiable; furthermore, the quantification procedures, at present, are only applicable to monominerallic clays. The optical m e t h o d of clay-fabric evaluation is based on the fact that the refractive indices for plate-shaped clay particles are approximately equal in the directions of the long axes {a and b axes), but significantly different from that in the direction of the short axis (c axis). Hence, if a group of oriented particles is viewed under cross-polarized light by looking down the short axis, the field exhibits a uniform darkness as the sample is rotated about the short axis. Alternatively, if a group of oriented particles is viewed normal to the short axis, four stages of illumination and extinction are observed as the sample is rotated through 360 ° . If the clay particles have a random arrangement, thin-sections oriented at different angles with respect to the reference axes appear similar, and there is no position of uniform darkening of extinction; rather, a uniform distribution of illuminated spots or a uniform grayness with a constant overall intensity is observed as the microscope stage is rotated through 360 ° between the crossed polarizers.
24
X-ray diffractometry X-ray diffraction analysis is a completely indirect method for studying particle orientation; this method has the strongest potential for providing a quantitative measure of particle orientation, and it can probably be extended to multiminerallic clays. Since the X-ray diffraction method averages the effects over an area about 1 mm in diameter, it is very useful in the study of uniform fabrics, but the results are inadequate to distinguish the t y p e of microfabric units present, such as single particles versus aggregates or singleparticle fabric versus domain fabric. X-ray diffraction data for the samples in this study were obtained by use of the m e t h o d developed by Baker, Wenk, and Christie (1969) and Tullis (1971). In this method the orientation of a given particle with respect to an orthogonal coordinate system is characterized by the angles between the normal to its (001) basal plane, which is termed a pole, and a set of reference axes; the rotation of the particle a b o u t its pole is neglected. The distribution of poles or pole figure for a particular specimen was measured by use of a modified Norelco pole-figure diffractometer. Mutually orthogonal thinsections a b o u t 100 microns thick were cut from each soil sample, X-rays were transmitted through the thin-sections, and those X-rays that satisfied the Bragg condition for the particular (001) plane were received at the detector. After fixing at the 20 angle for the (001) basal plane, the thin-section was rotated about its normal and simultaneously tilted at far slower speeds about an axis perpendicular to the plane of the X-rays. The net intensity (after the background correction) is proportional to the volume integral of all the particles at that particular orientation. By use of the intensity distributions from the three orthogonal planes, a complete pole figure of the normals can be obtained. The X-ray data are expressed in terms of (1) equal-area projection plots, which combine the information from orthogonal X-ray specimens and give the complete pole figure for the basal planes of particles (from this pole figure an evaluation of the axial s y m m e t r y of the particle orientation distribution can be made); {2) plots of Q(a) versus a, where Q(a) is defined as the ratio of the number of particles with poles in the a direction to the number of particles that would be oriented with poles in the a direction in an equivalent random sample, and a is the plane angle between the reference axis and the pole projected on to a plane containing the axis of symmetry; and (3) plots of N{a) versus a, where N(a) gives the two-dimensional particle orientation density distribution that is obtained by projecting the intensity at any angle a for an interval da by the total density. The last two plots are obtained after ascertaining the existence of axial symmetry. The details of the first and second methods have been reported by Chawla (1973), and the third method is described more fully by Ozaydin (1974). In the case of axial symmetry, the three-dimensional particle orientation distributions are adequately represented by the two-dimensional plots of Q(a} and N(a). The basic difference between these two plots lies in the fact
25
that the normalization of the measured intensity at any angle a is achieved in Q(a) with respect to an equivalent random sample and in N(a) with respect to the total intensity obtained by integration. Therefore, the Q(a) plot for a perfectly random sample is represented by a straight line that is parallel to the a axis and has a value of unity, whereas the N(a) plot for a similar sample has a value of 1/n = 0.318, since an N(a) plot conforms to a probability density function with the area under the curve being equal to unity.
Preparation of specimens for fabric analysis Specimen preparation techniques are extremely important in fabric studies and each technique has certain inherent advantages and disadvantages; since all procedures in current use result in some disruption of the fabric to be identified, the objective becomes one of minimizing this disruption. In this work the specimens for the scanning electron microscopy studies were prepared by fracturing air-dried cubes (about 1 cm on a side) to expose two or three orthogonal surfaces; then, these specimens were vacuum desiccated for at least 24 h, and the surfaces were coated first with a carbon layer approximately 50A thick and then with a gold layer between 1,000 and 2,000 A thick, For optical microscopy and X-ray diffraction in the transmission mode, thin sections were trimmed by use of a biological microtome from carbo-wax impregnated cubes (approximately 2 cm on a side). The thin sections were about 25p thick for the optical microscopy studies and about 100p thick for the X-ray diffraction work. FABRIC ANALYSIS OF CLAY SAMPLES
The afore-mentioned three independent procedures have been used to evaluate the fabric of 8 samples of kaolinite (Hydrite 10) prepared from slurries with two different chemistries and four different consolidation stress paths (see Table II). The following sections contain descriptions of the individual samples and discussions of the factors considered during preparation.
Anisotropically consolidated samples Figures 5 through 8 give the fabric descriptions of the four anisotropically consolidated samples (DA--2, DA--17, FA--2, and FA--17). For all samples the scanning micrographs of the planes normal to the major principal consolidation stress (horizontal planes) indicate a preponderance of particle faces with only a few edges, whereas corresponding micrographs of the planes parallel to the major principal consolidation stress (vertical planes) indicate a predominance of particle edges. In the latter case there is a general tendency for the particle edges to align in the horizontal direction, which is perpendicular to the direction of the major consolidation stress; this alignment
drainage
direction of
direction of major principal c o n s o l i d a t i o n stress
Fabric r e f e r e n c e axis
isotropic
anisotropic
Consolidation stress p a t h
2.3 17.5 2.3 17.5
17.5
flocculated NaOH dispersed CaCl 2 flocculated
2.3 17.5 2.3
Maximum effective c o n s o l i d a t i o n stress (kg/cm ~)
NaOH dispersed CaCl 2
Chemistry o f slurry
248 249 261 271
253
249 260 251
slurry
54 41 64 46
48
53 43 56
after consolidation
W a t e r c o n t e n t (%)
N o t a t i o n : D = Dispersed slurry; A = A n i s o t r o p i c c o n s o l i d a t i o n ; F = F l o c c u l a t e d slurry; I = I s o t r o p i c c o n s o l i d a t i o n . N u m b e r s a f t e r l e t t e r s refer to a p p r o x i m a t e value o f m a j o r p r i n c i p a l c o n s o l i d a t i o n stress in k g / c m 2 .
DI-2 DI-17 FI-2 FI-17
FA-2 FA-17
DA-2 DA-17
Sample designation
S u m m a r y of physical c o n d i t i o n s a n d p r o p e r t i e s o f s a m p l e s
T A B L E II
bO 05
27
i,.. 0.4
Fabric Reference Axis ( a , O )
:
J. \
Ok'ecfion of MQj0r P r i n c i ~ ConlOlidaflorl
Stress
~
0
18
~iO
41
40
"rs
110
Alqlle from AI~I| of Symmetry.cl(ClOip'eel)
IS
[ q N I ~ 0 Projection
30
44
e0
lS
SO
Angle from Axle of Symmetry,e(d~lrees)
X-ray Diffraction Results I Plane Normal to Fabric Reference Axis I
I Plane Parallel to Fabric Reference Axis
Scanning Electron Micrographs
~-200 ~ Extinction
~---200 ~-~ ~llumi~tion Extinction Optical Micrographs
Fig.5. Fabric description of Sample DA-2.
Illumination
28 Fabric Reference Axis ( a - O )
:
Direction of Major Principal Consolidation Stress
@ i' \
O!
1
IB
SO
4a
eo
75
90
i
O 0~
Angle from Ax~s of Symmetry, o (d4grees)
Equal Area Projecfl~
15
30
46
40
75
eQ
A~lgkD from A x i l of Symmetry,a (degreel)
X - r a y Diffraction R e s u l t s
I Plane Normal to Fabric Reference Axis ]
i0 ~
[ Plane Parallel to Fabric Reference Axis ]
~
K
I0 ~
SO~nning E l e c t r o n M i c r o g r a p h s
J ~( Extinction
200 ~
' )4 ]llumirmtaon
J< Extinction
Optical M i c r o g r a p h s
Fig.6. F a b r i c d e s c r i p t i o n o f S a m p l e D A - 1 7 .
200 nlumination
29
Fabric Reference Axis ( a - O ) i
:
Direction of Mojor Principol Consolidotion Stress
I.o:
i
~ 0.4 0.1 !
O0
le
30
48
EO
~
OO
gO
Al~91O from Axis of Symmetry, a(d~rees)
IrqUOI A~'O0Projection
15
30
4~
eo
75
90
Angh; from Axis of $ymmetry,a [degrtas)
X - r a y Diffraction Results
] Plane Normal to Fabric Reference Axis I
~ - lO ~
i Plane Parallel to Fabric Reference Axis I
~}
~
10 ~--*l
Sc~.nJng Electron Micrographs
~,--200 ~ Extinction
nl-mJr~tton
Extinction
Optical Micrographs
Fig.7. Fabric d e s c r i p t i o n o f S a m p l e FA-2.
~-200 i~---~ Illumination
30 Fabric Reference Axis (a = O)
:
Direction of Major Princ~oal Consolidotio~ Stress s
e~
o~ ~ 0,4
,
~
\
o o
15
Angle from Axis of Symmetry, a(de~'ees)
Equal Area Projection
Io
~lo
46
80
75
l en
Aflgf4 from Axls of Symmetry,a(degre4s)
X-ray Diffraction Results
[ Plane Normal to Fabric Reference Axis [
I Plane Parallel to Fabric Reference Axis I
IO~-A
~ - - 10 ~,.--,.t Scanning Electron Micrographs
1(--2oo u, ~
Extinction
Ie 200 ~
]llumtn~tion Extinction Optical Micrographs
Fig. 8. Fabric description of Sample FA-17.
Illumination
31 is most apparent in Samples DA-17 and DA-2, which were prepared from a dispersed slurry, b u t it is also discernible in the two samples prepared from a flocculated slurry and consolidated along corresponding stress paths. These observations verify the anticipated response based upon consideration of the initial chemistry of the slurries; the face-to-face particle associations that prevail in a dispersed slurry are more conducive to producing an oriented fabric upon anisotropic consolidation, whereas the initial edge-to-face associations of a flocculated slurry tend to restrain particle orientation. These qualitative assessments from the scanning micrographs are manifested quantitatively by the X-ray diffraction results. The X-ray data are presented in three different ways. The pole figures give a 3-dimensional map of the particle orientation distributions. As expected from the axially symmetric stress and strain conditions during slurry consolidation, an examination of these figures for the four anisotropically consolidated samples indicates that axial symmetry exists in the particle orientation distributions (where the axis of symmetry or fabric reference axis is in the direction of the major principal consolidation stress). Once axial symmetry is ascertained, the data in these pole figures can be reduced to Q(a) and N(a) plots, which can be used to evaluate the degree of particle orientation. A comparison of these plots indicates that Sample DA-17 is the most highly oriented and Sample FA-2 is the least oriented; however, Sample FA-2 still exhibits more orientation than the random samples to be discussed subsequently, and it can be reasonably characterized as having an intermediate particle orientation distribution. The optical micrographs for these samples show similar qualitative characteristics with some quantitative differences in light intensities. The optical micrographs of the horizontal sections exhibit a uniform grayness in both the extinction and the illumination positions; alternatively, the vertical sections show a uniform grayness in the extinction position, and they are illuminated when the microscope stage is rotated 45 ° from the extinction position. This technique substantiates the results of the previous t w o methods, and it confirms the picture displayed by the scanning micrograph over a larger area. The difference in the extinction and illumination light intensities of the vertical sections can be taken as a measure of the degree of orientation, but a qualitative comparison of such differences does not give the same trend in the degree of orientation. For example, Sample FA-2 seems to exhibit the greatest difference in extinction-illumination intensities, b u t this apparent discrepancy is proably due to other factors, such as the thickness of the thinsection or the density of the soil (the other parameters were essentially the same for all samples). A study of the optical micrographs of the horizontal planes of Samples DA-2 and FA-17 indicates that small illuminated spots are distributed uniformly over the micrograph; this suggests the presence of small groups of aligned particle edges uniformly distributed throughout the section. However, in the case of the vertical planes the particle edges are overwhelmingly aligned in the same direction, as indicated by the contrast in the light intensities between the illumination and the extinction positions.
32
Isotropically consolida ted samples Figures 9 through 12 give the fabric descriptions of the four isotropically consolidated samples (DI-2, DI-17, FI-2, and FI-17). Scanning micrographs of both the planes normal to the general direction of drainage (the horizontal planes) and the planes parallel to the general direction of drainage (vertical planes) show an essentially equal distribution of particle faces and particle edges, and there is no discernible alignment of particle edges in either plane for any of the four samples. However, it is possible to delineate particle groupings (termed domains) within which particles tend to group with some alignment, but the overall distribution of the domains (whether they consist of faces or edges) and their orientation is random. Over larger areas this is demonstrated by the uniform distribution of the illuminated spots in all four optical micrographs, thereby indicating the tendency for small groups of aligned particle edges to be uniformly distributed over the micrograph and simultaneously reach the illumination position as the stage of the microscope is rotated. This is particularly pronounced in the case of Sample FI-2; for the other three samples the optical micrographs at both positions for the two orthogonal planes manifest a more uniform grayness, and the illuminated spots are hardly noticeable. This can be interpreted as a random distribution of edges and faces, so that at every position of microscope stage only the edges oriented in one particular direction allow the light to be transmitted, whereas the faces and the edges oriented at different directions cause extinction. As contrasted to the oriented samples, the four optical micrographs for each random sample may be regarded as qualitatively similar. In the case of the isotropically consolidated samples, the optical thin-sections were cut in three mutually perpendicular planes. Even though the micrographs of only two planes are presented, the observations were confirmed by the micrographs of the third plane. The pole figures indicate that the four samples are reasonably symmetrical a b o u t an axis. However, the direction of the axis of symmetry coincides with the general drainage direction or fabric reference axis in the case of Samples DI-2 and FI-2, but it is perpendicular to that direction in the case of Samples DI-17 and FI-17. A comparison of the Q(a) and N(a) plots suggests that all four samples have very little preferred orientation of particles, and they exhibit a generally random fabric. As noted previously, Q(a) for a perfectly random sample is equal to unity and N(a) is equal to 1/~ ~- 0.318 for all values of a.
Comparison of fabrics Fabric information from the three techniques employed suggests that there is generally a preferred particle orientation for the anisotropically consolidated samples in a plane normal to the direction of the major principal consolidation stress and a random particle orientation for the isotropically consolidated samples. Furthermore, this is true regardless of the initial chemistry of the slurries. However, the effect of the initial chemistry is
33 Fabric Reference
Axis ( = , O )
: Direction of Droinoge
s 4 3 I 0.4 - 0.2 00
715
o
90
Angle from Axis of Symmotry, a(d~rNs)
Equal Area P,rojecflc~
O
II
30
4S
SO
75
110
Angle from Ax~ll of Symmetry,a(degrees)
X-ray Diffraction Results
[ Plane Normal to Fabric Reference Axis I
] Plane Parallel to Fabric Reference Axis I
Sc~.ning Electron Micrographs
1 t
~-2oo ~ --->1
Extinction
Illumination Extinction Optical Mlcrographs
Fig.9. Fabric description of Sample DI-2.
~ 2 0 0 ~ --,1 lllu~in~on
34 Fabric Reference Axis ( a = O )
:
Direction of 0roinags
6
~1 f.o
i
4
°,i
04 ~ 0.4
|°'
3
~
• Angl;
M
GO
2
~
'-
oo
18
~I0
I0
fr~
A x l l of Symmetry, a(deG'eN)
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IS
Zl~
M
SO
?l
$0
A~IIII from A]lJl of S y m m l t r y , a ( d l g r N | )
X-ray Diffraction Results
I Plane Normal to Fabric P~forenoe Axis I
[ Plane Parallel to Fabric Reference Axis I
i
~ - - 5 ~ --~
~--- 5 ~ ----~
Sc~nntng Electron Mlcrographs
g-- 200 ~ -~
~- 200 ~ -~ Extinction
Illttmlnation Extinction Optical Micrographs
Fig. 10. F a b r i c d e s c r i p t i o n o f S a m p l e DI-17.
Illumination
35
Fobric Reference Axis ( o = 0 )
:
Olrection of Droinoge A
hO:
8
4
I °"
o~1
S
0.2
t
O0
IS
30
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70
0 0
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Angle from Axis of Symmetry, a(degrm)
18
30
48
SO
~
90
Anglo from Ax6m o f Symme~ry,a(dlgroem)
ESUOl ,kre4 Projection
X-ray Diffraction Results
i Plane
Normal to Fabric Reference Axis
i
Plane Parallel to Fabric Reference Axis i
SO)nning Electroa Micrographs
1 i
Extinction
2o0 ~-~ Illumination Extinction Optical Micrographs
Fig.11. Fabric description of Sample FI-2.
200 ~ --~ Illumination
36
Fabric Referer, ce Axis ( a = O )
: Direction of Drainage
1 °" 04
J
0 0
IS
30
445
ID
7S
0 0
tO
'
Angle from A x l t of Symmetry,o(degrees)
[qugl Are4 I~*oJecttcm
IS
SO
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I0
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lO
Angle from Axle of Symmetry,a(~Qrees)
X-ray Diffraction Results [ Plane Normal to Fabric Reference Axis ]
[ Plane Parallel to Fabric Reference Axis ]
Scanning Electron Micrographs
200 ~ ~
Extinction
i..-2oo ~ -~
Illumination Extinction Optical Mlcrographs
Fig.12. F a b r i c d e s c r i p t i o n o f S a m p l e FI-17.
Illumination
37 manifested in the degree of orientation, with the dispersed samples having higher degrees of orientation than the flocculated ones for a given consolidation stress path. This is attributed to the initial presence of more face-toface associations in the dispersed slurries than in the flocculated ones. The degree of particle orientation tends to increase with increasing values of the major principal consolidation stress. Particles b e c o m e oriented in anisotropic consolidation at an effective vertical stress of approximately 2 kg/cm 2 and perhaps lower, even if they are initially flocculated. Although the dispersed samples yield a highly uniform fabric at the microscopic and submicroscopic levels, the flocculated samples indicate a somewhat nonuniform fabric at the submicroscopic level, as evidenced b y the scanning micrographs, b u t a more uniform fabric at the microscopic level, as indicated by the optical micrographs. Although data from all three fabric techniques demonstrate rather convincingly that the isotropically consolidated samples have a highly random fabric, minor deviations from perfect randomness are identifiable by the X-ray technique; however, even these deviations appear to be random. For example, based on the Q(a) plots for the isotropically consolidated samples, two of the samples (DI-2 and FI-2) exhibit a slight preferred orientation which is normal to that of the other two samples (DI-17 and FI-17). These deviations are probably due to u n k n o w n conditions that prevail during the preparation of these samples. Consequently, the homogeneity of the orientation characteristics very likely occurs at a higher level for the isotropically consolidated samples than for the anisotropically consolidated samples with a noticeable grouping of particles prevailing at the submicroscopic level, as demonstrated by the scanning micrographs. SUMMARY Techniques have been developed to prepare reasonably homogeneous reproducible bulk samples of kaolinite clay with predetermined microfabrics and to reliably identify these microfabrics both quantitatively and qualitatively, and these procedures have been used to study eight laboratory prepared samples with quite diverse histories. Two of the most important parameters that influence clay fabric are the chemistry of the environment and the subsequent consolidation stress path. The dispersion and flocculation characteristics of Hydrite 10 kaolinite (which was used in this work) were investigated by tube sedimentation tests, and it was found that concentrations of 0.01 M NaOH and 0.0001 M CaC12 at 250% water content were optimum for dispersion and flocculation, respectively. Consolidation of slurries was accomplished under both isotropic and anisotropic (zero lateral strain) stress conditions. The resulting fabric of these samples was identified and evaluated by the complementary use of scanning electron microscopy, optical microscopy, and X-ray diffraction; the combined use of all three methods allowed fabric to be studied at different levels of organization and provided a reasonably comprehensive appraisal of particle associations and orientation.
38 Based o n d a t a f r o m the 8 samples p r e p a r e d and e x a m i n e d in this p r o g r a m , it is seen t h a t a n i s o t r o p i c a l l y c o n s o l i d a t e d samples e x h i b i t e d a highly o r i e n t e d fabric, whereas isotropically c o n s o l i d a t e d samples t e n d e d to manifest a m o r e r a n d o m fabric. F o r the a n i s o t r o p i c a l l y c o n s o l i d a t e d samples orientation t e n d s to increase with increasing m a g n i t u d e s o f the m a j o r principal c o n s o l i d a t i o n stress, o r i e n t a t i o n is greater in samples p r e p a r e d f r o m dispersed slurries t h a n f r o m f l o c c u l a t e d slurries, and considerable preferred o r i e n t a t i o n o f particles is i n t r o d u c e d at low values (as low as 2 k g / c m 2 ) o f the m a j o r principal c o n s o l i d a t i o n stress. F o r the i s o t r o p i c a l l y c o n s o l i d a t e d samples the fabric is relatively r a n d o m with a slight p r e f e r r e d o r i e n t a t i o n p r o b a b l y being caused by u n k n o w n c o n d i t i o n s t h a t prevailed during the p l a c e m e n t o f the slurry. The presence o f small g r o u p s o f aligned particles is suggested in certain samples, especially in those c o n s o l i d a t e d isotropically. ACKNOWLEDGEMENTS This s t u d y was s u p p o r t e d in large part by the N a t i o n a l Science F o u n d a t i o n u n d e r G r a n t G K - 1 8 9 4 5 and by a N A T O Fellowship to Dr. T u n c e r Edil f r o m the Scientific and Technical Research Council o f T u r k e y . The X-ray diffraction w o r k was p e r f o r m e d by Dr. Kanwarjit S. Chawla with the help and coo p e r a t i o n o f Dr. David W. Baker at the University o f Illinois at Chicago Circle. REFERENCES Baker, D. W., Wenk, A. R. and Christie, J. M., 1969, X-Ray Analysis of preferred orientation in fine grained quartz aggregates. J. Geol., 77: 144--172. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. J Wiley, New York, N.Y., 000 pp. Chawla, K. S., 1973. Effect of Fabric on Creep Response of Kaolinite Clay. Thesis, Northwestern Univ., Dep. Civil Eng., Evanston, Ill. Edil, T. B., 1973. Influence of Fabric and Soil-Water Potential on Stress-Strain Response of Clay. Thesis, Northwestern Univ., Dep. Civil Eng., Evanston, Ill. Gillott, J. E., 1968. Clay in Engineering Geology. Elsevier, Amsterdam, 296 pp. Gillott, J. E., 1969. Study of fabric of fine grained sediments with the scanning electron microscope. J. Sediment. Petrogr., 35: 408--414. Lambe, T. W., 1953. The structure of inorganic soils. J. Soil Mech. Found. Div., ASCE, 79 (315): 1--49. Martin, R. T., 1965. Quantitative fabric of consolidated kaolinite, MITRes. Rept., Soils Publ., No. 179. Mitchell, J. K., 1956. The fabric of natural clays and its relation to engineering properties. Proc. Highway Res. Board, 35:693--713. Morgenstern, N. R. and Tchalenko, J. S., 1967. The optical determination of preferred orientation in clays and its application to the study of microstructure in consolidated Kaolin I and II. Proc. R. Soc. Lond., A-300:218--250. O'Brien, N. R and Harrison, W., 1969. Fabric of a non-fossile Pleistocene clay. Naturwissenschaften, 56:135--136. Ozaydin, I. K., 1974. A Micro-Mechanistic Analysis for Creep Response of Kaolin Clay. Thesis, Northwestern Univ. Dept. Civil Eng., Evanston, Ill. Rosenqvist, I. T., 1962. The influence of physico-chemical factors upon the mechanical properties of clays. Clays and Clay Minerals, Proc. Ninth Natl. Conf. on Clays and Clay Minerals, pp.12--27. Sheeran, D. E. and Krizek, R. J., 1971. Preparation of homogeneous soil samples by slurry consolidation. J. Mater. JMLSA, 6(2):356--373. Tullis, T. E., 1971. Experimental Development of Preferred Orientation of Mica During Recrystallization. Thesis, Univ. Calif., Los Angeles, Calif.
PREPARATION AND IDENTIFICATION OF CLAY SAMPLES WITH CONTROLLED FABRIC
RAYMOND J. KRIZEK, TUNCER B. EDIL and I. KUTAY OZAYDIN
The Technological Institute, Department of Civil Engineering, Northwestern University, Evanston, Ill. (U.S.A.) Department of Civil Engineering, University of Wisconsin, Madison, Wisc. (U.S.A.) Dames and Moore, Park Ridge, Ill. (U.S.A.) (Received December 5, 1973; revised and accepted July 23, 1974)
ABSTRACT Krizek, R. J., Edil, T. B. and Ozaydin, I. K., 1975. Preparation and identification of clay samples with controlled fabric. Eng. Geol., 9: 13--38. Techniques have been developed to prepare reasonably homogeneous, reproducible bulk samples of a kaolinite clay (Hydrite 10) with predetermined microfabrics and to reliably identify these microfabrics both quantitatively and qualitatively. Eight samples with quite diverse histories were produced in the laboratory by controlling the chemstry of the clay--water system, the consolidation stress path (either isotropic or anisotropic), and the magnitude of the consolidation stresses. The fabrics of these samples are identified and quantified by the combined use of scanning electron microscopy,optical microscopy, and X-ray diffractometry, and reasonably comprehensive appraisals of particle associations and orientation are obtained. Anisotropic consolidation was found to induce a preferred particle orientation, whereas isotropic consolidation tended to provide basically random samples. The anisotropically consolidated samples from dispersed slurries exhibited somewhat greater particle orientation than those from flocculated slurries, and, although considerable particle orientation occurred at low values of the consolidation stress, increases in the major principal consolidation stress did accentuate the particle orientation. The presence of domains or small groups of particles is suggested in certain samples, especially in those consolidated isotropically.
INTRODUCTION
The influence of microstructure on the macroscopic behavior of clays has long been recognized, and the problems associated with understanding this complex relationship have challenged researchers for several decades. Much of the difficulty involved in developing such an understanding has stemmed from the inability to systematically and quantitatively isolate and evaluate the various phenomena that contribute to the overall response. More specifically, the problem of (a) preparing reproducible bulk samples (from which specimens for various engineering tests can be trimmed) with a predetermined microfabric, and (b) reliably identifying both quantitatively and
14 qualitatively the fabric of these samples has proven to be an awesome task; these two aspects will be addressed in this work. FABRIC AND STRUCTURE As used herein, fabric is defined as the geometrical aspects of particle arrangements, and structure is the combination of fabric and the associated interparticle forces between adjacent particles. Since structure is extremely difficult to measure and quantify whereas fabric is more amenable to measurement and quantification, and since much can be inferred about structure from its associated fabric, the study of clay fabric constitutes a most important step in understanding the behavior of clays. However, soil fabric is not concerned with only the spatial arrangement of simple, discrete particles, but also with the arrangement of c o m p o u n d particles; consequently, the levels and units of fabric organization become important. Herein~ fabric is viewed at two general levels, namely, macrofabric and microfabric. Macrofabric refers basically to the fabric units which are observable by the unaided eye or by a low magnification microscope; terms like peds, crumbs, and aggregates have been used to describe these macroscopic units. For example, a p e d is defined as a soil aggregate consisting of a cluster of particles and separated from each other by surfaces of weakness, such as fissures, voids, or skins of different composition material (Brewer, 1964). Microfabric consists of units that are observable only under either an optical or an electron microscope. In this study emphasis is placed on this level of fabric, and the term "fabric" will be used to denote primarily microfabric. At this level units may range from single particles acting independently to domains which consist of two or more particles acting as a unit. In general, the levels of fabric are not sharply separated, but there may exist a rather continuous hierarchy of units ranging from single particles to domains to various size peds (primary, secondary, and tertiary). In microfabric analysis we are concerned with determining the geometrical aspects associated with particle arrangements; these are the orientation characteristics, particle shape and size distribution, porosity (or void ratio), and the distribution of pore sizes. The first of these parameters is a directional parameter, whereas the rest are scalar quantities. Particle orientation is defined by the angular relationship between the linear or planar elements (clay particles) and a set of chosen reference axes, and in this study fabric may further be considered essentially s y n o n y m o u s with orientation, since the other parameters, with the exception of porosity, were maintained nearly constant. The terms oriented and random are very important in describing the orientation characteristics of a clay. When there is parallel orientation between linear or planar elements, they are said to possess preferred orientation or to be oriented. The term random implies that the probability of particle orientation is equal in all directions and that a preferred direction of orientation does not exist. Most clays have fabrics which range between perfectly oriented and perfectly random.
15 CLAY INVESTIGATED Since the purpose of this work is to prepare reasonably homogeneous bulk samples with varying microfabrics and to identify the resulting fabrics, the factors controlling the selection of theclay to be studied are primarily (a) its suitability to induce various fabrics, and (b) the availability of procedures to identify and measure the resulting fabric. There has been developed within the last few years a considerable background of experience and technique for controlling and identifying the fabric of kaolinite; accordingly, kaolinite was selected as the clay mineral to be used in this investigation. However, the commercially available kaolinites, even though they are mineralogically the same, differ from each other in certain properties which are important in controlling fabric; some of these properties are grain-size distribution, grain morphology, nature of exchangeable ions, and presence of cementing agents. Three Georgia kaolinites (Hydrite R, Hydrite 10, and Hydrite UF) were investigated for possible use in this work. Hydrite R contains a b o u t 20% particles that are larger than 2 microns; these often form units consisting of stacks of platelets {also called crystallites) which exhibit a generally cubical shape (see Fig.lb), as opposed to the platelet shape of individual kaolinite particles (see Fig.la), and they are not amenable to inducing orientation. Furthermore, the presence of silt-size grains causes local disturbances in the overall fabric. As a consequence of these considerations and after some preliminary testing, Hydrite R was discarded as unsuitable for this investigation. Hydrite UF {ultra-fine) is available in very small particle sizes, but it is difficult to induce orientation because of such small sizes with relatively small differences in dimensions. Furthermore, Hydrite UF behaves much differently from most other kaolinites owing to its high specific surface and associated increased colloidal effects. Therefore, Hydrite 10 was selected as possessing the most desirable characteristics from the fabric point of view; this clay has the following physical properties: liquid limit, 62%; plastic limit, 34%; specific gravity, 2.64; silt fraction, 4%; and clay fraction (< 2 microns), 96%. SAMPLE PREPARATION Although the methods of sample preparation described herein are not intended to model or duplicate the soil-making processes of nature, they do produce a range of fabrics which includes the fabric of most naturally occurring sedimentary clays. Attention is given to the conditions which exist during the formation of sedimentary deposits, since the environmental conditions during deposition and the post-depositional load history play an important role in controlling the resultant fabric of sedimentary clays (Gillott, 1968). The fabric of clay deposited from an aqueous suspension is not only affected by the movements of the flowing medium, but also by the composition of the fluid phase, which determines the flocculation state of the clay minerals; also, the nature of the settling units, whether single particles or flocs, is important. As the overburden load increases, the water
16
5~
(a) H y d r i t e 10
2~ J
I
(b) H y d r i t e R Fig.1. Particle morphology of two hydrite kaolinites
17 content and the void ratio decrease, and this process of compression is accompanied by structural, physical, chemical, and possibly mineralogical changes in the soil skeleton and its included pore fluid. Accordingly, these two important factors (namely, the pore-fluid chemistry of the slurry and the stress path followed during the slurry consolidation) are used to produce samples with desired fabrics.
Dispersion and flocculation characteristics The dispersion of a kaolinite suspension is enhanced by a low cation concentration in the pore fluid, monovalent ions, a high pH to prevent positive charges on the edges of kaolinite particles, and a high water content to increase interparticle distances. When any of these conditions is n o t satisfied, the repulsive force regime is altered, the attraction becomes stronger between negative and positive double layers, which are associated with the faces and the edges of particles, respectively, and flocculation occurs. For the kaolinite used in this work, it was found that a critical pH less than approximately 4 was conducive to flocculation. Since the magnitude and nature of particle associations in the slurry state, whether in the field or in the laboratory, depend primarily on the clay mineralogy and the species and concentrations of ions present in the pore fluid, a measure of the relative effectiveness of different electrolytes with respect to floccultion or dispersion of a certain suspension was obtained from a series of sedimentation tests which were" c o n d u c t e d in test tubes on suspensions of the actual material used for sample preparation. Various amounts of an electrolyte were added to a specified a m o u n t of the clay, and the contents were mixed by shaking; then, the tubes were allowed to stand for approximately one week during which time the contents were observed visually and the thicknesses of the accumulated sediments were measured. In this way, the o p t i m u m flocculation or dispersion concentrations were b o u n d e d b e t w e e n upper and lower limits. This range was further narrowed in a second series of tests in which the concentrations were varied in smaller increments between the previously established limits. Although the results of these tube sedimentation tests are considered to have only relative significance, they allow a direct comparison of concentrations of different electrolytes to obtain the o p t i m u m flocculation or dispersion characteristics of a particular clay suspension with a specified clay concentration. In the dispersion tests the time to flocculation is a measure of the strength of the dispersion, that is, the magnitude of the repulsive force regime; longer times to flocculation are indicative of stronger repulsive forces. The appropriate concentration of dispersant is the one which maximizes the face-to-face particle associations. In the flocculation tests the volume of the sediment is used as a measure of effectiveness, since random particle associations yield an open structure with associated large volumes. Table I gives the dispersion characteristics of both Hydrite 10 and Hydrite R with NaOH solution; results are given in terms of the height ratio of the
18 TABLE I Dispersion characteristics of Hydrite 10 and Hydrite R Concentration of NaOH (moles/l) in solution
0 0.005 0.010 0.020 0.050
Ratio of height of supernatant to height of total suspension (1.5 g clay per 9.0 g solution) Hydrite 10
Hydrite R
1 day
2 days
0.142 0.185 0 0 0
0.175 0.164 0.040 0.062 0.059
1 week 0.148 0.246 0.097 0.031 0.110
1 day
2 days
1 week
0.091 0.055 0 0 0
0.127 0.154 0.076 0.060 0.029
0.058 0.698 0.092 0.061 0.156
supernatant solution to the total suspension at different times. In the case of both kaolinites the following observations can be made. T he 0.005 M NaOH suspension settled at the end of one day with a clear supernatant solution and sediment; however, the o t h e r three suspensions form ed very slight amounts of clear supernatant solutions and remained in suspension with density increasing with depth w i t h o u t a clearly defined sediment surface, even at the end of a week. Fr om these observations it can be concluded that the 0.005 M NaOH solution does n o t yield satisfactory dispersion, probably due to the insufficient supply o f OH ions to offset the attractive force regime, Although there is some difference bet w een the results for the ot her three concentrations, these differences are somewhat irregular and n o t considered significant. The observed t e n d e n c y for a decrease in the dispersion characteristics after one week with an increase in concent rat i on b e y o n d some o p t i m u m is pr oba bl y due to the fact t h at flocculating effects of the Na cation begin to overcome the dispersing effects of the OH group. Previous experience showed t hat high concentrations of dispersants tended to yield a high degree o f sensitivity in the consolidated blocks. With this fact in mind, the lowest o f the three concentrations t hat caused sufficient dispersion (0.01 M NaOH) was selected as the o p t i m u m dispersion c o n c e n t r a t i o n for b o th kaolinites and used in the preparation of dispersed slurries for slurry consolidation. This c o n c e n t r a t i o n corresponds to a pH o f 6 (which is above the critical pH o f 4) at the water c o n t e n t of 250%. Figure 2 illustrates the flocculation characteristics of the two types of clays with NaC1 and CaC12 ; results are expressed in terms of the height ratio o f the sediment to the total suspension as a function o f time for a period of one week. These data are summarized in Fig.3, where the same ratio is p l o tted versus the salt c o n c e n t r a t i o n at the end o f one week, when sedimentation practically reached equilibrium. An examination of these results indicates th at the 0.0001 M CaCl2 and the 0.01 M NaC1 concentrations for Hydrite 10 and the 0.001 M CaC12 and the 0.001 M NaC1 concent rat i ons for H y d r i t e R result in the largest normalized volume of sediment. T he decrease in sediment volume with high salt-concentration (2 M NaC1 or 2 M CaC12 ), as co mp ar e d with the sediment volume at lower concentrations, is
19
& 2M NoCI v O.IM NoCl
= O.OIM NoCl=
1.0
I Hydrite I0
l~L e-
Distilled° O.O01MwaterNaCI
L I t' 2M CoCle O.OIM CoCIz =O,O001MCoCl= I O.IM CoCI= ~ QOOIMCoCII • Distilled Water I Hydrite I0
0.9
E ~ D
0.8
O
~-
0.7
-
-
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/
o
/
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.,dr.e.
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~\
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I
0.6
r 0
<
2
4
r 6
-8
Oe ~ IOxlO 3
0
2
4
6
Elapsed Time (minutes) Fig.2. Flocculation characteristics of two hydrite kaolinites. 1.0
t
"5§
r----
+
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•"r 0 E ~ ~ ~
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j
L
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!
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I0 o
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Concentration of Solution (moles/liter)
Fig.3. Effect of salt concentration on equilibrium sediment volume of two hydrite kaolinites.
I0 x 103
20 believed to be due to the face-to-face aggregation of particles, which yields a " b o o k - h o u s e " particle arrangement wherein several particles aggregate faceto-face, but the aggregates exhibit a random association with each other. Suspensions with distilled water flocculate and give a height ratio lower than the height ratio in the case of the optimum concentration suspensions of Hydrite 10 with NaC1 and CaC12. However, in the case of Hydrite R, a distilled-water suspension had a height ratio higher than those of all saltflocculated suspensions with both NaC1 and CaC12. Flocculation of suspensions prepared with distilled water is basically due to the low pH. The pH of the Hydrite 10--distilled-water suspension at a b o u t 250 percent water content is 3.5 (the critical pH is 4). Figure 4 shows scanning electron micrographs of the final sediments of the Hydrite 10 suspensions prepared at the o p t i m u m dispersion and optimum flocculation concentrations, as well as that prepared with distilled water. The dominant face-to-face association of particles is very clearly illustrated in Fig.4a, which shows the sediment obtained from the dispersed slurry. However, due to the nature of the forces acting during the placement and drying processes, all particles do not necessarily orient in a certain direction. In the case of the sediments from the flocculated suspensions (due either to the low salt concentration or the low pH), the particles do not exhibit the dominant face-to-face associations illustrated in Fig.4a, but rather they exhibit random associations, as indicated in Figs.4b and 4c.
Effects of stress history One of the most important factors causing post
5~
I
I
5~
I
(b) From Salt-Flocculated Slurry
Fig.4. Scanning electron micrographs of Hydrite 10 sediments.
(a) From Dispersed Slurry
I~
5~
I
t~
(c) From Distilled Water Slurry
I
22
Techniques to control stress conditions during slurry consolidation Two techniques were developed to control the stress conditions during the process o f slurry consolidation. The anisotropic slurry consolidometers (Sheeran and Krizek, 1971) consist basically of rigid cylindrical chambers (20 cm in diameter and 50 cm in height) in which vertical stresses were applied by means of a piston, and vertical drainage was provided by porous stones located in the piston and base; at the end of anisotropic consolidation, a relativly h o m o g e n e o u s block of soil 20 cm in diameter and 10--15 cm in height was obtained. The isotropic slurry consolidometers (Edil, 1973) involve the use of double flexible rubber membranes filled with slurry to form a sphere (about 25 cm in diameter) which was floated in a liquid of slightly higher density within a pressure chamber, loaded hydrostatically, and drained at two diametrically opposite points; at the end of isotropic consolidation, a sphere of soil a p p r o x i m a t e l y 15--20 cm in diameter was obtained. In both systems, a back-pressure of 7 kg/cm 2 was applied to the pore fluid of the slurry to ensure saturation. Slurries were stored for a b o u t a m o n t h after mixing the kaolinite pow der with the appropriate fluid in order to achieve equilibrium in the distribution of the fluid and cations t h r o u g h o u t the slurry. Subsequently, the slurry was de-aired under vacuum for a b o u t 24 h before it was placed into the consolidometers. The stresses were applied incrementally, starting with 0.07 kg/cm 2 and doubling each previous load. In the case of anisotropically consolidated samples each increment was maintained until the com pl e t i on of primary consolidation or until the rate of vertical d e f o r m a t i o n became very slow (less than 0.25 cm/day); for the isotropically consolidated samples, either a comparable time was allowed or the pore fluid out-flow rate was m o n i t o r e d to establish the completion of primary consolidation. After the consolidation of the slurry was complete in both cases, the sample was allowed access to its respective pore fluid as the load was removed incrementally, following the loading path in reverse and equilibrating after each increment. R ebound with access to the emitted pore fluid prevents the d e v e l o p m e n t of excessive soil suctions and the associated disruptive influence on the fabric of the sample. All specimens had a degree o f saturation (ratio of the volume of water to the volume of voids in the soil) on the order of 98--100%. Although other investigators have p r o d u c e d small-size specimens with controlled fabrics, the techniques utilized herein were successful in producing extreme, as well as intermediate, fabric samples in sufficiently large sizes so that a num ber of conventionalsize compression or consolidation specimens could be trimmed w i t h o u t e n co u n ter in g b o u n d a r y disturbances. METHODS OF FABRIC ANALYSES Various techniques, bot h direct and indirect, have been developed to allow the qualitative and quantitative investigation of clay fabric; those e m p l o y e d in this work are scanning electron microscopy, polarized light
23 microscopy, and X-ray diffractometry. Since each of these techniques has its advantages and disadvantages, the combined use of all three seems essential to obtain a reasonably comprehensive appraisal of the clay fabric.
Scanning electron microscopy The scanning electron microscope provides a three
Optical microscopy Polarized-light microscopy is an indirect m e t h o d in which fabric interpretations are based on the properties of the transmitted polarized light that passes through a clay section. It gives the average effect of a group of particles in a certain area, and, while suffering from some of the disadvantages of electron microscopy, it has the advantage of revealing the variabilities that occur above the submicroscopic level; moreover, the optical m e t h o d provides a fast and easy determination of fabric homogeneity in a sample. Although there have been attempts to quantify the results of this procedure by measuring the light intensity with a photomultiplier tube, the extension of precise single-crystal optics to a fabric consisting of many crystals requires some assumptions regarding the superposition of the individual particle effects, and these assumptions are not readily verifiable; furthermore, the quantification procedures, at present, are only applicable to monominerallic clays. The optical m e t h o d of clay-fabric evaluation is based on the fact that the refractive indices for plate-shaped clay particles are approximately equal in the directions of the long axes {a and b axes), but significantly different from that in the direction of the short axis (c axis). Hence, if a group of oriented particles is viewed under cross-polarized light by looking down the short axis, the field exhibits a uniform darkness as the sample is rotated about the short axis. Alternatively, if a group of oriented particles is viewed normal to the short axis, four stages of illumination and extinction are observed as the sample is rotated through 360 ° . If the clay particles have a random arrangement, thin-sections oriented at different angles with respect to the reference axes appear similar, and there is no position of uniform darkening of extinction; rather, a uniform distribution of illuminated spots or a uniform grayness with a constant overall intensity is observed as the microscope stage is rotated through 360 ° between the crossed polarizers.
24
X-ray diffractometry X-ray diffraction analysis is a completely indirect method for studying particle orientation; this method has the strongest potential for providing a quantitative measure of particle orientation, and it can probably be extended to multiminerallic clays. Since the X-ray diffraction method averages the effects over an area about 1 mm in diameter, it is very useful in the study of uniform fabrics, but the results are inadequate to distinguish the t y p e of microfabric units present, such as single particles versus aggregates or singleparticle fabric versus domain fabric. X-ray diffraction data for the samples in this study were obtained by use of the m e t h o d developed by Baker, Wenk, and Christie (1969) and Tullis (1971). In this method the orientation of a given particle with respect to an orthogonal coordinate system is characterized by the angles between the normal to its (001) basal plane, which is termed a pole, and a set of reference axes; the rotation of the particle a b o u t its pole is neglected. The distribution of poles or pole figure for a particular specimen was measured by use of a modified Norelco pole-figure diffractometer. Mutually orthogonal thinsections a b o u t 100 microns thick were cut from each soil sample, X-rays were transmitted through the thin-sections, and those X-rays that satisfied the Bragg condition for the particular (001) plane were received at the detector. After fixing at the 20 angle for the (001) basal plane, the thin-section was rotated about its normal and simultaneously tilted at far slower speeds about an axis perpendicular to the plane of the X-rays. The net intensity (after the background correction) is proportional to the volume integral of all the particles at that particular orientation. By use of the intensity distributions from the three orthogonal planes, a complete pole figure of the normals can be obtained. The X-ray data are expressed in terms of (1) equal-area projection plots, which combine the information from orthogonal X-ray specimens and give the complete pole figure for the basal planes of particles (from this pole figure an evaluation of the axial s y m m e t r y of the particle orientation distribution can be made); {2) plots of Q(a) versus a, where Q(a) is defined as the ratio of the number of particles with poles in the a direction to the number of particles that would be oriented with poles in the a direction in an equivalent random sample, and a is the plane angle between the reference axis and the pole projected on to a plane containing the axis of symmetry; and (3) plots of N{a) versus a, where N(a) gives the two-dimensional particle orientation density distribution that is obtained by projecting the intensity at any angle a for an interval da by the total density. The last two plots are obtained after ascertaining the existence of axial symmetry. The details of the first and second methods have been reported by Chawla (1973), and the third method is described more fully by Ozaydin (1974). In the case of axial symmetry, the three-dimensional particle orientation distributions are adequately represented by the two-dimensional plots of Q(a} and N(a). The basic difference between these two plots lies in the fact
25
that the normalization of the measured intensity at any angle a is achieved in Q(a) with respect to an equivalent random sample and in N(a) with respect to the total intensity obtained by integration. Therefore, the Q(a) plot for a perfectly random sample is represented by a straight line that is parallel to the a axis and has a value of unity, whereas the N(a) plot for a similar sample has a value of 1/n = 0.318, since an N(a) plot conforms to a probability density function with the area under the curve being equal to unity.
Preparation of specimens for fabric analysis Specimen preparation techniques are extremely important in fabric studies and each technique has certain inherent advantages and disadvantages; since all procedures in current use result in some disruption of the fabric to be identified, the objective becomes one of minimizing this disruption. In this work the specimens for the scanning electron microscopy studies were prepared by fracturing air-dried cubes (about 1 cm on a side) to expose two or three orthogonal surfaces; then, these specimens were vacuum desiccated for at least 24 h, and the surfaces were coated first with a carbon layer approximately 50A thick and then with a gold layer between 1,000 and 2,000 A thick, For optical microscopy and X-ray diffraction in the transmission mode, thin sections were trimmed by use of a biological microtome from carbo-wax impregnated cubes (approximately 2 cm on a side). The thin sections were about 25p thick for the optical microscopy studies and about 100p thick for the X-ray diffraction work. FABRIC ANALYSIS OF CLAY SAMPLES
The afore-mentioned three independent procedures have been used to evaluate the fabric of 8 samples of kaolinite (Hydrite 10) prepared from slurries with two different chemistries and four different consolidation stress paths (see Table II). The following sections contain descriptions of the individual samples and discussions of the factors considered during preparation.
Anisotropically consolidated samples Figures 5 through 8 give the fabric descriptions of the four anisotropically consolidated samples (DA--2, DA--17, FA--2, and FA--17). For all samples the scanning micrographs of the planes normal to the major principal consolidation stress (horizontal planes) indicate a preponderance of particle faces with only a few edges, whereas corresponding micrographs of the planes parallel to the major principal consolidation stress (vertical planes) indicate a predominance of particle edges. In the latter case there is a general tendency for the particle edges to align in the horizontal direction, which is perpendicular to the direction of the major consolidation stress; this alignment
drainage
direction of
direction of major principal c o n s o l i d a t i o n stress
Fabric r e f e r e n c e axis
isotropic
anisotropic
Consolidation stress p a t h
2.3 17.5 2.3 17.5
17.5
flocculated NaOH dispersed CaCl 2 flocculated
2.3 17.5 2.3
Maximum effective c o n s o l i d a t i o n stress (kg/cm ~)
NaOH dispersed CaCl 2
Chemistry o f slurry
248 249 261 271
253
249 260 251
slurry
54 41 64 46
48
53 43 56
after consolidation
W a t e r c o n t e n t (%)
N o t a t i o n : D = Dispersed slurry; A = A n i s o t r o p i c c o n s o l i d a t i o n ; F = F l o c c u l a t e d slurry; I = I s o t r o p i c c o n s o l i d a t i o n . N u m b e r s a f t e r l e t t e r s refer to a p p r o x i m a t e value o f m a j o r p r i n c i p a l c o n s o l i d a t i o n stress in k g / c m 2 .
DI-2 DI-17 FI-2 FI-17
FA-2 FA-17
DA-2 DA-17
Sample designation
S u m m a r y of physical c o n d i t i o n s a n d p r o p e r t i e s o f s a m p l e s
T A B L E II
bO 05
27
i,.. 0.4
Fabric Reference Axis ( a , O )
:
J. \
Ok'ecfion of MQj0r P r i n c i ~ ConlOlidaflorl
Stress
~
0
18
~iO
41
40
"rs
110
Alqlle from AI~I| of Symmetry.cl(ClOip'eel)
IS
[ q N I ~ 0 Projection
30
44
e0
lS
SO
Angle from Axle of Symmetry,e(d~lrees)
X-ray Diffraction Results I Plane Normal to Fabric Reference Axis I
I Plane Parallel to Fabric Reference Axis
Scanning Electron Micrographs
~-200 ~ Extinction
~---200 ~-~ ~llumi~tion Extinction Optical Micrographs
Fig.5. Fabric description of Sample DA-2.
Illumination
28 Fabric Reference Axis ( a - O )
:
Direction of Major Principal Consolidation Stress
@ i' \
O!
1
IB
SO
4a
eo
75
90
i
O 0~
Angle from Ax~s of Symmetry, o (d4grees)
Equal Area Projecfl~
15
30
46
40
75
eQ
A~lgkD from A x i l of Symmetry,a (degreel)
X - r a y Diffraction R e s u l t s
I Plane Normal to Fabric Reference Axis ]
i0 ~
[ Plane Parallel to Fabric Reference Axis ]
~
K
I0 ~
SO~nning E l e c t r o n M i c r o g r a p h s
J ~( Extinction
200 ~
' )4 ]llumirmtaon
J< Extinction
Optical M i c r o g r a p h s
Fig.6. F a b r i c d e s c r i p t i o n o f S a m p l e D A - 1 7 .
200 nlumination
29
Fabric Reference Axis ( a - O ) i
:
Direction of Mojor Principol Consolidotion Stress
I.o:
i
~ 0.4 0.1 !
O0
le
30
48
EO
~
OO
gO
Al~91O from Axis of Symmetry, a(d~rees)
IrqUOI A~'O0Projection
15
30
4~
eo
75
90
Angh; from Axis of $ymmetry,a [degrtas)
X - r a y Diffraction Results
] Plane Normal to Fabric Reference Axis I
~ - lO ~
i Plane Parallel to Fabric Reference Axis I
~}
~
10 ~--*l
Sc~.nJng Electron Micrographs
~,--200 ~ Extinction
nl-mJr~tton
Extinction
Optical Micrographs
Fig.7. Fabric d e s c r i p t i o n o f S a m p l e FA-2.
~-200 i~---~ Illumination
30 Fabric Reference Axis (a = O)
:
Direction of Major Princ~oal Consolidotio~ Stress s
e~
o~ ~ 0,4
,
~
\
o o
15
Angle from Axis of Symmetry, a(de~'ees)
Equal Area Projection
Io
~lo
46
80
75
l en
Aflgf4 from Axls of Symmetry,a(degre4s)
X-ray Diffraction Results
[ Plane Normal to Fabric Reference Axis [
I Plane Parallel to Fabric Reference Axis I
IO~-A
~ - - 10 ~,.--,.t Scanning Electron Micrographs
1(--2oo u, ~
Extinction
Ie 200 ~
]llumtn~tion Extinction Optical Micrographs
Fig. 8. Fabric description of Sample FA-17.
Illumination
31 is most apparent in Samples DA-17 and DA-2, which were prepared from a dispersed slurry, b u t it is also discernible in the two samples prepared from a flocculated slurry and consolidated along corresponding stress paths. These observations verify the anticipated response based upon consideration of the initial chemistry of the slurries; the face-to-face particle associations that prevail in a dispersed slurry are more conducive to producing an oriented fabric upon anisotropic consolidation, whereas the initial edge-to-face associations of a flocculated slurry tend to restrain particle orientation. These qualitative assessments from the scanning micrographs are manifested quantitatively by the X-ray diffraction results. The X-ray data are presented in three different ways. The pole figures give a 3-dimensional map of the particle orientation distributions. As expected from the axially symmetric stress and strain conditions during slurry consolidation, an examination of these figures for the four anisotropically consolidated samples indicates that axial symmetry exists in the particle orientation distributions (where the axis of symmetry or fabric reference axis is in the direction of the major principal consolidation stress). Once axial symmetry is ascertained, the data in these pole figures can be reduced to Q(a) and N(a) plots, which can be used to evaluate the degree of particle orientation. A comparison of these plots indicates that Sample DA-17 is the most highly oriented and Sample FA-2 is the least oriented; however, Sample FA-2 still exhibits more orientation than the random samples to be discussed subsequently, and it can be reasonably characterized as having an intermediate particle orientation distribution. The optical micrographs for these samples show similar qualitative characteristics with some quantitative differences in light intensities. The optical micrographs of the horizontal sections exhibit a uniform grayness in both the extinction and the illumination positions; alternatively, the vertical sections show a uniform grayness in the extinction position, and they are illuminated when the microscope stage is rotated 45 ° from the extinction position. This technique substantiates the results of the previous t w o methods, and it confirms the picture displayed by the scanning micrograph over a larger area. The difference in the extinction and illumination light intensities of the vertical sections can be taken as a measure of the degree of orientation, but a qualitative comparison of such differences does not give the same trend in the degree of orientation. For example, Sample FA-2 seems to exhibit the greatest difference in extinction-illumination intensities, b u t this apparent discrepancy is proably due to other factors, such as the thickness of the thinsection or the density of the soil (the other parameters were essentially the same for all samples). A study of the optical micrographs of the horizontal planes of Samples DA-2 and FA-17 indicates that small illuminated spots are distributed uniformly over the micrograph; this suggests the presence of small groups of aligned particle edges uniformly distributed throughout the section. However, in the case of the vertical planes the particle edges are overwhelmingly aligned in the same direction, as indicated by the contrast in the light intensities between the illumination and the extinction positions.
32
Isotropically consolida ted samples Figures 9 through 12 give the fabric descriptions of the four isotropically consolidated samples (DI-2, DI-17, FI-2, and FI-17). Scanning micrographs of both the planes normal to the general direction of drainage (the horizontal planes) and the planes parallel to the general direction of drainage (vertical planes) show an essentially equal distribution of particle faces and particle edges, and there is no discernible alignment of particle edges in either plane for any of the four samples. However, it is possible to delineate particle groupings (termed domains) within which particles tend to group with some alignment, but the overall distribution of the domains (whether they consist of faces or edges) and their orientation is random. Over larger areas this is demonstrated by the uniform distribution of the illuminated spots in all four optical micrographs, thereby indicating the tendency for small groups of aligned particle edges to be uniformly distributed over the micrograph and simultaneously reach the illumination position as the stage of the microscope is rotated. This is particularly pronounced in the case of Sample FI-2; for the other three samples the optical micrographs at both positions for the two orthogonal planes manifest a more uniform grayness, and the illuminated spots are hardly noticeable. This can be interpreted as a random distribution of edges and faces, so that at every position of microscope stage only the edges oriented in one particular direction allow the light to be transmitted, whereas the faces and the edges oriented at different directions cause extinction. As contrasted to the oriented samples, the four optical micrographs for each random sample may be regarded as qualitatively similar. In the case of the isotropically consolidated samples, the optical thin-sections were cut in three mutually perpendicular planes. Even though the micrographs of only two planes are presented, the observations were confirmed by the micrographs of the third plane. The pole figures indicate that the four samples are reasonably symmetrical a b o u t an axis. However, the direction of the axis of symmetry coincides with the general drainage direction or fabric reference axis in the case of Samples DI-2 and FI-2, but it is perpendicular to that direction in the case of Samples DI-17 and FI-17. A comparison of the Q(a) and N(a) plots suggests that all four samples have very little preferred orientation of particles, and they exhibit a generally random fabric. As noted previously, Q(a) for a perfectly random sample is equal to unity and N(a) is equal to 1/~ ~- 0.318 for all values of a.
Comparison of fabrics Fabric information from the three techniques employed suggests that there is generally a preferred particle orientation for the anisotropically consolidated samples in a plane normal to the direction of the major principal consolidation stress and a random particle orientation for the isotropically consolidated samples. Furthermore, this is true regardless of the initial chemistry of the slurries. However, the effect of the initial chemistry is
33 Fabric Reference
Axis ( = , O )
: Direction of Droinoge
s 4 3 I 0.4 - 0.2 00
715
o
90
Angle from Axis of Symmotry, a(d~rNs)
Equal Area P,rojecflc~
O
II
30
4S
SO
75
110
Angle from Ax~ll of Symmetry,a(degrees)
X-ray Diffraction Results
[ Plane Normal to Fabric Reference Axis I
] Plane Parallel to Fabric Reference Axis I
Sc~.ning Electron Micrographs
1 t
~-2oo ~ --->1
Extinction
Illumination Extinction Optical Mlcrographs
Fig.9. Fabric description of Sample DI-2.
~ 2 0 0 ~ --,1 lllu~in~on
34 Fabric Reference Axis ( a = O )
:
Direction of 0roinags
6
~1 f.o
i
4
°,i
04 ~ 0.4
|°'
3
~
• Angl;
M
GO
2
~
'-
oo
18
~I0
I0
fr~
A x l l of Symmetry, a(deG'eN)
[quel k'eo Pmje=tlm
IS
Zl~
M
SO
?l
$0
A~IIII from A]lJl of S y m m l t r y , a ( d l g r N | )
X-ray Diffraction Results
I Plane Normal to Fabric P~forenoe Axis I
[ Plane Parallel to Fabric Reference Axis I
i
~ - - 5 ~ --~
~--- 5 ~ ----~
Sc~nntng Electron Mlcrographs
g-- 200 ~ -~
~- 200 ~ -~ Extinction
Illttmlnation Extinction Optical Micrographs
Fig. 10. F a b r i c d e s c r i p t i o n o f S a m p l e DI-17.
Illumination
35
Fobric Reference Axis ( o = 0 )
:
Olrection of Droinoge A
hO:
8
4
I °"
o~1
S
0.2
t
O0
IS
30
48
gO
70
0 0
90
Angle from Axis of Symmetry, a(degrm)
18
30
48
SO
~
90
Anglo from Ax6m o f Symme~ry,a(dlgroem)
ESUOl ,kre4 Projection
X-ray Diffraction Results
i Plane
Normal to Fabric Reference Axis
i
Plane Parallel to Fabric Reference Axis i
SO)nning Electroa Micrographs
1 i
Extinction
2o0 ~-~ Illumination Extinction Optical Micrographs
Fig.11. Fabric description of Sample FI-2.
200 ~ --~ Illumination
36
Fabric Referer, ce Axis ( a = O )
: Direction of Drainage
1 °" 04
J
0 0
IS
30
445
ID
7S
0 0
tO
'
Angle from A x l t of Symmetry,o(degrees)
[qugl Are4 I~*oJecttcm
IS
SO
45
I0
"PJ
lO
Angle from Axle of Symmetry,a(~Qrees)
X-ray Diffraction Results [ Plane Normal to Fabric Reference Axis ]
[ Plane Parallel to Fabric Reference Axis ]
Scanning Electron Micrographs
200 ~ ~
Extinction
i..-2oo ~ -~
Illumination Extinction Optical Mlcrographs
Fig.12. F a b r i c d e s c r i p t i o n o f S a m p l e FI-17.
Illumination
37 manifested in the degree of orientation, with the dispersed samples having higher degrees of orientation than the flocculated ones for a given consolidation stress path. This is attributed to the initial presence of more face-toface associations in the dispersed slurries than in the flocculated ones. The degree of particle orientation tends to increase with increasing values of the major principal consolidation stress. Particles b e c o m e oriented in anisotropic consolidation at an effective vertical stress of approximately 2 kg/cm 2 and perhaps lower, even if they are initially flocculated. Although the dispersed samples yield a highly uniform fabric at the microscopic and submicroscopic levels, the flocculated samples indicate a somewhat nonuniform fabric at the submicroscopic level, as evidenced b y the scanning micrographs, b u t a more uniform fabric at the microscopic level, as indicated by the optical micrographs. Although data from all three fabric techniques demonstrate rather convincingly that the isotropically consolidated samples have a highly random fabric, minor deviations from perfect randomness are identifiable by the X-ray technique; however, even these deviations appear to be random. For example, based on the Q(a) plots for the isotropically consolidated samples, two of the samples (DI-2 and FI-2) exhibit a slight preferred orientation which is normal to that of the other two samples (DI-17 and FI-17). These deviations are probably due to u n k n o w n conditions that prevail during the preparation of these samples. Consequently, the homogeneity of the orientation characteristics very likely occurs at a higher level for the isotropically consolidated samples than for the anisotropically consolidated samples with a noticeable grouping of particles prevailing at the submicroscopic level, as demonstrated by the scanning micrographs. SUMMARY Techniques have been developed to prepare reasonably homogeneous reproducible bulk samples of kaolinite clay with predetermined microfabrics and to reliably identify these microfabrics both quantitatively and qualitatively, and these procedures have been used to study eight laboratory prepared samples with quite diverse histories. Two of the most important parameters that influence clay fabric are the chemistry of the environment and the subsequent consolidation stress path. The dispersion and flocculation characteristics of Hydrite 10 kaolinite (which was used in this work) were investigated by tube sedimentation tests, and it was found that concentrations of 0.01 M NaOH and 0.0001 M CaC12 at 250% water content were optimum for dispersion and flocculation, respectively. Consolidation of slurries was accomplished under both isotropic and anisotropic (zero lateral strain) stress conditions. The resulting fabric of these samples was identified and evaluated by the complementary use of scanning electron microscopy, optical microscopy, and X-ray diffraction; the combined use of all three methods allowed fabric to be studied at different levels of organization and provided a reasonably comprehensive appraisal of particle associations and orientation.
38 Based o n d a t a f r o m the 8 samples p r e p a r e d and e x a m i n e d in this p r o g r a m , it is seen t h a t a n i s o t r o p i c a l l y c o n s o l i d a t e d samples e x h i b i t e d a highly o r i e n t e d fabric, whereas isotropically c o n s o l i d a t e d samples t e n d e d to manifest a m o r e r a n d o m fabric. F o r the a n i s o t r o p i c a l l y c o n s o l i d a t e d samples orientation t e n d s to increase with increasing m a g n i t u d e s o f the m a j o r principal c o n s o l i d a t i o n stress, o r i e n t a t i o n is greater in samples p r e p a r e d f r o m dispersed slurries t h a n f r o m f l o c c u l a t e d slurries, and considerable preferred o r i e n t a t i o n o f particles is i n t r o d u c e d at low values (as low as 2 k g / c m 2 ) o f the m a j o r principal c o n s o l i d a t i o n stress. F o r the i s o t r o p i c a l l y c o n s o l i d a t e d samples the fabric is relatively r a n d o m with a slight p r e f e r r e d o r i e n t a t i o n p r o b a b l y being caused by u n k n o w n c o n d i t i o n s t h a t prevailed during the p l a c e m e n t o f the slurry. The presence o f small g r o u p s o f aligned particles is suggested in certain samples, especially in those c o n s o l i d a t e d isotropically. ACKNOWLEDGEMENTS This s t u d y was s u p p o r t e d in large part by the N a t i o n a l Science F o u n d a t i o n u n d e r G r a n t G K - 1 8 9 4 5 and by a N A T O Fellowship to Dr. T u n c e r Edil f r o m the Scientific and Technical Research Council o f T u r k e y . The X-ray diffraction w o r k was p e r f o r m e d by Dr. Kanwarjit S. Chawla with the help and coo p e r a t i o n o f Dr. David W. Baker at the University o f Illinois at Chicago Circle. REFERENCES Baker, D. W., Wenk, A. R. and Christie, J. M., 1969, X-Ray Analysis of preferred orientation in fine grained quartz aggregates. J. Geol., 77: 144--172. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. J Wiley, New York, N.Y., 000 pp. Chawla, K. S., 1973. Effect of Fabric on Creep Response of Kaolinite Clay. Thesis, Northwestern Univ., Dep. Civil Eng., Evanston, Ill. Edil, T. B., 1973. Influence of Fabric and Soil-Water Potential on Stress-Strain Response of Clay. Thesis, Northwestern Univ., Dep. Civil Eng., Evanston, Ill. Gillott, J. E., 1968. Clay in Engineering Geology. Elsevier, Amsterdam, 296 pp. Gillott, J. E., 1969. Study of fabric of fine grained sediments with the scanning electron microscope. J. Sediment. Petrogr., 35: 408--414. Lambe, T. W., 1953. The structure of inorganic soils. J. Soil Mech. Found. Div., ASCE, 79 (315): 1--49. Martin, R. T., 1965. Quantitative fabric of consolidated kaolinite, MITRes. Rept., Soils Publ., No. 179. Mitchell, J. K., 1956. The fabric of natural clays and its relation to engineering properties. Proc. Highway Res. Board, 35:693--713. Morgenstern, N. R. and Tchalenko, J. S., 1967. The optical determination of preferred orientation in clays and its application to the study of microstructure in consolidated Kaolin I and II. Proc. R. Soc. Lond., A-300:218--250. O'Brien, N. R and Harrison, W., 1969. Fabric of a non-fossile Pleistocene clay. Naturwissenschaften, 56:135--136. Ozaydin, I. K., 1974. A Micro-Mechanistic Analysis for Creep Response of Kaolin Clay. Thesis, Northwestern Univ. Dept. Civil Eng., Evanston, Ill. Rosenqvist, I. T., 1962. The influence of physico-chemical factors upon the mechanical properties of clays. Clays and Clay Minerals, Proc. Ninth Natl. Conf. on Clays and Clay Minerals, pp.12--27. Sheeran, D. E. and Krizek, R. J., 1971. Preparation of homogeneous soil samples by slurry consolidation. J. Mater. JMLSA, 6(2):356--373. Tullis, T. E., 1971. Experimental Development of Preferred Orientation of Mica During Recrystallization. Thesis, Univ. Calif., Los Angeles, Calif.