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The fabric of a clay soil under controlled mechanical and hydraulic stress states
ELSEVIER
Applied Clay Science 11 (1996) 99-115
The fabric of a clay soil under controlled mechanical and hydraulic stress states M. A1-Mukhtar a,1 N. Belanteur a,2 D. Tessier b,3 S.K. Vanapalli c,4 a University of Orleans, ESEM-CNRS, CRMD, 45072 Orleans Cedex 2, France b 1NRA, Station de Science du Sol, 78026 Versailles, France c Department of Civil Engineering, University of Saskatchewan, Saskatoon, Canada
Received 11 July 1995; revised 10 September 1996; accepted 10 September 1996
Abstract Fabric determination is fundamental to the understanding of several mechanisms controlling a clayey soil behavior. Two techniques; namely, mercury intrusion tests and transmission electron microscopy (TEM) were used to study the soil fabric of a remoulded Boom clay. The effect of overburden pressure, excavation and desaturation on the soil fabric are studied. Results demonstrate that the nature of soil fabric is dependent on both the applied mechanical and hydraulic stress state conditions. Keywords: remoulded clay; soil fabric; mechanical and hydraulic stress state; mercury porosimetric; transmission electron microscopy tests
1. Introduction O n a b r o a d scale, it is w i d e l y a c c e p t e d in the literature t h a t t h e r e are t w o l e v e l s o f soil f a b r i c f o r c l a y e y soils. T h e s e are the m a c r o l e v e l f a b r i c a n d t h e m i c r o l e v e l fabric.
i Associate Professor. 2 Research Engineer. 3 Director of Research. 4 Research Engineer. 0169-1317/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 13 1 7 ( 9 6 ) 0 0 0 2 3 - 3
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The soil microfabric is described as the elementary particle associations within the soil aggregates, whereas the arrangement of these soil aggregates is referred to as the macrofabric (Mitchell, 1976). Several parameters such as the exchange capacity, the type (nature) of cations, the size of clay particles, the stress history, the hydraulic state, the soil chemistry, and the mineral history influence both the macro- and microfabric of a clayey soil (Van Olphen, 1977; Rosenqvist, 1984; Venial, 1985; Griffiths and Joshi, 1990). Collins and McGown (1974) and Collins (1984) demonstrated, based on transmission electron microscope (TEM) and porosimetry studies on natural soils, that two principal types of microfabric dominate in clayey soils. They are the matrix microfabric integrated by elementary particle arrangement of the clay platelets and the aggregation of microfabric integrated by arrays of elementary particle configuration. Microfabric studies have demonstrated that integrated elementary particle arrangement of clay platelets has large amounts of adsorbed water in low porosity clays (Push and Carlsson, 1985; Baldi et al., 1990). The behaviour of adsorbed water and consequently that of low porosity clays is significantly altered by electro-chemical interaction (Push et al., 1991). Three different types of pore spaces were identified within the microfabric based on size classification (Pusch, 1982; Ben Rhaiem et al., 1986, Touret et al., 1990; Tessier et al., 1992). The pore space size ranging between 10 to 25 A (0.001-0.0025 /~m) is called the intra-particle pore space or inter-platelet pore space. The inter-particle pore space or intra-aggregate pore space is between 25 A to 1500 A (0.0025-0.15 /~m). Giiven et al. (1992); Giiven et al. (1992), however suggested 2000 A (0.2 /_~m)for intra-aggregate pore space. The inter-aggregates have pore sizes higher than 1500 A or 2000 A. The soil fabric is dependent on the soil-water interactions at all levels of arrangement from the elementary platelets (layers) level to the association of particles and aggregates (Aylmore and Quirk, 1959; Tessier, 1984; Giiven et al., 1992). Water in clay mineral pores is bonded to the solid mineral surface by electro-chemical forces (Rosenqvist, 1959). The physico-chemical properties are translated into different thermodynamic potentials permitting the definition of three major types of water; namely, bulk water, capillary water and bounded (adsorbed) water characterized by strong links with solid matrix. Highly compacted soils are used as barriers for nuclear waste management because of their high sorption and low hydraulic conductivity. The study of the soil fabric is useful to understand the behavior of highly compacted soils under various mechanical and hydraulic stress state conditions. Such studies provide valuable information related to the storage of high level nuclear wastes. These wastes would be placed in a disposal vault 500-1000 m underground and would be surrounded by a buffer material, likely containing clay. In this paper soil fabric of a remoulded Boom clay is described using both mercury intrusion porosimetry and transmission electron microscope (TEM) studies. The influence of various hydraulic and mechanical stress state conditions on the pore size distribution are studied. The phenomenological studies of Boom clay soil fabric provides valuable information about the natural soil behaviour. Previous studies have shown that a good agreement can be obtained between the remoulded and natural soil behaviors (Rhattas, 1994). o
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2. Materials and methods 2.1. Materials
The research study for this paper was carried out on Boom clay, a soil formation in the north-east of Belgium. Several field and laboratory studies are in progress to assess the impact of burying radioactive wastes in an underground facility. The soil properties and mineralogical composition of the Boom clay tested are summarized in Table 1. Mineralogical analyses from X-diffraction show that the coarser fractions are dominated by quartz, whereas; the finer fractions are dominated by interstratified illite-smectite, illite, mica and kaolinite. The chemical composition of the Boom clay is presented in Table 2, 2.2. M e t h o d o l o g y
The soil specimens used for testing were prepared from remoulded Boom clay. The remoulded soil was prepared adding distilled water equal to twice the liquid limit value. Both saturated and unsaturated specimens were used in the study. Saturated specimens were prepared applying high axial stress in a oedometer device. Unsaturated specimen was prepared applying a controlled suction using high axial stress and high suction oedometer device. The suction in the unsaturated specimen is achieved by indirectly controlling the relative humidity using saturated salt solutions. The oedometer device has a facility to apply loads up to 40 MPa on the soil specimens. More details of the specimen preparation and testing procedures are available in A1-Mukhtar et al. (1993). The specimen dimensions for oedometer testing were 65 m m in diameter and initial height of 65 ram.
Table 1 Soil properties and mineralogical composition of Boom clay soil tested Soil properties
Liquid limit Plastic limit Plasticity index Specific gravity Specific surface area Cation exchange capacity BET surface
70% 25% 45% 2.67 100 m 2/g 20-40 meq/100 g 42 m2/g
Mineralogical composition
Quartz Interstratified illite-smectite Illite Kaolinite Microcline Plagioclase Pyrite Carbon sulphate
20-25% 33% 16% 13% 4-5% 4-5% 4-5% traces
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Table 2 Chemical composition of Boom clay soil Composition
(%)
SiO 2 AI203 Fe203 CaO MgO Na20 K 20 MnO TiO 2 P205 Loss due to ignition
57.58 12.93 7.57 2.22 2.40 0.12 1.96 0.01 0.88 0.18 14.15
Four saturated specimens were tested for this research program. Three saturated specimens were consolidated with axial stresses of 1, 5 and 15 MPa respectively with an open drainage during testing (i.e., conventional oedometer tests). This loading condition simulates the effect of over burden pressure on the soil fabric. Four saturated specimen were tested with an axial stress of 15 MPa with an undrained condition during unloading. This testing condition simulates the behavior of soil during the excavation operation (i.e., quick unloading). The last specimen was an unsaturated specimen compacted at 15 MPa with controlled suction of 110 MPa (i.e., relative humidity of 44%). This test simulates the behavior of a buffer material around the waste packages. Such a loading condition approximates the behavior of the soil under ventilation conditions in the underground facilities. The porosity of the unsaturated specimen tested was similar to the porosity of the saturated specimen consolidated at 5 MPa. Specimens prepared in the odometer were used for the mercury-intrusion porosimetry tests and in transmission electron microscopy (TEM). For the mercury-intrusion porosimetry tests, lyophilisation technique as suggested by Tessier and Berrier (1978); Delage and Lefebvre (1984) was used in the preparation of specimens to minimize microfabric changes. Mercury intrusion tests were performed using a 'Poresizer 9320' porosimeter. Washburn (1921) equation was used for estimating entrance pore diameter of the soil. This apparatus has a maximum loading capacity of 210 MPa on the mercur), and facilitates investigation of pore diameters between 350 × 104 A ( 3 5 0 ~ m ) and 36 A (0.0036 /xm). Preparation of the specimens for TEM analysis follows the method of Tessier (1984). Specimens were carefully cut so as to preserve their orientation and placed in dissolved Agar, which helps preserve the microstructure during the ensuing solvent exchange. After solidification, the excess Agar was removed and the sample placed in a Reichert-Jung-Lynx el sample embedder. Water was first replaced by methanol, then propylene oxide, and finally Spurr's resin. The specimens were then placed in a 65°C oven for 24 h to polymerize the resin. Thin sections were then cut with a diamond knife on a Reichert-Jung Ultracut E microtome to a thickness of 50-60 nm and a width of about 100 /xm. They were then placed on collodion coated Cu grids. Observations in the TEM were carried out on the surface perpendicular to the direction
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of the applied mechanical stress. Electron micrographs were obtained on a Philips model 420 TEM.
3. Test results and analysis 3.1. Oedometer test results Fig. 1 shows the variation of void ratio with respect to axial stress o f the saturated and unsaturated specimens tested. Void ratio of saturated specimens increased during unloading (i.e., specimens swelled during unloading). In the unsaturated specimen, no swelling was observed during unloading. Therefore, the saturated specimen shows elastoplastic (partially irreversible) behavior and the unsaturated specimen shows a plastic (completely irreversible) behavior. Small modification in the void ratio is however observed during unloading for the compacted unsaturated specimen at 15 MPa without drainage condition. Table 3 summarizes the hydraulic and mechanical stress state conditions of the specimens tested. 3.2. Mercury intrusion and T E M test results 3.2.1. Comparison o f saturated specimens consolidated at 1, 5 and 15 M P a Fig. 2 shows the results obtained from mercury intrusion tests for the pore size distribution in three specimens consolidated at 1, 5 and 15 MPa. Two distinguished families of entrance pore diameters 2 0 0 - 4 0 0 A and 4 0 0 - 1 0 0 0 A were observed at 1 MPa axial stress. These pores can be categorized as intra-aggregate (inter-particle) pores as the entrance pore diameters are less than 2000 A.
1,1
[ #
0,9
specimens
~:~ Unsaturatedspectmen with controlledrelative humidityof 44 %)
.o ~
Saturated
0,7
>
o
~
...
,~
0,5 i i
I I
0,3 0,1
1
10
100
Axial stress, MPa Fig. 1. Oedometer curves for the soil specimens tested in saturated and unsaturated conditions.
M. A1-Mukhtar et al./ Applied Clay Science 11 (1996) 99-115
104
Table 3 Hydraulic and mechanical stress state conditions of tested specimens Hydraulic state of specimens and drainage conditions
Mechanical stress (MPa)
Porosity
Saturated drained Saturated drained Saturated drained Saturated but undrained during unloading Unsaturated with controlled suction of 110 MPa (relative humidity of 44%)
1 5 15 15
0,45 0.38 0.35 0.31
10
0.38
With increased applied stresses, the family of pores having high entrance pore diameter are eliminated. For stresses 5 and 15 MPa, the pore size distribution seems to be very close (i.e., there is no further reduction in entrance pore diameter for loads higher than 5 MPa). At these stresses, only one family of pore size distribution having a range of entrance pore diameter of 800 to 60 A can be identified in the specimens. Fig. 3 presents the changes in the average entrance pore diameter for the tested specimens. The relationship between the average entrance pore diameter and the logarithmic applied stress is linear. This demonstrates that the modification in the pore size distribution between 0 and 1 MPa loading is higher in comparison to 1 and 10 MPa loading. TEM images for the consolidated specimens are shown in Figs. 4-6. Fig. 4 shows that the arrangement of soil fabric is not homogeneous for the specimens consolidated at 1 MPa. Organic materials can also be observed. In Fig. 5, the specimen consolidated at 5 MPa is more homogenous in comparison to the specimen consolidated at 1 MPa. This could be attributed to the reduction in pores spaces with the higher stresses applied.
0,06 -<>- Specimen consolidated at 1 MPa ----o--Specimen consolidatcd at 5 MPa t:l Specimen consolidated at 15 MPa ~
,04
>
0,o2
0,00 ~ 10
. . . . . . . . . . . . . . . 100
1000 Entrance pore diameter (A °)
10000
Fig. 2. Pore size distribution in the saturated specimens.
100000
105
M. Al-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
400 . . . . . . .
,. . . . .
..
.....
....
.. ....
o 300 -
~ ~ 200
..... . . . .
~
i. . . . . . . .
: . . . .
?
....
]00 I0 Axial stress, M P a
Fig. 3. Changes in the average entrance pore diameter as a function of axial stress in the saturated specimens.
T h u s , the p a r t i c l e s a r r a n g e m e n t a n d o r i e n t a t i o n is s t r o n g l y r e l a t e d to the a p p l i e d axial stress. In Fig. 6, for t h e s p e c i m e n c o n s o l i d a t e d at 15 M P a , the a r r a n g e m e n t is m o r e d e n s e a n d t h e p a r t i c l e s are m o r e o r i e n t e d in c o m p a r i s o n to the p r e v i o u s s p e c i m e n s . T h e s h a p e o f p a r t i c l e s f r o m t h e p r e v i o u s f i g u r e s (i.e., Figs. 4 - 6 ) is v a r i a b l e a n d n o t c i r c u l a r as s u p p o s e d f r o m m e r c u r y i n t r u s i o n tests.
Fig. 4. TEM photograph for a specimen consolidated at 1 MPa with a magnification of 10500 X 2.
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M. Al-Mukhtar et al. / Applied Clay Science 11 (1996) 99 115
Fig. 5. TEM photograph for a specimen consolidated at 5 MPa with a magnificationof 10500× 2.
Figs. 7 - 9 show the previous specimens with a higher magnification. The curvature of some clay layers and particles increases with increases in the applied axial stress. The reduction in the pore spaces due to forcing of the particles under applied stresses can also be observed. Some clay layers seems to have been ruptured under the applied stress while undergoing face-to-face clay alignment. 3.2.2. Comparison o f two specimens compacted at 15 M P a with and without drainage during unloading Fig. 10 shows mercury intrusion test results for the pore size distribution in two specimens compacted at 15 MPa with and without drainage conditions during unloading. In the specimen subjectedto undrained condition, some inter-aggregate pores (entrance pore diameter > 2000 A) can be observed. No such pores are observed in the consolidated specimen where drainage was allowed during unloading. It is also observed that the curve of unsaturated specimen is gradually shifting towards the smaller pores. This behavior can be attributed to the effect of suction developed due to the undrained loading condition. Rearrangement of the pore spaces may have occurred in the specimen under the action of suction and resulted in non-uniform pore spaces in the specimen.
M. Al-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
Fig. 6. TEM photograph for a specimen consolidated at 15 MPa with a magnification of 10500 X 2.
I
[ 196 A °
Fig. 7. TEM photograph for specimen consolidated at 1 MPa with a magnification of 51000 × 10.
107
108
M. Al-Mukhtar et al./ Applied Clay Science 11 (1996) 99-115
r
f
17o
i-it -
Fig. 8. TEM photograph for specimen consolidated at 5 MPa with a magnification of 51000 × 10.
,--..6~o
, ~ , iii~i~i~i¸ ; i l i l i ~ ......
Fig. 9. TEM photograph for specimen consolidated at 15 MPa with a magnification of 51000 × 10.
0,03 t~ Specimen consolidated at 15 MPa
~ 0,02
-.
i
~ ~
~
.
.
---~-Specimen compacted at 15 MPa [
a~un,oad~withoud~i t ~g0
¢~ 0,01
0,00
. 10
.
. 100
.
1000
10000
100000
Entrance pore diameter (A °) Fig. 10. Pore size distribution in the specimens consolidated at 15 MPa with and without drainage during unloading.
M. Al-Mukhtar et al. /Applied Clay Science 11 (1996) 99-115
109
Fig. 11. TEM photograph for the specimen consolidated at 15 MPa (but unloaded without drainage) with a magnification of 51000× 10.
TEM image for the undrained saturated specimen with 15 MPa loading is presented in Fig. 1 I. The number of clay layers per particle is greater in undrained loading condition in Fig. 11 in comparison to the specimen consolidated with drained loading condition in Fig. 12. The effect of undrained condition seems to reorient the particles and bring them face-to-face in alignment. The bending of particles (curvature in particles) decreased in the undrained specimens tested. Thus, the particle arrangement in the undrained specimen may be attributed to the combined effect of mechanical (axial stress, positive pressure) and hydraulic (suction, negative pressure) effects. 3.2.3. Comparison o f two specimens hm~ing the same oedometric porosity but different mechanical and hydraulic stress state conditions Fig. 13 shows the pore size distribution of the specimens having the same oedometric porosity but different mechanical and hydraulic stress states. The specimen compacted at lower axial stress shows smaller pore spaces. The effect of mechanical stress is to reduce the larger pores first and the smaller pores later. A narrow pore size distribution can be observed for the unsaturated specimen tested. The pore size increases and its peak shifts towards larger pore diameters with suction due to the influence of the hydraulic stress. Therefore, the effect of hydraulic stress appears to be mainly on the smallest size pores and results in clay layers within the particles to come closer. In summary, the effect of hydraulic stress dominates the arrangement of particles in the tested clay soil. From TEM viewing (Fig. 14 for the consolidated specimen with 5 MPa and Fig. 15 for the unsaturated specimen compacted at 10 MPa) it can be observed that the unsaturated specimen has more space between particles. The size of the soil particles in the unsaturated specimen is greater in comparison to the saturated specimen tested. The
1lO
M. AI-Mukhtar et al./Applied Clay Science 11 (1996) 99-115
196 A °
Fig. 12. TEM photograph for specimenconsolidatedat 15 MPa with a magnificationof 51000 × I0.
saturated specimen presents more homogenous organization between particles and pore spaces. Fig. 16 shows TEM image for the unsaturated specimen with higher magnification. From Figs. 8 and 15, the previous observations can be further substantiated. Each particle of unsaturated specimen is composed of more number of clay layers in comparison to that of saturated specimen. In the unsaturated specimen, no distorted particles are observed and more particles are in face-to-face arrangement or orientation.
3.2.4. Total p o r e volume in the specimens
Fig. 17, shows the cumulative pore volume obtained from mercury porosimetry of all the specimens tested. For the specimens consolidated at 1, 5 and 15 MPa, the total pore
0,06
[
+
t
--X--Specimencompactedat 10 MPa with
i'
controled relative humidity o f 4 4 %
"S 0,04 1
'
0,02
Specimenconsolidatedat 5 MPa
i
0,00
. 10
•
~ 100
~
1000
~ 10000
x
100000
Entrance pore diameter (A°) Fig. 13. Pore size distributionin two specimenshaving similaroedometer porosity.
M. AI-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
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Fig. 14. TEM photograph for the saturated specimen consolidated at 5 MPa with a magnification of 51000 × 2.
volume decreases in the soil with increasing axial stress. For the specimens compacted at 15 MPa, the total pore volume is higher when the drainage is allowed in comparison to the total pore volume obtained from the specimen without drainage. The swelling during unloading with permitted drainage condition explains such a behavior. The unsaturated specimen presents a total pore volume close to the saturated specimen having the same oedometric porosity.
Table 4 Results of mercury intrusion tests on Boom clay soil Axial stress applied (MPa)
Cumulative intrusion volume (ml/g)
Average pore diameter (A)
1 5 15
0.1317 0.1035 0.0924 0.0832 0.1055
326 251 233 211 264
15 (undrained-unloading)
10 (unsaturated, suction = 110 MPa)
112
M. AI-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
Fig. 15. TEM photograph lot the unsaturated specimen compacted at 10 MPa with a suction control of 110 MPa (relative humidity of 44%), with a magnification of 51000 × 2.
Fig. 16. TEM photograph fl)r the unsaturated specimen compacted at 10 MPa with a suction control of 110 MPa (relative humidity of 44%), magnification of 51000X 10.
113
M. AI-Mukhtar et aL /Applied Clay Science 11 (1996) 99-115
0,16 q-
~_
/
i+
~ O,12 "~
, i ] [
~ ..--
i ----o--Specimenconsohdatedat 1 MPa
~
Specimenconsolidatedat 5 MPa Specimenconsolidatedat 15 MPa
~
Specimencompactedat 15 MPa (Undrained ' unloading) ! x UnsaturaIedspecimencompactedatl0MPa
tl " ~ " ~ ~l~_~N,. "~
-~ 0,04 t..) 0,00 4 10
100
1000
10000
100000
1000000
Entrance pore radius (A °) Fig. 17. Pore volume in the specimen from mercury-intrusion tests.
Table 4 presents the cumulative and the average pore volume of the tested specimens. The porosity decreases as the consolidation stress increases. The average pore diameter and the total volume of pores also decreases. However, in the unsaturated specimen, the average pore diameter is higher in the specimen compacted at 5 MPa at a similar porosity. The lowest value is obtained in the specimen where the drainage was not permitted. Mercury intrusion test results are generally influenced by the sample preparation procedures. However, the results obtained from the two series of tests conducted with and without using lyophilisation technique demonstrate that the preparation procedures have not significantly influenced the porosities. The porosities of the B o o m clay soil samples compacted at three axial stresses 1, 5 and 10 MPa are summarized in Table 5. The results demonstrate that the microfabric of highly compacted clays is not affected by the preparation technique or affected in the same manner with and without using lyophilisation technique.
Table 5 Porosity of Boom clay soil samples obtained from mercury intrusion tests with and without using lyophilisation technique Applied load (MPa)
Porosity obtained without using lyophilisation technique (%)
Porosity obtained using lyophilisation technique (%)
1 5 10
25.42 24.01 22.79
25.05 24.27 23.27
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4. Summary and conelusions Two techniques were used in the study of a Boom clay soil fabric. Mercury intrusion gives the pore size distribution and the total volume of pores measured by this method. TEM reveals information about the size, shape, and arrangement of pores in the soil fabric. Results obtained show that the macroscopic behavior is related to the soil fabric. The applied mechanical load reduces the larger pores (i.e., inter-aggregate pores) before reducing the smaller size pores. Suction influences essentially the small size pores (i.e., intra-particle and inter-particle pore space). For the three loading situations studied, the following conclusions are derived: 1. For the applied mechanical stresses of 1 MPa on the consolidated specimen, no inter-aggregate pores were observed. For higher loads (i.e., 5 to 15 MPa), the size of pore spaces decreased and the particle arrangement was homogenous. The entrance pore diameter appears to be linearly related to the logarithm of the overburden pressure. Such results support a very low permeability for the soil. The likely mode of water transport through the soil is by diffusion. 2. The excavation of a clayey soil changes the hydraulic stress state conditions in the soil. Rearrangement of soil fabric occurs both due to mechanical and hydraulic stress states. A non uniform pore size distribution arises as a result of unloading condition. 3. Hydraulic stress state condition has a significant influence on the unsaturated soil behavior. Under such situation, soil fabric will be composed of particles with several clay layers and aggregates with a large number of particles. As the particles within the aggregates are close to each other, the rigidity of the unsaturated soil will be higher in comparison to a saturated soil at identical porosity values. Hence, the hydraulic conductivity of an unsaturated soil can be higher. Further studies using fractal techniques and image processing techniques may be necessary for better understanding the relationship between microscopic parameters and macroscopic variables of highly compacted clays. Other techniques like X-ray diffraction, thermal analysis and infrared methods can also provide more information,
Acknowledgements The authors acknowledge the assistance provided by Mrs. A.M. Jaunet for TEM tests. A N D R A (Agence National pour le stockage des Drchets Radioactifs) is acknowledged for providing the soil samples.
References A1-Mukhtar, M., Robinet, J.C, Liu, C.W. and Plas, F., 1993. Hydromechanicalbehaviour of partially saturated low porosity clays. Int. Conf. on Engineering Fills~ Sept. 1993, Newcastle Upon Tyne, pp. 87-98. Aylmore, L.A.G. and Quirk, J.P., 1959. Swelling of clay water systems. Nature, 183: 1752-1753.
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Baldi, G., Hueckel, T., Peano, A. and Pellegrini, 1990. Developments in Modelling of Thermo-Hydro-Geomechanical Behavior of Boom Clay and Clay Based Buffer Materials. Final Rep. CEC, ISMES, Bergamo. Ben Rhaiem, H., Tessier, D. and Puns, C.H., 1986. Comportement hydrique et Evolution structurale et texturale des montmorillonites au cours d'un cycle de dessication-humectation. Partie I. Cas des montmorillonites calciques. Clay Miner., 21: 9-19. Collins, K. and McGown, A., 1974. The form and function of microfabric features in a variety of natural soils. GEotechnique, 14(2): 222-254. Collins, K., 1984. Characterization of expansive soil microfabric. Proc. 5th. Int. Conf. Expansive Soils, Adelaide, pp. 37-43. Delage, P. and Lefebvre, G., 1984. Study of the structure of a sensitive Champlain clay and its evolution during consolidation. Can. Geotechn. J., 21: 21-35. Griffiths, F.J. and Joshi, R.C., 1990. Clay fabric response to consolidation. Appl. Clay Sci., 4: 37-66. Giiven, N., Low, P.F., Mitchell, J.K., Sposito, G. and Van Olphen, H., 1992. Clay-water interface and its rheological implications. In: N. GiJven and R.M. Pollastro (Editors), Clay Miner. Soc. Workshop Lect., 4. Mitchell, J.K., 1976. Fundamentals of Soil Mechanics. Wiley, New York, NY, 422 pp. Pusch, R., 1982. Mineral-water interactions and their influence on the physical behaviour of highly compacted Na bentonite. Can. Geotechn. J., 19: 381-387. Push, R. and Carlsson, T., 1985. The physical state of pore water of Na-smectite used as barrier component. Eng. Geol., 21: 257-265. Push, R., Ola Karnland and Hrkmark, H., 1991. The nature of expanding clays as exemplified by the multifaced smectite mineral montmorillonite. Int. Workshop Stress Partitioning in Engineering Clay Barriers, Duke Univ., pp. 1-23. Rhattas, A., 1994. Transfert de masse dans les argiles ?~ faible porositr: analyse thEorique et r~sultats expErimentaux. Th~se de Doctorat, OrlEans. Rosenqvist, T., 1959. Physico-chemical properties of soils in soil water system. ASCE J. Soil Mech. Found. Div., 85 (SM2): 31-53. Rosenqvist, T., 1984. The importance of pore water chemistry on mechanical and engineering properties of clay soils. Philos. Trans. R. Soc. London A, 311: 369-373. Tessier, D. and Berrier, J., 1978. Observations d'argiles hydratEes en microscopie Electronique h balayage: importance et choix de la technique de preparation. Proc. 5th Int. Working Meeting Soil Micromorphology, Granada, pp. 117-135. Tessier, D., 1984. Etude Experimentale de l'Organisation des MatEriaux Argileux. Th~se Sci., Paris VII. Tessier, D., Lajudi, A. and Petit, J.C., 1992. Relation between the macroscopic behavior of clays and their microstructural properties. Appl. Geochem., Suppl. 1: 151-161. Touret, O., Puns, C.H., Tessier, D. and Tardy, Y., 1990. Etude de la repartition de l'eau dans des argiles saturEes Mg 2+ aux fortes teneurs en eau. Clay Miner., 25: 217-233. Van Olphen, H., 1977. An Introduction to Clay Colloid Chemistry, 2nd ed. Wiley, New York, NY. Venial, F., 1985. The role of microfabric in clay soil stability. Mineral. Petrogr. Acta, 29, pp. 110-119. Washburn, E.W., 1921. Note on a method of determining the distribution of pore sizes in a porous material. Natl. Acad. Sci. Proc., 7, pp. 115-116.
Applied Clay Science 11 (1996) 99-115
The fabric of a clay soil under controlled mechanical and hydraulic stress states M. A1-Mukhtar a,1 N. Belanteur a,2 D. Tessier b,3 S.K. Vanapalli c,4 a University of Orleans, ESEM-CNRS, CRMD, 45072 Orleans Cedex 2, France b 1NRA, Station de Science du Sol, 78026 Versailles, France c Department of Civil Engineering, University of Saskatchewan, Saskatoon, Canada
Received 11 July 1995; revised 10 September 1996; accepted 10 September 1996
Abstract Fabric determination is fundamental to the understanding of several mechanisms controlling a clayey soil behavior. Two techniques; namely, mercury intrusion tests and transmission electron microscopy (TEM) were used to study the soil fabric of a remoulded Boom clay. The effect of overburden pressure, excavation and desaturation on the soil fabric are studied. Results demonstrate that the nature of soil fabric is dependent on both the applied mechanical and hydraulic stress state conditions. Keywords: remoulded clay; soil fabric; mechanical and hydraulic stress state; mercury porosimetric; transmission electron microscopy tests
1. Introduction O n a b r o a d scale, it is w i d e l y a c c e p t e d in the literature t h a t t h e r e are t w o l e v e l s o f soil f a b r i c f o r c l a y e y soils. T h e s e are the m a c r o l e v e l f a b r i c a n d t h e m i c r o l e v e l fabric.
i Associate Professor. 2 Research Engineer. 3 Director of Research. 4 Research Engineer. 0169-1317/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 13 1 7 ( 9 6 ) 0 0 0 2 3 - 3
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M. AI-Mukhtar et al. / Applied Clay Science 11 (1996) 99 115
The soil microfabric is described as the elementary particle associations within the soil aggregates, whereas the arrangement of these soil aggregates is referred to as the macrofabric (Mitchell, 1976). Several parameters such as the exchange capacity, the type (nature) of cations, the size of clay particles, the stress history, the hydraulic state, the soil chemistry, and the mineral history influence both the macro- and microfabric of a clayey soil (Van Olphen, 1977; Rosenqvist, 1984; Venial, 1985; Griffiths and Joshi, 1990). Collins and McGown (1974) and Collins (1984) demonstrated, based on transmission electron microscope (TEM) and porosimetry studies on natural soils, that two principal types of microfabric dominate in clayey soils. They are the matrix microfabric integrated by elementary particle arrangement of the clay platelets and the aggregation of microfabric integrated by arrays of elementary particle configuration. Microfabric studies have demonstrated that integrated elementary particle arrangement of clay platelets has large amounts of adsorbed water in low porosity clays (Push and Carlsson, 1985; Baldi et al., 1990). The behaviour of adsorbed water and consequently that of low porosity clays is significantly altered by electro-chemical interaction (Push et al., 1991). Three different types of pore spaces were identified within the microfabric based on size classification (Pusch, 1982; Ben Rhaiem et al., 1986, Touret et al., 1990; Tessier et al., 1992). The pore space size ranging between 10 to 25 A (0.001-0.0025 /~m) is called the intra-particle pore space or inter-platelet pore space. The inter-particle pore space or intra-aggregate pore space is between 25 A to 1500 A (0.0025-0.15 /~m). Giiven et al. (1992); Giiven et al. (1992), however suggested 2000 A (0.2 /_~m)for intra-aggregate pore space. The inter-aggregates have pore sizes higher than 1500 A or 2000 A. The soil fabric is dependent on the soil-water interactions at all levels of arrangement from the elementary platelets (layers) level to the association of particles and aggregates (Aylmore and Quirk, 1959; Tessier, 1984; Giiven et al., 1992). Water in clay mineral pores is bonded to the solid mineral surface by electro-chemical forces (Rosenqvist, 1959). The physico-chemical properties are translated into different thermodynamic potentials permitting the definition of three major types of water; namely, bulk water, capillary water and bounded (adsorbed) water characterized by strong links with solid matrix. Highly compacted soils are used as barriers for nuclear waste management because of their high sorption and low hydraulic conductivity. The study of the soil fabric is useful to understand the behavior of highly compacted soils under various mechanical and hydraulic stress state conditions. Such studies provide valuable information related to the storage of high level nuclear wastes. These wastes would be placed in a disposal vault 500-1000 m underground and would be surrounded by a buffer material, likely containing clay. In this paper soil fabric of a remoulded Boom clay is described using both mercury intrusion porosimetry and transmission electron microscope (TEM) studies. The influence of various hydraulic and mechanical stress state conditions on the pore size distribution are studied. The phenomenological studies of Boom clay soil fabric provides valuable information about the natural soil behaviour. Previous studies have shown that a good agreement can be obtained between the remoulded and natural soil behaviors (Rhattas, 1994). o
M. AI-Mukhtar et al, /Applied Clay Science 11 (1996) 99-115
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2. Materials and methods 2.1. Materials
The research study for this paper was carried out on Boom clay, a soil formation in the north-east of Belgium. Several field and laboratory studies are in progress to assess the impact of burying radioactive wastes in an underground facility. The soil properties and mineralogical composition of the Boom clay tested are summarized in Table 1. Mineralogical analyses from X-diffraction show that the coarser fractions are dominated by quartz, whereas; the finer fractions are dominated by interstratified illite-smectite, illite, mica and kaolinite. The chemical composition of the Boom clay is presented in Table 2, 2.2. M e t h o d o l o g y
The soil specimens used for testing were prepared from remoulded Boom clay. The remoulded soil was prepared adding distilled water equal to twice the liquid limit value. Both saturated and unsaturated specimens were used in the study. Saturated specimens were prepared applying high axial stress in a oedometer device. Unsaturated specimen was prepared applying a controlled suction using high axial stress and high suction oedometer device. The suction in the unsaturated specimen is achieved by indirectly controlling the relative humidity using saturated salt solutions. The oedometer device has a facility to apply loads up to 40 MPa on the soil specimens. More details of the specimen preparation and testing procedures are available in A1-Mukhtar et al. (1993). The specimen dimensions for oedometer testing were 65 m m in diameter and initial height of 65 ram.
Table 1 Soil properties and mineralogical composition of Boom clay soil tested Soil properties
Liquid limit Plastic limit Plasticity index Specific gravity Specific surface area Cation exchange capacity BET surface
70% 25% 45% 2.67 100 m 2/g 20-40 meq/100 g 42 m2/g
Mineralogical composition
Quartz Interstratified illite-smectite Illite Kaolinite Microcline Plagioclase Pyrite Carbon sulphate
20-25% 33% 16% 13% 4-5% 4-5% 4-5% traces
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M. AI-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
Table 2 Chemical composition of Boom clay soil Composition
(%)
SiO 2 AI203 Fe203 CaO MgO Na20 K 20 MnO TiO 2 P205 Loss due to ignition
57.58 12.93 7.57 2.22 2.40 0.12 1.96 0.01 0.88 0.18 14.15
Four saturated specimens were tested for this research program. Three saturated specimens were consolidated with axial stresses of 1, 5 and 15 MPa respectively with an open drainage during testing (i.e., conventional oedometer tests). This loading condition simulates the effect of over burden pressure on the soil fabric. Four saturated specimen were tested with an axial stress of 15 MPa with an undrained condition during unloading. This testing condition simulates the behavior of soil during the excavation operation (i.e., quick unloading). The last specimen was an unsaturated specimen compacted at 15 MPa with controlled suction of 110 MPa (i.e., relative humidity of 44%). This test simulates the behavior of a buffer material around the waste packages. Such a loading condition approximates the behavior of the soil under ventilation conditions in the underground facilities. The porosity of the unsaturated specimen tested was similar to the porosity of the saturated specimen consolidated at 5 MPa. Specimens prepared in the odometer were used for the mercury-intrusion porosimetry tests and in transmission electron microscopy (TEM). For the mercury-intrusion porosimetry tests, lyophilisation technique as suggested by Tessier and Berrier (1978); Delage and Lefebvre (1984) was used in the preparation of specimens to minimize microfabric changes. Mercury intrusion tests were performed using a 'Poresizer 9320' porosimeter. Washburn (1921) equation was used for estimating entrance pore diameter of the soil. This apparatus has a maximum loading capacity of 210 MPa on the mercur), and facilitates investigation of pore diameters between 350 × 104 A ( 3 5 0 ~ m ) and 36 A (0.0036 /xm). Preparation of the specimens for TEM analysis follows the method of Tessier (1984). Specimens were carefully cut so as to preserve their orientation and placed in dissolved Agar, which helps preserve the microstructure during the ensuing solvent exchange. After solidification, the excess Agar was removed and the sample placed in a Reichert-Jung-Lynx el sample embedder. Water was first replaced by methanol, then propylene oxide, and finally Spurr's resin. The specimens were then placed in a 65°C oven for 24 h to polymerize the resin. Thin sections were then cut with a diamond knife on a Reichert-Jung Ultracut E microtome to a thickness of 50-60 nm and a width of about 100 /xm. They were then placed on collodion coated Cu grids. Observations in the TEM were carried out on the surface perpendicular to the direction
M. A1-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
103
of the applied mechanical stress. Electron micrographs were obtained on a Philips model 420 TEM.
3. Test results and analysis 3.1. Oedometer test results Fig. 1 shows the variation of void ratio with respect to axial stress o f the saturated and unsaturated specimens tested. Void ratio of saturated specimens increased during unloading (i.e., specimens swelled during unloading). In the unsaturated specimen, no swelling was observed during unloading. Therefore, the saturated specimen shows elastoplastic (partially irreversible) behavior and the unsaturated specimen shows a plastic (completely irreversible) behavior. Small modification in the void ratio is however observed during unloading for the compacted unsaturated specimen at 15 MPa without drainage condition. Table 3 summarizes the hydraulic and mechanical stress state conditions of the specimens tested. 3.2. Mercury intrusion and T E M test results 3.2.1. Comparison o f saturated specimens consolidated at 1, 5 and 15 M P a Fig. 2 shows the results obtained from mercury intrusion tests for the pore size distribution in three specimens consolidated at 1, 5 and 15 MPa. Two distinguished families of entrance pore diameters 2 0 0 - 4 0 0 A and 4 0 0 - 1 0 0 0 A were observed at 1 MPa axial stress. These pores can be categorized as intra-aggregate (inter-particle) pores as the entrance pore diameters are less than 2000 A.
1,1
[ #
0,9
specimens
~:~ Unsaturatedspectmen with controlledrelative humidityof 44 %)
.o ~
Saturated
0,7
>
o
~
...
,~
0,5 i i
I I
0,3 0,1
1
10
100
Axial stress, MPa Fig. 1. Oedometer curves for the soil specimens tested in saturated and unsaturated conditions.
M. A1-Mukhtar et al./ Applied Clay Science 11 (1996) 99-115
104
Table 3 Hydraulic and mechanical stress state conditions of tested specimens Hydraulic state of specimens and drainage conditions
Mechanical stress (MPa)
Porosity
Saturated drained Saturated drained Saturated drained Saturated but undrained during unloading Unsaturated with controlled suction of 110 MPa (relative humidity of 44%)
1 5 15 15
0,45 0.38 0.35 0.31
10
0.38
With increased applied stresses, the family of pores having high entrance pore diameter are eliminated. For stresses 5 and 15 MPa, the pore size distribution seems to be very close (i.e., there is no further reduction in entrance pore diameter for loads higher than 5 MPa). At these stresses, only one family of pore size distribution having a range of entrance pore diameter of 800 to 60 A can be identified in the specimens. Fig. 3 presents the changes in the average entrance pore diameter for the tested specimens. The relationship between the average entrance pore diameter and the logarithmic applied stress is linear. This demonstrates that the modification in the pore size distribution between 0 and 1 MPa loading is higher in comparison to 1 and 10 MPa loading. TEM images for the consolidated specimens are shown in Figs. 4-6. Fig. 4 shows that the arrangement of soil fabric is not homogeneous for the specimens consolidated at 1 MPa. Organic materials can also be observed. In Fig. 5, the specimen consolidated at 5 MPa is more homogenous in comparison to the specimen consolidated at 1 MPa. This could be attributed to the reduction in pores spaces with the higher stresses applied.
0,06 -<>- Specimen consolidated at 1 MPa ----o--Specimen consolidatcd at 5 MPa t:l Specimen consolidated at 15 MPa ~
,04
>
0,o2
0,00 ~ 10
. . . . . . . . . . . . . . . 100
1000 Entrance pore diameter (A °)
10000
Fig. 2. Pore size distribution in the saturated specimens.
100000
105
M. Al-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
400 . . . . . . .
,. . . . .
..
.....
....
.. ....
o 300 -
~ ~ 200
..... . . . .
~
i. . . . . . . .
: . . . .
?
....
]00 I0 Axial stress, M P a
Fig. 3. Changes in the average entrance pore diameter as a function of axial stress in the saturated specimens.
T h u s , the p a r t i c l e s a r r a n g e m e n t a n d o r i e n t a t i o n is s t r o n g l y r e l a t e d to the a p p l i e d axial stress. In Fig. 6, for t h e s p e c i m e n c o n s o l i d a t e d at 15 M P a , the a r r a n g e m e n t is m o r e d e n s e a n d t h e p a r t i c l e s are m o r e o r i e n t e d in c o m p a r i s o n to the p r e v i o u s s p e c i m e n s . T h e s h a p e o f p a r t i c l e s f r o m t h e p r e v i o u s f i g u r e s (i.e., Figs. 4 - 6 ) is v a r i a b l e a n d n o t c i r c u l a r as s u p p o s e d f r o m m e r c u r y i n t r u s i o n tests.
Fig. 4. TEM photograph for a specimen consolidated at 1 MPa with a magnification of 10500 X 2.
106
M. Al-Mukhtar et al. / Applied Clay Science 11 (1996) 99 115
Fig. 5. TEM photograph for a specimen consolidated at 5 MPa with a magnificationof 10500× 2.
Figs. 7 - 9 show the previous specimens with a higher magnification. The curvature of some clay layers and particles increases with increases in the applied axial stress. The reduction in the pore spaces due to forcing of the particles under applied stresses can also be observed. Some clay layers seems to have been ruptured under the applied stress while undergoing face-to-face clay alignment. 3.2.2. Comparison o f two specimens compacted at 15 M P a with and without drainage during unloading Fig. 10 shows mercury intrusion test results for the pore size distribution in two specimens compacted at 15 MPa with and without drainage conditions during unloading. In the specimen subjectedto undrained condition, some inter-aggregate pores (entrance pore diameter > 2000 A) can be observed. No such pores are observed in the consolidated specimen where drainage was allowed during unloading. It is also observed that the curve of unsaturated specimen is gradually shifting towards the smaller pores. This behavior can be attributed to the effect of suction developed due to the undrained loading condition. Rearrangement of the pore spaces may have occurred in the specimen under the action of suction and resulted in non-uniform pore spaces in the specimen.
M. Al-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
Fig. 6. TEM photograph for a specimen consolidated at 15 MPa with a magnification of 10500 X 2.
I
[ 196 A °
Fig. 7. TEM photograph for specimen consolidated at 1 MPa with a magnification of 51000 × 10.
107
108
M. Al-Mukhtar et al./ Applied Clay Science 11 (1996) 99-115
r
f
17o
i-it -
Fig. 8. TEM photograph for specimen consolidated at 5 MPa with a magnification of 51000 × 10.
,--..6~o
, ~ , iii~i~i~i¸ ; i l i l i ~ ......
Fig. 9. TEM photograph for specimen consolidated at 15 MPa with a magnification of 51000 × 10.
0,03 t~ Specimen consolidated at 15 MPa
~ 0,02
-.
i
~ ~
~
.
.
---~-Specimen compacted at 15 MPa [
a~un,oad~withoud~i t ~g0
¢~ 0,01
0,00
. 10
.
. 100
.
1000
10000
100000
Entrance pore diameter (A °) Fig. 10. Pore size distribution in the specimens consolidated at 15 MPa with and without drainage during unloading.
M. Al-Mukhtar et al. /Applied Clay Science 11 (1996) 99-115
109
Fig. 11. TEM photograph for the specimen consolidated at 15 MPa (but unloaded without drainage) with a magnification of 51000× 10.
TEM image for the undrained saturated specimen with 15 MPa loading is presented in Fig. 1 I. The number of clay layers per particle is greater in undrained loading condition in Fig. 11 in comparison to the specimen consolidated with drained loading condition in Fig. 12. The effect of undrained condition seems to reorient the particles and bring them face-to-face in alignment. The bending of particles (curvature in particles) decreased in the undrained specimens tested. Thus, the particle arrangement in the undrained specimen may be attributed to the combined effect of mechanical (axial stress, positive pressure) and hydraulic (suction, negative pressure) effects. 3.2.3. Comparison o f two specimens hm~ing the same oedometric porosity but different mechanical and hydraulic stress state conditions Fig. 13 shows the pore size distribution of the specimens having the same oedometric porosity but different mechanical and hydraulic stress states. The specimen compacted at lower axial stress shows smaller pore spaces. The effect of mechanical stress is to reduce the larger pores first and the smaller pores later. A narrow pore size distribution can be observed for the unsaturated specimen tested. The pore size increases and its peak shifts towards larger pore diameters with suction due to the influence of the hydraulic stress. Therefore, the effect of hydraulic stress appears to be mainly on the smallest size pores and results in clay layers within the particles to come closer. In summary, the effect of hydraulic stress dominates the arrangement of particles in the tested clay soil. From TEM viewing (Fig. 14 for the consolidated specimen with 5 MPa and Fig. 15 for the unsaturated specimen compacted at 10 MPa) it can be observed that the unsaturated specimen has more space between particles. The size of the soil particles in the unsaturated specimen is greater in comparison to the saturated specimen tested. The
1lO
M. AI-Mukhtar et al./Applied Clay Science 11 (1996) 99-115
196 A °
Fig. 12. TEM photograph for specimenconsolidatedat 15 MPa with a magnificationof 51000 × I0.
saturated specimen presents more homogenous organization between particles and pore spaces. Fig. 16 shows TEM image for the unsaturated specimen with higher magnification. From Figs. 8 and 15, the previous observations can be further substantiated. Each particle of unsaturated specimen is composed of more number of clay layers in comparison to that of saturated specimen. In the unsaturated specimen, no distorted particles are observed and more particles are in face-to-face arrangement or orientation.
3.2.4. Total p o r e volume in the specimens
Fig. 17, shows the cumulative pore volume obtained from mercury porosimetry of all the specimens tested. For the specimens consolidated at 1, 5 and 15 MPa, the total pore
0,06
[
+
t
--X--Specimencompactedat 10 MPa with
i'
controled relative humidity o f 4 4 %
"S 0,04 1
'
0,02
Specimenconsolidatedat 5 MPa
i
0,00
. 10
•
~ 100
~
1000
~ 10000
x
100000
Entrance pore diameter (A°) Fig. 13. Pore size distributionin two specimenshaving similaroedometer porosity.
M. AI-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
111
Fig. 14. TEM photograph for the saturated specimen consolidated at 5 MPa with a magnification of 51000 × 2.
volume decreases in the soil with increasing axial stress. For the specimens compacted at 15 MPa, the total pore volume is higher when the drainage is allowed in comparison to the total pore volume obtained from the specimen without drainage. The swelling during unloading with permitted drainage condition explains such a behavior. The unsaturated specimen presents a total pore volume close to the saturated specimen having the same oedometric porosity.
Table 4 Results of mercury intrusion tests on Boom clay soil Axial stress applied (MPa)
Cumulative intrusion volume (ml/g)
Average pore diameter (A)
1 5 15
0.1317 0.1035 0.0924 0.0832 0.1055
326 251 233 211 264
15 (undrained-unloading)
10 (unsaturated, suction = 110 MPa)
112
M. AI-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
Fig. 15. TEM photograph lot the unsaturated specimen compacted at 10 MPa with a suction control of 110 MPa (relative humidity of 44%), with a magnification of 51000 × 2.
Fig. 16. TEM photograph fl)r the unsaturated specimen compacted at 10 MPa with a suction control of 110 MPa (relative humidity of 44%), magnification of 51000X 10.
113
M. AI-Mukhtar et aL /Applied Clay Science 11 (1996) 99-115
0,16 q-
~_
/
i+
~ O,12 "~
, i ] [
~ ..--
i ----o--Specimenconsohdatedat 1 MPa
~
Specimenconsolidatedat 5 MPa Specimenconsolidatedat 15 MPa
~
Specimencompactedat 15 MPa (Undrained ' unloading) ! x UnsaturaIedspecimencompactedatl0MPa
tl " ~ " ~ ~l~_~N,. "~
-~ 0,04 t..) 0,00 4 10
100
1000
10000
100000
1000000
Entrance pore radius (A °) Fig. 17. Pore volume in the specimen from mercury-intrusion tests.
Table 4 presents the cumulative and the average pore volume of the tested specimens. The porosity decreases as the consolidation stress increases. The average pore diameter and the total volume of pores also decreases. However, in the unsaturated specimen, the average pore diameter is higher in the specimen compacted at 5 MPa at a similar porosity. The lowest value is obtained in the specimen where the drainage was not permitted. Mercury intrusion test results are generally influenced by the sample preparation procedures. However, the results obtained from the two series of tests conducted with and without using lyophilisation technique demonstrate that the preparation procedures have not significantly influenced the porosities. The porosities of the B o o m clay soil samples compacted at three axial stresses 1, 5 and 10 MPa are summarized in Table 5. The results demonstrate that the microfabric of highly compacted clays is not affected by the preparation technique or affected in the same manner with and without using lyophilisation technique.
Table 5 Porosity of Boom clay soil samples obtained from mercury intrusion tests with and without using lyophilisation technique Applied load (MPa)
Porosity obtained without using lyophilisation technique (%)
Porosity obtained using lyophilisation technique (%)
1 5 10
25.42 24.01 22.79
25.05 24.27 23.27
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M. AI-Mukhtar et al. / Applied Clay Science 11 (1996) 99-115
4. Summary and conelusions Two techniques were used in the study of a Boom clay soil fabric. Mercury intrusion gives the pore size distribution and the total volume of pores measured by this method. TEM reveals information about the size, shape, and arrangement of pores in the soil fabric. Results obtained show that the macroscopic behavior is related to the soil fabric. The applied mechanical load reduces the larger pores (i.e., inter-aggregate pores) before reducing the smaller size pores. Suction influences essentially the small size pores (i.e., intra-particle and inter-particle pore space). For the three loading situations studied, the following conclusions are derived: 1. For the applied mechanical stresses of 1 MPa on the consolidated specimen, no inter-aggregate pores were observed. For higher loads (i.e., 5 to 15 MPa), the size of pore spaces decreased and the particle arrangement was homogenous. The entrance pore diameter appears to be linearly related to the logarithm of the overburden pressure. Such results support a very low permeability for the soil. The likely mode of water transport through the soil is by diffusion. 2. The excavation of a clayey soil changes the hydraulic stress state conditions in the soil. Rearrangement of soil fabric occurs both due to mechanical and hydraulic stress states. A non uniform pore size distribution arises as a result of unloading condition. 3. Hydraulic stress state condition has a significant influence on the unsaturated soil behavior. Under such situation, soil fabric will be composed of particles with several clay layers and aggregates with a large number of particles. As the particles within the aggregates are close to each other, the rigidity of the unsaturated soil will be higher in comparison to a saturated soil at identical porosity values. Hence, the hydraulic conductivity of an unsaturated soil can be higher. Further studies using fractal techniques and image processing techniques may be necessary for better understanding the relationship between microscopic parameters and macroscopic variables of highly compacted clays. Other techniques like X-ray diffraction, thermal analysis and infrared methods can also provide more information,
Acknowledgements The authors acknowledge the assistance provided by Mrs. A.M. Jaunet for TEM tests. A N D R A (Agence National pour le stockage des Drchets Radioactifs) is acknowledged for providing the soil samples.
References A1-Mukhtar, M., Robinet, J.C, Liu, C.W. and Plas, F., 1993. Hydromechanicalbehaviour of partially saturated low porosity clays. Int. Conf. on Engineering Fills~ Sept. 1993, Newcastle Upon Tyne, pp. 87-98. Aylmore, L.A.G. and Quirk, J.P., 1959. Swelling of clay water systems. Nature, 183: 1752-1753.
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Baldi, G., Hueckel, T., Peano, A. and Pellegrini, 1990. Developments in Modelling of Thermo-Hydro-Geomechanical Behavior of Boom Clay and Clay Based Buffer Materials. Final Rep. CEC, ISMES, Bergamo. Ben Rhaiem, H., Tessier, D. and Puns, C.H., 1986. Comportement hydrique et Evolution structurale et texturale des montmorillonites au cours d'un cycle de dessication-humectation. Partie I. Cas des montmorillonites calciques. Clay Miner., 21: 9-19. Collins, K. and McGown, A., 1974. The form and function of microfabric features in a variety of natural soils. GEotechnique, 14(2): 222-254. Collins, K., 1984. Characterization of expansive soil microfabric. Proc. 5th. Int. Conf. Expansive Soils, Adelaide, pp. 37-43. Delage, P. and Lefebvre, G., 1984. Study of the structure of a sensitive Champlain clay and its evolution during consolidation. Can. Geotechn. J., 21: 21-35. Griffiths, F.J. and Joshi, R.C., 1990. Clay fabric response to consolidation. Appl. Clay Sci., 4: 37-66. Giiven, N., Low, P.F., Mitchell, J.K., Sposito, G. and Van Olphen, H., 1992. Clay-water interface and its rheological implications. In: N. GiJven and R.M. Pollastro (Editors), Clay Miner. Soc. Workshop Lect., 4. Mitchell, J.K., 1976. Fundamentals of Soil Mechanics. Wiley, New York, NY, 422 pp. Pusch, R., 1982. Mineral-water interactions and their influence on the physical behaviour of highly compacted Na bentonite. Can. Geotechn. J., 19: 381-387. Push, R. and Carlsson, T., 1985. The physical state of pore water of Na-smectite used as barrier component. Eng. Geol., 21: 257-265. Push, R., Ola Karnland and Hrkmark, H., 1991. The nature of expanding clays as exemplified by the multifaced smectite mineral montmorillonite. Int. Workshop Stress Partitioning in Engineering Clay Barriers, Duke Univ., pp. 1-23. Rhattas, A., 1994. Transfert de masse dans les argiles ?~ faible porositr: analyse thEorique et r~sultats expErimentaux. Th~se de Doctorat, OrlEans. Rosenqvist, T., 1959. Physico-chemical properties of soils in soil water system. ASCE J. Soil Mech. Found. Div., 85 (SM2): 31-53. Rosenqvist, T., 1984. The importance of pore water chemistry on mechanical and engineering properties of clay soils. Philos. Trans. R. Soc. London A, 311: 369-373. Tessier, D. and Berrier, J., 1978. Observations d'argiles hydratEes en microscopie Electronique h balayage: importance et choix de la technique de preparation. Proc. 5th Int. Working Meeting Soil Micromorphology, Granada, pp. 117-135. Tessier, D., 1984. Etude Experimentale de l'Organisation des MatEriaux Argileux. Th~se Sci., Paris VII. Tessier, D., Lajudi, A. and Petit, J.C., 1992. Relation between the macroscopic behavior of clays and their microstructural properties. Appl. Geochem., Suppl. 1: 151-161. Touret, O., Puns, C.H., Tessier, D. and Tardy, Y., 1990. Etude de la repartition de l'eau dans des argiles saturEes Mg 2+ aux fortes teneurs en eau. Clay Miner., 25: 217-233. Van Olphen, H., 1977. An Introduction to Clay Colloid Chemistry, 2nd ed. Wiley, New York, NY. Venial, F., 1985. The role of microfabric in clay soil stability. Mineral. Petrogr. Acta, 29, pp. 110-119. Washburn, E.W., 1921. Note on a method of determining the distribution of pore sizes in a porous material. Natl. Acad. Sci. Proc., 7, pp. 115-116.