Published in Soil Sci. Soc. Am. J. 68:1145-1153 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-1—SOIL PHYSICS
Relationship between Clay Content, Clay Type, and Shrinkage Properties of Soil Samples
Pascal Boivina,*,
Patricia Garnierb and
Daniel Tessierc
a Lab. of Soil Science, Swiss Federal Institute for Technology, EPFL-ENAC-ISTE, 1015 Lausanne, Switzerland
b INRA, rue Fernand Christ, 02000 Laon, France
c Institut National de la Recherche Agronomique, Route de StCyr, 78026 Versailles Cedex, France
* Corresponding author (pascal.boivin{at}epfl.ch).
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ABSTRACT
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The availability of methods for quasi-continuous measurements of soil shrinkage curves allowed the development of new models. The exponential (XP) model allows the calculation of the volume of two pore phases in the soil, namely macro and micropore volumes. The micropore volume is identified with the pore volume of the soil clay matrix according to the assumption that the maximum swelling of the clay matrix (MS) is the point of minimum water content of the structural shrinkage (Point D). This is discussed using undisturbed and repacked soil samples with various clay contents and clay types. The slope of the shrinkage curves as a function of equivalent saturated-pore radius show a transition in pore type around a 10-µm pore radius, where smaller and more deformable pores start to desaturate. This corresponds to the fitted D point and is close to the size of the largest pores in the clay matrix or clay–silt phase reported in the literature. The calculated micropore volume and micropore swelling properties are close to clay paste properties reported in the literature. At low water content, the specific micropore volume is independent from clay content. At Point D, the shrinking capacity of the specific micropore volume decreases with increasing clay content for clay contents below 40%. Our results show that Point D can be identified with the MS of the clay matrix, and that the XP model can be used to calculate the swelling properties of clays in the soil, without extraction.
Abbreviations: AE, air-entry point • LM, macroporosity limit • MS, maximum swelling point • SL, shrinkage limit • XP, exponential
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INTRODUCTION
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CLAY CONTENT AND CLAY TYPE influence many physical properties of soils. Among them, soil shrinkage properties are widely used for characterizing soil structure. Measurement and modeling of soil shrinkage is also of growing interest due to the development of new pore-space approaches (Tuller and Or, 2003). The soil shrinkage is defined as the specific volume change of soil relative to its water content (Haines, 1923; Stirk, 1954) and is mainly due to clay swelling properties.
The shrinkage of clay paste occurs in two phases (Sposito and Giraldez, 1976; Chertkov, 2000). First, the shrinkage curve follows a 1:1 line called load line (Sposito, 1973) from a MS to the air entry point (AE). In this phase, the clay paste remains saturated and each cubic centimeter of water lost corresponds to a 1-cm3 volume decrease. After air entry, the shrinkage decreases, and a minimum clay-paste volume called the shrinkage limit (SL) is reached. As reviewed in McGarry and Malafant (1987) and Tariq and Durnford (1993a), shrinkage of structured soil samples was studied in the infancy of soil science (e.g., Tempany, 1917). Continuous measurement of shrinkage curves on undisturbed core samples allowed improvement of the knowledge of soil shrinking behavior, and to develop shrinkage curve models with different sets of parameters, as reviewed in Braudeau et al. (1999).
Shrinkage curves generally present a typical sigmoid shape, with linear and curvilinear parts separated by transition points as drawn in Fig. 1. Similar to clay paste, a SL and an AE are observed on structured soil samples (e.g., McGarry and Malafant, 1987; Tariq and Durnford, 1993b). However, the slope of the shrinkage curve for water content greater than AE, called normal shrinkage or basic shrinkage as discussed by Mitchell (1992), is generally not equal to one (e.g., Lauritzen and Stewart, 1941; Stirk, 1954; Reeve and Hall, 1978; Boivin, 1990; Coquet et al., 1998). Braudeau (1988) proposed a conceptual model of soil shrinkage derived from the model of Sposito and Giraldez (1976), where clay aggregates in the soil are assumed to shrink like clay paste, and where the slope of the normal shrinkage is related to aggregate fabric and aggregate stability. Therefore the model assumes that the soil shrinkage is the addition of the shrinkage of two pore volumes: the micropore volume, assumed to be the pore volume of clay matrix organized in clay aggregates, and the interaggregates pore volume. In this model, the SL, AE, and MS points of clay aggregates are fitted on the soil shrinkage curve and denoted A, B and D, respectively (Fig. 1). A fourth point named C is the transition point between the basic shrinkage and the following curvilinear part at higher water content. It is related to macropore volume properties and has thus no equivalent point on a clay paste shrinkage curve. The silt and sand fractions are accounted for as part of the solid phase, whatever their location inside or outside the aggregates. This model was later called XP model and its mathematical and fitting properties were compared with other shrinkage models (Braudeau et al., 1999). If the conceptual assumptions of Braudeau (1988) are verified, the XP model allows us therefore to calculate the air and water content of both clay-matrix and interaggregate pore volumes at each soil water content from direct measurement of the undisturbed soil sample shrinkage curve. Moreover, the physical properties of the clay matrix can be studied without its extraction, that is, from direct measurement on the soil, as proposed by Braudeau and Bruand (1993). These authors showed that the micropore volume estimated using XP model at Point A was equal to clay pore volume estimated on dry soil samples using mercury porosimetry on Oxisols and Ferruginous soil samples. Equality of A (soil) and SL (clay matrix) points is therefore demonstrated in that case. The correspondance of the soil AE (B) to the clay matrix AE was proposed by many authors like Sposito and Giraldez (1976). Both points occur at low water pressure head, close to wilting point (Boivin, 1990). The related pores are very thin and obviously because clay particle arrangement.
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Fig. 1. The soil shrinkage curve and transition points according to the exponential model (Braudeau et al., 1999). A, B, C, and D are transition points between linear and exponential phases, they are defined as the shrinkage limit point, the air entry point, the limit of macroporosity and the maximum swelling point, respectively. Dashed line is the saturation line or load line.
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The assumption on Point D (i.e., D is MS of clay aggregates) was not discussed. Studying the physical properties of the clay matrix without removing the clay from the soil is a very important goal, as extraction always sharply modifies the clay. Point D is used together with Points A and B to calculate the swelling of the clay matrix (Braudeau and Bruand, 1993), and the assumption on this point is thus very important to check. The aims of this paper are (i) to study the relationship between Point D and the maximum swelling of clay aggregates (MS) in the soil; and (ii) to study the influence of clay content and clay type on the shrinkage properties of repacked and undisturbed soil samples.
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MATERIALS AND METHODS
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Repacked Soil
To control the clay content and clay type, and to minimize other sources of variability between soil samples, the initial experiments were performed on repacked soil samples. The soil samples were made from a mixing of a vertisol and sand collected in the Senegal River Middel-Valley in North Senegal, Podor region, and an industrial kaolinite. The textural composition of the vertisol, sand, and kaolinite are given in Table 1. Kaolinite contained 55% silt-size particles that we did not quantify as clay. The X-ray diffraction analysis for the vertisol is: 60% smectite, 30% kaolinite, 5% illite, and 5% chlorite. The chemical analysis of vertisol and kaolinite is given in Table 2. The exchangeable cations were mostly Ca and Mg.
Sample Preparations
The vertisol was passed through a 2-mm sieve and ground to powder to have the same particle size as kaolinite. The sand was washed with HCl and passed through a 2-mm sieve. Two sets of 15 samples were repacked with varying clay content between the samples of each set and with two different ratio of swelling/nonswelling clay in the two sets (Table 3). This ratio is higher for Set 1 than for Set 2. Set 1 had a clay content ranging from 18 to 56% with 37% kaolinite, 54% smectite, and 9% illite and chlorite in the clay fraction, and Set 2 had a clay content ranging from 15 to 47% with 48% kaolinite, 44% smectite, and 8% illite and chlorite in clay fraction (Table 3). The clay content and mixture ratios of vertisol and kaolinite we used are close to the range of clay content and clay type found in Senegal alluvial deposits.
Therefore, for each sample, the required ratio of Vertisol, sand, and kaolinite, totaling 150 g of dry mass, was mixed together. The dry mixture was poured in a polyvinyl chloride (PVC) cylinder of 5.5 cm in height and 6.5 cm in diameter. The vertical side of the cylinder was covered inside with a 1-cm thick sponge and a thin plastic film to avoid contact between sponge and soil. This allowed the sample to swell upon rewetting, without lateral compaction of the soil. The mixture was poured in 1-cm homogenized layers and each layer was compacted into the cylinder to a dry bulk density of 1.7 g cm–3, which is the dry bulk density of vertisol B horizons in north Senegal. Though a mixed slurry might have been more homogeneous, it would have produced a clay paste with skeleton included (quartz grains), without interaggregate structure, and would thus have shown a unit slope shrinkage for higher content than AE, which is a particular case in soils, while we wanted to reproduce a structure, that is, aggregates of clay matrix (with some skeleton inside) with interaggregate porosity. We didn't observe heterogeneity in our samples. The samples were submitted to five wetting and drying cycles to restore cohesion between aggregates and structure. Demineralized water was applied gently from the bottom through a porous plate over 3 d. The samples were then subjected to evaporation from the top during 4 d at 20°C. After the fifth drying cycle, the samples were removed from the cylinders and saturated again.
Shrinkage and Retention Curve Measurement
Simultaneous shrinkage and retention curves were obtained using the methods developed by Boivin (1990). The experiment was performed in a climatic chamber at 20°C. The samples were placed on a balance to record the evaporative water loss with time (Fig. 2). Linear displacement transducers were used to measure the changes in sample height upon drying in the vertical direction. Ceramic cups (2 cm long and 0.2 cm diameter) connected to pressure transducers were inserted in the middle of the samples to record the water pressure head. The experiment was stopped when the sample weight remained constant; this was observed after approximately 4 d of gentle evaporation. Recorded sample weight and dry sample weight were used to calculate the gravimetric water content. The bulk density was measured twice, at the beginning and at the end of the experiment, from hydrostatic weighing using the plastic bag method (Boivin et al., 1991; Tariq and Durnford, 1993b). Changes in sample height, were converted in sample volume using:
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with
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where V0, Vf, V, L0, Lf, and L are volume and height of the sample at the beginning, at the end and during the experiment, respectively. The calculated n values were close to 3, showing that the shrinkage of the samples was isotropic in that experiment.
Shrinkage Modeling
Experimental shrinkage curves were fitted with the XP model (Braudeau et al., 1999). This general shrinkage model adds the behavior of two pore volumes, namely micropore (intraaggregates) and macropore (interaggregates) volume, and the micropore volume is assumed to shrink like a clay paste. The XP model combines linear and exponential equations on segments delimited by four fitted points, named A, B, C, and D, coordinates of which are the parameters of the model. The Points A, B, and D are the SL, AE, and MS points of the micropore shrinkage submodel (Fig. 1). Point C is assumed to be a transition point, where the water content of macropore volume becomes null and was called macroporosity limit (LM) in Braudeau (1988). The A, B, C, and D points delimit five zones in the shrinkage curve that correspond to the five different stages of the shrinkage process: three linear zones that are the structural (Stirk, 1954), basic (Mitchell, 1992), and residual (Tariq and Durnford, 1993a) shrinkage zones separated by curvilinear zones. The transition points of the shrinkage curves were identified by many authors but denoted differently. The main notations are summarized in Table 4. In this paper, we shall use the notations of Braudeau et al. (1999) for the soil shrinkage curve and the notations SL, AE, and MS introduced above for the clay paste or clay matrix shrinkage curve.
The shrinkage curve of the micropore volume, Vmi, as a function of the micropore water content, Wmi, can be calculated from the soil shrinkage curve as shown in Braudeau and Bruand (1993). The equations of the micropore shrinkage submodel and the XP model used are reported in Table 5. For lower water content than Point C, the amount of water in the soil sample is equal to the amount of water in micropores if macropores are empty of water. This is certainly verified for most of the basic shrinkage phase, as the water pressure head is close to the wilting point pressure value –150 m at Point B and close to –10 m at Point C as found by Boivin (1990), and only very fine pore are thus saturated. The corresponding Wmi and Vmi values are thus calculated from the sample water content. Between saturation and Point B, Wmi is equal to Vmi because micropores are supposed to be saturated with water in this water content range, as B is assumed to be the AE of the micropore volume. For higher water content than maximum swelling of clay aggregates, Wmi and Vmi do not change any more. It is however possible that some nonconnected clay-matrix pores were not saturated. This was not reported by the authors working on clay paste, who found a shrinkage curve equal to the 1:1 load line between AE and MS (Sposito and Giraldez, 1976). However, if this occurs in the soil clay-matrix, it wouldn't change the slope of the clay-matrix shrinkage curve, but would introduce a shift, leading to a shrinkage curve parallel to the load line like presented in McGarry and Malafant (1987). The micropore volume calculated with XP model neglects this volume (called hidden pores in Fies and Bruand [1998]), which is thus included in the air volume of the macroporosity.
Shrinkage of Undisturbed Soil Core Samples
Two sets of undisturbed soil core samples were collected in the Senegal River Valley and Casamance River Valley (south Senegal), respectively, on soils developed on the river deposits. Both rivers come from the Fouta Djalon Mountains (Guinea Conakry) and both deposits are made from a mixture of kaolinite and smectites (Michel, 1973). The Casamance soil samples were collected in an 8-ha field 90 km from the seaside and the Senegal soil samples were collected in a 30000-ha area 250 km from the seaside. The clay fraction of Casamance River soils is dominated by kaolinite (Boivin 1990). Shrinkage curves were measured on 31 samples with clay content ranging from 4 to 64%. The clay fraction of Senegal River soils is dominated by smectite, which represents approximately 60% of the clay fraction (Boivin, 1997). Shrinkage curves were established on 343 samples. The clay content ranged from 31 to 86%. For the most clayey soils, some large cracks may appear in the samples, upon drying. This induces a shrinkage anisotropy. Most cracking was prevented by smoothing the faces of the cores before experiments. When macroscopic cracks appeared, or when the n exponent (Eq. [2]) was different from 3, the results were discarded. Note that the occurring of small cracks between aggregates in the sample belongs to the shrinkage process and is not a limitation for the setup we used, provided that the size of the cracks is small compared with the size of the samples, and that cracks have no preferential orientation. Intense cracking at small scale will result in an increase in the macropore volume of the soil upon drying and will be related to soil structure, as discussed below. All shrinkage curves were fitted with the XP model.
Precision of Measurements
We repacked samples with small changes in clay content between them, and a slight change in clay type between the two sets. Changing from 100% kaolinite to 100% smectite would have induced huge changes in shrinkage properties. The experimental setup was designed to characterize small changes in the sample properties. The clay percentage in the soil samples was determined by weighting the fractions with a precision of 0.01 g. The water content is estimated by continuous measurement of the sample weight with a precision of 0.01 g. The sample height is estimated with a precision of 1 µm. The change in diameter is converted to change in volume by using dry and wet volume calibrations, with a precision of 1 cm3 each (Boivin et al., 1991). The measurement error on the soil sample volume is thus about 2%, while the measurement error on the soil sample water content is <1
. The transition points on the shrinkage curve are fitted as the transition between linear and curvilinear phases (Braudeau et al., 1999). Their water content (first coordinate) is not influenced by the volume calibration. The micropore volume is determined by the water content at Points A, B, and D, and by the water content of the sample between these points. Thus error measurements on volume calibration do not affect the estimation of the micropore volumes, but only the estimation of the air content of the macropore volumes in the soil sample. Similarly, heterogeneities in the sample packing would lead to variations in estimated macropore volumes, but not on estimated micropore volumes.
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RESULTS AND DISCUSSION
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Shrinkage and Retention Curves
Figure 3 shows the shrinkage and retention curves measured simultaneously on two samples from Sets 1 and 2 with 29.4 and 22.6% clay content, respectively. Both shrinkage and retention curves show a similar S shape. Figure 4 shows the shrinkage curves measured on samples of sets 1 and 2. For clarity, a subset of 15 curves is presented in Fig. 4, 6, and 7. The specific volume was divided by the specific volume at Point A to negate the differences in initial specific volumes between samples. The shrinkage magnitude increases with increasing clay content. The shapes of the curves are slightly different between the two sets, with steeper slopes in Set 2. The shrinkage limit is reached for higher specific water contents in Set 2. This is in agreement with the higher kaolinite content (Tessier, 1980).
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Fig. 3. Shrinkage and retention curves of (a) a sample from Set 1 with 29.4% clay content and (b) a sample from Set 2 with 22.6% clay content.
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The XP model was fitted using the nonlinear regression simplex method. The model fits perfectly the experimental curves. Figure 5 shows the fitted and experimental curves for the 29.4% (Set 1) and the 22.6% clay content (Set 2) samples.
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Fig. 5. Soil shrinkage curve measured and fitted with the exponential model for (a) a sample of Set 1 with 29.4% clay content and for (b) a sample of Set 2 with 22.6% clay content.
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Relation between Equivalent Pore Radius and the Slope of the Shrinkage Curve
The slope of the shrinkage curve, that is, the ratio between specific volume variation and specific water content variation, can be related to the maximum radius of pores containing water. The equivalent pore radius was calculated from the measured soil pressure head using Jurin's law (e.g., Hillel, 1980). The slope of the shrinkage curve was expressed as a function of the equivalent pore radius for the soil samples of Sets 1 and 2 as shown in Fig. 6a and 6b, respectively. The D points fitted on the shrinkage curves are reported on the slopes of the shrinkage curves. Figure 6 shows that the slopes of the shrinkage curves are translated higher on the Y-axis when the clay content increases, corresponding to higher volume change with water content. A change in the slopes of shrinkage curves appears for pore radius of about 10 µm. For largest pore radii, the curves are nearly constant or slowly sloping. The slope values show a rapid increase for pore radius <10 µm. This behavior is noticeable for all the curves and the change in the slopes is close to the fitted value of the Point D. This is a consequence of the XP model structure, showing a transition between a linear and an exponential equation at Point D. Tensiometer readings were stopped for 2-µm equivalent pore radii and there was probably no more water in macropore volume for such pore radii, which is in agreement with the assumption of Braudeau (1988) on C point. The change in shrinkage slope values is less abrupt for the samples with higher clay contents, and the corresponding D value was fitted for larger pore size. The corresponding shrinkage curves show little changes in slope from structural to basic shrinkage, and thus the determination of Point D is less precise. These samples show shrinkage slope values close to one in basic phase and their behavior is thus quite similar to a clay paste.
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Fig. 6. Slope of the shrinkage curve as a function of maximum equivalent radius of saturated pore and Point D fitted with the exponential model for Sets (a) 1 and (b) 2.
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The water pressure head and pore radius at Point D seem therefore related to a transition in pore type. At lower water content (lower pressure head), the shrinkage curve shows high and increasing changes in soil volume with water content compared with the shrinkage associated with the drainage of larger pores. This shows that the pores <10 µm shrink highly upon drying, compared with larger pores. A similar observation is reported by Bruand and Prost (1987) on the basis of porosity, retention curves and shrinkage curves measurements on small soil aggregates. Fies and Bruand (1998), using mercury porosimetry and scanning electron microscopy, found that in ternary mixtures (clay/silt/sand) the clay fraction could be considered as associated with the silt fraction, and that independently of the sand fraction the silt–clay phase has lacunar and interparticle pores with pore diameter 
10 µm on dry samples.
Diamond (1970) found that the 10-µm transition pore radius was the largest pore size at which significant porosity was found in the clay matrix of natural soil clays.
Since our results showed that a transition in pore type occurred at pore radius equal or lower than 10 µm, it is reasonable to deduce that Point D represents the MS point of clay aggregates in the soil.
This result can be relevant as according to Tessier (1984) the pore radius is related to clay fabric and clay domain or tactoid size, and this kind of structure can be useful to create a pore-scale model of swelling soil (Tuller and Or, 2003).
Microporal Water Retention Curves
The micropore shrinkage curve is the specific micropore volume Vmi (cm3 g–1) as a function of the specific micropore water content Wmi (cm3 g–1) calculated using equations in Table 5. The micropore volume was divided by the clay content in the soil sample and was thus expressed in micropore volume per gram of clay, referred to as specific micropore volume in the following. The micropore retention curve is the pressure head, measured by the pressure transducer, as a function of the specific micropore volume. These curves are presented in Fig. 7. The pressure head is plotted for values lower than 150 cm, the specific micropore volume being constant for higher pressure-head values. Figure 7 shows parallel lines, with greater spacing for lowest clay content. Shrinking capacity and water retention of the specific micropore volume is higher for lower clay content. For clay content lower than 30%, there is an inverse proportionality between the clay content and the expansion of the specific micropore volume at a given pressure head. The retention curves are different for Sets 1 and 2, curves are less interspaced and have steeper slopes in Set 2. This behavior can be attributed to the higher kaolinite content in Set 2, as kaolinite is more rigid than smectite (e.g., Tessier and Pedro, 1980).
Micropore Volumes during Shrinkage
Figure 8 shows the specific micropore volume in the soil samples (per gram of clay mass), at Points A, B, C, and D, as a function of clay content. For the Points A, B, and C, the specific micropore volumes are constant whatever the clay content. At Point D, the specific micropore volume decreases with clay content for clay content lower than 35%. Therefore between Points C and D, that is, for equivalent maximum pore radii ranging from 2 to 10 µm, the shrinking capacity of the specific micropore volume decreases with increasing clay content. Conversely from Point A to Point C, the shrinking capacity of the specific micropore volume is independent of clay content. The fact that the calculated micropore specific volumes are constant regardless of the clay content for the lower water contents is remarkable, and can be related to the high precision of the micropore volume estimation, which depends only on the weighting precision in the method we used. This is however not surprising regarding the developments of pedotransfer functions, which generally showed high correlations between (i) water content and (ii) clay content and clay fabric in soil samples, mostly for low water contents (e.g., Bruand and Tessier 2000). The corresponding porosities at Point A, that is, the SL point of the clay matrix, are in good agreement with measurements of Diamond (1970) on clay pastes. The specific micropore volumes at Points A and B are lower in Set 1 than in Set 2. This is also in agreement with the literature. Tessier and Pedro (1980) showed lower shrinkage for kaolinite paste than for smectite paste, because of increasing order and parallel stacking of clay particles in the case of smectite, and because of disorder and size of domains in the case of kaolinite. Diamond (1970) reported smaller pore sizes and porosities for smectite paste than for kaolinite paste. The decrease in micropore specific volume at Point D with increasing clay content can be attributed to limitations in clay swelling at higher pressure heads. This is not surprising as the repacked dry bulk density (1.7 g cm–3) was quite high, and the clayey samples had thus a low macropore space available for clay-matrix swelling.
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Fig. 8. Micropore specific volume at the transition points fitted with the exponential model as a function of clay content for (a) Set 1 and (b) Set 2.
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The shrinking capacity of the soil increases with clay content (Fig. 4), though shrinking capacity of the specific micropore volume decreases with clay content (Fig. 8). This apparent contradiction can be interpreted as follows. The clay phase is the swelling agent. Thus the shrink-swell capacity of the soil tends to increase with clay content. The micropore volume in the soil is also increasing with clay content. But at high clay content, the swelling of the clay is limited by available space in the soil, mostly at high water content when the clay is close to its maximum swelling and has a low swelling pressure. Thus, the specific micropore volume (i.e., micropore volume per gram of clay) tends to decrease with clay content, at high water content. This steric limitation is probably related to soil structure and structure stability as discussed below.
Comparison between Behavior of Micropore Volume in Repacked and Undisturbed Soil Samples
Figure 9 presents the specific micropore volumes calculated via the XP model at Points A and D for the samples collected in the Senegal River Valley (Fig. 9a) and the Casamance River Valley (Fig. 9b), respectively. Senegal soil samples behaved like the repacked soil samples, with a constant specific micropore volume at Point A, and a slightly decreasing specific micropore volume with increasing clay content at Point D. Most of the soil samples had more than 40% clay content, and the decrease in D-specific micropore volume was thus low. The A-specific micropore volumes found were close to the A-specific micropore volume of samples repacked with the larger amount of Senegal vertisol clay (Set 1). The clay mineralogy of the Senegal River Valley soils is very homogeneous and the soil samples were collected in a noncropped area (Boivin, 1997), it is thus not surprising to find such homogeneity in specific micropore volumes. The Casamance soils showed higher A-specific micropore volumes, close to those of soil samples repacked with higher amount of kaolinite (Set 2), which is consistent with the higher amount of kaolinite in the clay fraction of Casamance soils. D-specific micropore volumes are decreasing with increasing clay content, and A-specific micropore volumes are highly variable and seem to decrease slightly with increasing clay content. This observation might be due to (i) the low clay contents of these samples, which were not comparable with the repacked samples, and (ii) a higher variability in clay type than for Senegal soils. This hypothesis is supported by the variability of the cation-exchange capacity per gram of clay of these soil samples, which is ranging from 0.1 to 0.35 (Boivin, 1990). Semi-quantitative clay percentages were not determined on these soils.
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Fig. 9. Micropore specific volume at Points A and D fitted with the exponential model as a function of (a) clay content for Senegal soil samples and (b) Casamance soil samples.
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The shrinking capacity of the specific micropore volume calculated for samples of Set 1 and 2 and for Casamance and Senegal undisturbed soil samples are reported in Fig. 10. The shrinking capacity of the specific micropore volume was calculated as the difference in the specific micropore volumes between the Points D and A. For both repacked and undisturbed soil samples, it seems that the shrinking capacity of the micropore volume increases with decreasing clay content, particularly for clay contents <40%. The swelling of clay aggregates appears thus to be limited for higher clay content. The Casamance valley soil samples have lower smectite content and thus show lower shrinking capacity than the Senegal valley soil samples and the repacked samples.
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Fig. 10. Shrinking capacity of samples from Sets 1 and 2 and of undisturbed samples from Senegal and Casamance.
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CONCLUSIONS
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Results of this investigation indicated that the transition Points A, B, and D can be considered as estimations of the SL, AE, and MS points of the clay-matrix within the soil. The micropore volume as estimated with XP model (Braudeau and Bruand, 1993) can be considered as the porosity of the clay–silt phase as described by Fies and Bruand (1998).
The soil samples showed different shrinkage properties according to clay type and clay content. The changes in kaolinite/smectite ratio are associated with changes in calculated micropore volumes and swelling capacity in good agreement with previous studies on clay pastes (e.g., Tessier and Pedro, 1980; Diamond, 1970). The decreasing swelling capacity of the micropore volume with increasing clay content was also described by Braudeau and Bruand (1993) and is in agreement with the observations of Fies and Bruand (1998) on the clay–silt phase. These authors showed on clay/silt/sand mixtures that with increasing clay content, the interconnected lacunar pores were decreasing in volume and continued to exist as hidden (not interconnected) pores, thus limiting the space available for clay–silt phase swelling.
According to our results, the clay-matrix can be considered as the swelling agent, but the bulk soil shrinkage also reflects the structure of the soil. In a rigid structure, most of the clay swelling will not induce soil swelling. Therefore, clay and soil fabric, as well as structural stability, could be further studied together with clay-matrix swelling properties, using shrinkage curves modeling.
Our results, which need further verification, confirm that shrinkage curves measured on undisturbed soil and fitted with the XP model can be used to calculate the clay matrix pore volume and water content in undisturbed soil samples (Braudeau and Bruand, 1993), and that this pore volume corresponds with the pore volume of the clay–silt phase (Fies and Bruand, 1998). The relevance of these findings is represented by the possibility of characterizing soil and clay physical properties without the need for clay extraction, which always sharply modifies the clay.
Received for publication April 4, 2003.
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ABSTRACT
INTRODUCTION
