Effect of Compaction on Soil Physical and Hydraulic

Effect of Compaction on Soil Physical and Hydraulic Properties: Experimental Results and Modeling S. Assouline, J. Tavares-Filho, and D. Tessier* ABSTRACT

in aggregate stability of soils with similar texture and mineralogy can be related to additional factors such as the degree of particle orientation. However, little attention has been paid to the compactive behavior of Oxisols comparatively with other soil types. The first objective of this study is thus to investigate the compaction process in the soils of Cascavel and Palotina to explain their differential behavior in the field. Another aspect of the study of soil compaction is the modeling of its effect on soil hydraulic properties. In an overview of research needs in soil compaction, Schafer et al. (1992) have stated that "significant knowledge gaps exist in the description and modeling of soil compaction behavior, in relating soil compaction behavior to agronomic responses (biological and physical) and to conservation of soil and water resources". Models describing the increase of soil bulk density with applied stress in nonoverconsolidated soils are available. The earliest model of soil compressibility described the volume strain of soil as a function of the logarithm of effective stress (Terzaghi and Peck, 1967). A two-parameter logarithmic model was proposed by Bailey and Vanden Berg (1967): = [mlog(o) [1] where p is the soil bulk density, o, the applied stress, and m and d, coefficients determined by least-squares regression techniques. Larson et al. (1980) have also adopted the logarithmic model to describe the compressibility of partly saturated soils. A three-parameter multiplicative model was proposed by Bailey et al. (1986): ln(p) = ln(Po) - (a + bo) (1 - e-c°) [2] where po is the bulk density at zero stress, and a, b, and c, coefficients determined by nonlinear curve fitting techniques. This model was extended to account for the effect of initial bulk density (McNabb and Boersma, 1993) and water content (McNabb and Boersma, 1996) on soil compression. The models in Eq. [1] and [2] differ in their boundary conditions for very low stress (including zero) and for very high stress. The model in Eq. [1] is undefined for a zero stress and is not adequate for a very low stress (Bailey et al., 1986). For G-*oo, this model assumes that p-*oo at a decreasing rate. The model in Eq. [2] satisfies the boundary condition of p = p0 for a zero stress. For a-*•«>, this model assumes that dp/do = -&p 0 e- (a+6o) [3] Since a and b are negative values (Bailey et al., 1986), the result is that when a-*«>, p-»-oo with an exponentially increasing rate, which is dependent upon PO. As stated by Bailey et al. (1986), one of the disadvantages of Eq. [2] is that it has three parameters. However, the main disadvantage of the two models, from our point of view,

Soil compaction affects soil physical properties and, eventually, crop production. A severe drop in the productivity of the state of Parana, southern Brazil, was observed due to soil compaction. Two oxisols from this region, a Haplic Acrothox from the site of Cascavel and a Haplic Eutrothox from the site of Palotina, presenting different compaction behaviors in the field, are studied under laboratory conditions. Uniaxial compressive pressures, from 50 to 1000 kPa, are applied to soil samples at different initial matric potentials, varying from - 0.1 to -1000 kPa. The bulk density of the Palotina soil is always higher than that of the Cascavel soil and is the highest when the initial matric tension is -32 kPa. Differences in pH, cation-exchange capacity, organic matter, and clay particle thickness also tend to explain the different compaction behaviors. A model of the soil bulk density increase during compaction is proposed and compared with a multiplicative model and a logarithmic model. The performances of the proposed and the multiplicative models are practically similar and better than those of the logarithmic model. The major advantage of the proposed model is that it has one fitting parameter less than the multiplicative model. Compaction affects the soil water retention curves for the whole range of matric tensions, up to —100 MPa. An approach that allows the evaluation of the hydraulic conductivity functions of the compacted samples is proposed. Applied to the Brooks and Corey relationship, the main drying curves of the compacted samples are well reproduced using one fitting parameter only.

OIL COMPACTION is practically inevitable in modern agronomy. It has been shown that soil compaction S affects water, heat, and gas exchange (Warkentin, 1971; Willis and Raney, 1971; Grable and Siemer, 1968; Linn and Doran, 1984), root penetration (Taylor et al., 1966), and consequently crop production (Hakansson et al., 1988). The productivity of the state of Parana, in southern Brazil, is seriously affected by compaction resulting from 30 yr of intensive cultivation (Tavares-Filho, 1995). The soil types in the region are Latosols, a type of Oxisol that developed from basaltic rocks. The term Latosol refers to leached kaolinitic clayey soils, having a particular micropedic structure, with millimeter-sized aggregates. Their iron oxide content is generally =20%. These soils present a similar textural analysis. However, different effects of compaction have been observed in two different sites of the region of Parana, Cascavel and Palotina. This difference is unexpected since soil compactibility is generally related to the soil textural composition, especially the percentage and the type of clay (Faure, 1981). Oxisols, and particularly their aggregate stability, have been studied in the past (Cagauan and Uehara, 1965). It has been shown that the variation S. Assouline and D. Tessier, I.N.R. A. Centre de Recherches de Versailles. Unite de science du sol, Route de St-Cyr, 78026 Versailles cedex, France; and J. Tavares-Filho, Universidade Estadual de Londrina, CCAAgronomia, Londrina, Brazil. Received 27 Nov. 1995. *Corresponding author ([email protected]).

Abbreviations: WRC, water retention curve; CEC, cation-exchange capacity; OM, organic matter.

Published in Soil Sci. Soc. Am. J. 61:390-398 (1997).

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ASSOULINE ET AL.: EXPERIMENTAL RESULTS AND MODELING OF COMPACTION EFFECT ON SOIL

is the assumption that p-*-oo when V|/a

with

& = (6 - er)/(e, - er)

[ii]

where 0S and 0r are the saturated and residual water contents, \l/a is the air entry value, and X is the pore-size distribution index. The saturated water content, 0S, is generally assumed to be equal or very close to the soil porosity. The residual water content, 0r, is defined as the water content at which the water capacity C(n/) = d0/dv|/-K) and the soil water conductivity K(Qr) = 0. In some studies, it is defined as the water content at the wilting point, identified in practice by \|/ = -1500 kPa (Rogowski, 1972; van Genuchten and Nielsen, 1985). Usually, it is regarded as an additional fitting parameter. The constants v|/a and X are also fitting parameters. The unsaturated hydraulic conductivity function of the soil, K(Se), can be defined, in terms of Mualem's (1976) model: t>vc\ _ v cn+2+2/X f*-\^e) — ^s'-^e

MOI L^^J

where Ks is the soil-saturated hydraulic conductivity, and n, a parameter accounting for the correlation between pores and the flow path tortuosity. Considering data from 45 soils, Mualem (1976) suggested that the best value for n might be 0.5. When a compressive pressure P is applied to a homogeneous soil of initial bulk density Pi at an initial water content 0i, leading to a bulk density p(P,6i), the WRC of the compacted soil is (6sc - 0rc) (V|//V|/ac)

-Xc

[13]

[5] [6]

Vac = V|/aH[p(/ ) ,0i)/Pi] P

[7]

where |x and (3 are positive constants, found to be equal to 0.99 and 3.72, respectively. (iii) The residual water content is considered to be mainly a function of the surface area of the soil particles and, thus, to be practically not affected by the soil compaction when expressed on a weight basis. This leads to the relation between volumetric water content and bulk density:

[4]

the solution of Eq. [4] becomes

where pmax(6i) is the highest bulk density reachable at the specific 0i. For each soil type, a specific water content permits compacting the soil to a maximal bulk density. Denoting, in that case, the specific values of pmax(0i) and £(6i) by piL, and £*, Eq. [7] becomes = pi+ (p*» - Pi)d -

\-\.

where the subscript c denotes the new parameters which characterize the compacted state. According to Mualem and Assouline (1989), the different parameters for the compacted state can be denned in terms of p(P,Q\): (i) Considering that the volumetric water content at saturation of compacted soils is equal to the porosity, the saturated water content is 0SC = 1 - p(P,6i)/p, [14] where ps is the solid particle density. (ii) Based on the data of Laliberte et al. (1966) and Smith and Woolhiser (1971), the air entry value is given by the relationships

where r\ and £ are parameters dependent on the specific soil and water content. For the initial and boundary conditions P = 0; p = Pi(0i)

The Model for the Hydraulic Properties of the Compacted Soil The soil hydraulic properties consist of the WRC, which describes the relationships between the volumetric water content, 0, and the matric potential V|/, and the hydraulic conductivity function which relates 0 to the hydraulic conductivity, K. According to the Brooks and Corey (1964) model, the WRC is expressed by

[8]

[15]

[16] (iv) Based on experimental data, the pore-size distribution index appears to decrease with increasing bulk density. How0rc = 6r [p(P,0i)/pJ

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ever, no quantitative relationship describing this relationship is available. Therefore, it is assumed, as a first approximation, that this index decreases linearly with the change in bulk density: where y is a positive constant dependent on the specific soil. Another widely used model for the WRC is the van Genuchten (1980) relationship: Se(V|/) = 1/[1 + (a\\V\b}m [18] where a, b, and m are fitting parameters. In the particular case where m is related to b by the equation m = 1 - I/b, a = \|/a~' and b = K + 1. As a result, the relationships proposed above are also directly applicable when the van Genuchten model is preferred. Once the WRC is defined for the compacted state, the hydraulic conductivity at saturation, Kx, can be estimated by applyifig principles similar to those suggested by Mualem (1986). This leads to n+2 6sc — 6rc

[19J

e s -e r

Substitution of Eq. [13] into Eq. [19] yields, after integration Asc

_— Ac

«_!_*>

1

T

e s -e r

The unsaturated hydraulic conductivity of the compacted soil can be defined as in Eq. [12]. The relationships presented above define the hydraulic properties of the compacted soil both in saturated and unsaturated conditions and express them in terms of properties of the initial uncompacted soil.

the samples at y, values of -1.0, -3.2, -10.0, -32.0, -100.0, -320.0, and -1000.0 kPa. Soil aggregates from the samples at »|/j = —3.2 kPa were embedded in an epoxy resin (Tessier, 1984). After hardening, thin sections were cut with a diamond knife. Electron micrographs at low (lOOOOx) magnification were made using a Philips (Model 420, Eindhoven, the Netherlands) transmission electron microscope, to characterize the soil constituents. A series of one-dimensional compressive pressures was applied to the samples that had been pre-equilibrated at the different \|/i. A piston was installed in the filtration apparatus, and pressures of 50, 100, 150, 200, 250, 300, 400, 500, 600, 800, and 1000 kPa were applied. Few minutes were generally necessary to reach equilibrium. The volume change of the sample was evaluated by measuring the piston displacement. Water was allowed to drain out of the filtration apparatus during the compression experiments. After equilibrium was reached, and when no drainage was observed, the samples were extracted from the apparatus. Water retention curves and saturated hydraulic conductivity were measured on the samples pre-equilibrated at \\i, = -32.0 kPa, which had been compacted at 1000 kPa pressure. The main drying curves of the saturated samples were obtained using pressure cells for matric tensions up to —1.6 MPa. Dessicators with a range of relative humidity between 95 and 50% were used for matric tensions between —2 and —100 MPa. The main wetting curves were obtained using dry samples at 50% relative humidity and applying the sample procedure as for the drying curves but with increasing matric potentials. Volume change of the samples at the different matric potential steps of the WRC were measured according to the method presented in Tessier and Berrier (1979), to account for possible swelling and shrinking of the samples. As a result, the specific bulk density of each sample at every step was monitored, and the volumetric water content corresponding to the matric potential at each step determined with accuracy. The saturated hydraulic conductivity, K^, of the compacted samples was measured using constant-head permeameters.

MATERIALS AND METHODS The two soil types were a Haplic Acrothox from the site of Cascavel and a Haplic Eutrothox from the site of Palotina. Comparative studies in the region have shown that the B horizon (120-150 cm deep) was not affected by external mechanical stresses from tillage or traffic (Tavares-Filho, 1995). It could represent the soil properties before intensive cultivation. Therefore, the 120 to 150-cm depth zone of each soil was selected to study its compaction behavior. Some characteristics of the two soils are given in Table 1. Undisturbed blocks, of = 5000 cm3, were collected in the field. They were put into plastic bags to preserve their humidity and kept in a refrigerator ( = 4°C) before treatment. Coarse fragments of the blocks were gently fragmented and sieved. The < 5-mm fraction was kept, and samples of 40 cm3 were placed in a filtration apparatus for water retention measurements and for mechanical compaction (Sala and Tessier, 1993). The samples were fully rehydrated and brought to equilibrium at various matric potentials, \\i,. For this purpose, gas pressure was applied to the filtration apparatus and selected filter pore sizes were used to prepare

All the compression experiments as well as the WRC and Ks measurements were taken in 10 replicates. The fitting procedure used was an iterative nonlinear regression using the Marquardt-Levenberg algorithm to find the values of the parameters of the independent variable that gives, the best fit between the model and the data, i.e., that minimize the mean square errors between the observed and predicted values of the dependent variable (Glantz and Slinker, 1990). The square root of this mean is defined as the norm index, which is an indicator of the goodness of the fit reached.

RESULTS AND DISCUSSION The Compaction Behavior of the Two Soils The relationships between the final bulk density (p) and initial matric pressure (\|/i) are depicted in Fig. la and Ib for four different compressive pressures. The initial bulk density of the two soils is approximately the same. In spite of the high amount of clay, shrinking

Table 1. The physical and chemical properties of the soils of Cascavel and Palotina. Soils Cascavel Palotina

Clay

0.81 0.83

Silt _,

Sand

CECt

0.17 0.11

0.02 0.06

meqlOOg-' 4.6 6.6

PH 4.9 6.5

OMt

W

P,t

8.0 4.0

m s~' 4 x 10-' 3 x 10-'

Mgm-3 2.88 2.94

t CEC = cation-exchange capacity; OM = organic matter; K, = saturated hydraulic property; p, = solid particle density.

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-LOkPa) (-10.0kPa) - MOO.OkPa) 200

400

600

(-3.2 kPa)

(-32.0 kPa) (-1000.0 kPa) 800

1000

COMPRESSIVE PRESSURE (kPa) Fig. 2. Ratio between the bulk density of the Palotina and Cascavel soils at different applied pressures, for different \)»i conditions.

10.0

100.0

1000.0

-(INITIAL MATRIC POTENTIAL) (kPa) Fig. 1. Bulk densities obtained at different initial matric potentials, vj/i, for four of the eleven compressive pressures applied: (a) Palotina soil, (b) Cascavel soil. The dashed lines represent the bulk density of the uncompacted samples at each \|>i.

resulting from drying is limited and only slight changes in p are measured with the decrease in \|/j. For the lowest pressure applied (50 kPa), the same compaction behavior is observed for both soils. The highest p is achieved for V|/i = — 1.0 kPa, in other words, when the soil is saturated. For the two intermediate pressures presented (200 and 500 kPa), a different behavior is observed for each soil. For the Palotina soil, the highest p at 200 kPa applied pressure is achieved for v|/j = -10.0 kPa, and the highest p at 500 kPa is achieved for \j/j = —32.0 kPa, demonstrating clear effect of V|/i on p. On the contrary, for the Cascavel soil, the highest p is still achieved in both cases for \\i\ = —1.0 kPa, with practically no effect of \\i, on p, especially in the P = 500 kPa case. For the highest pressure applied (1000 kPa), the maximal p of the two soils is obtained when \i/j = -32.0 kPa. However, the bulk density achieved in the Palotina soil (~ 1.55 Mg m~3) is higher than that of the Cascavel soil (~ 1.30 Mg m~3). In Fig. 2, the results obtained for all compressive pressures applied at each \\it are presented in terms of ratio between the bulk density of the Palotina soil and that of the Cascavel soil. For the whole range of pressures, the ratio is greater than one, indicating that for any given W and P, the Palotina soil reaches a higher bulk density than the Cascavel soil. The highest ratios, around 1.2,

are achieved when \|/j = -10.0 kPa for 100 kPa < P < 300 kPa, and when \\i, = -32.0 kPa, for P > 400 kPa. It is interesting to note the similarity in the relative compactibility of these two soils when \\i\ = —1.0 and -1000.0 kPa and when \\it = -3.2 and -100.0 kPa. Beyond the similarity in textural analysis, the two soils present differences in physico-chemical properties, which are known to affect soil stability. The two soils differ in particle thickness and crystallinity of the respective clay fractions. Electron micrographs of thin sections of the two soils (Fig. 3) show that the clay particles, including kaolinite and oxides, are coarser in the Palotina soil (A) than in the Cascavel soil (B). It has been shown that the thinner the clay particles, the larger the surface area in contact between soil constituents, thus inferring a higher stability to the fabric (Tessier, 1991; Van Damme and Ben Ohoud, 1989). The soils also differ in pH, CEC, and OM content (Table 1). As shown by Guerif and Faure (1979) and O'Sullivan (1992), the presence of OM decreases the soil compaction sensitivity to initial water content and decreases the bulk density reached after compaction. The pH also affects the structure stability of strongly weathered soils such as Oxisols where kaolinite and oxides are dominant (El Swaifi, 1980; Me Bride, 1989; Schwertmann and Taylor, 1989). Under the acidic conditions of the Cascavel soil, the surfaces of iron oxides are mainly positively charged, while kaolinite surfaces are negatively charged. The resulting attraction forces between the oppositely charged soil constituents impart some physical stability to the clay aggregates. By contrast, in die Palotina soil, where pH is close to neutrality, iron oxides have very low charges and therefore a lower stability is obtained. Also, because of the low pH of the Cascavel soil, the presence of free aluminum hydroxides was only observed in this soil (Fig. 3). Free aluminum hydroxides act as a ligand and are more effective than iron oxides in maintaining the stability of soil aggregates (El Swaifi and Emerson, 1975). The effectiveness of the free aluminum hydroxides and the iron oxides in stability is increased by the presence of OM (Edwards and Bremner, 1967).

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SOIL SCI. SOC. AM. J., VOL. 61, MARCH-APRIL 1997

Fig. 3. Transmission electron micrographs of thin sections of the two uncompacted soils: (a) Palotina, (b) Cascavel (black surfaces represent particles; arrows indicate aluminum hydroxides).

395


All these differences in soil composition, which determine the chemical bonding strength, are related to the sensibility of compressibility to the initial matric pressure. They provide potential explanations for the higher compactibility observed in the Palotina soil. It is worth noting that, when differences in compaction behavior resulting from intensive cultivation are considered, the practical aspect of the sensitivity to initial matric pressure can play a preponderant role. Cultivation practices are usually carried out only when the upper soil layer is dry enough to allow the use of heavy machines, that is, when the matric potential in this layer reaches values between —10 and —30 kPa. This is precisely the range of matric potentials where the compaction of the Palotina soil is maximal within a wide range of compressive pressures (Fig. 2).

The Dynamic Change of the Soil Bulk Density during Compaction The measured bulk densities obtained at increasing P, for the samples initially at V|/, = —32 kPa, are depicted in Fig. 4. The fitted curves corresponding to the logarithmic model of Bailey and Vanden Berg (1967) and Larson et al. (1980) (Eq. [1]), the multiplicative model of Bailey et al. (1986) (Eq. [2]), and the proposed model (Eq. [8]) are also presented. The values of the fitting parameters and of the corresponding norm index are given in Table 2. The logarithmic model, which represents the measured data very roughly, leads to the less satisfactory results. The multiplicative model and the proposed model have similar performances. However, on a conceptual basis, the multiplicative model assumes that p-*oo with an exponentially increasing rate when P-*oo, which does not occur experimentally, while the proposed model yields the experimentally correct value p = pmax(6i) as P-*oo. Thus, one of the two parameters of the proposed model [Pmax(6i)] has a physical meaning, effectively transforming the model into an expression with only one empirical fitting parameter. The multiplicative model, on the other hand, requires three fitting parameters. It has been shown that compression tests under laboratory

Table 2. The parameters, and the corresponding norm index, resulting from fitting the three compaction models (Eq. [1], [2], and [8]) to measured bulk density resulting from the application of increasing compressive pressures to samples initially at y, = -32kPa. Soil type Model type Parameter 1 Palotina Multiplicative Logarithmic Proposed Cascavel Multiplicative Logarithmic Proposed

a = -0.86 m = -0.35 p,™,* = 1.57 a = -0.70 m — —0.36 Pm** = 1.33

Parameter 2

Parameter 3 Norm

3 b = - 1.26 10-"