effect of compaction on soil physical and hydraulic properties: experimental results and modeling

Effect of Compaction on Soil Physical and Hydraulic Properties:Experimental Results and Modeling

S. Assouline, J. Tavares-Filho, and D. Tessier*

ABSTRACTSoil compaction affects soil physical properties and, eventually,

crop production. A severe drop in the productivity of the state ofParana, southern Brazil, was observed due to soil compaction. Twooxisols from this region, a Haplic Acrothox from the site of Cascaveland a Haplic Eutrothox from the site of Palotina, presenting differentcompaction behaviors in the field, are studied under laboratory condi-tions. Uniaxial compressive pressures, from 50 to 1000 kPa, are appliedto soil samples at different initial matric potentials, varying from - 0.1to -1000 kPa. The bulk density of the Palotina soil is always higherthan that of the Cascavel soil and is the highest when the initial matrictension is -32 kPa. Differences in pH, cation-exchange capacity,organic matter, and clay particle thickness also tend to explain thedifferent compaction behaviors. A model of the soil bulk densityincrease during compaction is proposed and compared with a multi-plicative model and a logarithmic model. The performances of theproposed and the multiplicative models are practically similar andbetter than those of the logarithmic model. The major advantage ofthe proposed model is that it has one fitting parameter less than themultiplicative model. Compaction affects the soil water retention curvesfor the whole range of matric tensions, up to —100 MPa. An approachthat allows the evaluation of the hydraulic conductivity functions ofthe compacted samples is proposed. Applied to the Brooks and Coreyrelationship, the main drying curves of the compacted samples arewell reproduced using one fitting parameter only.

SOIL COMPACTION is practically inevitable in modernagronomy. It has been shown that soil compaction

affects water, heat, and gas exchange (Warkentin, 1971;Willis and Raney, 1971; Grable and Siemer, 1968; Linnand Doran, 1984), root penetration (Taylor et al., 1966),and consequently crop production (Hakansson et al.,1988). The productivity of the state of Parana, in southernBrazil, is seriously affected by compaction resulting from30 yr of intensive cultivation (Tavares-Filho, 1995). Thesoil types in the region are Latosols, a type of Oxisolthat developed from basaltic rocks. The term Latosolrefers to leached kaolinitic clayey soils, having a particu-lar micropedic structure, with millimeter-sized aggre-gates. Their iron oxide content is generally =20%.These soils present a similar textural analysis. However,different effects of compaction have been observed intwo different sites of the region of Parana, Cascaveland Palotina. This difference is unexpected since soilcompactibility is generally related to the soil texturalcomposition, especially the percentage and the type ofclay (Faure, 1981). Oxisols, and particularly their aggre-gate stability, have been studied in the past (Cagauanand 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, CCA-Agronomia, Londrina, Brazil. Received 27 Nov. 1995. *Correspondingauthor ([email protected]).

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

in aggregate stability of soils with similar texture andmineralogy can be related to additional factors such asthe degree of particle orientation. However, little atten-tion has been paid to the compactive behavior of Oxisolscomparatively with other soil types. The first objectiveof this study is thus to investigate the compaction processin the soils of Cascavel and Palotina to explain theirdifferential behavior in the field.

Another aspect of the study of soil compaction is themodeling of its effect on soil hydraulic properties. In anoverview of research needs in soil compaction, Schaferet al. (1992) have stated that "significant knowledge gapsexist in the description and modeling of soil compactionbehavior, in relating soil compaction behavior to agro-nomic responses (biological and physical) and to conser-vation of soil and water resources". Models describingthe increase of soil bulk density with applied stressin nonoverconsolidated soils are available. The earliestmodel of soil compressibility described the volume strainof soil as a function of the logarithm of effective stress(Terzaghi and Peck, 1967). A two-parameter logarithmicmodel 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-squaresregression techniques. Larson et al. (1980) have alsoadopted the logarithmic model to describe the compress-ibility of partly saturated soils. A three-parameter multi-plicative 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 fittingtechniques. This model was extended to account for theeffect 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] differin their boundary conditions for very low stress (includingzero) and for very high stress. The model in Eq. [1] isundefined for a zero stress and is not adequate for avery low stress (Bailey et al., 1986). For G-*oo, thismodel assumes that p-*oo at a decreasing rate. The modelin Eq. [2] satisfies the boundary condition of p = p0for a zero stress. For a-*•«>, this model assumes that

dp/do = -&p0e-(a+6o) [3]Since a and b are negative values (Bailey et al., 1986),the result is that when a-*«>, p-»-oo with an exponentiallyincreasing rate, which is dependent upon PO. As statedby Bailey et al. (1986), one of the disadvantages of Eq.[2] is that it has three parameters. However, the maindisadvantage of the two models, from our point of view,

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

390

ASSOULINE ET AL.: EXPERIMENTAL RESULTS AND MODELING OF COMPACTION EFFECT ON SOIL 391

is the assumption that p-*-oo when <j-*oo, while experi-mental data indicates that soils can be compacted up toa finite maximal bulk density, depending on the specificsoil and its initial water content (Amir et al., 1976;Faure, 1981). The second objective of our study is,therefore, to present a more simple two-parameter modelfor p(o), based on a physical concept and satisfying adifferent boundary condition for a very high stress.

As stated earlier, compaction affects soil hydraulicproperties (Gill and Vanden Berg, 1967). Experimentaldata relating the effect of the increase in the soil bulkdensity on soil hydraulic properties is quite scarce. Thetrends observed are that soil compaction (i) decreasesthe saturated water content and increases the air entryvalue (Croney and Coleman, 1954; Laliberte et al. , 1966;Smith and Woolhiser, 1971; Libardi et al., 1982) and(ii) decreases the saturated hydraulic conductivity (Dawi-dowsky and Koolen, 1987) and consequently, the soilinfiltrability (Akram and Kemper, 1979).

Approaches towards modeling the effect of the increasein the soil bulk density on soil hydraulic propertiesare very limited (Baumhardt et al., 1990; Mualem andAssouline, 1989). They were developed to infer hydraulicproperties to the seal layer formed at the soil surface bythe action of raindrops. The advantage of the approachof Mualem and Assouline (1989) is that it is conceptualand addresses both the soil water retention curve andthe hydraulic conductivity function. The third objectiveof this study is to apply this approach to modeling theeffect of compaction on the soil water retention curveand the hydraulic conductivity function.

THEORYThe Dynamic Model of Compaction

Consider a homogeneous nonoverconsolidated soil volumeof initial bulk density pi at a given volumetric water content0j. One may expect that, as the soil is progressively compacted,it will be more difficult to compact it further. In other words,the higher the soil bulk density, the smaller the increase ofthe bulk density, Ap, resulting from the increase of the com-pressive pressure applied, AP. This can be expressed throughthe linearly decreasing function

dp/dP = Ti(0,) - [4]where r\ and £ are parameters dependent on the specific soiland water content. For the initial and boundary conditions

P = 0; p = Pi(0i)

the solution of Eq. [4] becomes

[5][6]

[7]where pmax(6i) is the highest bulk density reachable at thespecific 0i. For each soil type, a specific water content permitscompacting the soil to a maximal bulk density. Denoting, inthat case, the specific values of pmax(0i) and £(6i) by piL, and£*, Eq. [7] becomes

The Model for the Hydraulic Propertiesof the Compacted Soil

The soil hydraulic properties consist of the WRC, whichdescribes the relationships between the volumetric water con-tent, 0, and the matric potential V|/, and the hydraulic conductiv-ity function which relates 0 to the hydraulic conductivity, K.According to the Brooks and Corey (1964) model, the WRCis expressed by

J/a [9]

[10]

\-\.

with> V|/a

& = (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 distributionindex. The saturated water content, 0S, is generally assumedto be equal or very close to the soil porosity. The residualwater content, 0r, is defined as the water content at which thewater capacity C(n/) = d0/dv|/-K) and the soil water conductiv-ity K(Qr) = 0. In some studies, it is defined as the watercontent 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 MOIf*-\^e) — ^s'-^e L^^J

where Ks is the soil-saturated hydraulic conductivity, and n,a parameter accounting for the correlation between pores andthe flow path tortuosity. Considering data from 45 soils, Mua-lem (1976) suggested that the best value for n might be 0.5.

When a compressive pressure P is applied to a homogeneoussoil of initial bulk density Pi at an initial water content 0i,leading to a bulk density p(P,6i), the WRC of the compactedsoil is

(6sc - 0rc) (V|//V|/ac)-Xc [13]

where the subscript c denotes the new parameters which charac-terize the compacted state. According to Mualem and Assouline(1989), the different parameters for the compacted state canbe denned in terms of p(P,Q\):

(i) Considering that the volumetric water content at saturationof compacted soils is equal to the porosity, the saturated watercontent 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 Smithand Woolhiser (1971), the air entry value is given by therelationships

Vac = V|/aH[p(/),0i)/Pi]P [15]

where |x and (3 are positive constants, found to be equal to0.99 and 3.72, respectively.

(iii) The residual water content is considered to be mainlya function of the surface area of the soil particles and, thus,to be practically not affected by the soil compaction whenexpressed on a weight basis. This leads to the relation betweenvolumetric water content and bulk density:

0rc = 6r [p(P,0i)/pJ [16]

= pi+ (p*» - Pi)d - [8](iv) Based on experimental data, the pore-size distribution

index appears to decrease with increasing bulk density. How-

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

ever, no quantitative relationship describing this relationshipis available. Therefore, it is assumed, as a first approximation,that this index decreases linearly with the change in bulkdensity:

where y is a positive constant dependent on the specific soil.Another widely used model for the WRC is the van Genuch-

ten (1980) relationship:

Se(V|/) = 1/[1 + (a\\V\b}m [18]where a, b, and m are fitting parameters. In the particularcase where m is related to b by the equation m = 1 - I/b,a = \|/a~' and b = K + 1. As a result, the relationshipsproposed above are also directly applicable when the vanGenuchten model is preferred.

Once the WRC is defined for the compacted state, thehydraulic conductivity at saturation, Kx, can be estimated byapplyifig principles similar to those suggested by Mualem(1986). This leads to

6sc — 6rcn+2

e s-e r[19J

Substitution of Eq. [13] into Eq. [19] yields, after integration«_!_*> 1 T

_Asc — Ac e s-e r

The unsaturated hydraulic conductivity of the compacted soilcan be defined as in Eq. [12]. The relationships presentedabove define the hydraulic properties of the compacted soilboth in saturated and unsaturated conditions and express themin terms of properties of the initial uncompacted soil.

MATERIALS AND METHODSThe 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 Bhorizon (120-150 cm deep) was not affected by external me-chanical stresses from tillage or traffic (Tavares-Filho, 1995). Itcould represent the soil properties before intensive cultivation.Therefore, the 120 to 150-cm depth zone of each soil wasselected to study its compaction behavior. Some characteristicsof the two soils are given in Table 1. Undisturbed blocks, of= 5000 cm3, were collected in the field. They were put intoplastic bags to preserve their humidity and kept in a refrigerator( = 4°C) before treatment. Coarse fragments of the blocks weregently fragmented and sieved. The < 5-mm fraction was kept,and samples of 40 cm3 were placed in a filtration apparatus forwater retention measurements and for mechanical compaction(Sala and Tessier, 1993). The samples were fully rehydratedand brought to equilibrium at various matric potentials, \\i,.For this purpose, gas pressure was applied to the filtrationapparatus and selected filter pore sizes were used to prepare

the samples at y, values of -1.0, -3.2, -10.0, -32.0,-100.0, -320.0, and -1000.0 kPa. Soil aggregates fromthe samples at »|/j = —3.2 kPa were embedded in an epoxyresin (Tessier, 1984). After hardening, thin sections were cutwith a diamond knife. Electron micrographs at low (lOOOOx)magnification were made using a Philips (Model 420, Eindho-ven, the Netherlands) transmission electron microscope, tocharacterize the soil constituents. A series of one-dimensionalcompressive pressures was applied to the samples that hadbeen pre-equilibrated at the different \|/i. A piston was installedin 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 measuringthe piston displacement. Water was allowed to drain out ofthe filtration apparatus during the compression experiments.After equilibrium was reached, and when no drainage wasobserved, the samples were extracted from the apparatus.

Water retention curves and saturated hydraulic conductivitywere measured on the samples pre-equilibrated at \\i, = -32.0kPa, which had been compacted at 1000 kPa pressure. Themain drying curves of the saturated samples were obtainedusing pressure cells for matric tensions up to —1.6 MPa.Dessicators with a range of relative humidity between 95 and50% were used for matric tensions between —2 and —100MPa. The main wetting curves were obtained using dry samplesat 50% relative humidity and applying the sample procedureas for the drying curves but with increasing matric potentials.Volume change of the samples at the different matric potentialsteps of the WRC were measured according to the methodpresented in Tessier and Berrier (1979), to account for possibleswelling and shrinking of the samples. As a result, the specificbulk density of each sample at every step was monitored,and the volumetric water content corresponding to the matricpotential at each step determined with accuracy.

The saturated hydraulic conductivity, K^, of the compactedsamples was measured using constant-head permeameters.

All the compression experiments as well as the WRC andKs measurements were taken in 10 replicates.

The fitting procedure used was an iterative nonlinear regres-sion using the Marquardt-Levenberg algorithm to find thevalues of the parameters of the independent variable that gives,the best fit between the model and the data, i.e., that minimizethe mean square errors between the observed and predictedvalues 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 DISCUSSIONThe Compaction Behavior of the Two SoilsThe relationships between the final bulk density (p)

and initial matric pressure (\|/i) are depicted in Fig. laand Ib for four different compressive pressures. Theinitial bulk density of the two soils is approximately thesame. In spite of the high amount of clay, shrinking

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

Soils

CascavelPalotina

Clay

0.810.83

Silt_,

0.170.11

Sand

0.020.06

CECtmeqlOOg-'

4.66.6

PH

4.96.5

OMt

8.04.0

W

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

P,tMgm- 3

2.882.94

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


10.0 100.0-(INITIAL MATRIC POTENTIAL) (kPa)

1000.0

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

resulting from drying is limited and only slight changesin p are measured with the decrease in \|/j.

For the lowest pressure applied (50 kPa), the samecompaction behavior is observed for both soils. Thehighest p is achieved for V|/i = — 1.0 kPa, in other words,when the soil is saturated. For the two intermediatepressures presented (200 and 500 kPa), a different behav-ior is observed for each soil. For the Palotina soil, thehighest p at 200 kPa applied pressure is achieved forv|/j = -10.0 kPa, and the highest p at 500 kPa is achievedfor \j/j = —32.0 kPa, demonstrating clear effect of V|/ion p. On the contrary, for the Cascavel soil, the highestp is still achieved in both cases for \\i\ = —1.0 kPa,with practically no effect of \\i, on p, especially in theP = 500 kPa case. For the highest pressure applied(1000 kPa), the maximal p of the two soils is obtainedwhen \i/j = -32.0 kPa. However, the bulk densityachieved in the Palotina soil (~ 1.55 Mg m~3) is higherthan that of the Cascavel soil (~ 1.30 Mg m~3). In Fig.2, the results obtained for all compressive pressuresapplied at each \\it are presented in terms of ratio betweenthe bulk density of the Palotina soil and that of theCascavel soil. For the whole range of pressures, theratio is greater than one, indicating that for any givenW and P, the Palotina soil reaches a higher bulk densitythan the Cascavel soil. The highest ratios, around 1.2,

(-3.2 kPa)(-32.0 kPa)(-1000.0 kPa)

-LOkPa)(-10.0kPa)

- MOO.OkPa)

200 400 600 800 1000COMPRESSIVE PRESSURE (kPa)

Fig. 2. Ratio between the bulk density of the Palotina and Cascavelsoils at different applied pressures, for different \)»i conditions.

are achieved when \|/j = -10.0 kPa for 100 kPa < P< 300 kPa, and when \\i, = -32.0 kPa, for P > 400kPa. It is interesting to note the similarity in the relativecompactibility 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 soilspresent differences in physico-chemical properties, whichare known to affect soil stability.

The two soils differ in particle thickness and crystallin-ity of the respective clay fractions. Electron micrographsof thin sections of the two soils (Fig. 3) show that theclay particles, including kaolinite and oxides, are coarserin the Palotina soil (A) than in the Cascavel soil (B). Ithas been shown that the thinner the clay particles, thelarger the surface area in contact between soil constit-uents, thus inferring a higher stability to the fabric (Tes-sier, 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) andO'Sullivan (1992), the presence of OM decreases thesoil compaction sensitivity to initial water content anddecreases the bulk density reached after compaction.The pH also affects the structure stability of stronglyweathered soils such as Oxisols where kaolinite andoxides are dominant (El Swaifi, 1980; Me Bride, 1989;Schwertmann and Taylor, 1989). Under the acidic condi-tions of the Cascavel soil, the surfaces of iron oxidesare mainly positively charged, while kaolinite surfacesare negatively charged. The resulting attraction forcesbetween the oppositely charged soil constituents impartsome physical stability to the clay aggregates. By con-trast, in die Palotina soil, where pH is close to neutrality,iron oxides have very low charges and therefore a lowerstability is obtained.

Also, because of the low pH of the Cascavel soil, thepresence of free aluminum hydroxides was only observedin this soil (Fig. 3). Free aluminum hydroxides act asa ligand and are more effective than iron oxides inmaintaining the stability of soil aggregates (El Swaifi andEmerson, 1975). The effectiveness of the free aluminumhydroxides and the iron oxides in stability is increasedby the presence of OM (Edwards and Bremner, 1967).


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


All these differences in soil composition, which deter-mine the chemical bonding strength, are related to thesensibility of compressibility to the initial matric pres-sure. They provide potential explanations for the highercompactibility observed in the Palotina soil. It is worthnoting that, when differences in compaction behaviorresulting from intensive cultivation are considered, thepractical aspect of the sensitivity to initial matric pressurecan play a preponderant role. Cultivation practices areusually carried out only when the upper soil layer is dryenough to allow the use of heavy machines, that is, whenthe matric potential in this layer reaches values between—10 and —30 kPa. This is precisely the range of matricpotentials where the compaction of the Palotina soil ismaximal within a wide range of compressive pressures(Fig. 2).

The Dynamic Change of the Soil Bulk Densityduring Compaction

The measured bulk densities obtained at increasing P,for the samples initially at V|/, = —32 kPa, are depicted inFig. 4. The fitted curves corresponding to the logarithmicmodel of Bailey and Vanden Berg (1967) and Larson etal. (1980) (Eq. [1]), the multiplicative model of Baileyet al. (1986) (Eq. [2]), and the proposed model (Eq.[8]) are also presented. The values of the fitting parame-ters and of the corresponding norm index are given inTable 2. The logarithmic model, which represents themeasured data very roughly, leads to the less satisfactoryresults. The multiplicative model and the proposed modelhave similar performances. However, on a conceptualbasis, the multiplicative model assumes that p-*oo withan exponentially increasing rate when P-*oo, which doesnot occur experimentally, while the proposed modelyields the experimentally correct value p = pmax(6i) asP-*oo. Thus, one of the two parameters of the proposedmodel [Pmax(6i)] has a physical meaning, effectively trans-forming the model into an expression with only oneempirical fitting parameter. The multiplicative model,on the other hand, requires three fitting parameters. Ithas been shown that compression tests under laboratory

200 400 600 800 1000COMPRESSIVE PRESSURE (kPa)

Fig. 4. Measured bulk densities of the compacted samples (initiallyat <)/i = - 32 kPa), at each of the pressures applied (full circles)and the fitted curves representing the three dynamic models com-pared (Eq. [1], Eq. [2], and Eq. [8]).

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 applica-tion of increasing compressive pressures to samples initially aty, = -32kPa.

Soil type Model type Parameter 1Palotina Multiplicative a = -0.86 b

Logarithmic m = -0.35 dProposed p,™,* = 1.57 tf

Cascavel Multiplicative a = -0.70 6Logarithmic m — —0.36 dProposed Pm** = 1.33 !;"

Parameter 2 Parameter 3 Norm= - 1.26 10-" <= 1.67

i- = 4.1 10-'= -1.40 10-" <= 1.85' = 3.5 10-3

: = 7.010-3 0.060.390.04

: = 6.110-3 0.020.370.02

conditions can predict the maximal bulk density reachedin the field (O'Sullivan, 1992; Hartge, 1986). In ourcase, the bulk densities obtained by the application of acompressive pressure of 1000 kPa to samples at v|/i =— 32 kPa are similar to the respective bulk densitiesmeasured in the field in the upper layer of the cultivatedsoils. In such a case, p*ax can be evaluated independentlyso that only one parameter, £*, remains to be determinedto define the dynamics of the compaction of the specificsoil studied.

The Effect of Compaction on the WaterRetention Curve

The main drying and wetting curves of the uncom-pacted and the compacted soils of Palotina and Cascavelare presented in Fig. 5a and 5b.

- - 0 - INITIAL-drying—•—COMPAC - drying• - Q- - INITIAL - wetting—•—COMPAC - wetting

• - O - INITIAL - drying—•—COMPAC - drying• - n- - INITIAL - wetting—•—COMPAC - wetting

1.0 10.0 100.0 1000.0-(MATRIC POTENTIAL) (kPa)

10000.0 100000.0

Fig. 5. Main wetting curves (squares) and main drying curves (circles)for uncompacted and compacted samples of (a) Palotina, (b) Casca-vel. Empty symbols represent uncompacted soil and full symbolsdenote compacted soil. The compaction conditions are (\\f> = — 32kPa, P = 1000 kPa).


Table 3. The parameters resulting from fitting the Brooks andCorey relationship to the data of the main drying curve forthe uncompacted soils.t

Table 4. The parameters of the Brooks and Corey relationshipfor the main drying curve of the compacted soils (at P = 1000kPa), computed using the relationships in Eq. [14] to [17].t

Soils

CascavelPalotina

PiMg m~3

0.961.25

<t>

0.670.57

es

0.580.49

e,

0.330.32

V,kPa-2.2-2.5

X

0.610.45

t Pi = initial bulk density; <t> = soil porosity; O, = saturated water content;Or = residual water content; i|/a = air entry value; X = pore-size distributionindex.

For the uncompacted soils, 9S is smaller than the soilporosity, <j>, indicating that air is easily entrapped in thesoil samples (Table 3). Less air is entrapped duringwetting from dry initial state, so that 0sW > 9so for thetwo soils. After compaction, 9SC for the drying curve ispractically similar to 0C, indicating negligible entrappedair at 9S after compaction. At the end of the wettingprocess of the compacted samples, 9SC is also equal tothe respective porosity of each soil, but this porosity islower than that at the beginning of the drying phase.Therefore, porosity changes in a compacted soil, inducedby drying to \y = —100 MPa, are irreversible and aremuch more accentuated than in the uncompacted soil.

The unsaturated domain can be divided in two parts.In the first part, up to \\i = -1500 kPa, the watercapacity C(vy) = d9/d\|/ is lower for the compacted soils,reflecting a reduction in number of larger pores in thisrange due to compaction.

In the range between \\> = -1500 and -100 MPa,water is mostly retained in very small pores of the fabricand as films absorbed to particle surfaces. Here, the watercapacity C(v|/) appears to be higher for the compacted soilthan for the uncompacted one. From \|/ « -15 MPa,there are only minor differences between wetting anddrying curves, because pores are mostly filled with vaporat these vy values. For \|/ = -100 MPa, the water contentin the compacted soils is slightly lower than in the initialsoil. The reason might be that compaction reduces thepotential of surfaces on which water can absorb byincreasing the points of contact between particles.

Figures 5a and 5b show that the difference betweenthe WRC of the uncompacted and the compacted statesis larger for the soil of Cascavel, which is the lesscompactable soil. Therefore, changes in bulk density donot necessarily represent the effect of compaction on soilwater retention properties.

The model of Brooks and Corey (Eq. [9]) was fittedto the data corresponding to the drying curves of theuncompacted soils, for the first part of the unsaturateddomain, up to \\i = —1500 kPa. The resulting parametersare shown in Table 3. Based on these parameters andon the bulk density, pc, of each compacted soil, theparameters characterizing the drying curves of the com-pacted soils are computed using Eq. [14] to [17] (Table4). The fitting parameter y relating X, to Xc (Eq. [17])is 0.28 for the Palotina soil and 0.48 for the Cascavelsoil. The fitted curves and the measured data are shownin Fig. 6a and 6b. The agreement between the computedcurves and the data is good. Considering that this is theresult of three predetermined parameters and only one

Soils Pe ere Vac X,

CascavelPalotina

Mg m-3

1.271.50

0.550.49

- m3 m 3 -0.550.49

0.430.39

kPa-6.2-4.9

0.460.38

t PC = bulk density of compacted soil; <t>c = porosity of compacted soil;BSC = saturated water content of compacted soil; Grc = residual watercontent of compacted soil; <)»„ = air entry value of compacted soil;Xc = pore-size distribution index for compacted soil.

fitting parameter, the relationships proposed in Eq. [14]to [17] can give a relatively good first quantitative approx-imation of the effect of soil compaction on the WRC.

The sensitivity of the model to y and \yac are shownin Fig. 6a and 6b, respectively. The dashed lines showthe eifects of imposed relative errors of +25% on thefitting parameter and on the air entry value, respectively.Errors in y affect primarily the low water content range,while errors in \|/ac affect the high water content range.The goodness of the fit can be improved if the parametersin Eq. [15] are determined specifically for the soils underinterest.

The parameters of the WRC are used in Eq. [20],to estimate the saturated hydraulic conductivity of thecompacted state. The measured and the computed valuesare presented in Table 5. The Ksc values obtained assum-ing Mualem's (1976) suggested value of n = 0.5 areone order of magnitude higher than the measured ones.Measured Kx values are better reproduced using n =4.3 for the Cascavel soil and n = 4.5 for the Palotinasoil. It is likely that the specific values of n for thesesoils differ from the value of 0.5, but this cannot beverified since experimental hydraulic conductivity func-tions are not available. Further research is required todetermine the effect of compaction on unsaturated hy-draulic conductivity to improve the proposed model.

CONCLUSIONSSoil compaction behavior is not determined only by

soil texture. It is also affected by properties such as pH,CEC, clay particle thickness, and by the presence ofOM, iron oxides, and free aluminum hydroxides, whichdetermine the nature of the resulting cohesive forcesbetween the soil constituents. Two Oxisols, presentingpractically the same texture but having different pH,CEC, OM content, and free aluminum hydroxides, ex-hibit different compaction properties. Uniaxial compres-sive pressure tests show that the soil at Palotina, withhigher pH and CEC and lower OM content, is morecompactable and more sensitive to the initial matrictension than the soil at Cascavel. The highest bulk densityin the Palotina soil is reached for an initial matric tensionof —32 kPa, which is close to the matric tension atwhich practices are generally carried out. These resultscorrespond to field observations that indicate that dam-ages resulting from compaction, following 30 yr of inten-sive cultivation, are greater in the Palotina soil than inthe Cascavel soil.

A new model is proposed that reproduces soil compac-


1000.0 Table 5. The measured and computed saturated hydraulic conduc-tivity of the compacted samples (tfsc) (at P = 1000 kPa) of theCascavel and the Palotina soils.

0.35 0.40 0.45 0.50 0.55 0.60

WATER CONTENT (m3/m3)Fig. 6. Measured main drying curves of the compacted soils (full

circles) for y-, = - 32 kPa and P = 1000 kPA, and the curves(solid lines) resulting from the proposed approach using one fittingparameter, y. In Fig. 6a, the dashed lines represent a ±25% errorin the value of y used for evaluation X« (Eq. [17]), and in Fig. 6b,the dashed lines represent at ±25% error in the parameter <|/ac(Eq. [15]).

tion data very well. It is compared to two availablemodels: the logarithmic model of Bailey and VandenBerg (1967) and the multiplicative model of Bailey etal. (1986). Unlike these models, which assume that aninfinite compressive pressure leads to an infinite soilbulk density, the proposed model assumes that the soilbulk density tends toward a finite maximal value that

Kx estimatedSoils Kx measured n = 0.5 4.3 4.5

Cascavel 3.0 x Ifl-' 5.0 x Ifl-* 3.0 x 10-' 2.6 x 10~9

Palotina 2.0 x lO"8 2.0 x 10"' 2.2 x lO"8 2.0 x 1Q-*

depends upon the initial water content of the soil duringthe compaction process. When fitted to measured data,the proposed model has better performances than thelogarithmic model and practically the same performancesas the multiplicative model. Its main advantage is that itonly needs two fitting parameters while the multiplicativemodel needs three.

The compaction process deeply affects the microstruc-ture of the soil, as revealed by the water retention curvesof both soils for the whole range of matric tensions,from -0.1 to -100 MPa. The relative changes betweenthe initial and the compacted water retention curves aregreater for the less compactable Cascavel soil. Therefore,it is not only the absolute increase in bulk density thatindicates the effect of compaction. Previously publishedmodels expressing the effect of the increase in bulkdensity on the soil hydraulic properties are investigated.The model of Mualem and Assouline (1989) is appliedto the parameters of the Brooks and Corey (1964) relationfor WRC and to the saturated hydraulic conductivity.Such models open the way to the representation of theeffect of compaction on soil hydraulic properties. Thisapproach emphasizes the joint effect of both the initialsoil hydraulic properties and the bulk density increaseon the result of the compaction process. At this stage,one fitting parameter is still needed. The results of theapplication of the approach to the main drying curvesof the two compacted soils are satisfactory. The methodcan also be applied to the main wetting curves and thusdetermine the effect of compaction on the hysteresisdomain. The modeled water retention function can alsobe used to evaluate the effect of compaction on thesaturated hydraulic conductivity and on the hydraulicconductivity function, although further research is stillneeded in this field.

ACKNOWLEDGMENTS

The prime author thanks the Ministry of Science and theArts of Israel for the financial support during his stay in France.The authors thank V. Snyder for his constructive comments.

effect of compaction on soil physical and hydraulic properties: experimental results and modeling

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