Effect of Compaction on Soil Physical and Hydraulic Properties: Experimental Results and Modeling
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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 compactionaffects 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 (firstname.lastname@example.org).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) 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) 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.  and  differin their boundary conditions for very low stress (includingzero) and for very high stress. The model in Eq.  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.  satisfies the boundary condition of p = p0for a zero stress. For a-*>, this model assumes that
dp/do = -&p0e-(a+6o) 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. 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.
ASSOULINE ET AL.: EXPERIMENTAL RESULTS AND MODELING OF COMPACTION EFFECT ON SOIL 391
is the assumption that p-*-oo when 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^^Jwhere 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 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, 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 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 
= pi+ (p* - Pi)d - (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 
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
e s-e r[19J
Substitution of Eq.  into Eq.  yields, after integration_!_*> 1 T
Asc Ac e s-e rThe unsaturated hydraulic conductivity of the compact