Compression of agricultural soils from Quebec

Soil& Tillage Research, 18 (1990) 357-365

357

Elsevier Science Publishers B.V., Amsterdam

Compression of agricultural soils from Quebec* D.A. Angers Agriculture Canada Research Station, 2560 Hochelaga Blvd., Sainte-Foy, Que. G1 V 2J3 (Canada) (Accepted for publication 13 June 1990)

ABSTRACT Angers, D.A., 1990. Compression of agricultural soils from Quebec. Soil Tillage Res., 18:357-365. Static uniaxial compression tests were performed on 26 agricultural soils from Quebec. Compression lines (bulk density vs. applied load) were obtained at different water contents for each soil previously sieved to 6 ram. For soils with clay contents less than 35%, the compression index (slope of the compression line) was best correlated with the mineral fraction of the soil (r= 0.75** with clay and r = -0.78** with sand). For clay-rich soils, the compression index was best correlated with organic carbon content ( r = -0.75**). The bulk density under standard compression conditions ( 100 kPa load and 50% water saturation) was related to both clay ( r = - 0 . 8 0 * * ) and organic carbon (r= -0.77**). This parameter was also highly correlated with the soil lower plastic limit (r= -0.95**) which corroborates the observation that the consistency limits can be good predictors of other mechanical properties which are more difficult to determine. Results suggested that both the mineral and the organic fractions have much influence on the compressive behaviour of Quebec agricultural soils.

INTRODUCTION

Compaction of agricultural soils has become an increasing concern as the weight and utilization of agricultural machinery have increased. Raghavan and his collaborators have shown the detrimental effects of traffic-induced compaction on soil conditions and crop yields in Quebec (e.g. Raghavan et al., 1978 ). Wang et al. ( 1985 ) observed, in the Ottawa area, that shallow root growth and severely reduced yield (50%) were associated with severe compaction and low hydraulic conductivity. Compaction also results in reduced water infiltration and increased susceptibility to erosion by water (Fullen, 1985 ). Estimated costs of soil compaction vary from 30 to 100 million dollars per year in the province of Quebec (Agriculture Canada, 1985 ). Knowledge of the effects of compaction on soil processes and plant behaviour is a first step in the development of strategies for reducing soil compaction and its detrimental effects on crop yield and soil and water conservation. The relationship between applied stress and soil density (or total porosity) is, in general, poorly understood for agricultural soils. Because of the lack of a universal soil mechanics theory, researchers have used quasi-theoretical *Contribution No. 388, Agriculture Canada, Sainte-Foy, Que. G 1V 2J3, Canada.

0167-1987/90/$03.50

© 1990 - - Elsevier Science Publishers B.V.

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D.A. ANGERS

models to describe this relationship (Soane et al., 1981). McBride (1989) used a model proposed by Bailey et al. (1986) to describe the compression behaviour of 34 undisturbed soil horizons from Ontario (eqn. (1)). This model has the advantage of describing the entire compression line but the disadvantage of being multiplicative which makes the interpretation of the coefficients difficult. In (p) = I n (P0) -

(A+Bcr) [ 1 - e x p

(Ctr) ]

(l)

where p = post-compression dry bulk density; go = pre-compression (initial) dry bulk density; or=total normal stress applied; A, B and C = parameter coefficients. Larson et al. (1980) measured the uniaxial compression curves of a wide range of unsaturated soils. They proposed that the dry bulk density, p, of a soil at a given degree of water saturation, S~, and applied normal stress, era, can be described using the following equation: p = [pkq-ST ( S l - S k ) ] +Cp log (tra/trk)

(2)

where Pk = dry bulk density at a known applied stress, Crk,and at a known degree of water saturation, Sk; ST = slope of p vs. the degree of water saturation at a standard stress of 100 kPa; Cp=the compression index. For a series of soils from over-the world, Larson et al. (1980) and Gupta and Larson ( 1982 ) established statistical models to estimate the parameters ofeqn. (2). Estimations of the three unknown parameters Cp,ST and Pk were obtained by correlation with the particle-size distribution of the soils. Coefficients of correlation varying from 0.63 to 0.89 were obtained. In particular, clay content had a determinate effect on the compressive behaviour of the soils. Saini et al. (1984) measured the compression indices (Cp) of seven New Brunswick soils and suggested that the presence of gravel seemed to have a more important effect on Cpthan clay in these soils. This model (eqn. (2) ) has the disadvantage of not describing the entire stress range (McBride, 1989) but the advantage of taking into account the effect of water contefit and of being easy to interpret. The objectives of this work were to study the compression behaviour of Quebec agricultural soils and to relate the compression characteristics to soil properties. Compression tests were performed on sieved ( < 6 m m ) soils using a uniaxial compression device. MATERIALS AND METHODS

The Ap horizons of 26 soils representing a wide range in organic matter and clay contents were sampled from different regions of southern Quebec. The soils are essentially the same as those studied by de Kimpe et al. ( 1982 ) and

COMPRESSION OF AGRICULTURAL SOILS FROM QUEBEC

359

TABLE l Soil properties and model parameters Variables

Means

Std. Dev.

Min.

Max.

Soil property Clay (%) Silt (%) Sand (%) OC (%) LPL (%) UPL (%) WSA (%)

36.0 30.4 33.4 3.70 37.0 42.3 65.6

20.8 12.4 25.6 1.90 8.0 10.4 28.0

3.8 7.8 1.7 1.00 24.8 18.9 2.9

73.8 62.5 86.6 9.30 49.4 59.3 93.3

Model parameters Cp ST ( × 104) Pk (Mg m -3)

0.35 48.2 1.09

0.06 20.7 O. 17

0.23 8.6 0.76

0.45 85.3 1.53

their properties are summarized in Table 1. The samples were air-dried and passed through a 6-mm sieve. The principles and the methods for the determination of soil compressibility are described by Bradford and Gupta ( 1986 ). Compression tests were carried out on three to five subsamples from each soil which had been wetted to water contents varying from 0. l0 to 0.40 kg kg-~ with a fine water spray. The subsamples were allowed to equilibrate for at least 5 days. Static uniaxial compression was applied using a Rowe Consolidation Cell model EL28-356. About 100 g of moist soil was confined in the sample chamber and compressed by a piston driven by compressed air. The diameter of the cylinder was 7.2 cm and the initial height of the sample varied from 3.0 to 3.5 cm. Sample height after each pressure increment was recorded from a dial. Static loads ranged from 0 to 800 kPa. Incremental increases in load were applied every 60 s. The dry bulk density of soil samples under various loads was calculated from the sample dimensions and the oven-dry mass of the soil. The unknown coefficients of eqn. (2) were calculated as follows. From each compression test, a compression line relating dry bulk density to the logarithm of the applied pressure was obtained. The compression index, Cp, was computed as the slope of the compression line using the points along the linear portion of the curve for each water content. In order to account for the effect of water content, the dry bulk density values at 100-kPa load were regressed against the corresponding percent water saturation. The slope of this line (Sx) was calculated using regression techniques. A dry bulk density under standard compression conditions of 100-kPa load and 50% water saturation (Pk) was also calculated from this regression equation.

360

D.A.ANGERS

Particle-size distribution by the pipette method (Gee and Bauder, 1986), organic carbon (OC) by wet oxidation (Nelson and Sommers, 1982), and the lower (LPL) and upper plastic limits (UPL) (Sowers, 1965 ) were determined on each soil sample. The proportion of water-stable aggregates larger than l m m (WSA) was also determined (Angers and Mehuys, 1988). Correlation and regression analyses were performed according to Snedecor and Cochran (1980). RESULTS AND DISCUSSION

Compression curves for the De l'Anse clay at four water contents are plotted in Fig. 1. The shapes of the curves correspond to data previously reported by Larson et al. (1980), Saini et al. (1984) and Angers et al. (1987). The linear portions of the compression curves at different water contents were usually parallel and consequently the compression indices (Cp) were averaged over water contents for each soil. The relationship between dry bulk density and the degree of water saturation at a load of 100 kPa for the sample shown in Fig. l is illustrated in Fig. 2. The slope ST of the straight line can be calculated along with the dry bulk density at 50% saturation (Pk). Values for the compression characteristics obtained in this study (Table 1 ) fit into the range of values measured by Larson et al. (1980) and Gupta and Larson (1982). In general, when considering the entire data set, the compression index is weakly correlated with soil properties (Table 2 ). The soils were therefore sep-

1.4

Water content

E

o • b •

1.2

( k g kg -1)

0.184 0.254 0.312 0.397

eee e@ 0 0

o

•

o

•

oo

•

v

o

•

o @

1.0

o

•

o

ca

D

• •

0 D

•

0

0 0

D

D

-o o

_~

•

EB

o

[]

0.8

0.6

i

0

Applied

100 load

i

1000 (kPa)

Fig. 1. Compression curves at four water contents for the De l'Anse soil (58% clay, 5% sand, 3.6% OC).

361

COMPRESSION OF AGRICULTURALSOILS FROM QUEBEC

1.00

i

E 0.90 O3 v

"0

_~ 0.80 m

e~

R2=0-99

2'0

3'0 4'0 5'0 Degree of water saturation (%)

0.70 6'0

Fig. 2. Relationship between dry bulk density and degree of water saturation at 100-kPa load pressure for the De r A n s e soil.

TABLE 2 Correlation coefficients between soil constituents and the compression characteristics (n = 26 )

Cp ST p~

Clay

Silt

Sand

OC

0.07 NS - 0 . 1 5 NS -- 0.80**

0.51"* 0.36 NS -- 0.10 NS

- 0 . 2 9 NS - 0 . 0 5 NS 0.71"*

-0.39* -0.55** - 0.77"*

NS = non-significant.

TABLE 3 Correlation coefficients between soil constituents and the compression index (C) for two groups of soils Soil property

Soils with < 35% clay (n=14)

Soils with > 35% clay (n=12)

Clay Silt Sand OC

0.75** 0.65** -0.78** - 0 . 1 1 NS

- 0 . 5 6 NS 0.32 NS 0.37 NS -0.75**

NS = non-significant.

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D.A. ANGERS

arated into two groups, soils with low ( < 35%) clay contents and soils with high ( > 35%) clay contents (Table 3 ). For the soils with low clay contents, significant relationships are found between Ca and clay content and between Cp and sand content whereas for fine-textured soils, Cp is highly correlated with OC. Larson et al. (1980) showed that Ca increases up to a clay content of about 33% and then levels off. It has been hypothesized that these finetextured soils consist of a clay matrix with coarser material embedded in the clay (Mitchell, 1976; Larson et al., 1980). Our results support this hypothesis but also give supplemental information on the role of organic matter. Increasing levels of organic matter have a negative effect on the compressibility of clay-rich soils. Multiple regression analysis (eqns. (3) and (4)) shows that about 60% of the variability in Cp can be explained by soil constituents. For soils with < 35% clay Cp=0.381 +0.0019 (clay)-0.0014 (sand)

R2=0.64

(3)

For soils with > 35% clay Cp=0.489-0.017 ( O C ) - 0 . 0 0 1 0 (clay)

R2=0.58

(4)

The dry bulk density under standard compression conditions of 100 kPa load and 50% water saturation (ilk) ranges between 0.76 and 1.53 Mg m -3. This parameter is highly correlated with both OC and clay content (Table 2 ). Gupta and Larson (1982) also found a high correlation of flk with clay content ( r = - 0 . 8 2 ) but not with organic matter content. De Kimpe et al. (1982) performed Proctor compaction tests on soils from Quebec and found a strong negative relationship between the maximum dry bulk density and OC. These laboratory studies are confirmed by a field study in which Angers and Simard ( 1986 ) found significant correlations between field dry bulk density and OC for Quebec soils. This effect of organic matter on soil bulk density may be due in part to the effect of organic matter on soil specific gravity (particle density ), soil cohesion, and aggregate density. Particle density was found to vary little in our soils, ranging from 2.40 Mg m -3 for the organic-rich soil to 2.74 Mg m - 3 for the low-organic soil. Gupta and Larson ( 1982 ) indicated that predicted compression curves were particularly sensitive to errors in the estimate of Pk- They suggested that regression equations with lower variability for the estimation of ilk are needed and thus recommended laboratory measurements ofpk. Our estimates of this parameter by soil constituents are very good. Multiple regression analysis (eqn. (5) ) showed that together OC and clay accounted for 78% of the variability in bulk density (ilk). pk=1.401--0.0044 (clay)--0.041 (OC)

R2=0.78

(5)

flk was also found to be highly correlated with the lower plastic limit. Regression analysis (Fig. 3 ) shows that LPL can explain up to 90% of the variability

363

COMPRESSION OF AGRICULTURAL SOILS FROM QUEBEC

1.4

E t~

kU) Z UJ

1.2

1.0

E / D m

r= -0.95

e~'~,,

0.8 20

3'0 4'0 5'0 LOWER PLASTIC LIMIT (%) Fig. 3. Relationship between LPL and dry bulk density at 100-kPa load and 50% water saturation. TABLE 4

Correlation coefficients between soil constituents and selected soil physical properties ( n = 26 )

LPL UPL WSA NS =

Clay

Silt

Sand

OC

0.75"* 0.83** 0.55**

0.21 N S 0. l 0 N S 0.08 N S

- 0.54** - 0.73** - 0.50**

0.66** 0.69** 0.66**

not-significant.

in Pk. Other authors have found that consistency limits are good surrogates for other soil mechanical properties which are more difficult to measure (de Kimpe et al., 1982; McBride, 1989 ). Preliminary data by Bailey et al. (1986) and the more extensive work by McBride (1989) suggested that the model described by eqn. ( 1 ) could be used to predict the compressive behaviour of soils. When eqn. ( 1 ) was fitted to our compression data, a good fit was obtained but the correlations between the three model parameters and the soil properties were very low (r < 0.30 ). Stratification of the data as per McBride ( 1989 ) or by clay content did not improve the correlations. It is accepted that the clay mineralogy of a soil has much influence on its compressive behaviour (e.g. Larson et al., 1980). Semi-quantitative identification of the clay minerals of Quebec soils (Simard et al., 1989) suggests that the magnitude of the variations is probably not sufficient to account for the large variations in compressive behaviour observed. This study also provides supplemental information on the factors control-

364

D.A.ANGERS

ling the consistency limits and the aggregation of Quebec soils. The consistency limits are well correlated with both OC and clay contents (Table 4). These results agree with previous studies (Soane et al., 1972; de Kimpe et al., 1982 ). The percent water-stable aggregates was also correlated with both OC and clay (Table 4) which confirms the role of these two constituents on aggregation (e.g. Kemper and Koch, 1966 ). CONCLUSIONS

The results of the study confirm that the soil consistency limits can be good predictors of other soil mechanical properties which are often more difficult to determine. The data also show that the compression behaviour of Quebec soils can be predicted from soil constituents. Both the mineral (particle-size distribution) and organic fractions have much influence on the mechanical properties of these soils. Of prime interest is the fact that OC was correlated with all of the compression characteristics measured. Soil organic matter reduced soil compressibility (Cp) and the bulk density under standard compaction (Pk). McBride and Watson (1990) recently showed that organic matter also plays a major role in increasing soil resiliency to compaction by enhancing rebound of compacted soils. Cropping practices designed to maintain or restore soil organic matter should contribute to reduce soil susceptibility to compression. The lack of a universal soil mechanics theory has encouraged soil scientists to verify the applicability of the "critical state" soil mechanics model to agricultural unsaturated soils (Hettiaratchi, 1987). Further applications of this work and of other similar studies would be the measurement of characteristics of shear failure and expressions of "critical state" models. ACKNOWLEDGEMENTS

The author wishes to acknowledge the assistance of Patrice Jolicoeur who performed the laboratory analyses and Drs. John Culley and Ray McBride for reviewing an early draft of the manuscript.

REFERENCES Agriculture Canada, 1985. Agricultural soil and water resources in Canada: situation and outlook. Agriculture Canada, Ottawa. Angers, D.A. and Mehuys, G.R., 1988. Effects of cropping on macro-aggregation of a marine clay soil. Can. J. Soil Sci., 68: 723-732. Angers, D.A. and Simard, R.R., 1986. Relations entre la teneur en mati~re organique et la masse volumique apparente du sol. Can. J. Soil Sci., 66: 743-746.

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