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Quantification of physical ripening in an unripe marine clay soil
Geoderma, 58 (1993) 67-77 Elsevier Science Publishers B.V., Amsterdam
67
Quantification of physical ripening in an unripe marine clay soil D.J. K i m a, J. Feyen a, H. Vereecken b, D.
Bo el s c a n d J.J.B. B r o n s w i j k c
alnstitute for Land and Water Management, KUL, Vital Decosterstraat 102, 3000 Leuven, Belgium bForschungszentrum Jiilich, Erdrl und Geochemie. ICEt 5, Post 1913, D5170 Jiilich, Germany CDepartment of Soil Conservation and Technology and Department of Soil Physical Transport Phenomena of Winand Staring Centre, Wageningen, The Netherlands (Received September 16, 1991; accepted after revision September 1, 1992)
ABSTRACT Kim, D.J., Feyen, J., Vereecken, H., Boels, D. and Bronswijk, J.J.B., 1993. Quantification of physical ripening in an unripe marine clay soil. Geoderma, 58: 67-77. The process of physical ripening of a soil developed on recently exposed marine clay has been studied in detail by examining the swelling of soil clods subjected to a number of drying and wetting cycles. During the wetting cycle, solutions with three different CaCI2 concentrations were used to study the effect of salts on the swelling. The swelling behaviour was represented by a linear swelling characteristic showing that the concentration of solutions clearly influences the swelling. Based on the swelling characteristics, quantitative aspects of physical ripening were investigated in terms of the irreversibility of extracted soil water. A linear relationship was noted between the moisture ratio at the threshold of irreversible extraction and the initial moisture ratio. Based on the relationship, a physical ripening index (PRI) is proposed, which can be used together with the initial moisture ratio to indicate stages in the ripening process for these soil materials. A greater proportion of water could be reversibly extracted from soils which had undergone increasing degrees of ripening.
INTRODUCTION
Marine sedimentary materials, are pedogenetically undeveloped at the time they become exposed (Pons and Van der Molen, 1973 ). These materials normally undergo several changes which are related to the soil water regime. One of the most distinctive changes is physical ripening, which is regarded as an initial soil-forming process caused by drying and wetting. Upon drying the soil material shrinks resulting in the loss of volume whereas upon wetting it swells regaining volume. If the soil material recovers the initial volume upon rewetting, it is termed reversible, otherwise irreversible. After a series of shrinkage and swelling events, the soil materials exhibit a physically ripened state marked by structure development, and altered stability and permeabilCorrespondence to." D.J. Kim, Institute for Land and Water Management, KUL, Vital Decosterstraat 102, 3000 Leuven, Belgium. 0016-7061/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved.
68
D.J. KIM ETAL.
ity. Swelling and shrinking is a natural process which is generally considered to be responsible for the compaction and improvement of the soil structure (Wilding and Hallmark, 1984; Bronswijk, 1989). It would then be important to know how the degree of physical ripening is related to the swelling and reversibility. Not much is reported about the degree of physical ripening of marine sedimentary materials in the literature. Zuur (1958) derived the following empirical relationship between the water content and the clay and organic matter content:
A=20+n(L+ 3H)
(1)
where A is the number of grams of water/100 g of dry material, n is the water factor, L is the clay content (%), and H is the humus content (%). The parameter n, the so-called water factor, is a measure of degree of ripening of a soil. The value 20 represents the quantity of water bound to non-colloidal components. The value 3 indicates that depending on the degree of humification organic matter binds three times as much water as clay. Pons and Zonneveld ( 1965 ) proposed a method for determining the physical ripening stage from either the water factor or the consistency of the soil material. Their method, however, provides a rough idea about the ripening stage due to uncertainties in the constant of eq. ( 1 ). The other shortcoming of the method is that it does not include any information on the soil physical properties or soil structural phases, except for the water content. Rijniersce (1983) reported that physical ripening of a soil is a process of irreversible drying. In this respect, a completely reversible soil cannot undergo physical ripening. A partially reversible soil undergoes ripening with limited reversibility or irreversibility. However, the criteria of irreversibility were not explicity presented. Hence this study is aimed to investigate quantitative aspects of physical ripening by defining the concept of irreversible drying based on a swelling characteristics, and to propose new indices to assess the degree of physical ripening in comparison with the existing methods. SOILS A N D M E T H O D S
Soil A large bulk sample of an unripe marine clay soil was taken from near Termunten on the northern coastal area of the Netherlands. Since the site was frequently inundated by sea water, all sampling was done during low tide when the soil emerged. The soil is classified as an alluvial or fine-clayey over loamy, illitic, mesic Hydraquents (De Bakker, 1979). The soil contains 45.8% clay and 52.3% silt and has a bulk density of 0.49 g cm-3. The major clay minerals
QUANTIFICATION OF PHYSICAL RIPENING
69
in the soil were kaolinite and illite. The concentration of the sea water was 256 meq 1-1 with SAR of 35.
Determination of swelling and shrinkage behaviour The swelling behaviour of the ripening marine clay soil was characterised using the clod method proposed by Bronswijk (1986). In order to create a flexible coating around the soil, plasticizer was added at a 1:10 weight ratio to SARAN-F310 resin (resin to solvent ratio 1:5 by weight). Several holes (less than 0.5 m m in diameter) were made on the surface of the saran resin to allow water to enter the soil clod. Solutions with three different levels of CaCl2 concentrations (0.0N CaC12, 0.1N CaC12 and 0.6N CaC12) were prepared to investigate the effect of salts on swelling in a combined form of CaC12 and NaC1 since the soil solution contains a considerable amount of sodium owing to frequent saturation by sea water. Swelling was measured in terms of moisture ratio on small clods ranging between l0 and 20 g. Soil clods were dried in five replicates to seven different moisture ratios (3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5 ). Once each soil clod was dried to the specified drying limit, it was placed successively in each solution until the m a x i m u m swelling limit was reached. Each clod was subjected to five sequential drying and wetting cycles.
Methods of quantification The shrinkage behaviour of a soil material can generally be characterized by changes of its moisture and void ratios (Haines, 1923; Bronswijk, 1988), which are defined as:
O=VwlVs
(2)
e=V,,/Vs
(3)
where 0 is the moisture ratio, e is the void ratio, Vw is the volume of water, Vv is the volume of void (L3), and Vs is the volume of solid (L3). The swelling behaviour of the marine clay soil can be characterized by either its extensibility or reversibility. Grossman et al. (1968) proposed a parameter COLE (Coefficient Of Linear Extensibility) to estimate the swelling and shrinkage potential of a soil layer and PLE (Potential Linear Extensibility) for the potential of different soil layers. These parameters assume that the soil follows an equidimensional shrinkage. However, the soil used in this study shows shrinkage from unidimensional to equidimensional depending on the moisture ratio, ripening stage and load (Kim et al., 1992 ). In this study, a swelling degree index (SDI) is proposed to evaluate the vol-
70
D.J. K1M ET AL.
ume expansion of a ripening soil. The SDI can be defined as the ratio of soil volume at drying to that at swelling, and can be given by: SDI= Vz/VL
(4)
where SDI is the swelling degree index, Vl is the volume at dry condition (L 3), and V2 is the volume at swelling condition (L 3). Since a ripening soil shows dynamic changes in the shrinkage curve, its volume after swelling differs depending on the stage of ripening. This complicates the calculation of volume for any dry and wet condition. Therefore the swelling degree of a soil can firstly be interpreted by the swelling function which simply relates the moisture ratio at drying to that at swelling. In conjunction with the shrinkage characteristic, the volume at swelling can then be calculated. For any ripening stage, the SDi can be rewritten as: SDI----- V 2 / V t = ( 1
+ e w ) / ( 1 +Ca)
(5)
Ow=F(Od)
(6)
ew = q)(Ow)
(7)
where 0w is the moisture ratio at swelling, do is the moisture ratio at drying, ew is the void ratio at swelling, eo is the void ratio at drying, F is the swelling function, • is the shrinkage function, and SDI is the swelling degree index. Another important parameter evaluating the swelling process is the reversibility. The reversibility of a soil is defined as its capacity to regain an original volume by rewetting after the soil has been dried to a certain moisture ratio. The swelling funtion (Fig. 1 ) can be used to determine the quantity of water extraction at which te soil shows irreversibility of swelling on rewetting. The quantity of extracted water is found by: L~ext = L ~ i - - 0 d
(8)
where Oext is the quantity of water extraction in the moisture ratio, tgi is the moisture ratio at initial condition, and do is the moisture ratio at dry condition upon extraction. After rewetting, the reversible quantity of water can be found by: /grev= "l- (/}d) -- •d
(9)
The difference between the two quantities Oext and Orev is the irreversible quantity of extracted water Oirr and is given by:
O~rr= Oex~--0rev
(10)
If the difference is positive, then the extraction is irreversible. For a given initial moisture ratio, the moisture ratio at dry condition (do) which satisfies eq. ( 10 ) equal to zero, is the moisture ratio (Ooc) at the threshold point below which the irreversible extraction of water occurs. For a given swelling func-
QUANTIFICATION OF PHYSICAL RIPENING
71
5
== "~
4-
m
I
3
/
t3
function 0
Moisture ratio at ary limit
Fig. 1. Relationship between the quantity of water extraction (0ex,), and the reversible quantity (0re,) and the irreversible quantity (tg~rr) for a given initial moisture ratio using swelling function. Data demonstrate the results of terms when a soil is subjected to drying from 0~ (4.0) to Od (2.0).
tion, several Odc values can be found for different Oi. A functional relationship can be derived by plotting the Odc versus the Oi. A measure of ripening degree can be found from the concept of reversibility. As a soil becomes riper, the reversible quantity of extracted water becomes greater. Further, the progress of physical ripening can be measured by comparing the reversible extracted quantity with the total quantity. The reversible extracted quantity is determined by the difference between the initial moisture ratio (Oi) and the threshold moisture ratio (0dc). A physical ripening index (PRI) is derived from: ~1~--~-(L~i - - ~ d c ) / ( ~ q i )
( 11)
where 7 is the physical ripening index. If a soil material is completely unripened, the value of 7 will be zero. A completely ripened soil will have 7 equal to 1 which indicates that no degree of water extraction causes irreversibility. RESULTS AND DISCUSSION
For a given do, the Ow of the soil is influenced by the number of drying and wetting cycles. Repeated drying and wetting causes a slight decrease in the Ow which approaches a constant value. This phenomenon is, in this study, referred to as "cycle effect". The significance of the cycle effect was examined using the analysis of covariance (ANCOVA), with the number of cycles as the treatment variable. The results of the ANCOVAare shown in Table 1. The dependent variable Y is defined as the Ow and the covariate X as the do. In ad-
72
D.J. KIM ET AL.
TABLEI
Results of ANCOVA for solutions with three different salt concentrations Conc.
Variable
F value
Pr> F
0.0N
X cycle cycle*X
1440.49 14.75 0.77
0.0001 0.0001 0.5511
O. IN
X cycle cycle*X
471.85 0.45 0.46
0.0001 0.7744 0.7625
0.6N
X cycle cycle*X
862.42 3.39 0.52
0.0001 0.0135 0.7223
X: continuous covariate as the moisture ratio at the dry limit. cycle: treatment of drying-wetting cycle. TABLE2
The fitting model (Y=cX+d) used in the swelling function Conc.
cycle
c
d
R2
1 2 3 4 5
0.790 0.837 0.797 0.738 0.742 0.742
1.345 1.451 1.391 1.407 1.300 1.307
0.908 0.997 0.996 0.998 0.997 0.998
0.1N
0.793
1.227
0.897
0.6N
0.756
1.117
0.900
0.0N
Y: dependent variable representing moisture ratio at swelling. X: independent variable representing moisture ratio at drying limit. c: coefficient of slope. d: constant. R 2: coefficient of determination.
dition, the interaction effect was tested between treatment and covariate (cycle*X). The cycle effect is significant only for the solution with zero salt concentration. For all three analyses, the interaction term was not significant. Table 2 shows the regression equations fitted through the data of the swelling characteristic. For the zero concentration, equations were obtained from different treatment levels. However, only one equation was obtained for other solutions since the cycle effect is not significant. These regression equations are termed "swelling function". It appears that the moisture at swelling de-
73
Q U A N T I F I C A T I O N OF PHYSICAL R I P E N I N G
':t
(a) . . . . O.ON
1.
-
//"°-'"-,,
-
0 . i N
--
0.6N
....
O.ON O.tN 0.6N
t.4-
t.3" i.2" t.t" i.
HoistuPe R a t i o 1.6
(b)
1.5
-
-
-t.4
////
"'""",,
,,
t.3 1.2 1.1-
],
i
0
; HoistuPe R a t i o
t.6 (c)
.... t.5-
-
-
--
O.ON 0.iN 0.6N
1.4~
t.3-
t.
0
Moisture
Ratio
Fig. 2. Comparison of SDI influenced by the different salt concentration at a given ripening of (a) stage 1, (b) stage 2, (c) stage 3.
74
D.J. KIM ET AL.
creases with increasing salt concentrations. The results of swelling functions clearly indicate that the ripening soil differs from ripened swelling soils with respect to reversibility. The decrease in both the swelling capacity and the cycle effect with higher salt concentrations can be explained by the influence of osmotic pressure. Figure 2 shows the results of SD1 versus the moisture ratio at dry, for various stages of ripening. To calculate the void ratio at swelling, shrinkage characteristic curves (Kim et al., 1992 ) were obtained for the three different ripening stages. The results show distinct differences for all ranges of 0o. The higher the salt concentration, the smaller the SDI. A clear peak value is observed around the moisture ratio of 0.7 regardless of the salt concentration and the stage of ripening. Figure 3 shows the regression equations obtained from the relationship between the Ode and the 0~. The Ooc decreases linearly with the O i. However, the 5
Cone.
C2
j1,725
--
1.51 7 1.438
c-
5 .~
C1
..... 0.0N 1,274 -
-
O. iN 1.251 0 . 6 N 1.314
3-
."
o
"~ 2-
I /
7
,/" .-"~ ,-" . ~ " ,,. f
/ /
/
ul
0
Inltlal Moisture Ratio
Fig. 3. Linear regression equations (Ode= C1 O~- C2) obtained for the relation between the initial moisture ratio (O~) a n d the moisture ratio (0de) at the threshold of irreversibility. TABLE 3
Comparison of physical ripening index (PRI) with parameter n and consistency of the soil material for different initial moisture ratios O~
?(%)
n
Designation
Consistency
4.5 4.0 3.5 3.0 2.5 2.0 1.5
9 13 19 26 36 50 76
2.25 2.00 1.75 1.50 1.25 1.00 0.75
Unripened Unripened Practically unripened Practically unripened Half ripened Nearly ripened Nearly ripened
Very soft Very soft Soft and sticky Soft and sticky Fairly soft Fairly firm Fairly firm
Values of parameter n are based on the following equation proposed by Bouten (1978) in the case L + 3H> 20.' A = n(L + 3H); where ,4, L and H are the same variables as described in eq. ( 1 ).
QUANTIFICATION OF PHYSICAL RIPENING
75
magnitude of decrease in O0c becomes larger as the 0 i decreases. This indicates that a greater quantity of water can reversibly extracted with the lower Oi, a slight extraction may cause a irreversible drying. Irreversibility is more pronounced for the solutions with higher concentration. This implies that a soil with a greater amount of salts can be more ripened for a given dryness. Table 3 presents a comparison of PRZ with the parameter n and consistency of the soil material for the different initial moisture ratios. To determine y (PRI), the swelling function corresponding to 0.1N solution was used. The increases with ripening and with increasing firmness of consistency. The increase of ~ indicates that a greater proportion of water can be extracted without causing irreversible drying. This conforms to the statement given by Rijniersce ( 1983 ) - - the riper the soil, the larger the reversible quantity. It is noteworthy that the measures of the degree of ripening, ~, and n, are highly correlated. Especially between Lqti and n, a perfect equivalence exists. This seems to be reasonable since the basis of the parameters is nothing but the water content of the soil material. It is likely that the 0i identical with the moisture ratio (Os) at saturation or swelling can also be used for estimation 3~ Hoisture
13:
Ratio
MotsLuee R a t i o st lowest dry
=
D'
W'D' 1
W' 2
D'
W' 3
D ' W'D' 4
W'D' 5
W 6
Cycles
Fig. 4. The effect of the moisture ratio at the lowest drying limit on the moisutre ratio (0w) at swelling during the alternate drying and wetting cylces. The variable on the Y axis can be Od or O, depending on the drying ( D ) or wetting (W) cycle. TABLE 4 A suggested guideline for estimation of ripening stage from Oi, n and 7' ( PRI ) Stage
0~
n
7
Consistency
Designation
1 2 3
> 3.5 2.0-3.5 < 2.0
> 1.75 1.00-1.75 < 1.00
< 20 20-50 > 50
Soft Sticky to firm Fairly firm
Unripened Half ripened Nearly ripened
76
D.J. KIM ETAL.
of the degree of ripening. The advantage in the use of Q over the other parameters is that it provides an idea of how much water can reversibly be extracted for a given Oi. However, the determination of 7 requires a known swelling function. Quick estimation of the degree of ripening can be obtained from the Oi since the determination is simple and much easier. Figure 4 shows another specific behaviour of swelling. When a soil clod is subjected to various drying limits, do, the swelling is finally dominated by the lowest tgo. This indicates that the degree of swelling of a ripening soil is determined by the most intensive drying treatment. Together with the dependence of the degree of ripening on the Oi, this behaviour provides important hints for the process of soil physical ripening. Since physical ripening is a continuous process, distinction between ripening stages is bound to be arbitrary depending on the individual's perception. Despite the difficulties in distinguishing these stages an approximate boundary between the ripening stages is proposed based on the information in Table 3. The proposed guidlines are presented in Table 4. CONCLUSIONS
Quantitative aspects of physical ripening were examined in relation to swelling behaviour. A linear swelling function which characterizes a swelling capacity of the ripening soil has been derived. Physical ripening is defined in a quantitative manner using the linear swelling function. The threshold of irreversibility was related to the initial moisture ratio 0i. The relationship reveals that the reversibility increases with the progress of ripening. It is noted that an unripe soil can be more susceptible to ripening owing to its higher initial moisture ratio since irreversible drying is more likely to occur at that level of moisture content. Combination of the unique linear swelling function (no cycle effect) with the dependence of the final swelling degree on the lowest drying limit leads to the conclusion that the degree of physical ripening is controlled by the lowest value of do. Finally, the physical ripening index ( PRI ) is proposed as a measure of ripening degree. However, for practical purpose, the use of the initial moisture ratio Oi is recommended for estimation of the degree of ripening. ACKNOWLEDGEMENTS
The author is gratefully indebted to Dr. A.L.M. van Wijk (The Winand Staring Centre, Wageningen) for providing laboratory facilities and helpful discussions throughout this research. Thanks are also due to L. Geeraerts for typing and E. Mwendera for contribution to revision of the manuscript.
QUANTIFICATION OF PHYSICAL RIPENING
77
REFERENCES Bronswijk, J,J.B., 1986. Evaporation and cracking of a heavy clay soil. Rep. 14, Inst. for Land and Water Management Research (ICW). Wageningen. Bronswijk, J.J.B., 1988. Modeling of water balance, cracking and subsidence of clay soils. J. Hydrol., 97: 199-212. Bronswijk, J.J.B., 1989. Prediction of actual cracking and subsidence inclay soils. Soil Sci., 148: 87-93. Bronswijk, J.J.B. and Evers-Vermeer. J.J., 1990. Shrinkage of Dutch clay soil aggregates. Neth. J. Agric. Sci., 38: 175-194. Bouten, W., 1978. Over de structuur en her waterbindend vermogen van pure klei. Werkdocument RIJP 1978-187 Abw, Rijksdienst IJsselmeerpolders, Lelystad. De Bakker, H., 1979. Major soils and soil regions in the Netherlands. Soil Survey Institute, Wageningen. Grossman, R.B., Brasher, B.R., Franzmeier, D.P. and Walker. J.L., 1968. Linear extensibility as calculated from natural-clod bulk density measurements. Soil Sci. Soc. Am. Proc., 32: 570-573. Haines, W.B., 1923. The volume changes associated with variations of water content in soil. J. Agric. Sci. Can., 13:296-311. Kim, D.J., Vereecken, H., Feyen, J., Boels, D. and Bronswijk, J.J.B., 1992. Shrinkage processes of an unripe marine clay soil in relation with physical ripening. Soil Sci., 153 (6): 471-481. Pons, L.J. and Van der Molen, W.H., 1973. Soil genesis under dewatering regimes during 1000 years of polder development. Soil Sci., 116: 228-235. Pons, L.J. and Zonneveld, I.S., 1965. Soil ripening and soil classification. ILRI Publ. No. 13. Int. Inst. for Land Reclamation and Improvement, Wageningen. Rijniersce, K., 1983. A simulation model for physical soil ripening in the IJsselmeerpolders. Flevo-berichten 203, Rijksdienst ljsselmeerpolders, Lelystad, 216 pp. Wilding, L.P. and Hallmark, C.T., 1984. Development of structural and microfabric properties in shrinking and swelling clays. In: J. Bouma and P.A.C. Raats (Editors), Proc. ISSS Symp. on Water and Solute Movement in Heavy Clay Soils. ILRI-Publ. 37, Int. Inst. for Land Reclamation and Improvement, Wageningen, pp. 1-22. Zuur, A.J., 1958. Bodemkunde der Nederlandse bedijkingen en droogmakerijen. Deel C. Het watergehalte, de indroging en enkele daarmede samenhangende processen. Coll. dictaat. Directie van de Wieringermeer, Wageningen.
67
Quantification of physical ripening in an unripe marine clay soil D.J. K i m a, J. Feyen a, H. Vereecken b, D.
Bo el s c a n d J.J.B. B r o n s w i j k c
alnstitute for Land and Water Management, KUL, Vital Decosterstraat 102, 3000 Leuven, Belgium bForschungszentrum Jiilich, Erdrl und Geochemie. ICEt 5, Post 1913, D5170 Jiilich, Germany CDepartment of Soil Conservation and Technology and Department of Soil Physical Transport Phenomena of Winand Staring Centre, Wageningen, The Netherlands (Received September 16, 1991; accepted after revision September 1, 1992)
ABSTRACT Kim, D.J., Feyen, J., Vereecken, H., Boels, D. and Bronswijk, J.J.B., 1993. Quantification of physical ripening in an unripe marine clay soil. Geoderma, 58: 67-77. The process of physical ripening of a soil developed on recently exposed marine clay has been studied in detail by examining the swelling of soil clods subjected to a number of drying and wetting cycles. During the wetting cycle, solutions with three different CaCI2 concentrations were used to study the effect of salts on the swelling. The swelling behaviour was represented by a linear swelling characteristic showing that the concentration of solutions clearly influences the swelling. Based on the swelling characteristics, quantitative aspects of physical ripening were investigated in terms of the irreversibility of extracted soil water. A linear relationship was noted between the moisture ratio at the threshold of irreversible extraction and the initial moisture ratio. Based on the relationship, a physical ripening index (PRI) is proposed, which can be used together with the initial moisture ratio to indicate stages in the ripening process for these soil materials. A greater proportion of water could be reversibly extracted from soils which had undergone increasing degrees of ripening.
INTRODUCTION
Marine sedimentary materials, are pedogenetically undeveloped at the time they become exposed (Pons and Van der Molen, 1973 ). These materials normally undergo several changes which are related to the soil water regime. One of the most distinctive changes is physical ripening, which is regarded as an initial soil-forming process caused by drying and wetting. Upon drying the soil material shrinks resulting in the loss of volume whereas upon wetting it swells regaining volume. If the soil material recovers the initial volume upon rewetting, it is termed reversible, otherwise irreversible. After a series of shrinkage and swelling events, the soil materials exhibit a physically ripened state marked by structure development, and altered stability and permeabilCorrespondence to." D.J. Kim, Institute for Land and Water Management, KUL, Vital Decosterstraat 102, 3000 Leuven, Belgium. 0016-7061/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved.
68
D.J. KIM ETAL.
ity. Swelling and shrinking is a natural process which is generally considered to be responsible for the compaction and improvement of the soil structure (Wilding and Hallmark, 1984; Bronswijk, 1989). It would then be important to know how the degree of physical ripening is related to the swelling and reversibility. Not much is reported about the degree of physical ripening of marine sedimentary materials in the literature. Zuur (1958) derived the following empirical relationship between the water content and the clay and organic matter content:
A=20+n(L+ 3H)
(1)
where A is the number of grams of water/100 g of dry material, n is the water factor, L is the clay content (%), and H is the humus content (%). The parameter n, the so-called water factor, is a measure of degree of ripening of a soil. The value 20 represents the quantity of water bound to non-colloidal components. The value 3 indicates that depending on the degree of humification organic matter binds three times as much water as clay. Pons and Zonneveld ( 1965 ) proposed a method for determining the physical ripening stage from either the water factor or the consistency of the soil material. Their method, however, provides a rough idea about the ripening stage due to uncertainties in the constant of eq. ( 1 ). The other shortcoming of the method is that it does not include any information on the soil physical properties or soil structural phases, except for the water content. Rijniersce (1983) reported that physical ripening of a soil is a process of irreversible drying. In this respect, a completely reversible soil cannot undergo physical ripening. A partially reversible soil undergoes ripening with limited reversibility or irreversibility. However, the criteria of irreversibility were not explicity presented. Hence this study is aimed to investigate quantitative aspects of physical ripening by defining the concept of irreversible drying based on a swelling characteristics, and to propose new indices to assess the degree of physical ripening in comparison with the existing methods. SOILS A N D M E T H O D S
Soil A large bulk sample of an unripe marine clay soil was taken from near Termunten on the northern coastal area of the Netherlands. Since the site was frequently inundated by sea water, all sampling was done during low tide when the soil emerged. The soil is classified as an alluvial or fine-clayey over loamy, illitic, mesic Hydraquents (De Bakker, 1979). The soil contains 45.8% clay and 52.3% silt and has a bulk density of 0.49 g cm-3. The major clay minerals
QUANTIFICATION OF PHYSICAL RIPENING
69
in the soil were kaolinite and illite. The concentration of the sea water was 256 meq 1-1 with SAR of 35.
Determination of swelling and shrinkage behaviour The swelling behaviour of the ripening marine clay soil was characterised using the clod method proposed by Bronswijk (1986). In order to create a flexible coating around the soil, plasticizer was added at a 1:10 weight ratio to SARAN-F310 resin (resin to solvent ratio 1:5 by weight). Several holes (less than 0.5 m m in diameter) were made on the surface of the saran resin to allow water to enter the soil clod. Solutions with three different levels of CaCl2 concentrations (0.0N CaC12, 0.1N CaC12 and 0.6N CaC12) were prepared to investigate the effect of salts on swelling in a combined form of CaC12 and NaC1 since the soil solution contains a considerable amount of sodium owing to frequent saturation by sea water. Swelling was measured in terms of moisture ratio on small clods ranging between l0 and 20 g. Soil clods were dried in five replicates to seven different moisture ratios (3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5 ). Once each soil clod was dried to the specified drying limit, it was placed successively in each solution until the m a x i m u m swelling limit was reached. Each clod was subjected to five sequential drying and wetting cycles.
Methods of quantification The shrinkage behaviour of a soil material can generally be characterized by changes of its moisture and void ratios (Haines, 1923; Bronswijk, 1988), which are defined as:
O=VwlVs
(2)
e=V,,/Vs
(3)
where 0 is the moisture ratio, e is the void ratio, Vw is the volume of water, Vv is the volume of void (L3), and Vs is the volume of solid (L3). The swelling behaviour of the marine clay soil can be characterized by either its extensibility or reversibility. Grossman et al. (1968) proposed a parameter COLE (Coefficient Of Linear Extensibility) to estimate the swelling and shrinkage potential of a soil layer and PLE (Potential Linear Extensibility) for the potential of different soil layers. These parameters assume that the soil follows an equidimensional shrinkage. However, the soil used in this study shows shrinkage from unidimensional to equidimensional depending on the moisture ratio, ripening stage and load (Kim et al., 1992 ). In this study, a swelling degree index (SDI) is proposed to evaluate the vol-
70
D.J. K1M ET AL.
ume expansion of a ripening soil. The SDI can be defined as the ratio of soil volume at drying to that at swelling, and can be given by: SDI= Vz/VL
(4)
where SDI is the swelling degree index, Vl is the volume at dry condition (L 3), and V2 is the volume at swelling condition (L 3). Since a ripening soil shows dynamic changes in the shrinkage curve, its volume after swelling differs depending on the stage of ripening. This complicates the calculation of volume for any dry and wet condition. Therefore the swelling degree of a soil can firstly be interpreted by the swelling function which simply relates the moisture ratio at drying to that at swelling. In conjunction with the shrinkage characteristic, the volume at swelling can then be calculated. For any ripening stage, the SDi can be rewritten as: SDI----- V 2 / V t = ( 1
+ e w ) / ( 1 +Ca)
(5)
Ow=F(Od)
(6)
ew = q)(Ow)
(7)
where 0w is the moisture ratio at swelling, do is the moisture ratio at drying, ew is the void ratio at swelling, eo is the void ratio at drying, F is the swelling function, • is the shrinkage function, and SDI is the swelling degree index. Another important parameter evaluating the swelling process is the reversibility. The reversibility of a soil is defined as its capacity to regain an original volume by rewetting after the soil has been dried to a certain moisture ratio. The swelling funtion (Fig. 1 ) can be used to determine the quantity of water extraction at which te soil shows irreversibility of swelling on rewetting. The quantity of extracted water is found by: L~ext = L ~ i - - 0 d
(8)
where Oext is the quantity of water extraction in the moisture ratio, tgi is the moisture ratio at initial condition, and do is the moisture ratio at dry condition upon extraction. After rewetting, the reversible quantity of water can be found by: /grev= "l- (/}d) -- •d
(9)
The difference between the two quantities Oext and Orev is the irreversible quantity of extracted water Oirr and is given by:
O~rr= Oex~--0rev
(10)
If the difference is positive, then the extraction is irreversible. For a given initial moisture ratio, the moisture ratio at dry condition (do) which satisfies eq. ( 10 ) equal to zero, is the moisture ratio (Ooc) at the threshold point below which the irreversible extraction of water occurs. For a given swelling func-
QUANTIFICATION OF PHYSICAL RIPENING
71
5
== "~
4-
m
I
3
/
t3
function 0
Moisture ratio at ary limit
Fig. 1. Relationship between the quantity of water extraction (0ex,), and the reversible quantity (0re,) and the irreversible quantity (tg~rr) for a given initial moisture ratio using swelling function. Data demonstrate the results of terms when a soil is subjected to drying from 0~ (4.0) to Od (2.0).
tion, several Odc values can be found for different Oi. A functional relationship can be derived by plotting the Odc versus the Oi. A measure of ripening degree can be found from the concept of reversibility. As a soil becomes riper, the reversible quantity of extracted water becomes greater. Further, the progress of physical ripening can be measured by comparing the reversible extracted quantity with the total quantity. The reversible extracted quantity is determined by the difference between the initial moisture ratio (Oi) and the threshold moisture ratio (0dc). A physical ripening index (PRI) is derived from: ~1~--~-(L~i - - ~ d c ) / ( ~ q i )
( 11)
where 7 is the physical ripening index. If a soil material is completely unripened, the value of 7 will be zero. A completely ripened soil will have 7 equal to 1 which indicates that no degree of water extraction causes irreversibility. RESULTS AND DISCUSSION
For a given do, the Ow of the soil is influenced by the number of drying and wetting cycles. Repeated drying and wetting causes a slight decrease in the Ow which approaches a constant value. This phenomenon is, in this study, referred to as "cycle effect". The significance of the cycle effect was examined using the analysis of covariance (ANCOVA), with the number of cycles as the treatment variable. The results of the ANCOVAare shown in Table 1. The dependent variable Y is defined as the Ow and the covariate X as the do. In ad-
72
D.J. KIM ET AL.
TABLEI
Results of ANCOVA for solutions with three different salt concentrations Conc.
Variable
F value
Pr> F
0.0N
X cycle cycle*X
1440.49 14.75 0.77
0.0001 0.0001 0.5511
O. IN
X cycle cycle*X
471.85 0.45 0.46
0.0001 0.7744 0.7625
0.6N
X cycle cycle*X
862.42 3.39 0.52
0.0001 0.0135 0.7223
X: continuous covariate as the moisture ratio at the dry limit. cycle: treatment of drying-wetting cycle. TABLE2
The fitting model (Y=cX+d) used in the swelling function Conc.
cycle
c
d
R2
1 2 3 4 5
0.790 0.837 0.797 0.738 0.742 0.742
1.345 1.451 1.391 1.407 1.300 1.307
0.908 0.997 0.996 0.998 0.997 0.998
0.1N
0.793
1.227
0.897
0.6N
0.756
1.117
0.900
0.0N
Y: dependent variable representing moisture ratio at swelling. X: independent variable representing moisture ratio at drying limit. c: coefficient of slope. d: constant. R 2: coefficient of determination.
dition, the interaction effect was tested between treatment and covariate (cycle*X). The cycle effect is significant only for the solution with zero salt concentration. For all three analyses, the interaction term was not significant. Table 2 shows the regression equations fitted through the data of the swelling characteristic. For the zero concentration, equations were obtained from different treatment levels. However, only one equation was obtained for other solutions since the cycle effect is not significant. These regression equations are termed "swelling function". It appears that the moisture at swelling de-
73
Q U A N T I F I C A T I O N OF PHYSICAL R I P E N I N G
':t
(a) . . . . O.ON
1.
-
//"°-'"-,,
-
0 . i N
--
0.6N
....
O.ON O.tN 0.6N
t.4-
t.3" i.2" t.t" i.
HoistuPe R a t i o 1.6
(b)
1.5
-
-
-t.4
////
"'""",,
,,
t.3 1.2 1.1-
],
i
0
; HoistuPe R a t i o
t.6 (c)
.... t.5-
-
-
--
O.ON 0.iN 0.6N
1.4~
t.3-
t.
0
Moisture
Ratio
Fig. 2. Comparison of SDI influenced by the different salt concentration at a given ripening of (a) stage 1, (b) stage 2, (c) stage 3.
74
D.J. KIM ET AL.
creases with increasing salt concentrations. The results of swelling functions clearly indicate that the ripening soil differs from ripened swelling soils with respect to reversibility. The decrease in both the swelling capacity and the cycle effect with higher salt concentrations can be explained by the influence of osmotic pressure. Figure 2 shows the results of SD1 versus the moisture ratio at dry, for various stages of ripening. To calculate the void ratio at swelling, shrinkage characteristic curves (Kim et al., 1992 ) were obtained for the three different ripening stages. The results show distinct differences for all ranges of 0o. The higher the salt concentration, the smaller the SDI. A clear peak value is observed around the moisture ratio of 0.7 regardless of the salt concentration and the stage of ripening. Figure 3 shows the regression equations obtained from the relationship between the Ode and the 0~. The Ooc decreases linearly with the O i. However, the 5
Cone.
C2
j1,725
--
1.51 7 1.438
c-
5 .~
C1
..... 0.0N 1,274 -
-
O. iN 1.251 0 . 6 N 1.314
3-
."
o
"~ 2-
I /
7
,/" .-"~ ,-" . ~ " ,,. f
/ /
/
ul
0
Inltlal Moisture Ratio
Fig. 3. Linear regression equations (Ode= C1 O~- C2) obtained for the relation between the initial moisture ratio (O~) a n d the moisture ratio (0de) at the threshold of irreversibility. TABLE 3
Comparison of physical ripening index (PRI) with parameter n and consistency of the soil material for different initial moisture ratios O~
?(%)
n
Designation
Consistency
4.5 4.0 3.5 3.0 2.5 2.0 1.5
9 13 19 26 36 50 76
2.25 2.00 1.75 1.50 1.25 1.00 0.75
Unripened Unripened Practically unripened Practically unripened Half ripened Nearly ripened Nearly ripened
Very soft Very soft Soft and sticky Soft and sticky Fairly soft Fairly firm Fairly firm
Values of parameter n are based on the following equation proposed by Bouten (1978) in the case L + 3H> 20.' A = n(L + 3H); where ,4, L and H are the same variables as described in eq. ( 1 ).
QUANTIFICATION OF PHYSICAL RIPENING
75
magnitude of decrease in O0c becomes larger as the 0 i decreases. This indicates that a greater quantity of water can reversibly extracted with the lower Oi, a slight extraction may cause a irreversible drying. Irreversibility is more pronounced for the solutions with higher concentration. This implies that a soil with a greater amount of salts can be more ripened for a given dryness. Table 3 presents a comparison of PRZ with the parameter n and consistency of the soil material for the different initial moisture ratios. To determine y (PRI), the swelling function corresponding to 0.1N solution was used. The increases with ripening and with increasing firmness of consistency. The increase of ~ indicates that a greater proportion of water can be extracted without causing irreversible drying. This conforms to the statement given by Rijniersce ( 1983 ) - - the riper the soil, the larger the reversible quantity. It is noteworthy that the measures of the degree of ripening, ~, and n, are highly correlated. Especially between Lqti and n, a perfect equivalence exists. This seems to be reasonable since the basis of the parameters is nothing but the water content of the soil material. It is likely that the 0i identical with the moisture ratio (Os) at saturation or swelling can also be used for estimation 3~ Hoisture
13:
Ratio
MotsLuee R a t i o st lowest dry
=
D'
W'D' 1
W' 2
D'
W' 3
D ' W'D' 4
W'D' 5
W 6
Cycles
Fig. 4. The effect of the moisture ratio at the lowest drying limit on the moisutre ratio (0w) at swelling during the alternate drying and wetting cylces. The variable on the Y axis can be Od or O, depending on the drying ( D ) or wetting (W) cycle. TABLE 4 A suggested guideline for estimation of ripening stage from Oi, n and 7' ( PRI ) Stage
0~
n
7
Consistency
Designation
1 2 3
> 3.5 2.0-3.5 < 2.0
> 1.75 1.00-1.75 < 1.00
< 20 20-50 > 50
Soft Sticky to firm Fairly firm
Unripened Half ripened Nearly ripened
76
D.J. KIM ETAL.
of the degree of ripening. The advantage in the use of Q over the other parameters is that it provides an idea of how much water can reversibly be extracted for a given Oi. However, the determination of 7 requires a known swelling function. Quick estimation of the degree of ripening can be obtained from the Oi since the determination is simple and much easier. Figure 4 shows another specific behaviour of swelling. When a soil clod is subjected to various drying limits, do, the swelling is finally dominated by the lowest tgo. This indicates that the degree of swelling of a ripening soil is determined by the most intensive drying treatment. Together with the dependence of the degree of ripening on the Oi, this behaviour provides important hints for the process of soil physical ripening. Since physical ripening is a continuous process, distinction between ripening stages is bound to be arbitrary depending on the individual's perception. Despite the difficulties in distinguishing these stages an approximate boundary between the ripening stages is proposed based on the information in Table 3. The proposed guidlines are presented in Table 4. CONCLUSIONS
Quantitative aspects of physical ripening were examined in relation to swelling behaviour. A linear swelling function which characterizes a swelling capacity of the ripening soil has been derived. Physical ripening is defined in a quantitative manner using the linear swelling function. The threshold of irreversibility was related to the initial moisture ratio 0i. The relationship reveals that the reversibility increases with the progress of ripening. It is noted that an unripe soil can be more susceptible to ripening owing to its higher initial moisture ratio since irreversible drying is more likely to occur at that level of moisture content. Combination of the unique linear swelling function (no cycle effect) with the dependence of the final swelling degree on the lowest drying limit leads to the conclusion that the degree of physical ripening is controlled by the lowest value of do. Finally, the physical ripening index ( PRI ) is proposed as a measure of ripening degree. However, for practical purpose, the use of the initial moisture ratio Oi is recommended for estimation of the degree of ripening. ACKNOWLEDGEMENTS
The author is gratefully indebted to Dr. A.L.M. van Wijk (The Winand Staring Centre, Wageningen) for providing laboratory facilities and helpful discussions throughout this research. Thanks are also due to L. Geeraerts for typing and E. Mwendera for contribution to revision of the manuscript.
QUANTIFICATION OF PHYSICAL RIPENING
77
REFERENCES Bronswijk, J,J.B., 1986. Evaporation and cracking of a heavy clay soil. Rep. 14, Inst. for Land and Water Management Research (ICW). Wageningen. Bronswijk, J.J.B., 1988. Modeling of water balance, cracking and subsidence of clay soils. J. Hydrol., 97: 199-212. Bronswijk, J.J.B., 1989. Prediction of actual cracking and subsidence inclay soils. Soil Sci., 148: 87-93. Bronswijk, J.J.B. and Evers-Vermeer. J.J., 1990. Shrinkage of Dutch clay soil aggregates. Neth. J. Agric. Sci., 38: 175-194. Bouten, W., 1978. Over de structuur en her waterbindend vermogen van pure klei. Werkdocument RIJP 1978-187 Abw, Rijksdienst IJsselmeerpolders, Lelystad. De Bakker, H., 1979. Major soils and soil regions in the Netherlands. Soil Survey Institute, Wageningen. Grossman, R.B., Brasher, B.R., Franzmeier, D.P. and Walker. J.L., 1968. Linear extensibility as calculated from natural-clod bulk density measurements. Soil Sci. Soc. Am. Proc., 32: 570-573. Haines, W.B., 1923. The volume changes associated with variations of water content in soil. J. Agric. Sci. Can., 13:296-311. Kim, D.J., Vereecken, H., Feyen, J., Boels, D. and Bronswijk, J.J.B., 1992. Shrinkage processes of an unripe marine clay soil in relation with physical ripening. Soil Sci., 153 (6): 471-481. Pons, L.J. and Van der Molen, W.H., 1973. Soil genesis under dewatering regimes during 1000 years of polder development. Soil Sci., 116: 228-235. Pons, L.J. and Zonneveld, I.S., 1965. Soil ripening and soil classification. ILRI Publ. No. 13. Int. Inst. for Land Reclamation and Improvement, Wageningen. Rijniersce, K., 1983. A simulation model for physical soil ripening in the IJsselmeerpolders. Flevo-berichten 203, Rijksdienst ljsselmeerpolders, Lelystad, 216 pp. Wilding, L.P. and Hallmark, C.T., 1984. Development of structural and microfabric properties in shrinking and swelling clays. In: J. Bouma and P.A.C. Raats (Editors), Proc. ISSS Symp. on Water and Solute Movement in Heavy Clay Soils. ILRI-Publ. 37, Int. Inst. for Land Reclamation and Improvement, Wageningen, pp. 1-22. Zuur, A.J., 1958. Bodemkunde der Nederlandse bedijkingen en droogmakerijen. Deel C. Het watergehalte, de indroging en enkele daarmede samenhangende processen. Coll. dictaat. Directie van de Wieringermeer, Wageningen.