Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol55.Issue3-4.Jun2000:
Soil & Tillage Research 55 (2000) 117±126
The essential mechanics of capillary crumbling of
structured agricultural soils
O.B. Aluko*, A.J. Koolen
Soil Technology Group, Wageningen University, Bomenweg 4, 6703HD Wageningen, Netherlands
Received 5 July 1999; received in revised form 28 January 2000; accepted 17 February 2000
Abstract
In soil loosening processes like seedbed preparation, signi®cant soil crumbling is often desired. A better understanding of
the mechanics of crumbling is necessary to optimize crumbling operations, particularly in structured agricultural soils in
which capillary bonds are dominant. To this end, a model of the mechanics of capillary crumbling in structured agricultural
soils is developed. Four sets of experiments on cylindrical soil samples were carried out to investigate the validity of the model
and soil crumbling characteristics as in¯uenced by freezing, thawing and drying. After preparation and sample pre-treatment,
the samples were dried to different moisture contents and then tested to determine soil bonding strength. Soil water suction
was also monitored at testing as were sample dimensions at different stages of experimentation. The model was found to
account satisfactorily for the mechanics of capillary crumbling in structured agricultural soils. Freezing had the effect of
reducing the strength of inter-aggregate bonds whilst preserving the integrity of soil aggregates during crumbling.
# 2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil bonding strength; Soil structure; Capillary bonds; Soil moisture content; Mechanics of crumbling
1. Introduction
When such assessments as traf®cability, water
in®ltrability and workability of an agricultural soil
are desired for cultivation and agricultural production
purposes, two physical properties are of primary
importance: soil textural composition and soil structure. The textural composition of a soil is largely
®xed with possible but minor changes being made
in the organic matter content. Soil structure, on the
*
Corresponding author. Present address: Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife,
Nigeria. Tel.: 234-362-30290; fax: 234-362-32041.
E-mail address: oaluko@oauife.edu.ng (O.B. Aluko)
other hand, is a transient property which can be
altered by different processes. Such processes include
shrinking and swelling, drying and wetting, freezing
and thawing, human, animal and vehicular traf®c,
mechanical tillage operations and incorporation of
organic matter.
The importance of soil structure cannot be overemphasized. For example, the state of the soil structure can be a farmer's greatest asset or his greatest
risk. The integrity of aggregates, the type and strength
of aggregate bonds, the size and distribution of pore
spaces within the soil and the moisture content are
salient factors that determine the state of structure
of an agricultural soil. When aggregate bonding is
the decisive factor, Koolen and Kuipers (1989)
have suggested that a soil consisting of ®rm units
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 0 5 - 7
118
O.B. Aluko, A.J. Koolen / Soil & Tillage Research 55 (2000) 117±126
(aggregates) with relatively little bonding between
adjacent aggregates at the mutual points (or areas)
of contact, can be regarded as having a good structure.
The phenomenon of ``crumbling'' in agricultural
soils is very important in the development of soil
structure. Indeed, agricultural soils can be regarded
as having a crumb structure (Russell, 1973). This
crumb structure approach has been used by Chandler
(1985) to explain the crumbling of agricultural soils
as a fracture process which involves the breaking
of bonds between soil crumbs (or aggregates). When
the crumbs are hard and the bonding between crumbs
is weak, the soil cracks in a brittle manner. On the
other hand, when the bonding between the crumbs
is strong compared with its internal strength, it may
become distorted without breaking (i.e. it is ductile).
Snyder and Miller (1989) also attributed crumbling
in agricultural soils during tillage operations, to
failure by cracking due to tensile stresses. For structured agricultural soils where capillary bonds play a
dominant role, Koolen (1987) proposed a capillary
bonding stress, pw (N/cm2), which is a function of
the soil water suction and the degree of pore saturation.
This paper reports an experimental-analytical
investigation to examine the mechanics of capillary
crumbling in a structured agricultural soil. In particular, Koolen's model of capillary bonding was further
developed and the effects of freezing, thawing and
drying on crumbling as well as the integrity of soil
structure, were studied.
Fig. 1. Physical model of the contact zone between two aggregates
in a structured soil (after Koolen, 1987): pw, capillary bonding
stress; u, soil water suction; S, degree of pore saturation.
per unit area (i.e. capillary bonding stress), pw (N/
cm2), given by
pw u S
(1)
where u is the soil water pressure (i.e. soil water
suction in units of stress) and S is the degree of pore
saturation, which is the fraction of the total pore
volume within the bulk soil occupied by water. The
range of S is 0±1.
In a further analysis of Eq. (1) (see Fig. 2), Koolen
considered its implications in a structured clay soil
with inter-aggregate pore spaces that fall within one
narrow size class. He reasoned that as the soil dries out
from an initially wet state, the trend in the capillary
bonding stress (pw) within aggregates would differ
2. Theory
In general, soil strength is derived from two
sources: inter-aggregate bonds and intra-aggregate
bonds. The strength of a soil is a re¯ection of the
force required to break these bonds. Fig. 1 is a physical
model of two aggregates in a structured soil, hatched
vertically and horizontally, respectively, that are in
contact at positions 1, 2 and 3 in plane I. The pores
between the aggregates are relatively larger than those
within the aggregates. At contact positions 1, 2, and 3
the tension in the soil water ``clenches'' the particles
together. Based on this clenching force, Koolen (1987)
considered surface-elements within the soil such as
planes I and II and derived a capillary bonding force
Fig. 2. The pw±u±S diagram of capillary crumbling proposed by
Koolen (1987). Curves: A, pF (log u) curve within aggregates; B,
pF curve between aggregates; C, capillary bonding stress within
aggregates; D, capillary bonding stress between aggregates.
O.B. Aluko, A.J. Koolen / Soil & Tillage Research 55 (2000) 117±126
from that between aggregates. Thus in Fig. 2, curves
A and C illustrate the pF (log u) curve (where u is
expressed as a negative pressure head in cm water
column) and the trend of pw within aggregates, respectively, while curves B and D describe the pF curve
and the trend of pw between aggregates, respectively.
The horizontal portion of curve B refers to the emptying of inter-aggregate pores within a particular
size class with little or no increase in soil water
suction.
Koolen's model suggests that, corresponding to this
emptying, the capillary bonding stress between aggregates decreases (see curve D) and then begins to rise
again as soil water suction again increases.
In an investigation into the mechanics of cracking of
remoulded clay bars, Towner (1987) stated that the
tensile strength of a soil is a material property that
depends in general on both the soil water suction and
the water content. In another study of the tensile
strength of unsaturated soils, Snyder and Miller
(1985) used a modi®ed effective stress equation to
estimate tensile strength, st. Their equation can be
expressed in the form (Snyder and Miller, 1989):
st ÿw yuw
(2)
where the factor w(y) is a function of the degree of pore
saturation of the soil and uw is the negative hydrostatic
pore water pressure. It is interesting to note that the
terms on the right-hand side of Eq. (2) are similar to
those in Eq. (1). Now, Mullins and Panayiotopolous
(1984) have pointed out that in structured soils, only
the fraction of water in the inter-aggregate pore space
will contribute to the cohesion between adjacent
aggregates which ultimately determines the strength
of a structured soil. In view of this, Snyder and Miller
(1989) suggested that the use of an ``effective'' degree
of pore saturation, ``ye'', based only on the interaggregate porosity of soil, might give a better estimate
of the tensile strength of soil in Eq. (2).
Assuming that the foregoing arguments are valid, an
expression, similar to Eq. (1), can be written for the
soil tensile strength st (N/cm2) as follows:
s t u Si
(3)
where Si is the degree of inter-aggregate pore saturation, which is de®ned as the fraction of the interaggregate pore space occupied by water. Like S, Si
ranges from 0 to 1.
119
In conjunction with Koolen's pw±u±S diagram (see
Fig. 2), Eqs. (1) and (3) constitute the proposed
capillary crumbling model for structured agricultural
soils in which capillary bonds are decisive. In comparison with st, pw is an index of the bonding strength
of the bulk soil whereas st is an index of the bonding
strength at inter-aggregate locations. Series of experiments, in which soil tensile strength, soil water suction
and degree of pore saturation were measured, were
carried out to investigate the validity of the proposed
capillary crumbling model. The materials used and
experimental procedure will now be described.
3. Experimental work
3.1. Soil preparation
The soil used for this investigation was a Wageningen silty clay loam (36% of minerals
The essential mechanics of capillary crumbling of
structured agricultural soils
O.B. Aluko*, A.J. Koolen
Soil Technology Group, Wageningen University, Bomenweg 4, 6703HD Wageningen, Netherlands
Received 5 July 1999; received in revised form 28 January 2000; accepted 17 February 2000
Abstract
In soil loosening processes like seedbed preparation, signi®cant soil crumbling is often desired. A better understanding of
the mechanics of crumbling is necessary to optimize crumbling operations, particularly in structured agricultural soils in
which capillary bonds are dominant. To this end, a model of the mechanics of capillary crumbling in structured agricultural
soils is developed. Four sets of experiments on cylindrical soil samples were carried out to investigate the validity of the model
and soil crumbling characteristics as in¯uenced by freezing, thawing and drying. After preparation and sample pre-treatment,
the samples were dried to different moisture contents and then tested to determine soil bonding strength. Soil water suction
was also monitored at testing as were sample dimensions at different stages of experimentation. The model was found to
account satisfactorily for the mechanics of capillary crumbling in structured agricultural soils. Freezing had the effect of
reducing the strength of inter-aggregate bonds whilst preserving the integrity of soil aggregates during crumbling.
# 2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil bonding strength; Soil structure; Capillary bonds; Soil moisture content; Mechanics of crumbling
1. Introduction
When such assessments as traf®cability, water
in®ltrability and workability of an agricultural soil
are desired for cultivation and agricultural production
purposes, two physical properties are of primary
importance: soil textural composition and soil structure. The textural composition of a soil is largely
®xed with possible but minor changes being made
in the organic matter content. Soil structure, on the
*
Corresponding author. Present address: Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife,
Nigeria. Tel.: 234-362-30290; fax: 234-362-32041.
E-mail address: oaluko@oauife.edu.ng (O.B. Aluko)
other hand, is a transient property which can be
altered by different processes. Such processes include
shrinking and swelling, drying and wetting, freezing
and thawing, human, animal and vehicular traf®c,
mechanical tillage operations and incorporation of
organic matter.
The importance of soil structure cannot be overemphasized. For example, the state of the soil structure can be a farmer's greatest asset or his greatest
risk. The integrity of aggregates, the type and strength
of aggregate bonds, the size and distribution of pore
spaces within the soil and the moisture content are
salient factors that determine the state of structure
of an agricultural soil. When aggregate bonding is
the decisive factor, Koolen and Kuipers (1989)
have suggested that a soil consisting of ®rm units
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 0 5 - 7
118
O.B. Aluko, A.J. Koolen / Soil & Tillage Research 55 (2000) 117±126
(aggregates) with relatively little bonding between
adjacent aggregates at the mutual points (or areas)
of contact, can be regarded as having a good structure.
The phenomenon of ``crumbling'' in agricultural
soils is very important in the development of soil
structure. Indeed, agricultural soils can be regarded
as having a crumb structure (Russell, 1973). This
crumb structure approach has been used by Chandler
(1985) to explain the crumbling of agricultural soils
as a fracture process which involves the breaking
of bonds between soil crumbs (or aggregates). When
the crumbs are hard and the bonding between crumbs
is weak, the soil cracks in a brittle manner. On the
other hand, when the bonding between the crumbs
is strong compared with its internal strength, it may
become distorted without breaking (i.e. it is ductile).
Snyder and Miller (1989) also attributed crumbling
in agricultural soils during tillage operations, to
failure by cracking due to tensile stresses. For structured agricultural soils where capillary bonds play a
dominant role, Koolen (1987) proposed a capillary
bonding stress, pw (N/cm2), which is a function of
the soil water suction and the degree of pore saturation.
This paper reports an experimental-analytical
investigation to examine the mechanics of capillary
crumbling in a structured agricultural soil. In particular, Koolen's model of capillary bonding was further
developed and the effects of freezing, thawing and
drying on crumbling as well as the integrity of soil
structure, were studied.
Fig. 1. Physical model of the contact zone between two aggregates
in a structured soil (after Koolen, 1987): pw, capillary bonding
stress; u, soil water suction; S, degree of pore saturation.
per unit area (i.e. capillary bonding stress), pw (N/
cm2), given by
pw u S
(1)
where u is the soil water pressure (i.e. soil water
suction in units of stress) and S is the degree of pore
saturation, which is the fraction of the total pore
volume within the bulk soil occupied by water. The
range of S is 0±1.
In a further analysis of Eq. (1) (see Fig. 2), Koolen
considered its implications in a structured clay soil
with inter-aggregate pore spaces that fall within one
narrow size class. He reasoned that as the soil dries out
from an initially wet state, the trend in the capillary
bonding stress (pw) within aggregates would differ
2. Theory
In general, soil strength is derived from two
sources: inter-aggregate bonds and intra-aggregate
bonds. The strength of a soil is a re¯ection of the
force required to break these bonds. Fig. 1 is a physical
model of two aggregates in a structured soil, hatched
vertically and horizontally, respectively, that are in
contact at positions 1, 2 and 3 in plane I. The pores
between the aggregates are relatively larger than those
within the aggregates. At contact positions 1, 2, and 3
the tension in the soil water ``clenches'' the particles
together. Based on this clenching force, Koolen (1987)
considered surface-elements within the soil such as
planes I and II and derived a capillary bonding force
Fig. 2. The pw±u±S diagram of capillary crumbling proposed by
Koolen (1987). Curves: A, pF (log u) curve within aggregates; B,
pF curve between aggregates; C, capillary bonding stress within
aggregates; D, capillary bonding stress between aggregates.
O.B. Aluko, A.J. Koolen / Soil & Tillage Research 55 (2000) 117±126
from that between aggregates. Thus in Fig. 2, curves
A and C illustrate the pF (log u) curve (where u is
expressed as a negative pressure head in cm water
column) and the trend of pw within aggregates, respectively, while curves B and D describe the pF curve
and the trend of pw between aggregates, respectively.
The horizontal portion of curve B refers to the emptying of inter-aggregate pores within a particular
size class with little or no increase in soil water
suction.
Koolen's model suggests that, corresponding to this
emptying, the capillary bonding stress between aggregates decreases (see curve D) and then begins to rise
again as soil water suction again increases.
In an investigation into the mechanics of cracking of
remoulded clay bars, Towner (1987) stated that the
tensile strength of a soil is a material property that
depends in general on both the soil water suction and
the water content. In another study of the tensile
strength of unsaturated soils, Snyder and Miller
(1985) used a modi®ed effective stress equation to
estimate tensile strength, st. Their equation can be
expressed in the form (Snyder and Miller, 1989):
st ÿw yuw
(2)
where the factor w(y) is a function of the degree of pore
saturation of the soil and uw is the negative hydrostatic
pore water pressure. It is interesting to note that the
terms on the right-hand side of Eq. (2) are similar to
those in Eq. (1). Now, Mullins and Panayiotopolous
(1984) have pointed out that in structured soils, only
the fraction of water in the inter-aggregate pore space
will contribute to the cohesion between adjacent
aggregates which ultimately determines the strength
of a structured soil. In view of this, Snyder and Miller
(1989) suggested that the use of an ``effective'' degree
of pore saturation, ``ye'', based only on the interaggregate porosity of soil, might give a better estimate
of the tensile strength of soil in Eq. (2).
Assuming that the foregoing arguments are valid, an
expression, similar to Eq. (1), can be written for the
soil tensile strength st (N/cm2) as follows:
s t u Si
(3)
where Si is the degree of inter-aggregate pore saturation, which is de®ned as the fraction of the interaggregate pore space occupied by water. Like S, Si
ranges from 0 to 1.
119
In conjunction with Koolen's pw±u±S diagram (see
Fig. 2), Eqs. (1) and (3) constitute the proposed
capillary crumbling model for structured agricultural
soils in which capillary bonds are decisive. In comparison with st, pw is an index of the bonding strength
of the bulk soil whereas st is an index of the bonding
strength at inter-aggregate locations. Series of experiments, in which soil tensile strength, soil water suction
and degree of pore saturation were measured, were
carried out to investigate the validity of the proposed
capillary crumbling model. The materials used and
experimental procedure will now be described.
3. Experimental work
3.1. Soil preparation
The soil used for this investigation was a Wageningen silty clay loam (36% of minerals