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III. Soil Water Relations: Swelling and Shrinkage
J. P. QUIRK
don clay) can exist in a series of structural states depending on mechanical
disturbance and sample history. That is, the amount of water retained by clay at a
given suction can vary considerably depending on the amount of enmeshed water
which increases with disturbance, such as rapid wetting of a dry soil or mechanical working of the soil at low suctions (puddling). They reported that London
clay, when puddled in preparation for engineering testing, retained 105% water
at a suction of I kPa and that as the suction is successively increased the water
content suction curve followed is the normal consolidation curve. Each point on
this curve is a different structural state since a reproducible hysteresis loop is
produced on decreasing and then increasing the suction to that at the starting
point on the normal curve. When the suction reached the normal shrinkage limit
(suction 6.3 MPa) the reproducible hysteresis loop traveled between water contents of 18 and 30% (suction 1 kPa); this final hysteresis loop is referred to as the
overconsolidated curve and approximates closely the water content-suction
curve (moisture characteristic) of the soil in its natural or undisturbed state.
When a soil in its natural overconsolidated state is disturbed it tends to move
toward the suction-water content relationship of the normal consolidation curve
if water is freely available. If the water content is held constant, then the suction
increases in response to applied work. Soil aggregates which are stabilized by the
presence of organic matter, sesquioxides, and other materials resist this change to
varying degrees and this can be regarded as one manifestation of structural
The limit of shrinkage occurs when particles come into contact. Camontmorillonite (Wyoming) sustains a d(OO1) value of 15.5 li over the pIpo
range 0.91 (13 MPa) (Slade and Quirk, 1991) (Fig. 6, Section V) to 0.10
(Mooney et al., 1952). Magnesium-vermiculite (Llano) sustains a spacing of
14.8 8, from immersion in water to a pIpo value of 0.02 (van Olphen, 1969)
when the two-layer state within the crystal starts to transform to the one-layer
state. It is concluded that contact involves the separation of overlapping crystal
surfaces by two layers of water.
The air-entry value is determined principally by the pore size which in turn is
related to the thickness of the clay crystals. For a slit-shaped pore of 50 width,
the air-entry value would be 28.8 MPa and for pores 300 8, across the value
would be 4.8 MPa; these pore sizes would be expected in a fine-grained illite and
The swelling results of Holmes (1955) can be related to the swelling of the soil
profiles (Aitchison and Holmes, 1953). A red-brown earth subsoil followed
normal shrinkage over the water content range from 30 to 17%; with a particle
density of 2.65 g cm-3, this represents a volume change of 24%. Blocks of a
hydromorphic black earth followed normal shrinkage over the water content
range 39 to 16%which corresponds to a volume change of 44%.These soils exist
in a region of winter rainfall and summer drought, and from the end of summer to
the end of winter when the profile is fully wet the vertical movement measured is
4.4 cm for the red-brown earth soil and 7.9 cm for the black earth (Aitchison and
Holmes, 1953).The horizontal expression of swelling along the other two axes is
via extensive cracking, especially for the black earth; cracks are an important
avenue for the wetting of subsoils. Both these soils have an almost identical clay
content (64%) but their respective air-entry values correspond to pores sizes of
114 and 46 8, which reflects the finer particle size and hence greater swelling for
the black earth.
Table V shows that the residual shrinkage of the kaolinite (N2surface area of
36 m2/g) and the fine-grained illite (160 m2/g) is relatively small, being the
difference between the water content at the point of transition from normal to
residual shrinkage and the final porosity (oven dry).
The pore sizes at which air enters the clay, as shown in Table V, have been
calculated using the relation pgh = 2 y / r ,given earlier, for slit-shaped pores. The
pore size for air-entry reflects the relative particle thicknesses for kaolinite and
illite. Expressed on a volume basis the illite has a residual shrinkage of 2% which
contrasts with the rnontmorillonite which has a residual shrinkage of 15%; this
large value is due to the removal of the interlamellar water and possible particle
Water Content and Suction at the Transition from Normal to Residual Shrinkage+
the Slit-Shaped Pore Size Corresponding to the Suction, and the Porosity
Prior to Swelling for Ca Clay Coresa
(g H Z O K ' )
Experimental information is taken from Aylmore (1960).
Rocky Gully kaolinite is from the pallied zone of a laterite and Willalooka illite is from the B
horizon of a solodized solonetz. The nitrogen surface area of these three clays is, respectively, 36,
160, and 38 m2g-1.
J. P. QUIRK
The change in structural porosity of clay soil aggregates with increasing suction has been investigated by Lauritzen (1948) and Stirk (1954); they found that
there was an initial stage of shrinkage for naturally structured (undisturbed) soil
aggregates for which the decrease in volume of the soil was less than the change
in water content and concluded that this was because of air entering large pores
and cracks. This structural porosity extended to a suction of 0.03 MPa after
which the shrinkage became normal.
Structural pores are obviously critical in relation to water entry and ready
drainage after rainfall. For many practical purposes the soil water content attained a few days after the cessation of rainfall or irrigation is sensibly stationary,
and for a freely draining soil this water content is referred to as field capacity.
The attainment of field capacity depends on a high probability of continuity of
macropores or structural porosity since, if these pores were randomly distributed
throughout the soil mass, the approach to equilibrium would be very slow.
Millington and Quirk (1961, 1964) have discussed permeability in terms of the
probability of continuity of pore size classes. Although the total water content of
a soil increases when it is puddled, the permeability decreases markedly because
the structural pores are destroyed or lose their continuity by being randomly
distributed throughout the soil mass. It is for this reason that rice soils are
puddled to reduce percolation.
W. SWELLING OF SODIUM CLAYS
Considerable attention has been given to the extensive crystalline swelling,
d(001) >40 A, of Na clays because such swelling can be readily followed by
low-angle X-ray diffraction and also because of the possible implications that
such research has in relation to sodic soils. Sodic soils have been defined (Richards, 1954) as soils with an exchangeable sodium percentage in excess of 15;
however, adverse physical behavior may be encountered at lower percentages
The physical behavior of a soil may be more difficult than anticipated from the
exchangeable sodium percentage because of ion segregation. For instance, Ca2+
absorption on the internal surfaces of montmorillonite is favored, and as a result
the exchangeable sodium percentage on external surfaces of qu8sicrystals is
greater than the average for the whole material.
Another reason why the swelling of Na clays is considered important is that
advanced by Clapp and Emerson ( 1965) who proposed that the swelling pressure
of Na-saturated soils could be used as “a chemical hammer” to measure the effect
of organic matter in conferring different degrees of stability on soil aggregates in
water. This method has been used to assess the stability of soil aggregates of the
surface soil of a red-brown earth which had experienced varying periods in
pasture and in arable phases (Greenland et al., 1962).
Norrish (1954) and Norrish and Rausell-Colom (1963) measured the X-ray
spacing of oriented flakes of Na-montmorillonite (Wyoming) and single crystals
of Li-vermiculite (Kenya) as a function of NaCl and LiCl concentration in the
absence of a restraining pressure; that is, under conditions of free swelling since
there was no applied suction to restrain the swelling.
The following relationships were obtained between the basal spacing and the
electrolyte concentration, c, for Na-montmorillonite and Li-vermiculite:
d(001) = 11.3 c-0.5
( c from 0.3 to
( c from 0.15 to 2
This strong dependence of swelling on electrolyte concentration is indicative
of diffuse double layer behavior and especially so as the slope of the lines is
related to c-0.5 that is K - I in diffuse layer theory. The difference between the two
minerals is not due to the saturating cation since the slope of Li- and Namontmorillonite is almost the same (Norrish, 1954).
The intercepts shown in Eqs. (18) and (19) were interpreted as arising from the
aluminosilicate layer thickness (10 A) and the two layers of water (5.5 A)
associated with each of the opposing surfaces; this is, in effect, evidence for the
existence of Stem layers.
Since K - I is 3.04 c - O . ~ for a 1:l electrolyte, the slope of 23.8 [Eq. (19)]
converts to 7.8 K - I which is the separation of the Gouy planes at a given
Norrish and Raussel-Colom (1963) also obtained X-ray spacings for a Livermiculite crystal immersed in 0.03 M LiCl as a function of applied pressure.
Their results are shown in Fig. 4 in which the half-separation of the Gouy planes
$[d(OO1)-21] varies with applied load (pressure). Also shown in the figure is the
expected pressure-distance relationship for reduced Gouy electric potentials of
2, 3, 4, and 6 . If the full crystallographic charge density of 5.8 X 104 esu cm-2
(0.193 Cm-2 or one unit charge per 83 A2) is substituted into Eq. (13) for the
concentration of 0.03 M ,the reduced electric potential calculated is 5.9, corresponding to a surface potential of - 150 mV.
Figure 4 shows that the spacing-pressure relationship is reversible. Over the