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III. Soil Water Relations: Swelling and Shrinkage

III. Soil Water Relations: Swelling and Shrinkage

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144



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

stability.

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

kaolinite.

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



INTERPARTICLE FORCES



145



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.



B. RESIDUAL

SHFUNKAGE

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

bending.



Table V

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

Materialb



Water content

(g H Z O K ' )



Kaolinite

Illite

Montmorillorite



0.21

0.20

0.21



Suction

(MPa)

5.1



31

50



Pore size



(4



260

40

30



Dry porosity

(cm3 g-I)

0.185

0.188

0.135



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.

a



146



J. P. QUIRK



C. STRUCTURAL

POROSITY

AND SHRINKAGE

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

(McIntyre, 1979).

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



INTERPARTICLE FORCES



147



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).



A. EXTENSIVE

CRYSTALLINE

SWELLING

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



d(OO1)



=



23.8



c-0.5



+ 21

+ 22



M)



( c from 0.3 to

( c from 0.15 to 2



X



10-2M)



(18)

(19)



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

concentration.

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



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