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Dewatering and the hydraulic properties of soft, sulfidic, coastal clay soils

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Dewatering and consolidation of saturated swelling soils are governed by pressure-dependent soil hydraulic properties. Existing measurement techniques are difficult and slow. We illustrate a simple, rapid desorption technique, developed for industrial slurries, to measure hydraulic properties of a gel-like sulfidic, estuarine soil (∼70% water content). Measured hydraulic conductivities, K(ψ), were very small, ∼10−10 m/s, giving a representative capillary fringe thickness of ∼7 m and characteristic gravity drainage times around 40 years. Capillarity therefore dominates flow in these soils. Estimated times for dewatering this soil under surface loading with closely spaced, vertical wick drains, are 2 to 70 years, consistent with experience. A Netherlands marine clay soil, saturated with seawater, is unexpectedly wetter than the brackish estuarine soil here at the same matric potential, ψ. However, K(ψ) for both soils overlap, suggesting the engineering approximation, K(ψ) ∝ ∣ψ∣−1, for marine-deposited clays. The functional dependencies of hydraulic properties surprisingly are not consistent with similar-media or double-layer theories.
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Dewatering and the hydraulic properties of soft, sulfidic, coastal
clay soils
Ian White,
1
David E. Smiles,
2
Silvana Santomartino,
1,3
Pam van Oploo,
4
Bennett C. T. Macdonald,
1
and T. David Waite
5
Received 20 March 2002; revised 23 December 2002; accepted 2 May 2003; published 22 October 2003.
[1] Dewatering and consolidation of saturated swelling soils are governed by pressure-
dependent soil hydraulic properties. Existing measurement techniques are difficult and
slow. We illustrate a simple, rapid desorption technique, developed for industrial slurries,
to measure hydraulic properties of a gel-like sulfidic, estuarine soil (70% water content).
Measured hydraulic conductivities, K(y), were very small, 10
10
m/s, giving a
representative capillary fringe thickness of 7 m and characteristic gravity drainage times
around 40 years. Capillarity therefore dominates flow in these soils. Estimated times for
dewatering this soil under surface loading with closely spaced, vertical wick drains, are
2 to 70 years, consistent with experience. A Netherlands marine clay soil, saturated
with seawater, is unexpectedly wetter than the brackish estuarine soil here at the same
matric potential, y. However, K(y) for both soils overlap, suggesting the engineering
approximation, K(y) /jyj
1
, for marine-deposited clays. The functional dependencies of
hydraulic properties surprisingly are not consistent with similar-media or double-layer
theories.
INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 1866 Hydrology: Soil moisture;
1878 Hydrology: Water/energy interactions; 1894 Hydrology: Instruments and techniques; K
EYWORDS:
swelling soils, marine clays, hydraulic properties, dewatering, consolidation, soptivity
Citation: White, I., D. E. Smiles, S. Santomartino, P. van Oploo, B. C. T. Macdonald, and T. D. Waite, Dewatering and the hydraulic
properties of soft, sulfidic, coastal clay soils, Water Resour. Res., 39(10), 1295, doi:10.1029/2002WR001324, 2003.
1. Introduction
[2] The development of coastal lowlands is increasing
[White et al., 1997] with associated expansion of major
engineering works, particular roadways and embankments.
Many of these lowlands have unconsolidated, saturated,
estuarine- and marine-origin sulfidic, Holocene sediments
with shallow groundwater. Large areas [10
8
ha] of these
unoxidized (unripe), potential acid sulfate soils [Pons, 1973;
Kittrick et al., 1982] were deposited in low-energy marine
or estuarine environments throughout the world following
the last major sea level rise [Dent, 1986]. These saturated
soils have low bulk densities and volumetric water contents
around 80%. They are frequently gel-like and pose consid-
erable engineering and environmental problems because
they shrink significantly on drying, deform and flow under
surface loads, and export highly acidic groundwaters fol-
lowing drainage and subsequent sulfide oxidation [Willett et
al., 1993; Tin and Wilander, 1995; Wilson et al., 1999].
Oxidation of sulfides and ensuing soil acidification produce
high concentrations of dissolved multivalent aluminum and
iron in pore waters that react with the soils exchange
complex. These exchange reactions, together with drying,
progressively alter the physical properties of the gel-like soil
in a process known as ‘ripening’ where soil s tructure
develops.
[
3] Some constructions on soft, potential acid sulfate soils
have subsided or failed [Smiles, 1973], wi th signific ant
consequences, and large costs. During failure (see
Figure 1), the surrounding subsoils heave and flow, moving
sulfidic, subsoils from anoxic to oxic zones. This can
increase oxidation of sulfides and the rate of export of
acidic oxidation products into neighboring coastal streams
[White and Melville, 1993]. A common construction strat-
egy on soft soils is to consolidate the soil to sufficient depth
by pre-loading the soil surface [Smiles and Poulos, 1969]
and to install drainage systems, such as closely spaced,
vertical, wick drains, to enhance dewatering of the loaded
soil. The rate of conso lidation of soft soils due to overbur-
den and applied loads depends on the soil’s hydraulic
properties [Terzaghi, 1923; Horn and Baumgartl, 1999].
The hydraulic properties of saturated, swelling soils differ
markedly from those of rigid soils [Giraldez and Sposito,
1985; Baumgartl and Horn, 1999]. They are dependent on
overburden pressure and not readily measured by conven-
tional techniques [Smiles, 2000]. Richards [1979, 1992] and
Gra¨sle et al. [1996] have explored the impact of overburden
and applied loads on the hydraulic properties on stiff and
unsaturated swelling and aggregated soils.
[
4] The soils of interest here fall between the wet, model
clay systems with moisture r atios [ratio of volume of
water to volume of solid] around 30 [Smiles and Harvey,
1
Centre for Resource and Environmental Studies, Institute of Advanced
Studies, Australian National University, Canberra, ACT, Australia.
2
Land and Water, Commonwealth Scientific and Industrial Research
Organisation, Canberra, ACT, Australia.
3
Presently at Department of Earth Sciences, La Trobe University,
Bundoora, Victoria, Australia.
4
Biological, Earth and Environmental Science, University of New South
Wales, Sydney, New South Wales, Australia.
5
School of Civil and Environmental Engineering, University of New
South Wales, Sydney, New South Wales, Australia.
Copyright 2003 by the American Geophysical Union.
0043-1397/03/2002WR001324
SBH 12 - 1
WATER RESOURCES RESEARCH, VOL. 39, NO. 10, 1295, doi:10.1029/2002WR001324, 2003
1973; Smiles, 1976] and stiff clay soils with moisture ratios
around1[Baumgartl and Horn,1999].Therearefew
measurements in the intermediate range. Kim et al.
[1992a, 1992b] measured the hydraulic conductivity and
moisture characteristic of shrinking, unripe marine cl ay
samples, with initial moisture ratio around 4, using a
modification of an evaporation method [Wind, 1966]. They
compared this technique with the one-step outflow method
in which an instantaneous suction is applied to the bottom
of the sample and the soil moisture diffusivity is determined
from the rate of change of outflow with soil water content
remaining in the sample [Passioura , 1976]. Their preferred
modified evaporation technique required the insertion of
tensiometers in the consolidating soil, and although auto-
mated, ran for almost six weeks. That duration would
preclude its use i n routine testing during development
projects. As well, evaporation concentrates p ore water
electro lytes, which could alter soil hydraulic properties.
The hydraulic properties of clay slurries depend strongly
on soil solution concentration [Smiles et al., 1985]. While
the modified evaporation technique may be relevant to the
ripening of marine sediments, it may not be applicable to
dewatering during consolidation, where no increase in soil
water electrolyte occurs.
[
5] In very wet, swelling materials such as bentonite and
red mud slurries [a clay-iron oxide by-product of the
processing of bauxite] measurement of the early stage of
pressure-driven outflow take only a few hours to complete
[Smiles and Harvey, 1973; Smiles, 1976]. If this is also the
case for saturated soft soils, then the technique is attractive
for predicting consolidation and dewatering. The technique
has been thoroughly tested for the uniqueness of the resultant
measured hydraulic properties against transient and steady
state techniques for clay slurries [Smiles, 1976, 1978].
[
6] In this work w e explore the application of that
technique to soft, potential acid sulfate field soils, deposited
in low energy shallow lakes in a brackish, estuarine envi-
ronment and with initial moisture ratios about one tenth that
of the slurries. The dependence of hydraulic properties on
initial soil moisture ratio is considered. We use the results to
estimate the impact of capillarity on flow and dewatering of
the soil and compare hydraulic properties of the estuarine
soil here with those of Kim et al. [1992b] for a marine-
derived soi l, in equili brium with seawater. Finally we
examine the functional dependencies of the hydraulic prop-
erties and compare them with those expected from similar
media and double-layer theory.
2. Theory
[7] The theory of one-dimensional flow in saturated
swelling materials together with the fundamental differences
between flow in swelling or consolidating and rigid systems
are reviewed by Smiles [1986, 2000]. There are two critical
differences between swelling and rigid soils. The first is in
the energetics of soil water and the second is the coordinate
system used to describe water flow.
[
8] In swelling and shrinking soils, the total potential of
water (here all potential are expressed as work per unit
weight of water, L) is the sum of the position in the
gravitation al field [L], the unloaded matric potential, y
[L], and the overburden potential [L]. Overburden potential
includes the load due to the weight of soil above the point of
interest plus any imposed stresses [Croney and Coleman,
1961]. In swelling soils, water flow is described by Darcy’s
law with flow relative to the soil particles, in a material or
Lagrarian coordinate system, not relative to fixed spatial or
Eulerian coordinates, as in rigid materials [Smiles and
Rosenthal, 1968]. The water transport parameters that appear
in Darcys law for swelling systems [Smiles and Rosenthal,
1968] are the material hydraulic conductivity, k
m
(y)[LT
1
],
and the material moisture diffusivity, D
m
(J)[L
2
T
1
], with J
the moisture ratio, [J = q/q
S
= q/(1 q) with q and q
S
the
volume fractions of water and solid]. In geotechnical appli-
cations [Narasimham and Witherspoon, 1977], D
m
(J), has
long been known as the consolidation coefficient, a term that
shall be used here, and is related to k
m
and y by:
D
m
JðÞ¼k
m
JðÞdy=dJ ð1Þ
In two phase systems, the material properties are related to
the better-known, spatially defined hydraulic conductivity
K(q) and moisture diffusivity D(q) by:
K qðÞ¼ 1 þ JðÞk
m
JðÞ ð2Þ
D qðÞ¼ 1 þ JðÞ
3
D
m
JðÞ ð3Þ
We use here the ‘early stages’ of outflow during desorption
of water from thin samples of saturated, consolidating soil
under a range of imposed gas pressures to determine
hydraulic properties [Smiles and Harvey, 1973]. The
desorptivity, S [LT
1=2
], of the soil is determined from the
rate of outflow of water from thin samples of soil, under a
range of imposed gas pressures that are equivalent to
unloaded matric potentials, y [L]. Thin samples with large
area to height ratios are used so that friction between the
sample and the wall of the apparatus is minimized [Horn
and Baumgartl, 1999] and the effect of soil overburden is
negligible relative to the imposed stress. By using thin
samples we ensure that the matric potential at the end of
outflow equals the imposed gas pressure. The equilibrium
volumetric water content, q [m
3
m
3
], or moisture ratio, at
the end of each outflow measurement provides a point on
Figure 1. Failure of a major highway embankment
constructed on unripe potential acid sulfate soils. The site
is less than 1 km from the soil sampling site for this work,
Mcleods Creek, Tweed River, NSW, eastern Australia.
SBH 12 - 2 WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS
the moisture characteristic, J(y) of the soil. Each point on
the moisture characteristic corresponds to a separate soil
sample. This outflow technique has been thoroughly tested
against transient and steady state techniques [Smiles, 1976,
1978]. Differentiation of the desorptivity data with respect
to matric potential provides an estimate of the material
hydraulic conductivity, k
m
(y), while differentiation with
respect to moist ure ratio gives the consolidation coefficient,
D
m
(J)[L
2
T
1
].
[
9] Using the terminology of Philip [1957], cumulative
outflow, i [L
3
L
2
] in the early stages of outflow under an
imposed gas pressure is:
i ¼ S
0
t
1=2
ð4Þ
where S
0
= S(y
0
, y
n
) is the sorptivity of the sample [Philip,
1957], defined as positive for adsorption and negative for
desorption and y
n
is the initial unloaded matric potential of
the sample and y
0
is the surface matric potential or final soil
matric potential in equilibrium with the imposed gas
pressure. Sorptivity contains in an integral sense all the
information about the changes in the pore system between
the initial moisture ratio and the final equilibrium moisture
ratio relevant to water flow [White and Sully, 1987].
[
10] Integral techniques for solving the flow equation
subject to a step change of imposed pressure [Parlange,
1971; Philip, 1973; Philip and Knight, 1974] provide
relations between sorptivity and soil water hydraulic prop-
erties [Smiles and Harvey, 1973; Parlange, 1975a; White
and Perroux, 1989]:
S
2
0
¼ J=bðÞ
Z
y
0
y
n
k
m
yðÞdy ¼ J=bðÞ
Z
J
0
J
n
D
m
JðÞdJ ð5Þ
with J = J
0
J
n
, the difference in moisture ratio between
final, J
0
, and initial, J
n
, moisture ratios and b is:
b ¼ J
Z
y
0
y
n
k
m
yðÞdy=2
Z
y
0
y
n
J J
n
ðÞk
m
yðÞ
F y; y
0
; y
n
ðÞ
dy ð6Þ
In (6), F is the flux-concentration relation [Philip, 1973].
The factor b has relatively narrow limits and its value is
determined by the ‘shape’ of k
m
(y)dy/dJ and by y
0
, y
n
[White and Sully, 1987]. This approach assumes that k
m
and
D
m
are single valued functions of y and J, which is
appropriate for saturated, consolidating soils. Differentiation
of (5) with respect to the supply pressure y
0
[White and
Perroux, 1989] provides an estimate of k
m
:
k
m
y
0
ðÞ¼2 b=JðÞS
0
@S
0
=@y
0
þ S
2
0
@ b=JðÞ=@y
0
ð7Þ
Differentiation of (5) with respect to J
0
[Smiles and Harvey,
1973] gives D
m
:
D
m
J
0
ðÞ¼2 b=JðÞS
0
@S
0
=@J
0
þ S
2
0
@ b=JðÞ=@J
0
ð8Þ
The useful and quite general ‘optimal’ case of Parlange
[1975a], in which b =1=2 and F =[(J J
n
)/J]
1/2
, leads
to:
k
m
y
0
ðÞ¼S
0
=JðÞ@S
0
=@y
0
S
0
=4JðÞ@J
0
=@y
0
½ð9Þ
D
m
J
0
ðÞ¼S
0
=JðÞ@S
0
=@J
0
S
0
=4J½ð10Þ
Parlange’s approximations (9) and (10) are sufficiently
accurate for practical purposes in both swelling and rigid
systems [Smiles, 1976; White and Perroux, 1989] and we
use them here. To determine k
m
and D
m
from outflow
experiments using (9) and (10), S
0
and J
0
are measured for
different soil samples over a range of supply potentials, y
0
by imposing a range of gas pressures p [p = y
0
]tothe
samples and determining S
0
from the early stages of outflow
through (4).
3. Methods
3.1. Soil Samples
[
11] Bulk samples of potential acid sulfate soil with a field
moisture ratio of 3.3 were taken at depth of 1.5 m below the
soil surface in a sugarcane farm at a site on McLeods Creek,
Tweed River, northern NSW, in eastern Australia. The
Holocene soils (Thionic Fluvisols) at this depth have the
field texture of a light clay. The road site in Figure 1 is within
1 km of the soil sample site. Oxidized, actual acid sulfate
soils at the site occur to a depth of about 0.8 m. Below this
depth, unconsolidated, Holocene potential acid soils at the
site extend to a depth of about 9 m, beneath which they are
underlain by highly consolidated Pleistocene clays.
[
12] About 30 kg monoliths of the structureless gel soil
were extracted from the profile and sealed in double plastic
bags to exclude air. These were transported under refriger-
ation and stored at 4C to minimize oxidation. The physical
properties of the soil are given in Table 1 and the compo-
sition of the clay exchange complex is given in Table 2,
which reveals that magnesium dominates the exchan ge
complex. Table 3 shows the concentration of major ions
in the soil solution. The ratio of Na to Cl in the pore water
solution [Na/Cl = 0.7 with concentrations in mg/L] shows
the marine origin of these waters (seawater Na/Cl = 0.55)
but the Cl concentration is only about 4% of that of
seawater, reflecting perhaps the estuarine, back-swamp
origin of these soils.
3.2. Sorptivity and the Moisture Characteristic
[
13] Approximately 10 mL of the potential acid sulfate
soil were transferred into a pressure filtration cell [Smiles et
al., 1985] which was filled to a depth of about 9 mm. The
cell had a cross-sectional area of 1.14 10
3
m
2
and was
fitted with a 0.45 mm membrane at its outflow surface to
retain clay. Air was excluded from the cell and a selected
constant gas pressure, p, in excess of atmospheric pressure
was imposed on the sample at time t = 0. This pressure was
Table 1. Physical Properties at Sampling of the McLeods Creek
Potential Acid Sulfate Soil at Depth 1.5 m
Property Value
Clay content (wt %) 48
Silt content (wt%) 51
Fine sand content (wt %) 1
Expandable lattice clay, Smectite (% of clay fraction) 60
Kaolinite (% of clay fraction) 30
Illite (% of clay fraction) 10
Pyrite (wt %) 3.5
Bulk density, r (t/m
3
)0.6
Particle density, r
s
(t/m
3
) 2.545
Volumetric water content, q (m
3
/m
3
) 0.78
Moisture ratio in field, J (m
3
/m
3
)3.3
WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS SBH 12 - 3
maintained and the outflow of water from the cell was
weighed (to 10
5
kg) as a function of time. During outflow
the soil sample consolidates and shrinks one dimensionally
under saturated conditions. All measurements were at 20C.
[
14] Sorptivity was determined by plotting the measured
outflow as a function of the square root of time and
determining the slope of the initial period of desorption as
in (1). Measurement was continued until no further soil
water was expelled from the sample. The cell was then
disassembled and the water content of the soil in equilibrium
with the gas supply pressure, p, was determined by oven
drying at 105C. The moisture ratio in equilibrium with the
supply pressure was found from J
0
= q
g
r
S
/r
W
, with q
g
the gravimetric moisture content of the sample and r
W
the
density of the soil water solution. We emphasize that the soil
remains saturated throughout these measurements and that
each measurement requires a fresh soil sample.
[
15] It is important to examine the uniqueness of the
hydraulic properties produced by this technique. One way
to do this is to use samples with different initial moisture
contents [Smiles, 1976]. I f the hydra ulic properties are
unique they should be independent of initial moisture
content. In order to investigate the effect of initial moisture
content, a diluted soil sample (5 soil:1 water) was prepared
by adding approximately 40 ml of distilled water to 200 ml
of soil. The sample was mixed and allowed to equilibrate.
The original sample had an initial moisture ratio of 3.13 and
the diluted sample had J
n
= 3.36. The hydraulic properties
of the diluted sample were then measured in the same way as
those of the original soil sample described above.
4. Results
4.1. Outflow and Sorptivity
[
16] Figure 2 shows the measured outflow, for an imposed
gas pressure of 5.44 m H
2
O. The linear dependence of early
stages of outflow on the square root of time predicted in (1)
and typical of the results here, is evident in Figure 2 as is the
relatively rapid approach to equilibrium.
[
17] Figure 3 shows the measured sorptivities as a
function of the imposed gas pressure, p [= y
0
]. As
Table 2. Composition of the Exchange Complex of the McLeods
Creek Potential Acid Sulfate Soil at Depth 1.5 m
a
Exchangeable Cations Abundance on Exchange Complex, %
Mg
2+
55
Ca
2+
21.5
Na
2+
13
K
+
6.5
Al
3+
3.0
Fe
3+
0.5
Mn
2+
0.5
a
Total exchange capacity 59 mM(+)/kg.
Table 3. Concentration of the Major Ions in the Soil Solution,
McLeods Creek Potential Acid Sulfate Soil
a
Ion Concentration, mg/L
Na
+
547
K
+
41
Mg
2+
260
Ca
2+
129
Mn
2+
19
Fe
2+
4
Al
3+
0.9
Cl
785
SO
4
2
1480
a
Electrical conductivity 4.1 dS/m; pH 6.7.
Figure 2. Typical results for the cumulative outflow from
an estuarine clay soil sample showing the early stage square
root time behavior expected from (1) and the approach to
equilibrium. Here the imposed gas pressure was 5.44 m
H
2
O.
Figure 3. Dependence of sorptivity during dewatering on
the imposed gas pressure [ p = y
0
], for both the original
estuarine clay soil sample (solid symbols) and the dilute d
sample (open symbols). The lines are the fit of the data
to (11).
SBH 12 - 4 WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS
expected, the sorptivities of the diluted, initially wetter
samples are larger than those of the original, drier soil
samples.
[
18] Measured sorptivities of mud and clay slurries
determined during desorption have been found to follow:
ln S y
0
ðÞ½¼G þ H ln y
0
jj ð11Þ
with G and H constants [Smiles, 1976]. Figure 3 also shows
the fitted relations (11) and values for the constants, H and
G are listed in Table 4 together with the square of the
correlation coefficient, r. The logarithmic relationship (11)
appears to describe the data adequately. The measured
exponents, H, in Table 4, for the two samples are 0.4 and
0.46. Red mud slurries have H close to 0.4 [Smiles, 1976]
while for bentonite slurries H 0.2 [Smiles and Harvey,
1973].
4.2. Moisture Characteristic
[
19] The moisture characteristics, J(y), for the original
and diluted samples are plotted in Figure 4. Within the
scatter of data, there is no significant difference between
the original and diluted samples. This is consistent with the
measurements on bentonite and red mud slurries [Smiles
and Harvey, 1973; Smiles, 1976]. Also plotted in Figure 4 is
the moisture characteristic measured by Kim et al. [1992b]
for an unripe, marine clay soil saturated with seawater. We
defer discussion of this comparison to section 5.5 below.
[
20] The theory of weakly interacting planar double
layers [Collis-George and Bozeman, 1970; Smiles et al.,
1985; Sposito, 1989], suggests that J(y) in clay systems
should follow:
J ¼ A B ln y
jj
ð12Þ
with B a parameter that depends on surface charge density
on the clay, soil water electrolyte composition and
concentration and temperature. For clay slurries, however,
the magnitude of the dependence of B on electrolyte
concentration is not predicted by simple double-layer theory
[Smiles et al., 1985] and A and B must be considered as
empirical parameters. The fit of the combined J(y) data to
(12) is also shown in Figure 4 and values of the constants A
and B in (12) are listed in Table 4, together with the value of
r
2
. The semilogarithmic relation (12) gives a reasonable fit
to the data.
4.3. Hydraulic Conductivity
[
21] The calculated material hydraulic conductivities,
k
m
(y), for the two samples are shown in Figure 5. Again,
both the original and diluted samples are identical within the
scatter of results. Material hydraulic conductivities of slur-
ries [Smiles and Harvey, 1973; Smiles, 1975, 1995], appear
to follo w k
m
/jyj
N
or:
ln k
m
yðÞ½¼M N ln y
jj
ð13Þ
with M and N empirical constants. Figure 5 shows the
combined data fits the logarithmic relation (13) quite well.
Table 4. Functional Dependencies of the Hydraulic Properties S(y
0
), J(y
0
), k
m
(y), K(y), D
m
(J) and D(q) of the Estuarine
Soil Together With Values of the Square of the Correlation Coefficient r Compared With Those of a Marine Soil
a
Equation Source of Data Intercept Slope r
2
ln(S
0
)=G + H ln jy
0
j this work, original sample 11.04 0.46 0.98
ln(S
0
)=G + H ln jy
0
j this work, diluted sample 10.76 0.40 0.99
J = A B ln jy
0
j this work, combined samples 2.34 0.30 0.90
J = A B ln jy
0
j Kim et al. [1992b], combined data
b
3.24 0.25
ln(k
m
)=M N ln jyj this work, combined samples 22.43 0.39 0.86
ln(k
m
)=M N ln jyj Kim et al. [1992b], combined data
b
21.76 1.09
ln(k
m
)=M N ln jyj all data 21.72 0.82 0.87
ln(k)=m mlnjyj this work, combined samples 21.21 0.50 0.98
ln(k)=m mlnjyj Kim et al. [1992b], combined data
b
20.32 1.2
ln(K)=m mlnjyj all data 20.42 1.0 0.90
ln(D
m
)=P QJ this work, combined samples 16.46 2.0
ln(D
m
)=P QJ Kim et al. [1992b], combined data
b
21.54 0.36
a
See Kim et al. [1992b].
b
Data from Kim et al. [1992b] estimated from the ‘standard’ results in their Figures 7 and 10.
Figure 4. Moisture characteristics, J(y), for the original
estuarine clay soil sample (solid symbols) and the dilute d
sample (open symbols). Also shown are results estimated
from Kim et al. [1992b] for a marine-origin clay soil. The
lines show the fit of the data to (12).
WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS SBH 12 - 5
Values for the parameters M and N in (14) and r
2
are also
listed in Table 4 where we see a value of N 0.4 for the
estuarine soil. Red mud slurries have N 0.3 [Smiles, 1976]
but N 1.1 appears more appropriate for bentonite slurries
[Smiles, 1995].
[
22] Values of the more familiar hydraulic conductivity
K(y), evaluated using (9), are plotted in Figure 6. The
combined data from both sets of samples are described very
well by:
ln K yðÞ½¼m n ln y
jj
ð14Þ
Equation (14) is plotted in Figure 6 and values for the
constants, m and n are in Table 4. We note that the
magnitude of K( y) in Figure 6 is quite small, typically of
order 0.01 mm/day, close to the range where materials are
considered impermeable, despite the high water content of
these soils.
4.4. Consolidation Coefficient
[
23] The consoli dation coe fficient [Narasimham and
Witherspoon, 1977] follows from the definition (8) and
the empirical relations (12) and (13):
ln D
m
JðÞ½¼M ln B
N 1
B
A JðÞ
which can be written as:
ln D
m
JðÞ½¼P QJ ð15Þ
Consolidation coefficients, D
m
(J), estimated from (15) and
the better-known moisture diffusivity D(q), calculated
using (3), are plotted in Figures 7 and 8. Again, results for
the original and diluted samples in both figures are identical
within the scatter of data. The D
m
(J) data in Figure 7 show
the semi-logarithmic dependence on moisture ratio expected
Figure 5. Dependence of the material hydraulic conduc-
tivity, k
m
(y), on unloaded matric potential for both the
original estuarine clay soil sample (solid symbols) and the
diluted sample (open symbols) of the potential acid sulfate
soil. Also shown is the fit of the data to (13).
Figure 6. Dependence of the hydraulic conductivity,
K(y), on unloaded matric potential for both the original
estuarine clay soil (solid symbols) and the diluted sample
(open symbols) of the potential acid sulfate soil. The line is
the data fitted to (14).
Figure 7. Dependence of the consolidation coefficient on
the moisture ratio for both the original estuarine clay soil
sample (solid symbols) and the diluted sample (open
symbols) of the potential acid sulfate soil. Also shown is
the fit of the data to (15).
SBH 12 - 6 WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS
from (15). Values for the parameters P and Q in (15)
are listed in Table 4. The value of Q = 2.0 measured
here for the estuarine soil is similar to that for red mud
slurries, Q = 1.5 [Smiles, 1976] but is much larger than
that estimated for bentonite slurries, Q =0.06[Smiles and
Harvey, 1973]. Again diluted and original samples are
indistinguishable.
[
24] No significant dependence of D on q is evident
within the scatter of the data in Figure 8 over the small
range of volumetric moisture content sampled here. This
means that D(q) can be conveniently represented by a
constant mean value of 3.8 10
8
m
2
/s. Bentonite slurries
appear to have a similar, weak or nonmonotonic dependence
of D on water content [Smiles and Harvey, 1973].
5. Discussion
5.1. Uniqueness of Hydraulic Properties
[
25] A concern with the desorption technique used
here is that it may yield nonunique hydraulic properties
[Parlange, 1975b]. If this were so then we expect that
hydraulic properties should depend on the initial moisture
content of the sample. Previous measurements using the
desorption technique on clay slurries have shown that the
moisture characteristic, J(y), the consolidation coefficient,
D
m
(J) the material hydraulic conductivity k
m
(y) and actual
hydraulic conductivity, K(y) are unique and independent
of the initial moisture content of the sample [Smiles, 1976,
1978]. In a limited way, and within the scatter of the
experimental data, we have also found that here for the
more complex soft, potential acid sulfate soil. The hydrau-
lic properties for the 20% diluted sample in Figures 4 to
8 are identical to those of the original sample. Exchange
with the clay complex in the soil here means that the 20%
dilution we have used has little impact on soil solution
concentration.
5.2. Effect of Gravity and Capillarity
[
26] Swelling reduces the impact of gravity and extends
the ‘early stages’’, capillarity-dominated period of transient
flow [Smiles, 2000]. Philip [1969] identified a time, t
grav
when the influence of gravity compared with capillarity,
starts to be significant during infiltration. White and Sully
[1987] suggested that a consistent measure of t
grav
is:
t
grav
¼ bS
0
= K y
0
ðÞK y
n
ðÞ½
fg
2
ð16Þ
Figure 9 shows t
grav
calculated using (16) with b = 1/2 and
the sorptivities and hydraulic conductivities measured in
this work. From our results here we estimate K(y
n
)=1.1
10
9
m/s for the undiluted sample and K(y
n
)=1.2
10
9
m/s for the diluted sample at their respective initial
moisture contents. Values of t
grav
range from about 12 years
to 67 years, and are approximately linearly dependent on the
matric potential, with, t
grav
= 15.6 + 2.65 y
0
. In bentonite
slurries, t
grav
can be as large as 400 years [Smiles, 1986].
[
27] The macroscopic capillary length l
c
of a soil is a
measure of the representative capillary fringe thickness
above a water table and, consequently, the characteristic
pore size of the soil [Myers and van Bavel, 1963; Bouwer,
1964; White and Sully, 1987]. It can be estimated from
[White and Sully, 1987]:
l
c
¼ bS
2
0
= q K y
0
ðÞK y
n
ðÞ½
fg
ð17Þ
where q = q(y
0
) q(y
n
) and b is given by (9). Values for
l
c
calculated from (17) with b =1=2, and are plotted in
Figure 8. Soil moisture diffusivity as a function of
volumetric soil water content for both the original estuarine
clay soil sample (solid symbols) and the diluted sample
(open symbols) of the potential acid sulfate soil. The line
indicates the mean value.
Figure 9. Values of the gravity time, estimated from (16),
for the estuarine clay soil for both the original estuarine clay
soil sample (solid symbols) and the diluted sample (open
symbols).
WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS SBH 12 - 7
Figure 10 for the combined sample s. The es timated
representative capillary fringe thicknesses show consider-
able scatter but span a range from 3 to 12 m. These large
values suggest that the influence of the water table in these
soils, particularly on evaporation, will be propagated over
considerable depths in the profile.
[
28] The ratio L/l
c
, where L is a characteristic length of a
source or sink of water, determines whether the dimension-
ality of flow needs to be considered [Bouwer, 1964]. When
L/l
c
1, flows can be treated as one-dimensional. The large
values of l
c
found here indicate that the dimensionality of
the flow from surface sources or sinks will be important in
the early stages of flow in these soils. Together, the results
for l
c
and t
grav
suggest that most flows in these materials can
be treated as capillarity-driven (gravity free) but that the
dimensionality of flow will need to be considered for source
or sink characteristic lengths of order 10 m or less.
5.3. Dewatering Times for Vertically Drained Soils
[
29] The actual magnitudes of the material hydraulic
conductivity, the hydraulic conductivity and the consolida-
tion coefficient measured here for the estuarine soil are very
small. They indicate that dewatering these soils will be a
slow process. To accelerate dewatering of soft coastal sedi-
ments, closely spaced, vertical wick drains are inserted 10 to
20 m into the soil and surface loads are gradually applied.
Because gravity effects are negligible, the time, t, to dewater
these soils can be estimated from [Smiles, 1973]:
J J
n
J
¼ erfc
S
0
2JL
D
ffiffiffiffiffi
p t
p

ð18Þ
Here J = J
0
J
n
and L
D
is the half-spacing between
vertical drains. The usual goal is to drain the sediment to a
relative water content of (J
0
J
n
)/J = 0.5.
[
30] Figure 11 shows the estimated time for dewatering as
a function of the drain half spacing, for a total overburden
load of 5 m (water potentials here are expressed in work per
unit weight of water so that the unit of potential and hence
overburden is meters of water). For a typical value of L
D
of
1.0 m (18) predicts that over 2 years are required to dewater
these sediments using verti cal wick drains with a 5 m
applied surface load. A major highway close to the soil
sample site without surface drainage has continued to settle
by 4 to 6 m over the past 40 years. Because dewatering is a
desorption process, outflow and consolidation will be rapid
at first but will decline as the square root of time (as in (4)).
The early, rapid dewatering may lead to unfounded expect-
ations of quick consolidation.
[
31] There is the prospect that the long dewatering times
estimated here could be reduced by one to two orders of
magnitude. The injection or in situ generation of higher
solution concentrations of salt or cations of higher valence
such as calcium, iron or aluminum in wick drains prior to
dewatering may increase the consolidation coefficient mark-
edly, close to outflow surfaces. Rosenqvist [1953] reported
dramatic ‘stiffening’ of quick clays in the field after the
injection of 2% brine solution.
5.4. Relevance of Laboratory Measurements
to Field Situations
[
32] The question of the relevance of these laboratory
measurements of the hydraulic properties of small samples
to field situations is important. The measurements here have
revealed very small hydraulic conductivities which give rise
to low dewatering times even with relatively closely spaced
vertical wick drains. These back swamp gel soils were
mainly deposited in shallow, low energy, barrier lakes
where sedimentation of fine particles occurred slowly. They
are remarkably uniform. Recent highway construction has
Figure 10. Values of the representative capillary fringe
thickness, l
c
, estimated from (17) for the combined samples
of estuarine clay soil.
Figure 11. Predicted dependence of the dewatering time
on the half-spacing between vertical wick drains from (18)
for a total overburden load of 5 m.
SBH 12 - 8 WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS
occu rred along nearly 400 km of the eastern Australia
coastal floodplains covering nearly 50 km of unconsolidated
sulfidic back swamp areas with depths of up to 30 m. Wick
drains, with half-spacing of less than 0.7 m, were required
to provide adequate dewatering over periods of 1 to 2 years
(D. Cramer, personal communication, February 2002). This
is entirely consistent with predictions made here from the
laboratory measured hydraulic prope rties.
5.5. Comparison With Previous Measurements of
Unripe, Marine-Origin Soils
[
33] The hydraulic properties of the soil studied here falls
between the properties of wet clay slurries with moisture
ratios around 30 and stiff clay soils with moisture ratios less
than 1 [Baumgartl and Horn, 1999]. There are few com-
prehensive st udies on soil systems wit h moisture ratios
between 1 and 10 with which to compare our results. Kim
et al. [1992 b] measured the hydraulic properties of two
samples of an unripe, marine clay soil with initial moisture
ratios around 4. This soil is saturated with seawater and
deposited under a similar low energy environment to the
soil examined here. There are some discrepancies between
the results from different measurement techniques used in
their work. We will compare our measurement here to their
‘standard technique.’
[
34] Netherlands soil, had a bulk density of 490 kg/m
3
,
with clay and silt contents of approximately 46% and 53%
respectively [Kim et al., 1992a]. The clay minerals were
mainly kaolinite and illite with a small amount of montmo-
rillonite, reflecting the marine origin of the soil. The soil
solution was in equilibrium with seawater. Our eastern
Australian clay soil had similar amounts of clay and silt
(Table 1) but has smecti te as the dominant clay followed by
kaolinite with a minor amount of illite, showing its estuarine
origin. It has a higher bulk density, lower moisture ratio at
sampling and a smaller soil solution electrolyte concentra-
tion (1/25 seawater, based on chloride concentrations) than
the Netherlands soil. The Australian soil was deposited
around 6000 BP. It is presumed that the Netherlands soil
is much younger than this.
5.5.1. Moisture Characteristics
[
35] We have combined the moisture characteristic data of
the two samples from Figure 7 of Kim et al. [1992b] for jyj >
0.1 m H
2
O and have fitted the data to (12). Moisture ratios
were calculated using their plotted values of q. The resulting
moisture characteristic is compared with that found here in
Figure 4. Values of the A and B parameters, found by fitting
their data to (12), are also listed in Table 4. We do not list a
value for r
2
since it would merely be a measure of our ability
to extract information from their Figure 7.
[
36] The moisture ratios of Kim et al.’s marine clay i n
equilibrium with seawater (Figure 4) are significantly larger,
at the same matric potential, than those of the estuarine clay
studied here. The ambient soil solution concentration of our
estuarine soil was approximately 1/25 of that of the marine
clay. Double-layer theory for weakly interacting, dilute clay
systems indicates that the unloaded matric potential at fixed
moisture ratio should be a function of the ambient solution
electrolyte concentration and temperature [see, e.g., Sposito,
1989]. If these two soils were similar materials, both theory
and experience with bentonite slurries [Smiles et al., 1985;
Smiles, 1995] suggest that the moisture ratios at the higher
salt concentration in the Netherlands’ marine sediment
should be smaller than those of our estuarine soil at the
same matric potential. The value of the slope parameter B =
0.25 in (12) listed in Table 4 for the marine sample of Kim
et al. is more than 80% of that for our estuarine sample.
[
37] The soils from the Netherlands and eastern Australia
differ in two important aspects, their clay chemistry and the
age of the soils. Kaolinite and illite are the dominant clays
in the Netherlands soil compared with smectites in the
Australian soil. At the same ambient soil solution concen-
trations the Netherlands soil would be expected to swell less
than the Australian soil. The evaporation method used by
Kim et al. [1992b] increased the soil solution concentration
above that of the initial seawater saturating the samples.
Under these higher electrolyte concentrations we should
also expect the Netherlands soil to be drier than the
Austral ian soil at the same matric potential. The soils,
however, differ in age. It is quite possible that the Australian
soil has experienced some consolidation under its 1.5 m of
soil overburden in the 6,000 years since its deposition,
whereas the younger Netherlands soil (from an unspecified
but possible shallow depth) has not. We also note here that
both soils contain over 50% silt. The role of this silt fraction
in determining the moisture characteristic may be important.
5.5.2. Hydraulic Conductivity Moisture
Potential Relations
[
38] Smiles et al. [1985] found large differences between
the moisture characteristics and the k
m
(J) relations of
bentonite slurries when pore water electrolyte concentra-
tions were varied. When, however, material conductivities
were expressed as a function of matric potential, k
m
(y), the
measurements for different electrolyte concentrations unex-
pectedly collapsed to a single curve [Smiles, 1995] and the
empirical equation (13) provided a single, apparently
unique, relation for K
m
(y) for bentonite slurries over a wide
range of pore water electrolyte concentrations. The theoret-
ical principles underpinning this collapse have yet to be
discovered. We examine here if such a collapse is also
possible for both the Netherlands and Australian soils.
[
39] Kim et al. [1992b, Figure 10] report measured K(y)
for two samples of the marine-origin sediment. Figure 12
compares K(y) for their marine-origin soil with results from
this work. The two soils, whose solution concentrations
differ 25-fold, were measured using different techniques
mainly over different y ranges. The larger K( y) for the
Netherlands marine samples were determined at generally
smaller values of jyj. Where the jyj measurement ranges
overlap, the hydraulic conductivities of both soils also
overlap. The low values of hydraulic conductivity found
for estuarine soil here are therefore consistent with those
found for the Netherlands marine-origin soils.
[
40] We have fitted the combined data of Kim et al.
[1992b] to (14). The values found for the parameters m
and n are listed in Table 4. The slope of n = 1.2 for the
marine-origin soil is similar to that for bentonite slurries but
differs from that found here for the estuarine clay, n = 0.5
[Table 4]. If both sets of K(y) data for the two soils are
combined and fitted to (14), a plausible ‘engineering’
approximation is suggested for marine-origin clays as
shown in Figure 12 with K(y) 1.4 10
9
jyj
1
.
[
41] We have also calculated the material conductivity,
k
m
(y) for the combined samples of Kim et al. [1992b].
WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS SBH 12 - 9
Table 4 shows the corresponding parameters M and N when
these results are fitted to (13). The results for k
m
are similar
to K(y). Where matric potentials are similar, the k
m
for the
Netherlands marine-origin soil and those of the Australian
estuarine soil overlap. Again, combining both sets of data
and fitting them to (13) results in an ‘engineering’ approx-
imation with parameters listed in Table 4.
5.5.3. Consolidation Coefficient
[
42] Finally, we can use the values of A, B, M and N for
the marine soil of Kim et al. in Table 4 to calculate the
consolidation coefficient, D
m
(J), from their data using (15).
Values for t he parameters P and Q for their soil are
compared in Table 4 with those found in this work. We
note that the slope parameter, Q, for the marine-origin soil is
of different sign to that of the soil investigated here and also
to those for red mud and bentonite slurries. Results for
bentonite [Smiles and Harvey, 1973], however, suggest that
the functional dependence of D
m
(J) may be nonmonotonic.
5.6. Functional Dependence of Hydraulic Properties
for Saturated Clays
[
43] Philip [1970] assumed in idealized swelling materials
at equilibrium there was a balance between sedimentation
and soil particle Brownian motion. For dilute, saturated,
swelling soil pastes he derived the moisture characteristic
J þ 1 ¼
kT
r
W
g
v
1
y
ð19Þ
Here k is Boltzmann’s constant, T the absolute temperature,
r
W
the specific gravity of the soil water solution, g the
acceleration due to gravity, and
v the mean particle volume.
Equation (19) suggests that the value of B in (12) should be
close to 1.0. Here, we found B 0.3 for the estuarine soil
and B 0.25 for the marine-origin soil of Kim et al.
[1992b]. For red mud slurries B 0.2 [Smiles, 1976], while
for bentonite slurries the value is much larger, with B
ranging from approximately 3.2 to 5.6 dependent on soil
solution electrolyte concentration [ Smiles et al., 1985].
Equation (19) embodies the notions that a swelling
suspension can expand without limit on the continued
addition of water and soil particles are free to move under
Brownian motion. Both appear unrealistic for field soils.
[
44] The saturated, swelling clay soils examined here are
relatively simple, two-phase materials. It is reasonable to
suspect that their hydraulic properties during dewatering,
when normal volume change occurs [Croney and Coleman,
1961], may behave in a self-similar way during dewatering.
If so, similar media concepts [Miller and Miller, 1956;
Philip,1969] suggest that y / 1/l and S / l
1/2
, with l a
characteristic pore size of the material. We would then
expect S / y
1/2
. It must be recognized, however, that
the dependence of S on y enters as the upper and lower
limits of integration in (8) and (9). If we concentrate on the
supply potential, y
0
, for desorption, where larger imposed
jy
0
j mean that additional finer pores drain and more water is
expressed, we might expect that S
0
/ y
0
1/2
or more
generally, S
0
/jy
0
j
H
, as in (11). Here we have found H in
the range 0.4 to 0.46, slightly larger than that for red mud
slurries of 0.36.
[
45] Similar media theory also dictates that K / l
2
,so
for a self-similar, dewatering clay soil we would expect K /
1/y
2
. If this is so, then (9) and (19) suggest that, for
dewatering, k
m
/ 1/y. The estuarine soil here and slurries
[Smiles and Harvey, 1973; Smiles, 1975, 1995] however,
appear to fol low more generally k
m
/jyj
N
, as in (13). We
have found N 0.4 for the estuarine soil, but the Nether-
lands soil had N 1.1, close to that for bentonite slurries,
and to similar media expectations. Red mud slurries, how-
ever, have N 0.3, similar to the estuarine soil studied here.
[
46] Smiles [1995] attempted to fit the Kozeny-Carman
equation [ Carman, 1939] to the material hydraulic conduc-
tivity of bentonite slurries. This equa tion can be expressed as:
k
m
JðÞ/J
3
= J þ 1ðÞ
2
ð20Þ
If (19) is valid, then (20) also suggests that k
m
is
approximately proportional to 1/y. The functional depen-
dence of k
m
(J) predicted by (20), however, gives a poor fit
to our measurements for the estuarine soil here and also for
bentonite slurries.
[
47] The theory of weakly interacting planar double
layers [Collis-George and Bozeman, 1970; Smiles et al.,
1985; Sposito, 1989], suggests that J(y) in clay slurries
should follow J /jyj
B
, as in (12). If we set A = aB in
(12), then, at a fixed temperature, double-layer theory
predicts for a single electrolyte species of ambient solution
concentration, C:
a ¼ a
r
þ ln C=C
r
ðÞ ð21Þ
and
B ¼ B
r
ffiffiffiffiffiffiffiffiffiffi
C
r
=C
p
where
B
r
¼ r
S
d=2
Figure 12. Comparison of the hydraulic conductivity for
the estuarine clay soil of this work (circles) with that of the
marine-origin clay soil (crosses) of Kim et al. [1992b]. Also
shown is the fit of the combined data for both soils to (14).
SBH 12 - 10 WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS
Here a
r
and B
r
are constants at reference concentration C
r
at
the same temperature, is the specific surface area of the
solid particles and d the Debye length
.
The values of a
r
and
B
r
also depend on the valence of the cations in the soil
solution. The dependence of a on solution concentration in
(21) is less than that of B. Both a and B should decrease
with increasing soil solution concentratio n. We therefore
expect that A in (12) should decrease by about the same
relative amount as B with increasing solution concentration.
[
48] If it is assumed that the estuarine soil here and the
marine soil of Kim et al. [1992b] behave similarly, double-
layer theory qualitatively predicts that the Netherlands soil,
with it s larger, ambient pore water salt concentrations
should be drier and denser than our estuarine-origin soil.
Rosenqvist [1953] believed that gel natu re of marine-
deposited quick-clays only persists in situations where
saline formation pore waters have been exchanged with
fresh water. The clay fraction morphology in these soils is in
itself complex [Sposito, 1989], their geologic history and
the fine silt fraction may also play important roles.
[
49] If the two soils had behaved similarly then equation
(21) would predict that the value of B for the marine
samples of Kim et al. should be approximately 1/5 that of
the estuarine samples here. Instead, B for the marine
samples of Kim et al. is about 80% of the estuarine soils.
In bentonite slurries, B decreased by a f actor of only 1.7 for
a 37-fold increase in pore water electrolyte concentration,
much lower than expected from (18) [Smiles et al., 1985].
This posses a dilemma. It is well known that the model,
parallel-plate, clay systems in the laboratory follow closely
double-layer theory well [Israelachivili, 1985; Sposito,
1989; Kjellander et al., 1990; Quirk, 1994]. Perhaps in
slurries and materials with ‘card-house’ clay structures,
the combination of edge and surface charges gives rise to
different behavior.
[
50] The expanding lattice clays in of these soils, under
suitable conditions, form expanded, gel-like structures held
together by long-range attractive forces. In these expanded
structures, clay platelets are arranged in a complex mixture
of face-to-face, edge-to-face and edge-to-edge configura-
tions [Van Olphen, 1964; Sposito, 1989; Quirk, 1994; de
Krester et al., 1998]. Secondary minimum induced coagu-
lation results in long distance associations with hydrated
counter-ions taking up interlayer space [Sposito, 1989]. The
stability of these structures depends critically on the elec-
trolyte content of the pore water solution [Keren et al.,
1988], on the confining pressure and on shearing forces.
Increase in concentration of counter ions past a critical
concentration enables primary minimum aggregation, where
the strength of the face-face interactions increase and the
complete gel structure collapses. Dramatic, ‘quick-clay’’-
like changes in the rheological and physical properties of
these soils can occur when their pore water solution
compositions are altered or when stresses are applied,
causing the collapse of the gel-like structures [Rosenqvist,
1953, 1966; Lessard and Mitchell, 1985; Mitchell, 1986;
Luckham and Rossi, 1999].
[
51] Within the scatter of results here, no abrupt changes
in soil properties were observed with imposed stress. If,
however, the rearrangement of clay platelets under increas-
ing pressure does not follow a ‘self-similar path, with a
progression of simple, geometrically scaled pore dimen-
sions during dewatering [Miller and Miller, 1956], K(y)
may not follow the 1/y
2
behavior expected from scaling
theory. It is clear that the nature of the K(y) relation for
saturated swelling soil systems an d the impact of soil
solution electrolyte concentration on require investigation.
6. Conclusions
[52] We have demonstrated that the outflow technique,
developed for determining the hydraulic properties of in-
dustrial slurries with large moisture ratios during the early
stages of dewatering [Smiles and Harvey, 1973; Smiles,
1976], is directly applicable to unripe, saturated marine-
or estua rine-origin soils that occur in coas tal lowlands.
Measurements of outflow using samples that are thin
relative to typical overburden loads and imposed stresses,
and have large area to height ratios, avoids complications
caused by thick samples. It also allows dewatering to be
completed in a few hours, even when the hydraulic con-
ductivity is of order 10
10
m/s, as here. The technique is
rapid, as required in engineering applications, simple, and
does not require the insertion of tensi ometers or cause an
increase in the pore water electrolyte concentration as in the
slow, evaporation method of Kim et al. [1992b]. In addition,
it has been thorough ly tested for slurries against other
transient and steady state techniques. These rapid measure-
ments yield all the necessary hydraulic properties, th e
moisture characteristic, material hydraulic conductivity
and the consolidation coefficient, required for predicting
flow and dewatering of consolidating materials using the
macroscopic Darcy description of water flow.
[
53] Our measurements comparing 20% diluted samples
with samples taken at field moisture content showed that the
moisture characteristic, J (y), consolidation coefficient,
D
m
(J), the soil moisture diffusivity D(q), the mat erial
hydraulic conductivity k
m
(y) and actual hydraulic conduc-
tivity, K(y) are, within experimental error, unique and
independent of the initial moisture content of the sample.
[
54] The water transport properties of the gel soil mea-
sured here are small, close to being considered imperme-
able, despite their 70% water content. Associated with these
very small transport properties, are large representative
capillary fringe thicknesses, l
c
, betwe en 3 and 12 m.
Gravity times t
grav
[Philip,1969] are corresponding long,
between 12 and 67 years. We conclude that capillary forces
will therefore dominate water flow in these soils and the
dimensionality of flow from surface sources or to sinks will
be important. As a consequence of this, predicted dewater-
ing times for the soil, under surface loads and with vertical,
wick drains are large, typically in the range 2 to 70 years,
which is consistent with experience. Times required for
consolidation during highway construction traversing long
sections of the coastal floodplains in eastern Australia are
consistent with the estimates based on the laboratory mea-
sured hydraulic properties here.
[
55] Previously used empirical or quasi-theoretical rela-
tionships proposed fo r the functional dependencies of
hydraulic properties adequately describe the hydraulic
properties measured here, within the scatter of the data.
How ever the functional dependencies of the hydraulic
properties during dewatering do not follow those expected
from self-similar media. The simple relationship for
WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS SBH 12 - 11
hydraulic conductivity K(y) / y
n
describes our measure-
ments very well. Hydraulic conductivities, K(y), deter-
mined here have an approximate 1/y
1/2
dependence and
not the 1/y
2
dependence expected from ‘self-similar
media arguments [Miller and Miller, 1956; Philip, 1969].
The question of the functional dependence of K(y) for soft
sediments and the impact of electrolyte concentrations on
that dependence, we believe, warrant further attention.
[
56] Comparison of the measurements of hydraulic prop-
erties here with previous results for an unripe, marine-origin
soil from the Netherlands is intriguing. The moisture content
of the Netherlands soils is wetter than the estuarine soil from
eastern Australia at the same matric potential. Simple dou-
ble-layer theory would suggest that the Netherlands soil,
with its 25 times larger pore water electrolyte concentration
should be drier than the eastern Australian soil. Electrolyte
effects on J(y) of bentonite slurries also do not appear
predictable from double-layer theory [Smiles et al., 1985].
Thisisapuzzlesincemodel,parallel-platemodelclay
systems obey theory well [Israelachivili, 1985]. We have
speculated that this behavior, as well as the failure of slurries
and the estuarine soil here to be self-similar during dewater-
ing, may be due perhaps to the collapse of ‘card-house’ clay
structures, or to the different geologic histories and miner-
alogy of the soil, or to interactions with the silt fraction.
[
57] Where y ranges overlapped for the Netherlands and
Australian soils, the K(y) for both soils also overlapped.
This suggests that the low values found here are consistent
with those measured for the similar clay and silt content soil
from the Netherlands. Although the slopes of the K(y)
relations differed for the two soils, the engineering approx-
imation K(y) 1.4 10
9
jyj
1
provides a first
approximation for both soils, despite the 25-fold difference
in pore water electrolyte concentrations. The collapse of
K(y) to a single relation independent of soil solution
concentration found by Smiles [1995] for bentonite slurries
and the overlap here of K(y) for the saturated, low-energy
marine and brackish water-deposited clays, with markedly
different pore water electrolyte concentrations, deser ve
further study. In addition, the potential to accelerate de-
watering of these soils through injection or in situ generation
of multivalent electrolytes requires additional research.
Notation
A constant in moisture characteristic (12).
B slope parameter in moisture characteristic (12).
b parameter for sorptivity relation (3).
C electrolyte concentration in soil water [ML
3
].
D soil moisture diffusivity [L
2
T
1
].
D
m
consolidation coefficient [L
2
T
1
].
F flux-concentration relation.
g gravitational acceleration [LT
2
].
G constant in sorptivity relation (11).
H slope parameter in sorptivity relation (11).
i cumulative desorption [L
3
L
2
].
K hydraulic conductivity [LT
1
].
k
m
material hydraulic conductivity [LT
1
].
L characteristic source or sink dimension [L].
L
D
half spacing between vertical drains [L].
M constant in material hydraulic conductivity relation
(13).
m constant in hydraulic conductivity relation (14).
N slope parameter in material hydraulic conductivity
relation (13).
n slope parameter in material hydraulic conductivity
relation (14).
p imposed gas pressure [ML
1
T
2
].
P constant in consolidation coefficient relation (15).
Q slope parameter in consolidation coefficient relation
(15).
r correlation coefficient.
S sorptvity [LT
1/2
].
t time [T].
t
grav
gravity time defined in (16) [T].
T absolute temperature [K].
v mean particle volume [L
3
].
a = A/B in (21).
d Debye length [L].
total potential per unit weight of water [L].
g surface tension of soil solution [MT
2
].
J moisture ratio = volume of water/volume of solid
[L
3
L
3
].
J = J
0
J
n
.
k Boltzmann constant.
l
c
representative capillary fringe thickness defined in
(17) [L].
q volumetric soil water content [L
3
L
3
].
q
S
volume fraction of solid [L
3
L
3
].
r bulk specific gravity of soil [ML
3
].
r
S
specific gravity of soil solids [ML
3
].
r
W
specific gravity of soil water [ML
3
].
specific surface area of solid particles [L
2
M
1
].
y unloaded matric potential [L].
Subscripts
0 value at supply potential y
0
.
m material value.
n value at initial conditions.
r value at reference conditions.
[
58] Acknowledgments. We thank Mike Melville of the University of
NSW for helpful discussions and cane farmers Robert and Allan Quirk and
Robert Hawken for support throughout this work. Support from the Water
Research Foundation of Australia, the NSW ASSPRO and the Australian
Research Council, under ARC Large Grant A39917105, ARC Linkage
Grant LP0219426 and Discovery Grant DP0345145 and are gratefully
acknowledged.
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WHITE ET AL.: HYDRAULIC PROPERTIES OF COASTAL CLAY SOILS SBH 12 - 13
... S 0 values provide the information about the changes in filter cake porosity which influences the permeate flow from the initial moisture ratio until the final equilibrium moisture ratio (Smiles, 1986). Consequently, Smiles and Harvey (1973) showed that D m can be estimated by differentiating the experimentally obtained S 0 () relation which, after coupling with the optimal case suggested by Parlange (White et al., 2003), leads to: ...
... White and co-workers (White et al., 2003) later suggested the estimation of K m by differentiating S 0 (P) and  0 (P) relations as: ...
... Equations (2.34) and (2.35) assume that k m and D m are single valued functions of P and  and, for swelling or rigid systems, Parlange's approximations are sufficiently accurate for the determination of k m (P) and D m () from experimental data (Parlange, 1975, White et al., 2003. ...
... Almost 50% w/w of PASS consists of clay minerals and more than 60 % of the clay-sized fraction is montmorillonite (Smiles, 2000;White et al., 2003). It is well-known that cations can compress the diffuse double layer of clay particles leading to changes in the modes of particle association (i.e., face-to-face, edge-to-face or edge-to-edge) (van Olphen, 1963;Stein, 1986;Wakeman, 1986;Mohan and Fogler, 1997;Tombacz and Szekeres, 2004) and, as a result, considerably alter the hydraulic conductivity and consolidation properties of consolidated assemblages of these materials (Mohan and Fogler, 1997;Mata and Ledesma, 2003;Santiwong et al., 2008). ...
... It is well-known that cations can compress the diffuse double layer of clay particles leading to changes in the modes of particle association (i.e., face-to-face, edge-to-face or edge-to-edge) (van Olphen, 1963;Stein, 1986;Wakeman, 1986;Mohan and Fogler, 1997;Tombacz and Szekeres, 2004) and, as a result, considerably alter the hydraulic conductivity and consolidation properties of consolidated assemblages of these materials (Mohan and Fogler, 1997;Mata and Ledesma, 2003;Santiwong et al., 2008). Nevertheless, in a previous study (Le et al., 2008) we observed that dissolved milli-molar concentrations of H + and Ca 2+ had little, if any, measureable impact on the hydraulic conductivity and consolidation of PASS in experiments employing a hydraulic consolidometer, despite theoretical calculations suggesting that changing pore water electrolyte concentrations should dramatically change the hydraulic properties of these soils (White et al., 2003(White et al., , 2005. However, it may reasonably be argued that the use of a bulk soil sample in the consolidometer studies as well as the short timescale ($7 days) of the experiments may not be representative of either the material's physical state in the natural environment or the long timescales (months to years) required to induce significant physical changes. ...
... Dead-end filtration cells have been employed for many industrial applications including investigation of membrane fouling in water and wastewater treatment, assessment of the permeability of landfill containers, examination of the permeability of substrates to drilling fluid and ease of sediment dewatering in civil constructions (Siyag et al., 1983;Petrov and Rowe, 1997;Shackelford et al., 2000;Graham et al., 2001;Vasco et al., 2001). The method has also previously been used for the examination of the hydraulic conductivity and consolidation properties of PASS (White et al., 2003). ...
Article
Acid sulfate soils (ASS) cover extensive areas of east Australian coastal floodplains. Upon oxidation, these hydromorphic pyritic sediments produce large quantities of sulfuric acid. In addition, due to their geographic location, these soils may also come in contact with high ionic strength estuarine tidal waters. As a result, there is typically a large variation in acidity (pH) and cation concentrations in soil porewaters and adjacent aquatic systems (e.g., agricultural field drains, rivers, estuaries, etc.). Acid sulfate soils, especially from the unoxidized gelatinous deeper layers, contain a relatively high proportion of montmorillonite, which is wellknown for its shrink-swell properties. Variations in cation concentrations, including H3O+, can influence montmorillonite platelet interactions and may, thus, also significantly affect the hydraulic conductivity of materials containing this clay. In this paper we report on the effect of four common cations, at reasonable environmental concentrations, on the hydraulic properties of potential (unoxidized) acid sulfate soil materials. The natural system was simplified by examining individually the effects of each cation (H+, Ca2+, Fe2+ and Na+) on a soil–water suspension in a filtration cell unit. Moisture ratio, hydraulic conductivity and the consolidation coefficient of the deposited filter cakes were calculated using material coordinates theory. The results indicate that the hydraulic conductivity of potential acid sulfate soils increases at low pH and with cation concentration. Although an increase in the charge of amphoteric edge groups on montmorillonite clays may result in some aggregation between individual clay platelets, we conclude that the extent of these changes are unlikely to cause significant increases in the transportation of acidity (and contaminants) through potential acid sulfate soils as the hydraulic conductivity of these materials remain low (<10−9 m/s) at pH and ionic conditions normally experienced in the field.
... The sub-catchment is hydrologically isolated from the adjacent land and Tweed River by natural levees, artificial bund walls and bordering tributaries [Green et al., 2006]. Local groundwater flow is shallow, as is typical of these coastal lowlands [White et al., 2003], and contained within the sub-catchment area that defines the water balance boundary in this study. The estuarine clay layer that exists at 1 m (20.4 m AHD) below the surface and extends for a further 10 m has an hydraulic conductivity of 0.03 m d 21 , which effectively impedes any rainfall transmitted through the upper soil horizons, and infiltration of seawater into the drains [White et al., 1993]. ...
Article
The water balance of drained floodplains is highly dynamic with complex groundwater-surface water interactions operating over varying spatial and temporal scales. Here, we hypothesise that the majority of groundwater discharge will follow flood events in a modified wetland. To test this hypothesis, we developed a detailed water balance that quantifies the contribution of groundwater discharge to the annual water budget of an extensively drained agricultural floodplain. A clear relationship between surface water radon measurements and groundwater level indicated alternating connection-disconnection dynamics between the drains and shallow groundwater. This relationship was used to develop a radon mass balance to quantitatively model groundwater discharge continuously throughout the year. Groundwater discharge varied by four orders of magnitude over the study period, with daily average rates ranging from 0 to 27,200 m3 d−1, peaking just a few hours after floods receded. Flood events occurred only 12% of the time yet contributed 72 to 76% of the total groundwater discharge. During flood recession periods, aerial groundwater discharge rates reached up to 325 cm d−1 which were some of the highest rates ever estimated. We proposed that the high drainage density of this site (12.4 km constructed drains km−2 catchment area) enhanced groundwater discharge during wet periods due to increased connectivity with the soil. Overall, groundwater discharge contributed 30-80% to the total surface water discharge. This study offers insight into the dynamic behaviour of groundwater within an extensively drained floodplain, and the importance of capturing flood events to quantify total groundwater contribution to floodplain water balances. This article is protected by copyright. All rights reserved.
... Samples were only taken from this depth as shallow groundwater is the major interacting groundwater source with the surface water due to the presence of a 10 m thick layer of gel-like marine/estuarine clay that exists N1.5 m from the surface. This deeper sediment has a very low hydraulic conductivity (White et al., 2003), and acts as a confining layer between any deeper groundwater aquifers. Known volumes of radium-free tap water equilibrated with the atmosphere was added to the sediments and incubated for 21 days to allow for radon source ( 226 Ra decay) and sink ( 222 Rn decay) to reach steady state equilibrium. ...
Article
Many coastal floodplains have been artificially drained for agriculture, altering hydrological connectivity and the delivery of groundwater-derived solutes including carbon dioxide (CO2) and methane (CH4) to surface waters. Here, we investigated the drivers of CO2 and CH4 within the artificial drains of a coastal floodplain under sugarcane plantation and quantify the contribution of groundwater discharge to CO2 and CH4 dynamics over a flood (290 mm of rainfall). High temporal resolution, in situ observations of dissolved CO2 and CH4, carbon stable isotope for CH4 (δ13C-CH4), and the natural groundwater tracer radon (222Rn) allowed us to quantify CO2, CH4 and groundwater dynamics during the rapid recession of a flood over a five day period. Extreme super-saturation of free CO2 ([CO2*]) up to 2,951 µM (25,480% of atmospheric equilibrium) was driven by large groundwater input into the drains (maximum 87 cm day-1), caused by a steep hydraulic head in the adjacent groundwater. Groundwater input sustained between 95-124% of the surface [CO2*] flux during the flood recession by delivering high carbonate alkalinity groundwater (DIC = 10,533 µM, ~pH = 7.05) to acidic surface water (pH <4), consequently transforming all groundwater-derived DIC to [CO2*]. In contrast, groundwater was not a major direct driver of CH4 contributing only 14% of total CH4 fluxes. A progressive increase in CH4 concentrations of up to ~2,400 nM day-1 occurred as a combination of increased substrate availability delivered by post-flood drainage water and longer residence times, which allowed for a biogenic CH4 signal to develop. The progressive enrichment in δ 13C-CH4 values (-70‰ to -48‰) and increase in CH4 concentrations (46-2,460 nM) support coupled production-oxidation, with concentrations and δ13C values remaining higher (-47‰ and 2,798 nM) than pre-flood conditions (-55‰ and 534 nM) three weeks after the flood. Our findings demonstrate how separate processes can drive the aquatic CO2 and CH4 response to a flood event in a drained coastal floodplain, and the key role groundwater had in post-flood [CO2*] evasion to the atmosphere, but not CH4.
... Tuller and Or (2003) capitalized on modeling liquid distribution in rigid angular pore spaces (Tuller and Or, 2001) to provide the framework for pore-scale geometrical changes in swelling porous media. Other modeling and measurement frameworks have been proposed for quantifying volume changes and soil hydraulic properties (Chertkov and Ravina, 2002;Chertkov, 2012;Garnier et al., 1997;White et al., 2003;among others). A key feature in these models is the need to combine physicochemical processes with mechanical and hydrodynamic considerations towards predicting mechanical state, volume changes, and constitutive hydraulic relationships for swelling porous media. ...
Article
The water retention curve (WRC) and the hydraulic conductivity function (HCF) are key ingredients in most analytical and numerical models for flow and transport in unsaturated porous media. Despite their formal derivation for a representative elementary volume (REV) of soil complex pore spaces, these two hydraulic functions are rooted in pore-scale capillarity and viscous flows that, in turn, are invoked to provide interpretation of measurements and processes, such as linking WRC with the more difficult to measure HCF. Numerous conceptual and parametric models were proposed for the representation of processes within soil pore spaces and inferences concerning the two hydraulic functions (WRC and HCF) from surrogate variables. We review some of the primary models and highlight their physical basis, assumptions, advantages, and limitations. The first part focuses on the representation and modeling of WRC, including recent advances such as capillarity in angular pores and film adsorption and present empirical models based on easy to measure surrogate properties (pedotransfer functions). In the second part, we review the HCF and focus on widely used models that use WRC information to predict the saturated and unsaturated hydraulic conductivity. In the third part, we briefly review issues related to parameter equivalence between models, hysteresis in WRC, and effects of structural changes on hydraulic functions. Recent technological advances and monitoring networks offer opportunities for extensive hydrological information of high quality. The increase in measurement capabilities highlights the urgent need for building a hierarchy of parameters and model structures suitable for different modeling objectives and predictions across spatial scales. Additionally, the commonly assumed links between WRC and HCF must be reevaluated and involve more direct measurements of HCF. The modeling of flow and transport through structured and special porous media may require special functions and reflecting modifications in the governing equations. Finally, the impact of dynamics and transient processes at fluid interfaces on flow regimes and hydraulic properties necessitate different modeling and representation strategies beyond the present REV-based framework.
Thesis
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Baghdad city is characterized by its high groundwater level with different locations and levels, for this reason, the need for water table lowering is on important activity to be handled before starting any construction project in which the need for dry work state is important to excavate the site. One of the basic concepts for geotechnical engineer is that dewatering will improve soil characteristics by increasing its strength and accelerating the consolidation rate. This may also have a side effect on nearby foundation, which have been built for a long time, or any other type of building since the process of dewatering will directly lead to instability of building or some damages may happen. This study aims to pinpoint on the importance on soil dewatering which is considered as a minor activity to a large number of contractors and site work engineers, as sometimes it leads to unexpected project budget, therefore, it is necessary to carry out a hydrological study before starting any excavation. To get more understanding about groundwater dewatering, a laboratory model has been made which is similar to the process of pumping of water by means of a well from a soil of different layers, water has been extracted with different quantities as well as laying a foundation at different distances from the well in order to record the settlement induced by dewatering. This simulation represents uncontrolled dewatering design. The study revealed that dewatering induces soil consolidation settlement in which the water draws gradually until the water level steady state and deformation takes for several days, which will stop after 2 weeks starting from water pumping then the deformation becomes very little that it can be neglected. The shear force had been checked for cohesive soil before and after dewatering and the results point of the increase in the strength of shear forces ii in the areas which surrounds the drawing well at a distance about 40% of the influence distance, a 10% reduction in water content and an increase also in the internal friction angle for the cohesionless soil. There has also been an analytical study to describe the tilting of the minarets in al-Kadhum sacredly using Plaxis 3D and visual MODFLOW programs. The hydrological site study found that the effect of dewatering from wells surrounding the holy site has a direct effect to tilt the minarets especially when the effective stresses are changed inside the soil as a result of 6 m drop in watertable at the base of minarets and a settlement adding to the change in the direction of the groundwater flow, lead to draw out the fine particles from the soil. These changes might make holes and cavities, which have been found through the site test by means of Ground Penetrating Radar (GPR) test. A virtual case study was carried out at Bab el-Mudham area at al-Rusafa side of Baghdad to evaluate the effect of groundwater lowering on pile. This study showed that the effect lowering groundwater level leads to the occurrence of negative skin friction on piles because of soil settlement around the piles. This leads to make additional forces on the piles and piles settlement, with continuous groundwater falling down until it reached the pile base plus the settlement of the soil under pile base which means that pile foundation building has more settlement when groundwater is lowered below pile base.
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Formation of highly impermeable assemblages on membranes in water treatment can result in significant problems in maintaining permeate flux and necessitate frequent cleaning of the membrane. In this study, we describe the hydraulic properties of montmorillonite clay particles and examine the link between microscopic particle-particle interactions and the bulk properties of the clay assemblage. In particular, we show that changes in the ionic composition of feed waters can have a dramatic impact on the fouling properties of montmorillonite particles with highly impermeable, voluminous filter cakes formed at low ionic concentrations and dramatically more porous assemblages formed at higher ionic strengths.
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