1071
Technical Committee 106 /
Comité technique 106
Suction (kPa)
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Volumetric water content
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Clay
Till
Silt
Figure 2. Soil-water characteristic curves (Ito and Hu 2011)
Suction (kPa)
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Coefficient of permeability (m/day)
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Clay
Silt
Till
Figure 3. Coefficient of permeability functions (Ito and Hu 2011)
The climate and the vegetation data for the period of study
were applied during the fully coupled transient analysis on the
vegetated area only.
A “no flow” natural boundary condition
was applied in VADOSE/W by default on the pavement to
represent the pavement as an impervious layer and moisture in
and out flow are occurring through the vegetated area (see
Figure 1)
.
In winter season (1 November, 2009 to 31 March,
2010), the precipitation was received as snow. The cumulative
snow precipitation was applied on a single day, when the
temperature rose and remained above 0 °C (1 April, 2010). The
climate data recorded at the Regina Airport weather station was
used for the modeling. The climate in winter months was
however set up to be constant. The temperature was assumed to
be -5
o
C, the relative humidity as 100%, and the remainder of the
climate data was zero. In other words, the model was not
intended to simulate soil movement activities during winter.
Because the site is located in a residential area with a park
that has mature trees, the daily wind speed, precipitation and net
radiation recorded at the weather station were multiplied by
scale factors of 0.3, 0.7, and 0.3, respectively, as suggested by
Ito and Hu 2011. Furthermore, a park watering rate of 1.8064 ×
10
-3
m/day was applied on every Monday and Friday for the
period from 23 June to 12 October as reported in Vu et al. 2007.
However,
water uptake by mature trees was not included in the
modeling.
Similar to Ito and Hu 2011, and Vu et al. 2007, the
vegetation was specified as good grass and the growing season
was assumed to start in April and end in October as suggested
by Vu et al. (2007). The LAI function for good vegetation with
a maximum LAI value of 2 was used as suggested in SoilCover
(Unsaturated Soils Group 1996). The root depth of 150 mm was
used as suggested and the root distribution was assumed to be
triangular. A plant moisture limiting point of 500 kPa and a
wilting point of 2500 kPa were used for this simulation.
Mass balance checking was performed on the VADOSE/W
run, and the model solved with a total mass balance error of less
than 1.5%.
Figure 4 shows the predicted soil suction response to a
changing surface boundary over the entire year under the centre
of the vegetation cover. The soil suction was found to vary with
depth and time. It can be seen that the fluctuations in suctions
correlated well with the environmental conditions on the surface
boundary. The suction at the ground surface fluctuated widely
and these fluctuations reduced with depth. The predicted
suctions for this study agreed well with the results of Ito and Hu
(2011). The correspondence between the suction values was
accomplished using the same meteorological data (e.g.,
precipitation, temperature), soils properties and initial boundary
conditions.
The corresponding suction profiles under the centre of the
vegetation cover for various times were also investigated.
However,
due to limitations of the paper length, suction profiles
are not provided in this paper
. In general, extreme changes in
suction (that vary between 600 and 2500 kPa) occurred at the
ground surface. The suction values are typically greater at the
surface during relatively dry periods. During infiltration, the
suction values decreased at the surface, and it continued to
decrease further as water infiltrated to greater depths. The
suction fluctuations were predominant at the surface and
approached minimum values at 3.4 m (which is the active zone
depth). According to Azam and Ito (2012), this behavior was
attributed to the surface soil layer that was initially at an
unsaturated state and imbibed any water available by the
infiltration. Likewise, the layer can rapidly lose water under
relatively dry conditions. With increasing depth, the overlaying
soil provides a cover and the geotechnical properties of the
underlying materials become progressively more significant.
The high water retention capability and the low coefficient of
permeability of the Regina clay, especially under unsaturated
conditions, impede the soil suction at higher depths to respond
to the variations of the surface boundary. This soil-atmospheric
interaction corroborated well with the suction values obtained
from Ito and Hu (2011) thereby validating the VADOSE/W
output that can be used for predicting the vertical movement of
the test site with Regina expansive clay.
4.2 Estimation of the vertical soil movements
To calculate the vertical soil movements at different depths
(0, 0.5, 1, 2, 3, and 6 m), the soil profile was divided into
several sub-layers up to 6.4 m depth (which is the thickness
of Regina expansive clay layer). The total vertical movement
of the soil at a certain depth for a given day was computed
by adding the vertical movements of all layers up to the
considered depth using Equation 2. The soil compressibility
modulus,
s
2
m
, was calculated using
the Poisson’s ratio (μ =
0.33) and the soil modulus of elasticity in terms of soil
suction which was calculated using Equations 3 and 4.
Vanapalli and Oh (2010) suggested the fitting paramete
r, β,
equals 2 for fine-grained soil, which was used for Regina
expansive clay. The fitting parameter,
α
, was assumed to be
1/12 in order to provide reasonable comparison between the
predicted and the published results of the vertical soil
under
, P
a
=
ree of
sibility
pect to
ionship
(4)
omplex
er, this
s been
Similar
Ltd. for
ion
igration
rtant in
ls. The
io, was
on with
rogram
airport was applied at the vegetative cover over a period of one
year (from 1 May, 2009 to 30 April, 2010). Figures 2 and 3
show the SWCCs and the coefficient of permeability functions,
respectively, for Regina and other materials used in the
numerical modelling. Ito and Hu (2011) provide more details
about the soil, the climate, and the vegetation data of the site.
4 RESULTS AND DISCUSSIONS
4.1 Estimation of the soil suctions
The soil profile shown in Figure 1 was modeled using the
fully coupled transient analysis with the 2-D software
package (VADOSE/W) to estimate the suction changes
associated with the environmental changes for a period of
one year. Beside the soil properties, the initial and boundary
conditions are needed as input data to run the program.
The
initial conditions for all nodes of the model domain, including
pressure and temperature, were derived from implementing a
steady-state analysis using the same model.
Based on the field
suction data measured by Vu et al. (2007),
the
initial
pressure head
during the steady-state analysis
was set up to be
-163.15 m for the top 3 m of the clay layer, -101.97 m for
the rest of the clay, -61.18 m for the silt, and -203.94 m for
the till. T
he temperatures of nodes at the lower boundary were
set up to be 10
o
C
.
Figure 1. Soil profile and soil properties (Ito and Hu 2011)
