Environmental Engineering Reference
In-Depth Information
Paydar, 1996; Tomasella and Hodnett, 1998; Zapata, 1999).
The correlation of SWCC fitting parameters published by
Zapata (1999) is for fitting parameters used in the Fredlund
and Xing (1994) equation. The correlations were based on
a statistical analysis of soil classification properties and the
SWCC fitting parameters.
D
60
=
grain-size diameter corresponding to 60% passing
by mass.
m
f
=
0
.
1772[ln
(D
60
)
]
+
0
.
7734
(5.98)
where:
5.14.1 Zapata (1999) Correlation Model for SWCC
Zapata (1999) used a database of approximately 190 soils
collected from research papers as well as information on
a number of soils found in the knowledge-based database
developed by SoilVision Systems. The soils collected were
divided into two categories: soils exhibiting plasticity (i.e.,
positive plasticity index, PI) and soils with a PI equal to zero.
The database used in the analysis consisted of approximately
70 soils with PI greater than zero and 120 soils with PI
equal to zero. The data collected for the soils with PI greater
than zero consisted of the percentage passing the No. 200
sieve and the Atterberg limits, in particular, the plasticity
index. The grain-size diameter
D
60
was used to represent
nonplastic soils. Each soil used in the statistical correlation
had a measured and well-defined SWCC.
The product of the percentage passing the No. 200 sieve
(used as a decimal value) was multiplied by the plasticity
index to form a weighted PI value (i.e.,
w
p
PI) for soils with a
plasticity index greater than zero. The weighted PI was used
as the primary variable for correlation. The
D
60
variable was
used as the primary variable for correlation purposes when the
soil was nonplastic. The assumption was made in the analysis
that all soils remained at a constant volume as soil suction
was increased. The Zapata (1999) correlations of SWCCwere
presented in terms of the degree of saturation of the soil.
The correlation study yielded a family of SWCCs for both
plastic and nonplastic soils. The results of the Zapata (1999)
study have been used as part of the Enhanced Integrated
Climatic Model (EICM, version 2.6) in the pavement design
guide (Houston et al., 2006). The family of curves has also
been used as an approximate guide to a potential desorption
curve for soils.
m
f
=
fitting parameter influencing the curvature of the
SWCC at low and high suctions.
n
f
=
7
.
5
(5.99)
where:
n
f
=
fitting parameter equal to the slope at the inflection
point of the SWCC.
a
f
9
.
7
e
−
4
1
D
60
+
h
r
=
(5.100)
where:
e
=
constant equal
to 2.71828, base of
the natural
logarithm.
The above suggested equations can be used to generate a
family of curves based on
D
60
values as shown in Fig. 5.119.
The grain-size
D
60
is limited to the range between 0.1 and
1.0 mm.
5.14.3 SWCC for Plastic Materials (Zapata, 1999)
Soils exhibiting plasticity were analyzed as a separate group
of soils. The fitting parameters (i.e.,
a
f
,m
f
,n
f
,
and
h
r
)
were found to correlate well with the percent passing the
No. 200 sieve (i.e.,
w
p
)
multiplied by the plasticity index of
the soil (i.e.,
w
p
PI). The following equations were generated
for the fitting parameters of plastic soils:
0
.
00364
(
w
PI
)
3
.
35
a
f
=
+
4
(
w
PI
)
+
11
(5.101)
5.14.2 SWCC for Granular Nonplastic Materials
(Zapata, 1999)
The grain-size distribution curve for each soil was analyzed
when the soil was classified as nonplastic. Each SWCC was
best fit with the Fredlund and Xing (1994) equation where
four fitting parameters were generated for each soil:
a
f
,n
f
,
m
f
, and
h
r
. The four parameters were correlated with the
particle diameters corresponding to 60% passing. The corre-
lations for nonplastic soils were represented by the following
equations:
0
.
0514
(
w
PI
)
0
.
465
m
f
=
+
0
.
5
(5.102)
2
.
313
(
w
PI
)
0
.
14
n
f
=
m
f
(
−
+
5
)
(5.103)
a
f
(
32
.
44
e
0
.
0186
(
w
PI
)
)
h
r
=
(5.104)
where:
e
=
constant equal to 2.71828, the base of the natural
logarithm.
0
.
8627
(D
60
)
−
0
.
751
a
f
=
(5.97)
where:
The above equations can be used to generate a family of
SWCCs for soils with
w
p
PI values ranging from 0.1 to 50,
as shown in Fig. 5.120.
a
f
=
fitting parameter related to the inflection point on
the SWCC and
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