1253
Technical Committee 202 /
Comité technique 202
(a)
(b)
(c)
Figure 6. (a) Shear moduli variation with strain level; (b) damping ratio according to cyclic shear strain; (c) cumulative strain with load repetitions.
Figure 7. Cumulative vertical displacement (rutting depth).
q
f
= static failure deviator stress; N = number of repeated load
applications; and a, m, n and b are constant largely dependant
on soil properties, which could be assumed for the soft soil
analyzed here, according to those values suggested by Li and
Selig (1996): a = 1.2; b = 0.18; m = 2.4; n = 1.
Considering the results of q
is
, q
d
and q
f
depicted in Figures
4b and 5, the cumulative plastic strain is calculated at different
depths according to the Equation 6. In Figure 6c is observed that
results from finite element model fit reasonably well with the
empirical equation proposed by Chai and Miura (2000)
considering N = 100. Otherwise, Figure 7 shows a cumulative
permanent deflection of about 45 mm (rut depth), given from
Equation 6 for a number of repeated load applications up to N =
5·10
5
, and considering the thickness of soft soil influenced by
the cyclic load. Such results suppose an unallowable level of
rutting failure, even for a low trafficked road as its typical
thresholds range from 10 to 15 mm for N greater than 10
6
.
Possible solutions for pavements based over such soft soil might
be achieved by means of deep ground improvement, which
could overcome the detrimental effects of the induced deviator
stress and excess pore pressure throughout the depth of load
influence (Elias et al. 2004, Sonderman and Wehr 2004).
Table 3. Results of subgrade soil stiffness at 1.5 m depth.
Number of load repetitions
Parameter Unit
10
30
50
100
OCR
-
1.2
1.4
1.47
1.5
-
2.90E-04
1.16E-04
1.20E-04
1.07E-04
G
S
/G
0
-
0.61
0.80
0.79
0.81
G
t
/G
0
-
0.37
0.63
0.63
0.66
G
S
kPa
27473
35851
35601
29485
-
0.891
0.844
0.822
0.794
E
ur
*
kPa
63661
78635
73191
58551
E
50
*
kPa
25464
31454
29276
23421
*final stiffness
4 CONCLUSION
The theoretical procedure presented by means of finite element
modelling has shown that deep soft soils might be decisive to
long term behavior of flexible pavements, especially in the
cases when shallow treatments of subgrade would be
uneconomic or inefficient. Deep soil treatments should be
applied to achieve an allowable capacity of soft soils up to
minimum depth of about 6 m, otherwise maintenance cost of
pavements might be excessive. The analysis presented has
included the response of soft subgrade layers under static and
cyclic loading taking into account the influence of small-strain
levels on the soil stiffness; the results fit reasonably well with
the analytical solution of hysteretic damping ratio presented by
Brinkgreve et al. (2007).
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