Aggregate
Surface
L
B
α
γγh
h
h
Geotextile
2a
Subgrade/Aggregate
Interface
Figure 10. Wheel load distribution by aggregate layer to subgrade (Giroud and Noiray 1981).
80 kN and 620 kPa tire pressure averaged 0.9, 2.1,
sign is the secant modulus obtained from ten-
and 5.2%, while the maximum strain in each of
sile tests.*
the geotextiles was 2.6, 3.3, and 7.4%, respectively.
The influence of the inaccuracy of this assump-
The first assumption is reasonable for
tion on the validity of the design technique is not
geotextiles that meet survivability requirements.
yet clear.
Measurements of test sections that were carefully
It is also not clear what modulus value should
trafficked to minimize the wander of wheels in
be used with this design method. Giroud and
lanes indicated that the shape of the deformed
Noiray (1981) recommend the use of a biaxial ten-
geotextile is approximately parabolic (e.g., Kinney
sile test, where the lateral deformation of the
and Barenberg 1979, Fannin and Sigurdsson 1996).
geotextile is prevented during testing, and that the
If the geotextileaggregate friction is inadequate,
secant modulus in the transverse direction of the
the aggregate can (and most likely does) slide
road be used. Biaxial testing would lead to esti-
along the surface of the geotextile (e.g., Kinney and
mates of modulus values that are higher than those
Barenberg 1979, Kinney 1982); however, use of a
determined in uniaxial tests by about 1.1 to 1.35
high-quality aggregate (as assumed by this
times (e.g., Giroud 1992, Soderman and Giroud
method) would probably prevent this from occur-
1995). However, Kinney (1982, 1998, personal com-
ring. Tests have also shown that the strain is not
completely uniform along the length of the
geotextile (e.g., Kinney and Barenberg 1979,
Table 2. Thickness of plastic zone in the subgrade
Fannin and Sigurdsson 1996). For example, Fannin
for dual-tired truck loading and aggregate layer
and Sigurdsson (1996) measured the strain in three
of thickness 0.15 m (6 in.).
geotextiles placed beneath a 0.25-m-thick layer of
sandy gravel and over a subgrade of average
Axle load
Plastic zone
Plastic zone
kN (lb)/
thickness, on-
thickness, off-
strength of 40kPa. The strain in the three
tire pressure
highway truck
highway truck
geotextiles at 80 passes of a standard axle load of
kPa (psi)
Hmin (m/in.)
Hmin (m/in.)
80 kN (18,000 lb)/
0.42/16.5
0.47/18.5
480 kPa (70 psi)
*The tensile modules of geotextiles can now be obtained
from ASTM D 4595 (1998) Standard test method for ten-
60 kN (13,500 lb)
0.38/15
0.43/17
sile properties of geotextiles by the wide-width strip
480 kPa (70 psi)
method.
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