surface is not necessarily best modeled as a rigid
Design curves for Army vehicles
Because the potential for aggregate and cost
circular area (as it is now). For example, the length
savings is of interest to the U.S. Army, and the
to width ratio for a HEMTT is estimated as L =
Giroud and Noiray (1981) method shows prom-
1.6B (e.g., Richmond et al. 1990). Thus, other
ise for large savings over the current Army design
wheel-load geometries should be considered.
method, design curves for Army vehicles were
The Giroud and Noiray (1981) design method
developed according to both methods for compari-
indicates that the geotextile may be able to pro-
son of aggregate thickness required. Design curves
vide reinforcement with no aggregate on the sur-
for the U.S. Army's 10- and 20-ton trucks are pre-
face. As discussed earlier, this is not a currently
sented in Figures 16 and 17, respectively. A geo-
recommended practice because of the increased
textile tensile modulus of 200 kN/m (1143 lb/in.)
risk of damage to the geotextile due to trafficking
was used for Figures 16 and 17 (bottom) because
and because of deterioration when exposed to
this value is easily obtained for commercially avail-
sunlight (e.g., Holtz et al. 1993). However, it is
able products (Table 3).
potentially of great interest to the U.S. Army for
Considerable aggregate savings can be realized
reinforcement of thawing soils, especially for ex-
if the Giroud and Noiray (1981) method is used.
pedient, temporary operations where ultraviolet
For the 10-ton truck, with a soil strength of 30 kPa
degradation due to exposure to sunlight is not a
(4.4 psi), the aggregate savings for the Giroud and
consideration (e.g., less than 10 days of exposure)
Noiray (1981) method over the current Army
and aggregate is not available. For this concept to
method is about 0.2 m (8 in.) with geotextile. For
be implemented, the geotextiles would likely have
the 20-ton truck at a soil strength of 40 kPa (5.8
to be anchored in some way in order for the ten-
psi), the aggregate savings for the Giroud and
sile properties to fully develop and provide the
Noiray (1981) method over the current Army
necessary reinforcement. (Even though the
method is about 0.2 m (8 in.) with geotextile. Thus,
geotextile is in a state of tension between the
accounting for the tensile support provided by the
wheels, the portion on the outside of each set of
geotextile provides considerable advantages of
wheels could easily slip into ruts formed by the
aggregate savings. It is important to remember that
vehicles.)
the aggregate used with this method should have
An important factor in the adoption of the
a minimum CBR of 80 (Giroud and Noiray 1981).
Giroud and Noiray (1981) design method for use
is knowledge of the appropriate geotextile modu-
lus values. Geotextile modulus values at 5% strain
are readily available. Based on limited field experi-
RECOMMENDATIONS FOR FUTURE WORK
ments, this appears to be a reasonable strain esti-
Using the Giroud and Noiray (1981) method as
mate for static loading of geotextiles performing
it is presented herein may lead to unconservative
reinforcement over low-bearing-capacity soils.
design and construction results because the stress
However, modulus values are higher in biaxial
distribution through the aggregate layer to the
tension, but possibly far lower for repeated load-
subgrade is less than the Boussinesq method,
ing than for monotonically loaded uniaxial tests--
which is widely accepted and well-supported.
the tests that are now performed to determine
However, it should be further investigated because
geotextile modulus values. In reinforcing low-
it promises large aggregate savings compared with
bearing-capacity soil, the geotextile is expected to
the current design method, due solely to its abil-
undergo both biaxial tension and repeated load-
ity to account for tensile properties of the geotextile
ing. Therefore, field or other experimental work
reinforcement at large rut depths, a situation that
is needed to help establish the effective modulus
can be tolerated by military vehicles on thawing
values of the geotextiles when they are being used
soils. Depending on the outcome of an investiga-
and to related them to the values measured in ten-
tion of the stress distribution through the aggre-
sile tests. Finally, the tensile modulus values of
gate layer to the subgrade, it may be worthwhile
some commercially available geotextiles far exceed
to develop a hybrid design method that uses a
those used in Figures 16 and 17 (bottom). Future
Boussinesq stress distribution through the sub-
work should consider the use of available prod-
grade with a membrane support mechanism as
ucts with appropriately high modulus values. This
presented by Giroud and Noiray (1981). For use
could result in substantial aggregate savings.
of a Boussinesq stress distribution, the load at the
Regardless of whether the Giroud and Noiray
18
Previous Page
