m
in.
70
Wheel Load, W*
194.6 kN (43,750 lb)
60
1.5
166.8 kN (37,500 lb)
106.8 kN (24,000 lb)
50
77.8 kN (17,500 lb)
35.6 kN ( 8,000 lb)
40
a = 0.66 (Barenberg et al. 1975)
1.0
*W = Sum of load on all four
wheels on one side of
30
2W
the axle
P=
a
20
0.5
x
10
0
0
10
50
100
500
kPa
1
5
10
50
100 psi
cNc
Figure 3. Aggregate thickness design curve for tandem (dual)-wheel load on gravel-surface
roads. (From TM 5-818-8.)
dual wheels on a single axle (Fig. 4a and 4b), the
soil cohesion of 52 kPa (7.5 psi) or CBR of 2.*,†
wheel load, W, is defined as the total load on ei-
There will be approximately 100 passes of this
ther the left or right side of the axle. The axle load,
vehicle, and a rut depth of 0.3 m (12 in.) can be
P, a quantity used in other design methods, is de-
tolerated. The tire inflation/contact pressure is
fined as the total load on the axle, or 2W. For tan-
equal to 552 kPa (80 psi).
dem axles (Fig. 4c and 4d), estimations of wheel
1. Calculate cNc as (52)(3.3) = 172 kPa (25 psi) with-
and axle loads vary. Barenberg et al. (1975) use a
out geotextile, and (52)(6) = 312 kPa (45 psi) with
wheel load of 0.66 times the total load on one side
geotextile.
of both of the tandem axles. Giroud and Noiray
2. Enter Figure 2 with a cNc value of 172 kPa (25
(1981) obtained the design axle load by multiply-
psi) for a wheel load of 40 kN (9,000 lb) to ob-
ing the sum of the two axle loads by 0.60. Most
U.S. states allow maximum loads on each pair of
tain a value of 0.32 m (13 in.) of aggregate re-
tandem axles equal to 0.563 times the maximum
quired without geotextile. Using a cNc value of
allowable single axle load (Yoder and Witczak
312 kPa (45 psi) for the same wheel load, 0.20 m
1975). Contact pressures for use in design are ap-
(8 in.) of aggregate is required with geotextile.
proximately 0.9 to 1.0 times the tire inflation pres-
3. Determine whether the cost of the geotextile
sure for single-tired vehicles and 0.70 to 0.75 times
exceeds the cost of 0.12 m (5 in.) aggregate,
the tire inflation pressure for dual tires (Barenberg
which would be saved by using the geotextile.
et al. 1975). However, for the design method pre-
sented in TM5-818-8, there is negligible difference
4. If it is advantageous to use a geotextile, specify
in the aggregate thickness design curves for ac-
one using Tables 2-2 through 2-4 in TM5-818-8
tual tire inflation pressure vs. contact pressure (e.g.,
(based on the need for the geotextile to survive
Fig. 5).
*The relationship between shear strength (cohesion) or
Design example
CBR and Cone Index is given in Figure 2-3 of TM5-818-
Given an 80-kN (18,000-lb) maximum expected
8 (1995).
single axle load (40-kN or 9,000-lb wheel load) on
†Saturated silts and clays are likely to have CBR values
a dual-tired vehicle, determine the aggregate thick-
of this order of magnitude. Thawing frost-susceptible
ness required with and without geotextile for a
soils may have even lower CBR values.
3