TM 5-818-8/AFJMAN 32-1030
2-5. Design for Separation
conditions and construction equipment; whereas,
table 2-3 relates survivability to cover material
When serving as a separator, the geotextile pre-
and construction equipment. Table 2-4 gives mini-
vents fines from migrating into the base course
mum geotextile grab, puncture, burst, and tear
and/or prevents base course aggregate from pene-
strengths for the survivability required for the
trating into, the subgrade. The soil retention prop-
conditions indicated in tables 2-2 and 2-3.
erties of the geotextile are basically the same as
those required for drainage or filtration. Therefore,
2-7. Design for Reinforcement
the retention and permeability criteria required
for drainage should be met. In addition, the geo-
surfaced roads is generally limited to use over soft
textile should withstand the stresses resulting
cohesive soils (CBR < 4). One procedure for
from the load applied to the pavement. The nature
determining the thickness requirements of aggre-
of these stresses depend on the condition of the
gate above the geotextile was developed by the US
subgrade, type of construction equipment, and the
Forest Service (Steward, et al. 1977) and is as
cover over the subgrade. Since the geotextile
serves to prevent aggregate from penetrating the
a. Determine In-Situ Soil Strength. Determine
subgrade, it must meet puncture, burst, grab and
the in-situ soil strength using the field California
tear strengths specified in the following para-
Bearing Ratio (CBR), cone penetrometer, or Vane
Shear device. Make several readings and use the
lower quartile value.
b. Convert Soil Strength. Convert the soil
strength to an equivalent cohesion (C) value using
Table 2-2 has been developed for the Federal
the correlation shown in figure 2-3. The shear
Highway Administration (FHWA) to consider sur-
strength is equal to the C value.
vivability requirements as related to subgrade
Table 2-2. Construction Survivability Ratings (FHWA 1989)
Site Soil CBR
H = High, M = Medium, NR = Not recommended.
'Maximum aggregate size not to exceed one half the compacted cover
For low volume unpaved road (ADT 200 vehicles).
The four inch minimum cover is limited to existing road bases and
not intended for use in new construction.