UFC 3-260-02
30 June 2001
CHAPTER 6
SUBGRADE
1. SUITABILITY OF SUBGRADE. The information obtained from the explorations and tests
previously described should be adequate to enable full consideration of all factors affecting the suitability
of the subgrade and subsoil. The primary factors are as follows:
a. The general characteristics of the subgrade soils.
b. Depth to bedrock.
c. Depth to water table (including perched water table).
d. The compaction that can be attained in the subgrade and the adequacy of the existing density in
the layers below the zone of compaction requirements.
e. The strength that the compacted subgrade, uncompacted subgrade, and subsoil will have under
local environmental conditions.
f. The presence of weak or soft layers in the subsoil.
g. Susceptibility to detrimental frost action.
h. Settlement potential.
I. Expansion potential.
j. Drainage characteristics.
2. GRADE LINE. The soil type together with information on the drainage requirements, balancing cut
and fill, flooding potential, depth to water table, depth to bedrock, and the compaction and strength
characteristics should be considered in locating the grade line of the top of the subgrade. Generally, this
grade line should be established to obtain the best possible subgrade material consistent with the proper
utilization of available materials; however, economics of plans for construction must be given prime
consideration.
3. SUBGRADE CBR. The strength of the subgrade may be expressed in terms of the CBR for flexible
pavement design. The CBR test is described in CRD-C 654. It includes procedures for making tests on
samples compacted to the design density in test molds and is soaked 4 days for making in-place CBR
tests and for making tests on undisturbed samples. These tests are used to estimate the CBR that will
develop in the pavement structure. However, a subgrade design CBR value above 20 is not permitted
unless the subgrade meets the requirements for subbases. The CBR selected for the subgrade will be
based on the predominant moisture conditions occurring during the life of the pavement. This moisture
situation can be obtained from pavement evaluation reports and from soil tests under existing
pavements. Where long duration soil moisture conditions cannot be determined with confidence, the
soaked laboratory CBR will be selected for the subgrade soil.
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