Section 4. SUBGRADES
1. EXPLORATION. Sec Section 2 of this manual for criteria for subgrade explor-
ation, identification, and testing. Additional criteria for the determination
of the design subgrade strength is given below.
2. SUBGRADE STRENGTH.
a. Flexible Pavements. Flexible pavement thickness design procedures are
based on the CBR method of design, which requires an appraisal of the ultimate
moisture content and CBR of the subgrade soil. The objective of the procedure
is to design the pavement on the basis of the predominant subgrade moisture
content anticipated during the life of the pavement. To determine the design
subgrade CBR, use the following procedures:
(1) Field-in-place tests.
(a) Determine field-in-place CBR of the natural subgrade with
corresponding water content and density at proposed subgrade elevation
whenever possible.
(b) Conduct in-place CBR tests in subgrades under existing adjacent
pavements that may have reached equilibrium moisture conditions and may be
indicators of the long-term strength.
(c) Use in-place tests in coarse-grained subgrade soils where little
increase in moisture content is anticipated.
(d) Use in-place tests in clays which would lose strength when
excavated and remolded, in silts and fine sands which become quick or spongy
under high water table, and in subgrade soils that are near saturation in
their natural state.
(2) Tests on Laboratory Molded Samples. In the absence of reliable field
information, perform laboratory CBR tests on subgrade samples molded at
varying moisture contents and compactive effort and subjected to four days
soaking. Follow the procedure contained in MIL-STD 621, Method 101. Perform
one complete series of tests for each distinct type of soil.
(3) Determine design CBR. Where a range of soil types is encountered in
the proposed roadway and CBR test results are variable, judgment must be made
as to whether variable design thicknesses are necessary and economically
feasible. Where variable design thicknesses are not warranted, determine the
CBR value for design, as that value is equal to or less than 85 percent of the
CBR test values along the proposed roadway.
(4) Approximate Values. Use the following approximate values of CBR for
checking and estimating purposes and for preliminary design of minor paved
areas:
Subgrade Soil Type
Approx. CBR
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Well and poorly graded gravels, well graded sands
>18
Silty and clayey sands
12-18
Low plastic clays, inorganic silts, very fine sands
6-12
Highly plastic and organic clays, micaceous silts
1-6
b. Rigid Pavements. Use the subgrade modulus K for the design of rigid
pavements. See Section 2.
3. SUBGRADE DRAINAGE.
a. Conditions Requiring Subgrade Drainage. Consider the use of subgrade
drains whenever the following conditions exist:
(1) High ground-water levels which may reduce subgrade stability and
provide a source of water for frost action.
(2) In subgrade soils of silts and very fine sands which may become quick
or spongy when saturated.
(3) Consider intercepting drains where water seeps from underlying water-
bearing strata or from subgrades in cut areas.
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