UFC 3-260-02
30 June 2001
6. SUBGRADE COMPACTION FOR RIGID PAVEMENTS - NORMAL CASES. Compaction improves
soil strength and ensures that densification with resulting voids under the concrete slab does not occur.
Subgrade soils that gain strength when remolded and compacted will be prepared in accordance with
the following criteria.
Table 6-7
Compaction Requirements for Shoulders
1
Depth of
1
Depth of Compaction in inches
Compaction in inches for
for Cohesive Subgrades and
Cohesionless Subgrades
Select Materials
and Select Materials
(LL # 25; PI # 5)
Percent Compaction
(LL > 25; PI > 25)
85
17
29
90
14
23
95
10
16
100
6
10
1
Depth is measured from pavement surface.
Conversion Factor: Millimeters = 25.4 inches.
a. Compacting Fill Sections. Fills composed of soil having a plasticity index (PI) greater than 5 or a
liquid limit (LL) greater than 25 will be compacted to not less than 90 percent of ASTM D 1557 maximum
density. Fills composed of soil having a PI equal to or less than 5 and an LL equal to or less than 25 will
be compacted as follows: the top 152 millimeters (6 inches) will be 100 percent of ASTM D 1557
maximum density; the remaining depth of fill will be 95 percent of ASTM D 1557 maximum density.
Large fills on natural soil should be analyzed for bearing capacity and settlement using conventional soil
mechanics.
b. Compacting Cut Sections. The top 152 millimeters (6 inches) of subgrades composed of soil
having a PI greater than 5 or an LL greater than 25 will be compacted to not less than 90 percent of
ASTM D 1557 maximum density. If the natural subgrade exhibits densities equal to or greater than
90 percent of other ASTM D 1557 maximum density, no compaction is necessary other than that
required to provide a smooth surface. Soils having a PI equal to or less than 5 and an LL equal to or
less than 25 will be compacted as follows: the top 152 millimeters (6 inches) will be 100 percent of
ASTM D 1557 maximum density; the 455 millimeters (18 inches) below the top 152 millimeters
(6 inches) will be 95 percent of ASTM D 1557 maximum density. Again, if the natural subgrade exhibits
densities equal to or in excess of the specified densities, no compaction will be necessary other than that
required to provide a smooth surface; in most cases, these densities can be obtained by surface rolling
only.
c. Permissible Variations in Field Density. The above criteria should be considered as minimal
values. Also, it is emphasized that it is often difficult to correlate field densities with those obtained by
practical compaction procedures in the field. Higher densities should result in higher foundation
strengths and thus thinner pavements which may offset the added cost of compaction. Experience has
shown that the highest densities for all but the special cases (that is, soils that lose strength when
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