b. Design Details. Use subsurface drains (perforated collector pipes and
filters) in lieu of deep ditches for collecting and transporting ground water.
For typical subgrade drain installation, see Figure 5. For design criteria see
NAVFAC DM-7 and NAVFAC DM-21.
4. SUBGRADE COMPACTION.
a. Fill Sections. All granular fill materials should be compacted to at
lest 95 percent of maximum density. Cohesive fill material should be completed
to no less than 90 percent.
b. Cut Sections. In cut, the depth and degree of compaction required varies
with the pavement design index. Specific guidance on compaction depth is given
in DA TM 5-822-5. When existing subgrade soils do not meet minimum compaction
requirements, consider the following alternatives:
(1) Compact soils from the surface.
(2) Remove and process soil to attain the approximate optimum moisture
and replace and compact.
(3) Replace subgrade soil with suitable borrow material.
(4) Raise the grade so that existing natural densities meet required
values.
5. SETTLEMENT. When designing roadways in fill sections, examine soil profiles
to ascertain possible settlement due to the existence of compressible layers.
For settlement analysis, see NAVFAC DM-7.
6. SUBGRADE STABILIZATION. Native subgrade and lower quality borrow materials
may be improved for use in the pavement structure with the use of a cement,
bitumen, or lime stabilizing agent. For stabilization or modification of
cohesive subgrade soils, hydrated lime is the most widely used. Lime is
applicable in clay soils (CH, CL) and in granular soils containing clay binder
(GC, SC). Lime reduces the plasticity index (PI) and renders a clay soil less
sensitive to moisture changes. Consider the use of lime whenever the PI of the
soil is greater than 10.
a. Lime Treatment. Lime treatment or modification consists of the
application of from 1 to 3 percent hydrated lime to aid drying of the soil and
permit compaction. As such it is useful in the construction of a "working
platform" to expedite construction. Lime modification may also be considered
to condition a soil for follow-on stabilization with cement or bitumen. Lime
treatment of subgrade soils is intended to expedite construction, and no
reduction in the required pavement thickness should be made.
b. Lime Stabilization. For lime stabilization of clay subgrades, the lime
content should be from 3 percent to 8 percent of the dry weight of the soil,
and the cured mass should have an unconfined compressive strength of at least
50 psi in 28 days. The optimum lime content should be determined with the use
of unconfined compression and Atterburg Limits tests on laboratory lime-soil
mixtures molded at varying percentages of lime. Determine the laboratory CBR
of the optimum lime soil mixture for use in pavement thickness design
procedures.
Section 5. SUBBASE
1. FUNCTIONS AND REQUIREMENTS.
a. Rigid Pavements. Subbase normally is not used in rigid pavements. The
course directly beneath the concrete is termed "base."
b. Flexible Pavements. In flexible pavements, the subbase is composed of a
selected borrow or stabilized material used to reduce the thickness of the
base. The subbase is placed on the subgrade and serves as a load-distributing
medium to reduce subgrade stress.
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