UFC 3-260-02
30 June 2001
large stones exposed during subgrade preparation work must also be removed, down to the full
depth to which subgrade preparation is required. Failure to remove stones or large roots can result
in increasingly severe pavement roughness as the stones or roots are heaved gradually upward
toward the pavement surface. They eventually break through the surface in extreme cases,
necessitating complete reconstruction.
d. Soil Conditions. Abrupt changes in soil conditions must not be permitted. Where the
subgrade changes from a cut to a fill section, a wedge of subgrade soil in the cut section with the
dimensions shown in Figure 20-8 should be removed and replaced with fill material. Discontinuities
in subgrade conditions require the most careful attention of designers and construction inspection
personnel, as failure to enforce strict compliance with the requirements for transitions may result in
serious pavement distress.
e. Rock Excavation. In areas where rock excavation is required, the character of the rock and
seepage conditions should be considered. In any case, the excavation should be made so that
positive transverse drainage is provided and no pockets are left on the rock surface that will permit
ponding of water within the depth of freezing. The irregular ground water availability created by
such conditions may result in markedly irregular heaving under freezing conditions. It may be
necessary to fill drainage pockets with lean concrete. At intersections of fills with rock cuts, the
tapered transitions illustrated in Figure 20-8 are essential. Rock subgrades where large quantities
of seepage are involved should be blanketed with a highly pervious material to permit the escape of
water. Frequently, the fractures and joints in the rock contain frost susceptible soils. These
materials should be cleaned out of the joints to the depth of frost penetration and replaced with
nonfrost susceptible material. If this is impractical, it may be necessary to remove the rock to the
full depth of frost penetration. An alternative method of treating rock subgrades, in-place
fragmentation, has been used effectively in airfield construction. Blast holes 0.9 to 1.8 meters
(3 to 6 feet) deep are commonly used. They are spaced suitable for achieving thorough
fragmentation of the rock to permit effective drainage of water through the shattered rock and out
of the zone of freezing in the subgrade. A tapered transition should be provided between the
shattered rock cut and the adjacent fill. Underdrains are essential to quickly remove excess water.
11. CONTROL OF DIFFERENTIAL HEAVE AT DRAINS, CULVERTS, DUCTS, INLETS, HYDRANTS,
AND LIGHTS.
a. Design Details and Transitions for Drains, Culverts, and Ducts. Drains, culverts, or utility
ducts placed under pavements on frost-susceptible subgrades frequently experience differential
heaving. Wherever possible, the placing of such facilities beneath pavements should be avoided.
Where this cannot be avoided, construction of drains should be in accordance with the"correct"
method indicated in Figure 20-9, while treatment of culverts and large ducts should conform with
Figure 20-10. All drains of similar features should be placed first and the base and subbase course
materials carried across them without bread so as to obtain maximum uniformity of pavement
support. The practice of constructing the base and subbase course and then excavating back
through them to lay drains, pipes, etc. is unsatisfactory as a marked discontinuity in support will
result. It is almost impossible to compact material in a trench to the same degree as the
surrounding base and subbase course materials. Also, the amount of fines in the excavated and
backfilled material may be increased by incorporation of subgrade soil during the trench excavation
or by manufacture of fines by the added handling. The poor experience record of combination
drains--those intercepting both surface and subsurface water--indicates that the filter material
should never be carried to the surface as illustrated in the "incorrect" column in Figure 20-9. Under
winter conditions, this detail may allow thaw water accumulating at the edge of the pavement to
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