UFC 3-220-01N
15 AUGUST 2005
should not produce a differential settlement of either a smooth curved hump or sag of
25 mm in 15 m (1 inch in 50 f) or a uniform slope of 50 mm in 15 m (2 inches in 50 ft).
8-2.6
Hydraulic Fills. Hydraulic fills are placed on land or underwater by
pumping material through a pipeline from a dredge or by bottom dumping from barges.
Dredge materials vary from sands to silts and fine-grained silty clays and clays.
Extensive maintenance dredging in the United States has resulted in disposal areas for
dredge materials, which are especially attractive from an economic standpoint for
development purposes. Dikes are usually required to retain hydraulic fills on land and
may be feasible for underwater fills. Underwater dikes may be constructed of large
stones and gravel.
8-2.6.1
Pervious Fills. Hydraulically placed pervious fills with less than 10
percent fines will generally be at a relative density of 50 to 60 percent but locally may be
lower. Controlled placement is necessary to avoid silt concentrations. Compaction can
be used to produce relative densities sufficient for foundation support (Table 8-2.1).
Existing uncompacted hydraulic fills of pervious materials in seismic areas are subject
to liquefaction, and densification will be required if important structures are to be
founded on such deposits. Rough estimates of relative density may be obtained using
standard penetration resistance. Undisturbed borings will be required to obtain more
precise evaluation of in situ density and to obtain undisturbed samples for cyclic triaxial
testing, if required. For new fills, the coarsest materials economically available should
be used. Unless special provisions are made for removal of fines, borrow containing
more than 10 percent fines passing the 0.075 Micron (No. 200) sieve should be
avoided, and even then controlled placement is necessary to avoid local silt
concentrations.
8-2.6.2
Fine-Grained Fills. Hydraulically placed overconsolidated clays
excavated by suction dredges produce a fill of clay balls if fines in the wash water are
permitted to run off. The slope of such fills will be extremely flat ranging from about 12
to 16H on 1V. These fills will undergo large immediate consolidation for about the first 6
months until the clay balls distort to close void spaces. Additional settlements for a one-
year period after this time will total about 3 to 5 percent of the fill height.
Maintenance dredgings and hydraulically placed normally consolidated clays will initially
be at water contents between 4 and 5 times the liquid limit. Depending on measures
taken to induce surface drainage, it will take approximately 2 years before a crust is
formed sufficient to support light equipment and the water content of the underlying
materials approaches the liquid limit. Placing 305 mm to 1 m (1 to 3 ft) of additional
cohesionless borrow can be used to improve these areas rapidly so that they can
support surcharge fills, with or without vertical sand drains to accelerate consolidation.
After consolidation, substantial one- or two-story buildings and spread foundations can
be used without objectionable settlement. Use considerable care in applying the
surcharge so that the shear strength of the soil is not exceeded (e.g., use light
equipment).
8-35
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