UFC 3-220-01N
15 AUGUST 2005
construction of subsurface structures. If seepage of groundwater into the excavation is not
adequately controlled, backfilling operations will be extremely difficult. The groundwater level
must be lowered sufficiently (at least 0.6 0.9 m (2 to 3 ft) for granular soils and as much as
1.6 - 3 m (5 to 10 ft) for fine-grained soils below the lowest level of backfilling) so that a firm
or compaction equipment may pump seepage water through the backfill, or the initial backfill
layers may be difficult to compact because of an unstable foundation. Since the proper water
content of the backfill is essential for achieving proper compaction, prevention of groundwater
seepage into the excavation during backfilling operations is mandatory.
The contractor is generally responsible for the design, installation, and operation
of dewatering equipment. Inadequate dewatering efforts can be minimized by
adequate planning and implementation of groundwater investigations
The possibility of hydraulic heave in cohesive material must also be investigated
to ensure stability of the excavation floor. Hydraulic heave may occur where an
excavation overlies a confined permeable stratum below the groundwater table
8-3.3a. If the upward hydrostatic pressure acting at the bottom of the confining
layer exceeds the weight of overburden between the bottom of the excavation
and the confining layer, the bottom of the excavation will rise bodily even though
the design of the dewatering system is adequate for control of groundwater into
the excavation. To prevent heave, the hydrostatic pressure beneath the confined
stratum must be relieved
Subsurface structures located in part or wholly below the groundwater table
require permanent protection against groundwater seepage. The type of
protection may range from simple impermeable barriers to complex permanent
dewatering systems
Dewatering and groundwater control procedures are described in Chapter 9
8-3.2.3.7
Gradation and Filter Criteria for Drainage Materials. Groundwater control is
often accomplished by ditches positioned to intercept the flow of groundwater and filled with
permeable granular material. The water is generally collected in perforated pipes located at
the bottom of the ditch and pumped to a suitable discharge area. Such drainage systems are
referred to as filter drains. The gradation of the granular filter material is critical for the
functioning of the system. Selection of the proper gradation for the filter material is dependent
quires a graded filter made up of several layers of granular material with each layer having
specific requirements for maximum grain size and gradation. Details on the design of filter
drains are presented in Chapter 9.
8-3.2.3.7.1 Selected Material. If materials at the jobsite do not meet the designed filter
requirements, select material must be purchased from commercial sources and shipped to the
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