TM 5-818-4/AFM 88-5, Chap. 5
Figure 2-4. Excavation subject to bottom heave.
mized by adequate planning and implementation of
mized by requiring higher backfill densities adjacent
groundwater investigations.
to the structure.
(2) The possibility of hydraulic heave in cohesive
b. Groundwater. Groundwater is an important
material must also be investigated to ensure stability
consideration in planning for construction of subsur-
of the excavation floor. Hydraulic heave may occur
face structures. If seepage of groundwater into the ex-
where an excavation overlies a confined permeable
cavation is not adequately controlled, backfilling
stratum below the groundwater table (fig. 2-4). If the
operations will be extremely difficult. The ground-
upward hydrostatic pressure acting at the bottom of
water level must be lowered sufficiently (at least 2 to 3
the confining layer exceeds the weight of overburden
feet for granular soils and as much as 5 to 10 feet for
between the bottom of the excavation and the confin-
fine-grained soils below the lowest level of backfilling)
ing layer, the bottom of the excavation will rise bodily
so that a firm foundation for backfill can be estab-
even though the design of the dewatering system is
lished. If the level is not lowered, the movement of
adequate for control of groundwater into the excava-
hauling or compaction equipment may pump seepage
tion. To prevent heave, the hydrostatic pressure be-
water through the backfill, or the initial backfill layers
neath the confined stratum must be relieved.
may be difficult to compact because of an unstable
(3) Subsurface structures located in part or wholly
foundation. Since the proper water content of the
below the groundwater table require permanent pro-
backfill is essential for achieving proper compaction,
tection against groundwater seepage. The type of pro-
prevention of groundwater seepage into the excava-
tection may range from simple impermeable barriers
tion during backfilling operations is mandatory.
to complex permanent dewatering systems.
Figure 3-14 of EM 1110-2-1911 shows a method for
(4) Dewatering and groundwater control proce-
dewatering rock foundations.
dures are described in TM 5-818-5/NACFAC
(1) The contractor is generally responsible for the
P-418/AFM 88-5, Chapter 6.
design, installation, and operation of dewatering
equipment. The Corps of Engineers is responsible for
specifying the type of dewatering system and evaluat-
als. Groundwater control is often accomplished by
ing the contractor's proposed dewatering plan. Since
ditches positioned to intercept the flow of groundwa-
the dual responsibility of the contractor and the Corps
ter and filled with permeable granular material. The
relies on a thorough understanding of groundwater
water is generally collected in perforated pipes located
conditions, inadequate dewatering efforts can be mini-
at the bottom of the ditch and pumped to a suitable
2-5
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