TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
where H - he represents vertical distance from the
acteristics of the subsurface soils, stratification, and
groundwater table; the incompleteness of the data and
groundwater table to the bottom of the excavation.
accuracy of the formulations on which the design is
These criteria for piping are probably only applicable
based; the reduction in the efficiency of the dewater-
to dewatering of sheeted, cellular, or earth-dike coffer-
ing system with time; the frailties of machines and op-
dams founded on granular soils. Once piping develops,
erating personnel; and the criticality of failure of the
erosion of the soil may accelerate rapidly. As the
system with regard to safety, economics, and damage
length of seepage flow is reduced, the hydraulic gradi-
to the project. All of these factors should be considered
ent and seepage velocity increase, with a resultant ac-
in selecting the factor of safety. The less information
celeration in piping and erosion. Piping can be con-
on which the design is based and the more critical the
trolled by lowering the groundwater table in the exca-
dewatering is to the success of the project, the higher
vated slopes or bottom of an excavation, or in either
the required factor of safety. Suggested factors of
less critical situations or emergencies by placement of
safety and design procedures are as follows:
filters over the seepage exit surface to prevent erosion
of the soil but still permit free flow of the seepage. The
(1) Select or determine the design parameters as
gradation of the filter material should be such that the
accurately as possible from existing information.
permeability is high compared with the aquifer, yet
(2) Use applicable design procedures and equa-
fine enough that aquifer materials will not migrate
tions set forth in this manual.
into or through the filter. The filter should be designed
(3) Consider the above enumerated factors in
on the basis of criteria given in paragraph 4-6c. More
selecting a factor of safety.
than one layer of filter material may be required to
(4) Evaluate the experience of the designer.
stabilize a seeping slope or bottom of an excavation in
(5) After having considered steps l-4, the follow-
order to meet these criteria.
ing factors of safety are considered appropriate for
modifying computed design values for flow, draw-
d. Dewatering systems. As in the design of any
down, well spacing, and required "wetted screen
works, the design of a dewatering system should in-
length."
clude a factor of safety to cover the variations in char-
Factor of Safety for Design (FS = 1.0 + (a+ b+c))
Factor to be Added
to 1.0
(a) Design Data
0.25
Poor
0.20
Fair
0.10
Good
0.05
Excellent
Experience of Designer
(b)
0.25
Little
0.20
Some
0.10
Good
0.05
Excellent
(c) Criticality
0.25
Great
0.20
Moderate
0.15
Little
Application of Factor of Safety to Computed
Values or System Design Features
Computed Value
Design Procedure
Remarks
System Design Feature
Increase Q to FS
Pump capacity, header, and dis-
.....................
charge pipe (Q)
Adjust either drawdown or
Drawdown (Ah)
well spacing, but not
Well spacing (a)
Decrease a by 10 percent
both
Wetted screen length hws)
h
.....................
ws
Note: In initially computing drawdown, well spacing, and wetted screen, use flow and other pa-
rameters unadjusted for factor of safety.
4-41
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