In essence, water that comes out of the ground at the toe starts a
process of erosion (if the exit gradient is high enough) that culminates in
the formation of a tunnel-shaped passage (or "pipe") beneath the structure.
When the passage finally works backward to meet the free water, a mixture of
soil and water rushes through the passage, undermining the structure and
flooding the channel below the dam. It has been shown that the danger of a
piping failure due to subsurface erosion increases with decreasing grain
size.
Similar subsurface erosion problems can occur in relieved drydocks,
where water is seeping from a free source to a drainage or filter blanket
beneath the floor or behind the walls. If the filter fails or is defective
and the hydraulic gradients are critical, serious concentrations of flow can
result in large voids and eroded channels.
Potential passageways for the initiation of piping include:
uniformly graded gravel deposits, conglomerate, open joints in bedrock,
cracks caused by earthquakes or crustal movements, open joints in pipelines,
hydraulic fracture, open voids in coarse boulder drains including French
drains, abandoned wellpoint holes, gopher holes, cavities formed in levee
foundations by rotting roots or buried wood, improper backfilling of
pipelines, pipes without antiseepage collars, etc.
Failure by piping requires progressive movement of soil particles
to a free exit surface. It can be controlled by adequately designed filters
or relief blankets. Guidelines for preventing piping beneath dams may be
found in Reference 1, Security from Under Seepage of Masonary Dams on Earth
Foundations, by Lee.
3. DEWATERING. Dewatering methods are discussed in Table 7,
DM-7.2, Chapter 1. Figures 13 and 14 in DM-7.2, Chapter 1 illustrate some
methods of construction dewatering and soil grain size limitations for
different dewatering methods. See NAVFAC P-418 for dewatering and
groundwater control systems.
4. THREE-DIMENSIONAL FLOW. For analysis of flow quantity and drawdown to
individual wells or to any array of wells, see Section 5.
Section 3.
SEEPAGE CONTROL BY CUTOFF
1. METHODS. Procedures for seepage control include cutoff walls
for decreasing the seepage quantity and reducing the exit gradients, and
drainage or relief structures that increase flow quantity but reduce seepage
pressures or cause drawdown in critical areas. See Table 1; Table 7 of
DM-7.2, Chapter 1; and DM-7.3, Chapter 3 (Diaphragm Walls) for methods of
creating partial or complete cutoff. See NAVFAC P-418 for construction
dewatering.
2. SHEETPILING. A driven line of interlocking steel sheeting may be
utilized for a cutoff as a construction expedient or as a part of the
completed structure.
a. Applicability.
The following considerations govern the use of
sheetpiling:
7.1-263
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