(4) An outside open water source may be blanketed with fines or
bentonite dumped through water or placed as a slurry. See Table 2.
Evaluate the effectiveness of these measures by flow net analysis.
3. GROUTED CUTOFF. For grouting methods and materials, see DM-7.3, Chapter
2. Complete grouted cutoff is frequently difficult and costly to attain.
Success of grouting requires careful evaluation of pervious strata for
selection of appropriate grout mix and procedures. These techniques, in
combination with other cutoff or drainage methods, are particularly useful
as a construction expedient to control local seepage.
4. IMPERVIOUS SOIL BARRIERS. Backfilling of cutoff trenches with selected
impervious material and placing impervious fills for embankment cores are
routine procedures for earth dams.
a. Compacted Impervious Fill. Properly constructed, these sections
permit negligible seepage compared to the flow through foundations or
abutments. Pervious layers or lenses in the compacted cutoff must be
avoided by blending of borrow materials and scarifying to bond successive
b. Mixed-in-Place Piles. Overlapping mixed-in-place piles of cement
and natural soil forms a cofferdam with some shear resistance around an
c. Slurry-filled Trench. Concurrent excavation of a straight sided
trench and backfilling with a slurry of bentonite with natural soil is done.
Alternatively, a cement bentonite mix can be used in a narrower trench where
coarser gravel occurs. In certain cases, tremie concrete may be placed,
working upward from the base of a slurry-filled trench, to form a permanent
peripheral wall (Diaphragm Wall, see DM-7.3, Chapter 3).
See Section 2, DM-7.3, Chapter 2, and Table 7, DM-7.2,
1. FILTERS. If water flows from a silt to a gravel, the silt will
wash into the interstices of the gravel. This could lead to the following,
which must be avoided:
(1) The loss of silt may continue, causing creation of a cavity.
(2) The silt may clog the gravel, stopping flow, and causing
hydrostatic pressure buildup.
The purpose of filters is to allow water to pass freely across the
interface (filter must be coarse enough to avoid head loss) but still be
sufficiently fine to prevent the migration of fines. The filter particles
must be durable, e.g., certain crushed limestones may dissolve. Filter
requirements apply to all permanent subdrainage structures in contact with
soil, including wells. See Figure 4 for protective filter design criteria.