TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
1 pound per square inch per foot of depth of the injec-
where the stability of the bottom of the excavation is
tion.
endangered by artesian pressures in an underlying
(a) Portland cement is best adapted to filling
aquifer. Complete relief of the artesian pressures to a
voids and fractures in rock and has the advantage of
level below the bottom of the excavation is not always
required depending on the thickness, uniformity, and
appreciably strengthening the formation, but it is inef-
permeability of the materials. For uniform tight shales
fective in penetrating the voids of sand with an effec-
or clays, an upward seepage gradient i as high as 0.5 to
tive grain size of 1 millimetre or less. To overcome this
0.6 may be safe, but clay silts or silts generally require
deficiency, chemical grouts have been developed that
lowering the groundwater 5 to 10 feet below the bot-
have nearly the viscosity of water, when mixed and in-
tom of the excavation to provide a dry, stable work
jected, and later react to form a gel which seals the for-
area.
mation Chemical grouts can be injected effectively in-
to soils with an effective grain size D10 that is less than
(2) The flow to a pressure relief system is artesian;
0.1 millimetre. Cement grout normally requires a day
therefore, such a system may be designed or evaluated
or two to hydrate and set, whereas chemical grout can
on the basis of the methods presented in paragraphs
be mixed to gel in a few minutes.
4-2 and 4-3 for artesian flow. The penetration of the
(b) Cement grouts are commonly mixed at wa-
wells or wellpoints need be no more than that required
ter-cement ratios of from 5:1 to 10:1 depending on the
to achieve the required drawdown to keep the flow to
grain size of the soils. However, the use of a high wa-
the system a minimum. If the aquifer is stratified and
ter-cement ratio will result in greater shrinkage of the
anisotropic, the penetration required should be deter-
cement, so it is desirable to use as little water as practi-
mined by computing the effective penetration into the
cal. Bentonite and screened fly ash may be added to a
transformed aquifer as described in appendix E. Ex-
cement grout to both improve the workability and re-
amples of the design of a wellpoint system and a deep-
duce the shrinkage of the cement. The setting time of a
well system for relieving pressure beneath an open ex-
cement grout can be accelerated by using a 1:1 mixture
cavation are presented in figures D-6 and D-7.
of gypsum-base plaster and cement or by adding not
more than 3 percent calcium chloride. High-early-
g. Cutoffs. Seepage cutoffs are used as barriers to
strength cement can be used when a short set time is
flow in highly permeable aquifers in which the quanti-
required.
ty of seepage would be too great to handle with deep-
well or wellpoint dewatering systems alone, or when
(c) Chemical grouts, both liquid and powder-
based, are diluted with water for injection, with the
pumping costs would be large and a cutoff is more eco-
nomical. The cutoff should be located far enough back
proportions of the chemicals and admixtures varied to
of the excavation slope to ensure that the hydrostatic
control the gel time.
pressure behind the cutoff does not endanger the sta-
(d) Injection patterns and techniques vary with
grout materials, character of the formation, and geom-
bility of the slope. If possible, a cutoff should pene-
trate several feet into an underlying impermeable stra-
etry of the grout curtains. (Grout holes are generally
tum. However, the depth of the aquifer or other condi-
spaced on 2- to 5-foot centers.) Grout curtains may be
tions may preclude full penetration of the cutoff, in
formed by successively regrouting an area at reduced
spacings until the curtain becomes tight. Grouting is
which case seepage beneath the cutoff must be consid-
ered. Figure 4-36 illustrates the effectiveness of a par-
usually done from the top of the formation downward.
tial cutoff for various penetrations into an aquifer.
(e) The most perplexing problem connected with
The figure also shows the soils to be homogeneous and
grouting is the uncertainty about continuity and effec-
isotropic with respect to permeability. If, however, the
tiveness of the seal. Grout injected under pressure will
soils are stratified or anisotropic with respect to per-
move in the direction of least resistance. If, for exam-
meability, they must be transformed into an isotropic
ple, a sand deposit contains a layer of gravel, the grav-
section and the equivalent penetration computed by
el may take all the grout injected while the sand re-
the method given in appendix E before the curves
mains untreated. Injection until the grout take dimin-
shown in the figure are applicable.
ishes is not an entirely satisfactory measure of the suc-
(1) Cement and chemical grout curtain. Pressure
cess of a grouting operation. The grout may block the
injection of grout into a soil or rock may be used to re-
injection hole or penetrate the formation only a short
duce the permeability of the formation in a zone and
distance, resulting in a discontinuous and ineffective
seal off the flow of water. The purpose of the injection
grout curtain. The success of a grouting operation is
of grout is to fill the void spaces with cement or chemi-
difficult to evaluate before the curtain is complete and
cals and thus form a solid mass through which no wa-
in operation, and a considerable construction delay can
ter can flow. Portland cement, fly ash, bentonite, and
result if the grout curtain is not effective. A single row
sodium silicate are commonly used as grout materials.
of grout holes is relatively ineffective for cutoff pur-
Generally, grouting pressures should not exceed about
poses compared with an effectiveness of 2 or 3 times
4-45
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