TM 5-818-1 / AFM 88-3, Chap. 7
which each of these grout types is effective is shown in
in diameter and are installed at spacings of 5 to 15 feet.
figure 16-6.
A sand blanket or a collector drain system is placed over
the surface to facilitate drainage. Other types of drains
available are special cardboard or combination plastic-
16-5.
Precompression.
cardboard drains. Provisions should be made to monitor
a. Preloading. Earth fill or other material is
pore pressures and settlements with time to determine
placed over the site to be stabilized in amounts sufficient
when the desired degree of precompression has been
to produce a stress in the soft soil equal to that
obtained.
anticipated from the final structures.
As the time
(2) Both
displacement
and
required for consolidation of the soft soil may be long
nondisplacement methods have been used for installing
(months to years), varying directly as the square of the
sand drains. Although driven, displacement drains are
layer thickness and inversely as the hydraulic
less expensive than augered or "bored" nondisplacement
conductivity, preloading alone is likely to be suitable only
drains; they should not be used in sensitive deposits or in
for stabilizing thin layers and with a long period of time
stratified soils that have higher hydraulic conductivity in
available prior to final development of the site.
the horizontal than in the vertical direction. Vertical
b. Surcharge fills. If the thickness of the fill
drains are not needed in fibrous organic deposits
placed for pre-loading is greater than that required to
because the hydraulic conductivity of these materials is
induce stresses corresponding to structure-induced
high, but they may be required in underlying soft clays.
stresses, the excess fill is termed a surcharge fill.
d. Dynamic consolidation (heavy tamping).
Although the rate of consolidation is essentially
Densification by heavy tamping has also been reported
independent of stress increase, the amount of
as an effective means for improving silts and clays, with
consolidation varies approximately in proportion to the
preconstruction settlements obtained about 2 to 3 times
stress increase. It follows, therefore, that the preloading
the predicted construction settlement. The time required
fill plus surcharge can cause a given amount of
for treatment is less than for surcharge loading with sand
settlement in shorter time than can the preloading fill
drains. The method is essentially the same as that used
alone. Thus, through the use of surcharge fills, the time
for cohesionless soils, except that more time is required.
required for preloading can be reduced significantly.
Several blows are applied at each location followed by a
(1) The required surcharge and loading
1- to 4-week rest period, then the process is repeated.
period can be determined using conventional theories of
Several cycles may be required. In each cycle the
consolidation. Both primary consolidation and most of
settlement is immediate, followed by drainage of pore
the secondary compression settlements can be taken
water. Drainage is facilitated by the radial fissures that
out in advance by surcharge fills.
Secondary
form around impact points and by the use of horizontal
compression settlements may be the major part of the
and peripheral drains. Because of the necessity for a
total settlement of highly organic deposits or old sanitary
time lapse between successive cycles of heavy tamping
landfill sites.
when treating silts and clays, a minimum treatment area
(2) Because the degree of consolidation
of 18,000 to 35,000 square yards (4 to 8 acres) is
and applied stress vary with depth, it is necessary to
necessary for economical use of the method. This
determine if excess pore pressures will remain at any
method is presently considered experimental in
depth after surcharge removal. If so, further primary
saturated clays.
consolidation settlement under permanent loadings
e. Electroosmosis.
Soil stabilization by
would occur. To avoid this occurrence, determine the
electroosmosis may be effective and economical under
duration of the surcharge loading required for points
the following conditions: (1) a saturated silt or silty clay
most distant from drainage boundaries.
soil, (2) a normally consolidated soil, and (3) a low pore
(3) The rate and amount of preload may
water electrolyte concentration. Gas generation and
be controlled by the strength of the underlying soft soil.
drying and fissuring at the electrodes can impair the
Use berms to maintain foundation stability and place fill
efficiency of the method and limit the magnitude of
in stages to permit the soil to gain strength from
consolidation pressures that develop. Treatment results
consolidation. Predictions of the rates of consolidation
in nonuniform changes in properties between electrodes
strength and strength gain should be checked during fill
because the induced consolidation depends on the
placement by means of piezometers, borings, laboratory
voltage, and the voltage varies between anode and
tests, and in situ strength tests.
cathode. Thus, reversal of electrode polarity may be
c. Vertical drains.
desirable to achieve a more uniform stress condition.
(1) The required preloading time for most
Electroosmosis may also be used to accelerate the
soft clay deposits more than about 5 to 10 feet thick will
consolidation under a preload or surcharge fill. The
be large. The consolidation time may be reduced by
method is relatively expensive.
providing a shorter drainage path by installing vertical
sand drains. Sand drains are typically 10 to 15 inches
16-14
Previous Page
