UFC 3-220-01N
15 AUGUST 2005
consolidation strength and strength gain should be checked during fill placement by
means of piezometers, borings, laboratory tests, and in-situ strength tests.
8-7.5.2
Vertical Drains. The required preloading time for most soft clay deposits
more than about 1.5 to 3 m (5 to 10 ft) thick will be large. Providing a shorter drainage
path by installing vertical sand drains may reduce the consolidation time. Sand drains
are typically 254 to 370 mm (10 to 15 in) in diameter and are installed at spacings of 1.5
to 4.5 m (5 to 15 ft). 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-cardboard drains. Provisions should be made to monitor pore
pressures and settlements with time to determine when the desired degree of pre-
compression has been obtained.
Both displacement and non-displacement methods have been used for installing sand
drains. Although driven, displacement drains are less expensive than augured or
"bored" non-displacement drains; they should not be used in sensitive deposits or in
stratified soils that have higher hydraulic conductivity in the horizontal than in the
vertical direction. Vertical drains are not needed in fibrous organic deposits because
the hydraulic conductivity of these materials is high, but they may be required in
underlying soft clays.
8-7.5.3
Dynamic Consolidation (Heavy Tamping). Densification by heavy
tamping has also been reported as an effective means for improving silts and clays, with
preconstruction settlements obtained about 2 to 3 times the predicted construction
settlement. The time required for treatment is less than for surcharge loading with sand
drains. The method is essentially the same as that used for cohesionless soils, except
that more time is required. Several blows are applied at each location followed by a 1-
to 4-week rest period, then the process is repeated. Several cycles may be required. In
each cycle the settlement is immediate, followed by drainage of pore water. Drainage is
facilitated by the radial fissures that form around impact points and by the use of
horizontal and peripheral drains. Because of the necessity for a time lapse between
successive cycles of heavy tamping when treating silts and clays, a minimum treatment
area of 15,000 to 30,000 m2 (18,000 to 35,000 sq yd) is necessary for economical use
of the method. This method is presently considered experimental in saturated clays.
8-7.5.4
Electro osmosis. Soil stabilization by electro osmosis may be effective
and economical under the following conditions: (1) a saturated silt or silty clay soil, (2) a
normally consolidated soil, and (3) a low pore water electrolyte concentration. Gas
generation and drying and fissuring at the electrodes can impair the efficiency of the
method and limit the magnitude of consolidation pressures that develop. Treatment
results in non-uniform changes in properties between electrodes because the induced
consolidation depends on the voltage, and the voltage varies between anode and
cathode. Thus, reversal of electrode polarity may be desirable to achieve a more
uniform stress condition. Electro osmosis may also be used to accelerate the
consolidation under a preload or surcharge fill. The method is relatively expensive.
8-99
Previous Page
