SUBGRADE SOILS: Precompression.

a. Preloading.   Earth fill or other material is placed over the site to be stabilized in amounts sufficient
to produce a stress in the soft soil equal to that anticipated from the final structures.  As the time required for consolidation of the soft soil may be long (months to years), varying directly as the square of the layer thickness and inversely as the hydraulic conductivity, preloading alone is likely to be suitable only for stabilizing thin layers and with a long period of time available prior to final development of the site.

b. Surcharge fills.   If the thickness of the fill placed for pre-loading is greater than that required to induce stresses corresponding to structure-induced stresses, the excess fill is termed a surcharge fill.

Although the rate of consolidation is essentially independent of stress increase, the amount of
consolidation varies approximately in proportion to the stress increase.  It follows, therefore, that the preloading fill plus surcharge can cause a given amount of settlement in shorter time than can the preloading fill alone.  Thus, through the use of surcharge fills, the time required for preloading can be reduced significantly.

(1) The required surcharge and loading period can be determined using conventional theories of
consolidation.  Both primary consolidation and most of the secondary compression settlements can be taken
out in advance by surcharge fills.  Secondary compression settlements may be the major part of the total settlement of highly organic deposits or old sanitary landfill sites.

(2) Because the degree of consolidation and applied stress vary with depth, it is necessary to determine if excess pore pressures will remain at any depth after surcharge removal.  If so, further primary consolidation settlement under permanent loadings would occur.  To avoid this occurrence, determine the duration of the surcharge loading required for points most distant from drainage boundaries.

(3) The rate and amount of preload may be controlled by the strength of the underlying soft soil.
Use berms to maintain foundation stability and place fill in stages to permit the soil to gain strength from consolidation.  Predictions of the rates of consolidation strength and strength gain should be checked during fill placement by means of piezometers, borings, laboratory tests, and in situ strength tests.

c. Vertical drains.

(1) The required preloading time for most soft clay deposits more than about 5 to 10 feet thick will be large.  The consolidation time may be reduced by providing a shorter drainage path by installing vertical sand drains.  Sand drains are typically 10 to 15 inches in diameter and are installed at spacings of 5 to 15 feet.

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 precompression has been obtained.

(2) Both displacement and nondisplacement methods have been used for installing sand drains.  Although driven, displacement drains are less expensive than augered or "bored" nondisplacement 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.

d. 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 18,000 to 35,000 square yards (4 to 8 acres) is necessary for economical use of the method.  This method is presently considered experimental in saturated clays.

e. Electroosmosis.   Soil stabilization by electroosmosis 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 nonuniform 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.

Electroosmosis may also be used to accelerate the consolidation under a preload or surcharge fill.  The method is relatively expensive.

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