SUBGRADE SOILS: Miscellaneous Methods.

a. Remove and replace.  Removal of poor soil and replacement with the same soil treated by compaction, with or without admixtures, or by a higher quality material offer an excellent opportunity for producing high-strength, relatively incompressible, uniform foundation conditions.  The cost of removal and replacement of thick deposits is high because of the need for excavation and materials handling, processing, and recompaction.  Occasionally, an expensive dewatering system also may be required.  Excluding highly organic soils, peats and sanitary landfills, virtually any inorganic soil can be processed and treated so as to form an acceptable structural fill material.

b. Lime treatment.   This treatment of plastic fine-grained soils can produce high-strength, durable
materials.  Lime treatment levels of 3 to 8 percent by weight of dry soil are typical.

c. Portland cement.  With treatment levels of 3 to 10 percent by dry weight, portland cement is particularly well suited for low-plasticity soils and sand soils.

d. Stabilization using fills.

(1) At sites underlain by soft, compressible soils and where filling is required or possible to establish the final ground elevation, load-bearing structural fills can be used to distribute the stresses from light structures.

Compacted sands and gravels are well suited for this application as are also fly ash, bottom ash, slag, and various lightweight aggregates, such as expended shale, clam and oyster shell, and incinerator ash. Admixture stabilizers may be incorporated in these materials to increase their strength and stiffness.

(2) Clam and oyster shells as a structural fill material over soft marsh deposits represent a new development.  The large deposits of clam and oyster or reef shells that are available in the Gulf States coastal areas can be mined and tasportd short distances economically.  Clam shells are 1/4, to 1/2 inch in diameter; whereas, oyster shells, which are coarser and more elongated, are 2 to 4 inches in size.  When dumped over soft ground, the shells interlock; if there are finmes and water present, some cementation develops owing to the high calcium carbonate (>90 percent) content.  In the loose state, the shell unit weight is about 63 pounds per square foot; after construction, it is about 95 pounds per square foot.  Shell embenkments "float" over very soft ground; whereas, conventional fills would sink out of sight.  About a 5-foot-thick layer is required to be placed in a single lift.  The only compaction used is from the top of the lift, so the upper several inches are more tightly knit and denser than the rest of the layer.

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