15 AUGUST 2005
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, pests, and sanitary landfills, virtually
any inorganic soil can be processed and treated so as to form and acceptable structural
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.
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.
Stabilization Using Fills. 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.
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 transported short
distances economically. Clamshells are 19 to 38 mm (, to 1 in) in diameter,
whereas, oyster shells, which are coarser and more elongated, are 50 to 100 mm (2 to
4 in) in size. When dumped over soft ground, the shells interlock; if there are fines 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 3 kPa (63 lbs per sq
ft); after construction, it is about 4.5 kPa (95 lbs per sq ft). Shell embankments "float"
over very soft ground; whereas, conventional fills would sink out of sight. An
approximately 1.5 m (5 ft) 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.