. Cast-in-place concrete piles may be usedwhen conditions are favorable. They are made bypouring concrete into a tapered hole or cylindricalform previously driven into the ground or into a holein the ground from which a driven mandrel has beenwithdrawn.The left-in-place form may be a steelshell heavy enough to be driven without a mandrel, orit may be a steel form designed for driving with amandrel that is removed on completion of driving (fig.12-77).20. Composite piles. Composite piles are formed ofone material in the lower section and another materialin the upper section (fig. 12-78). A composite pile thatis constructed of wood and concrete is used to supportloads of 20 to 30 tons.A composite pile that isconstructed of steel and concrete is used to support loadsup to 50 tons. As shown in figure 12-78, the first sectionof wood or steel is driven first, then a mandrel and steelcasing are driven on top of the first section. The mandrelis removed and the casing is filled with concrete.21. Sheet piles.Sheet piles are special shapes ofinterlocking piles that are made of steel, wood, or formedconcrete which are used to forma continuous wall to resisthorizontal pressures, resulting from earth or water loads.Figure 12-77.—Cast-in-place concrete piles.12-54
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