Stress zones near displacement piers: I. Plastic and liquefied behavior

Radial stresses measured with the K-0 stepped blade in saturated soils near Rammed Aggregate Piers indicate temporary liquefaction of soil in the vicinity of the rammer. Drainage and compaction proceed during ramming of succeeding layers of the pier, such that the final distribution of radial effective stresses follows plastic cavity expansion theory. A plastic zone and liquefaction can develop only if the radial stress from ramming exceeds the soil compressive strength. If that condition is met, the transient hydrostatic condition allows the zone to expand with additional ramming. A passive condition occurs where radial stresses cannot be fully contained by the overburden pressure, in which case they approximate Rankine passive pressures. The tangential intermediate principal stress then can be modified by ramming of adjacent piers. Stress-induced cracking in the elastic zone contributes to rapid drainage and is discussed in Part II of this two-part series.