The Bottom Feed Vibro-Replacement System represented a major advance in ground improvement technology with its dry construction of high integrity compacted stone columns in weak cohesive soils. Now the bottom feed concept has been extended to construct Vibro Concrete Columns (VCC) through very weak cohesive and organic soils which are unsuitable for the proper applications of conventional Vibro-Replacement techniques. Since being developed in 1976, one application of the VCC technique has been as a ground modification technique beneath embankments or structures with large floor loads.
The VCC technique combines the ground improvement advantages of the vibro systems with the load carrying characteristics of piles. Beneath large area loads such as embankments or structures with significant floor loads, the VCC technique is used to reduce settlement, increase bearing capacity and. if necessary, increase slope stability. This vibro-displacement technique will density granular soils and transfer loads through soft cohesive and organic soils.
The analysis and design of the VCC improved site is essentially the same as would be performed for a pile foundation, except that the improved soil parameters are used. For large area loads the VCC system can be overlain by a granular mat, sometimes reinforced by a geogrid,to evenly distribute the structural loads to the VCC system.
At the ground surface a slight mushrooming of the concrete column occurs which also assists with the load transfer. Therefore, the planned structure can be designed with a standard shallow foundation system or, in the case of embankments, with a uniform bearing pressure. The addition of the granular mat is not needed if a sufficient thickness of surface granular soil is present, which will be densified as a result of the VCC construction and act to distribute the loads to the VCC system.
In general, the Concrete Columns are used without reinforcement. However, if required, single bars for uplift loads or short cages (<20 ft) can be vibrated into the completed columns for lateral load conditions.