Related to Vibro-Compaction, Vibro-Replacement is the process of constructing stone columns using a vibratory probe (vibroflot). After the vibroflot penetrates to the desired depth of treatment, stone fill is deposited into the hole from the ground surface or through feed tubes to the tip of the vibroflot as it is withdrawn. Repeated raising and lowering ("plunging") of the vibroflot into the stone during withdrawal results in columns of stone measuring 30-42" (750-1100mm) in diameter. As with vibro-replacement, stone columns results in higher bearing capacities for shallow foundations, a reduction in total and differential settlement, increased safety factors for slope stability, and mitigation of liquefaction potential in seismically active areas. Vibro-replacement is often an economical alternative to deep foundations, especially when considering the added liquefaction protection in seismic areas.
Vibro Replacement is a method of constructing stone columns through fill material and weak soils, to improve their load bearing and settlement characteristics. Vibro Replacement provides an economic and technically sound solution to a wide range of geotechnical and foundation problems.
To construct stone columns, the vibrator is allowed to penetrate to the design depth and the resulting cavity is filled with hard inert stone, free of clay and silt fines. The required interaction between the stone columns and the surrounding soils is developed by the stone infill being introduced and compacted in stages, each charge of stone being thoroughly compacted.
The stone columns and the confining soils form an integrated foundation support system having low compressibility and improved load bearing capacity. In cohesive soils, relatively rapid consolidation is achieved through excess pore water pressures being readily dissipated by the stone columns.
The stone column layout can be arranged to suit varying combinations of load, soil type and required settlement performance. In general, column spacings are in the range of 1.2 to 2.3 meters beneath main load bearing foundations, and up to 3.0 meters beneath floor slabs.
There are two principal methods for constructing Vibro Replacement stone columns:
The conventional Top Feed process
This is where the hole formed by a purpose built base machine with a vibrator mounted on the mast or crane suspended vibrator remains open and the stone infill is added and compacted by the vibrator in stages, assisted by water jetting.
The Bottom Feed process
This is a 'dry' process used to treat unstable soils and/or soils with a high ground water level.
• The Vibroflot penetrates weak soil under action of vibrations and "compressed air" jetting meduim and forms a hole to design, usually to a competent bearing stratum.
• After being held at depth for a short time, the vibroflot is slightly elevated and stone is introduced via a Tremie pipe, attached to the vibroflot.
• The vibroflot is frequently raised and lowered to desired compaction and height of the stone column.
• By adding successive charges of stone and compacting each one, a column of very compact stone is built up to ground level. The reinforcing and compacting action of the stone columns serves to significantly improve the load bearing and settlement characteristics of the ground.