Geotechnical talks

Among my various other stone column projects one of the interesting project was installation in order to reduce the Soil liquefaction potential. We were working in the region which was prone to earthquake and comes in zone IV. We need to do stone column of various depths and lots of free boring. It means that we had to leave free bore for certain depth below ground level and below that we had to install actual stone column. This whole assignment was done with wet method.

Project Description:
Indian railways (Our Client) had identified two zones in the plant area which is susceptible to high liquefaction potential. First zone is at the main plant building and second location was at time office. At main plant area cast wheel manufacturing unit was about to set up. In order to mitigate high liquefaction potential at these two regions stone columns are to be installed. Stone column installation substantially increases the density of the soil and simultaneously increases the rate of excess pore water pressure dissipation. Finally stone column helps in increasing the stiffness of the soil which was earlier susceptible to high liquefaction potential.

Technical specification:
Vibro stone column of 750 mm dia is to be installed at 2.8m square spacing c/c. Top 6.7 m from the existing ground level is free boring and below this actual installation work is to be done up to the depth of 12m. Area replacement ratio was 5.6%.Above stone column 300 mm course sand will be laid and above that PCC will be done to start foundation work.

Vibro Replacement methods of construction of stone column through weak soil and filled up soil is basically used in order to increase the load bearing capacity of soil and reduce settlement characteristics. It provides an economical and technically sound solution of geotechnical and foundation problems. Other than taking care of bearing capacity of the soil, stone column is also sometimes used in order to reduce the Soil liquefaction potential of the soil.

For stone column construction vibrator is allowed to penetrate to the designed depth using its self weight with water pressure (Wet Method) or air pressure (dry method). Resulting cavity is filled up with stone which will be free of clay and silt fines. The filled up stones and surrounding soils is properly interacted and compacted in stages.

The stone column spacing and basic design is based on load applied, soil type and requirement of settlement. In general spacing of stone column varies from 1.2 to 3 meters.

The two methods of constructing Vibro replacement stone columns:
Wet method: In this method boring is done using water pressure and simultaneously compaction is carried on.
Dry method: In this method boring is done using air pressure.