Vibro Stone Columns

Vibro Stone Columns (VSCs) are the most commonly specified ground improvement method, due to the fact that they can be tailored to meet a wide variety of bearing capacity and settlement requirements and provide an economical and sustainable alternative to traditional piling and deep foundations.  

Dry Top Feed Method

In the simplest stone column installation technique - the dry top feed method - the vibroflot penetrates soils by its own mass, the vibratory action of the poker and air flush in order to form a bore hole. Once refusal or design depth is reached the flot is removed and stone fill is introduced into the bore, the 'charge' is typically 500-800mm deep. The flot is re-inserted and 'packs' the stone into the surrounding strata - successive charges of stone are added and compacted bringing the column up to working level. Typically the stone grading is 40-75mm.

Dry Bottom Feed Method

Alternatively, in weaker soils or situations where there is a high water table and the bore hole is liable to collapse between flot insertions, the dry bottom feed method is required. The flot again penetrates by mass, air flush and vibrations, but at design depth the stone is introduced via a hopper into a stone pipe fixed to the side of the vibroflot. The stone, usually of 40mm size, exits the pipe at the tip of the vibroflot and into the bottom of the bore hole. As with the 'top feed' method the stone is then compacted into the surrounding soil by repeated withdrawal and insertion of the poker.

Wet Bottom Feed Method

Water jetting from the tip of the vibroflot is used in order to maintain an open hole where fines and silts are present before successive stone 'charges' are introduced and packed. This process results in larger diameter stone columns with enhanced settlement control characteristics.

VSCs are ideal:

• In granular soils, mixed fills, and cohesive soils, with bearing pressures of between 100-200kN/m2 typically achievable
• To construct columns to depths of up to 12m (depending upon the loading and ground conditions)
• To improve settlement by a factor of 1.5 to 3 on most projects
• To accelerate the dissipation of excess pore water generated by loading pressures
• As an alternative to more expensive piling systems - due to speed of installation and cheaper materials 
• For a vast array of sectors and applications, including housing, road, rail, ports, commercial, retail, industrial, education, medical, wind farms, utilities, leisure and military projects