Stone column construction should be closely monitored and design adapted to suit actual soil conditions.

The shear stress is partially taken over by the stone column

Stone columns reinforce and compact the soil. Liquefaction can be prevented.

Design and Quality

The Design

The design comprises the assessment of bearing capacity, settlement, stability and liquefaction potential of a soil after Vibroflotation Ground Improvement.

For Vibro Compaction the design is very straightforward :
Assess the required improved soil strength and stiffness after compaction and calculate otherwise as for any unimproved soil. The only difficulty lies in the evaluation of the degree of compaction possible in different soils. This can only be assessed with extensive experience in local soil conditions and the use of appropriate sounding techniques.
For stone columns the design is much more complex. Stone columns are a ground reinforcement, and their behavior is closely linked to the behavior of the soil surrounding the columns. Specific calculation methods for stone columns have been developed and calibrated using full scale tests, allowing stone columns to be designed with the same confidence as piles but with the advantage of better resistance to earthquake loads.

The graph shows the settlement reduction with different percentages of soil being replaced by stone columns.

Reduction of Settlements

Screenshot of the DC software.

Based on Priebe’s design diagram and assuming 25 % of the soil being replaced by stone columns (factor A/Ac =4) and a friction angle for the column material of 40░, it follows that n = 2.6 , i.e. a 2.6-fold reduction in settlements as compared to the unimproved ground. DC-Software and Vibroflotation Group have jointly developed a Windows« program called DC-Vibro. This software is based on the Priebe method including the depth factor. A free trial version of this software can be downloaded from the Internet on DC-Software’s site With DC-Vibro any arrangement of structures can be input and for every cross section an individual multilayer soil model and varying column diameter over depth profile can be assumed.

A depth factor has been added to take into consideration the positive effect of larger confinement of the stone column with increased depth.

Increase in Slope Stability

Stone columns have a threefold effect in soil :

  1. Compaction of compatible layers.
    Layers of clean sand and gravel and slightly silty sand (up to 20 % fines content) are compacted during the process of stone column installation.
  2. Reinforcement of the soil.
    Column and soil form a reinforced matrix with instantly increased shear resistance and stiffness modulus. The high friction angle of the column material gives an immediate overall increase in shear strength to the composite of soil and column.
  3. Drainage.
    The columns accelerate drainage (consolidation) of the cohesive soil, i.e. the columns work as vertical drains and thereby accelerate reduction of harmful excess pore pressures
The screenshot (finite elements software: plaxis«) shows possible slope failure without stone columns.Safety factor against slope failure:
F = 1.06

After slope stabilisation with stone columns: F = 1.60

Mitigation of Soil Liquefaction

The compaction of the in-situ soil is the most important positive effect of vibrocompaction or stone columns for the prevention of liquefaction in granular soil layers.
In soils with a silt content of over 10 % the most important effect of the stone column is the reduction of the total cyclic shear stress in the soil. The graph left shows that the required sounding resistance can be variable over depth as long as a proper consideration of the varying fines content is made. In order to achieve the same safety factor against liquefaction in an SM-ML a SPT blow count of 11 is sufficient, while in the clean sand a value of around 20 is required