120 Ground improvement by deep vibratory methods
cell concept can be applied and where the stress concentration n can be
estimated with sufficient accuracy from Equation 4.12. With decreasing
numbers of stone columns in large groups the accuracy of the method
decreases.
The main effect of a ground improvement measure is settlement reduction
expressed by the improvement factor β as the ratio of the settlement without
stone columns and the settlement after ground improvement. The different
behavior of a stone column within a group is governed by its relative position within it and influencing its overall performance. With increasing numbers of columns, group behavior approaches that of the infinite column grid.
It is difficult to define the number of stone columns that no longer behave
as a column group since other factors such as improvement depth, size, and
rigidity of the foundation have an influence. For practical reasons, more
than 50 stone columns regularly arranged below a foundation slab may be
addressed as an infinite pattern of columns when the ratio of foundation
width to column depth is at least 3.
The effect of stone columns on the ground to be improved is manifold.
The higher stiffness of the column material in relation to that of the soil
leads to load concentrations on the columns, thereby reducing the settlement. Since the column material is considerably more permeable than the
soil, stone columns act as drains when constructed and loaded in waterbearing soils, reducing consolidation time substantially. The fill material also
has a much higher shear strength compared to the original soil; therefore,
also increasing its bearing capacity. The improvement effect is increased by
the load concentration on the columns also leading to increased stability
of structures when supported by vibro stone columns. In saturated soils,
the existence of stone columns reduces the liquefaction potential of silt and
sand deposits during dynamic loading or earthquakes. The combination of
densification, drainage, and increased shear strength prevents total loss of
strength during a seismic event. To summarize, the main effects of the vibro
replacement method are
• Reduction of settlements.
• Reduction of consolidation time.
• Increase of bearing capacity.
• Reduction of liquefaction potential.
In parallel to the large variety of applications of vibro stone columns, and
as a result of the technical development and better understanding of ground
improvement in general, various computational and design methods have been
proposed. These can be distinguished from each other by the computation
approach, by the column geometry that is being considered, and by the design
objective, which could be bearing capacity, settlement reduction or acceleration,
and earthquake risk mitigation. Some of these methods are exclusively based
on empirical findings, while others use analytical approaches of the cylindrical
cell concept can be applied and where the stress concentration n can be
estimated with sufficient accuracy from Equation 4.12. With decreasing
numbers of stone columns in large groups the accuracy of the method
decreases.
The main effect of a ground improvement measure is settlement reduction
expressed by the improvement factor β as the ratio of the settlement without
stone columns and the settlement after ground improvement. The different
behavior of a stone column within a group is governed by its relative position within it and influencing its overall performance. With increasing numbers of columns, group behavior approaches that of the infinite column grid.
It is difficult to define the number of stone columns that no longer behave
as a column group since other factors such as improvement depth, size, and
rigidity of the foundation have an influence. For practical reasons, more
than 50 stone columns regularly arranged below a foundation slab may be
addressed as an infinite pattern of columns when the ratio of foundation
width to column depth is at least 3.
The effect of stone columns on the ground to be improved is manifold.
The higher stiffness of the column material in relation to that of the soil
leads to load concentrations on the columns, thereby reducing the settlement. Since the column material is considerably more permeable than the
soil, stone columns act as drains when constructed and loaded in waterbearing soils, reducing consolidation time substantially. The fill material also
has a much higher shear strength compared to the original soil; therefore,
also increasing its bearing capacity. The improvement effect is increased by
the load concentration on the columns also leading to increased stability
of structures when supported by vibro stone columns. In saturated soils,
the existence of stone columns reduces the liquefaction potential of silt and
sand deposits during dynamic loading or earthquakes. The combination of
densification, drainage, and increased shear strength prevents total loss of
strength during a seismic event. To summarize, the main effects of the vibro
replacement method are
• Reduction of settlements.
• Reduction of consolidation time.
• Increase of bearing capacity.
• Reduction of liquefaction potential.
In parallel to the large variety of applications of vibro stone columns, and
as a result of the technical development and better understanding of ground
improvement in general, various computational and design methods have been
proposed. These can be distinguished from each other by the computation
approach, by the column geometry that is being considered, and by the design
objective, which could be bearing capacity, settlement reduction or acceleration,
and earthquake risk mitigation. Some of these methods are exclusively based
on empirical findings, while others use analytical approaches of the cylindrical