148 Ground improvement by deep vibratory methods
beyond λ = 0.75. If, however, the thickness of the soft soil is very large in comparison with the dimensions of the footing, stone column lengths in excess of
three times the footing width no longer contribute to the settlement reduction.
Any chosen design must finally be checked with regard to its bearing capacity
according to Section 4.3.3.
Generally, a load distribution layer consisting of gravel or crushed rock
with a thickness of at least 300 mm is placed over the vibro stone column
heads. This layer also serves to safeguard the drainage capability of the
stone columns in water-bearing soils when loaded, but requires effective
Competent layer
Soft layer
B
H
h
T
l
a c = A c /A F and φ c
A F
Competent
layer at depth T
approachable?
λ = (l/T) > 0.75
possible?
λ = 1 is to be
approached
λ > 0.5 to be approached,
but in any case l > B
Maximize area ratio,
preferably with
a c > 0.3
B < l < 3 ·B
Yes
No
No
φ′ c > 40° by selecting
adequate and well
compacted column
material
Optimize load
transfer layer
Maximize improvement factor
Optimize number
of columns
for economical design
High footing stiffness
supports
homogenization
of column loading
Optimize column layout
Yes
Figure 4.27 Flowchart for the design of a group of stone columns.