146 Ground improvement by deep vibratory methods
foreseen for this purpose, its abrasion resistance and its chemical characteristics require special consideration.
When adopting the displacement method of constructing stone columns,
the horizontal stresses increase in the soil adjacent to the stone columns during
their installation more significantly than with the wet replacement method.
This stress increase is permanent and results also in an elevated modulus in
those soils that are sufficiently stiff and do not tend to creep. Kirsch (2004)
has measured this effect in a field test on two groups of 25 stone columns
each in silty clay and sandy silt, respectively. Column diameter was d = 0.8 m
and column depth 6–9 m. Figure 4.25a shows the effective horizontal stresses
after column installation in relation to the initial stresses, expressed as the
ratio of the earth pressure factors at rest K 0 after and before (index i) column
installation, with a maximum of 160% (approx.) at a distance of between 4d
and 5d from column axis. Figure 4.25b shows the modulus increase measured with the Menard pressure meter with a maximum of about 2.5 times
the initial stiffness at distances of between 4 and 6 column diameters.
Figure 4.25 also shows the stress relief due to remolding caused by
dynamic excitations in the vicinity of the columns, which will normally
recover to original levels during reconsolidation of soils. The same paper also
provides the results of a numerical analysis that simulates this installation
effect on a foundation supported by 25 stone columns. Figure 4.26 summarizes the findings and compares the load settlement behavior of this footing,
which is characterized by an area replacement factor (total stone column
area/ footing area) of a c = 0.28, φ c = 45°, and l c = 6 m (λ = 0.5, floating situation), as derived from the numerical analysis with the results of standard
analytical computations according to Priebe (2003) and Goughnour and
Bayuk (1979b) (extended by Kirsch, 2004, to column groups).
It is interesting to see that the numerical method simulating the installation
effect with groups of stone columns ties in well with the Priebe method (a) leading to improvement factors β that are about 50% higher than those calculated
1.6
1.4
1.0
1.8
1.2
2.0
1.5
1.0
0.0
2.0
0.5
3.0
2.5
0
2
4
6
8
1 0
1 2
1 4
1 6
E/E
initial
(−)
K
0 /K
initial (−)
Distance from column axis a/d c (−)
0
2
4
6
8
10
12
Distance from column axis a/d c (−)
Sandy
silt
Sandy silt
Silty
clay
Silty clay
Installation sequence
Installation sequence
(a)
(b)
Figure 4.25 (a) Horizontal stress increase and (b) development of ground stiffness,
during installation of stone columns.
foreseen for this purpose, its abrasion resistance and its chemical characteristics require special consideration.
When adopting the displacement method of constructing stone columns,
the horizontal stresses increase in the soil adjacent to the stone columns during
their installation more significantly than with the wet replacement method.
This stress increase is permanent and results also in an elevated modulus in
those soils that are sufficiently stiff and do not tend to creep. Kirsch (2004)
has measured this effect in a field test on two groups of 25 stone columns
each in silty clay and sandy silt, respectively. Column diameter was d = 0.8 m
and column depth 6–9 m. Figure 4.25a shows the effective horizontal stresses
after column installation in relation to the initial stresses, expressed as the
ratio of the earth pressure factors at rest K 0 after and before (index i) column
installation, with a maximum of 160% (approx.) at a distance of between 4d
and 5d from column axis. Figure 4.25b shows the modulus increase measured with the Menard pressure meter with a maximum of about 2.5 times
the initial stiffness at distances of between 4 and 6 column diameters.
Figure 4.25 also shows the stress relief due to remolding caused by
dynamic excitations in the vicinity of the columns, which will normally
recover to original levels during reconsolidation of soils. The same paper also
provides the results of a numerical analysis that simulates this installation
effect on a foundation supported by 25 stone columns. Figure 4.26 summarizes the findings and compares the load settlement behavior of this footing,
which is characterized by an area replacement factor (total stone column
area/ footing area) of a c = 0.28, φ c = 45°, and l c = 6 m (λ = 0.5, floating situation), as derived from the numerical analysis with the results of standard
analytical computations according to Priebe (2003) and Goughnour and
Bayuk (1979b) (extended by Kirsch, 2004, to column groups).
It is interesting to see that the numerical method simulating the installation
effect with groups of stone columns ties in well with the Priebe method (a) leading to improvement factors β that are about 50% higher than those calculated
1.6
1.4
1.0
1.8
1.2
2.0
1.5
1.0
0.0
2.0
0.5
3.0
2.5
0
2
4
6
8
1 0
1 2
1 4
1 6
E/E
initial
(−)
K
0 /K
initial (−)
Distance from column axis a/d c (−)
0
2
4
6
8
10
12
Distance from column axis a/d c (−)
Sandy
silt
Sandy silt
Silty
clay
Silty clay
Installation sequence
Installation sequence
(a)
(b)
Figure 4.25 (a) Horizontal stress increase and (b) development of ground stiffness,
during installation of stone columns.