152 Ground improvement by deep vibratory methods
next load increment is applied. Load test results need to be theoretically
analyzed to predict as realistically as possible the actual performance of
the stone column-reinforced ground under load. Unfortunately, field tests
are expensive and very often for a true and realistic assessment of the effect
of ground improvement certain elements—for instance the settlement performance of the untreated soil—are missing. It is therefore strongly recommended to carefully design all field measurements to reflect not only the
relevant loading conditions as closely as possible but also the prevailing
geotechnical conditions on-site.
Occasionally, static cone or dynamic penetration tests are proposed
to measure the integrity (density and continuity) of the stone columns
installed, although test results obtained in this way are often questionable
if not misleading and generally difficult to interpret. Very often, the steel
rod of the testing equipment deviates from the vertical and leaves the relatively slender stone column without noticeable indication, thereby giving
room for incorrect interpretations. Scrutiny in checking the printouts of the
data collection system is the better, and most commonly used way to detect
deviations from the specified geometry of the constructed stone columns.
4.5 SUITABLE SOILS AND METHOD LIMITATIONS
In Figure 3.11, the application limits of the vibro compaction method are
basically restricted to areas A and B. Soils with grain size distribution falling into areas C and D cannot be compacted anymore by vibratory motions,
but their characteristics may be improved by the vibro replacement stone
column method, provided that their stiffness does not prevent the depth
vibrator penetrating further.
Degen (1997b) has provided useful comments for the applicability of the
vibro stone column method in these soils using the USCS classification system
(Table 4.3), which particularly take into account the plastic behavior of these
soils. To securely construct a stone column, the soils must have a minimum
strength, expressed by their undrained cohesion, which should not fall below
c u = 5 kPa, since otherwise it cannot provide sufficient containing pressure
for the column. If these soils extend to ground level, their softness generally
requires a competent gravel blanket of about 0.5 m thickness to be placed as a
working platform over the whole site. Into stiffer soils at lower water content
and cohesions of above c u = 50 kPa, even slender, powerful, high-frequency
depth vibrators may not be able to penetrate. Although their characteristics
may not need improvement, softer soils may necessitate the stone columns to
reach deep. In these cases, pre-drilling of the stiffer soils with suitable methods is advisable. Pre-drilling is often also necessary if a hard surface crust has
developed by drying out or through site traffic, or when stone and weathered
rock layers or other obstructions pose penetration problems.
In stiff soils with low water content, closely spaced vibro stone columns
at distances below 3d can cause substantial heave at ground surface during
next load increment is applied. Load test results need to be theoretically
analyzed to predict as realistically as possible the actual performance of
the stone column-reinforced ground under load. Unfortunately, field tests
are expensive and very often for a true and realistic assessment of the effect
of ground improvement certain elements—for instance the settlement performance of the untreated soil—are missing. It is therefore strongly recommended to carefully design all field measurements to reflect not only the
relevant loading conditions as closely as possible but also the prevailing
geotechnical conditions on-site.
Occasionally, static cone or dynamic penetration tests are proposed
to measure the integrity (density and continuity) of the stone columns
installed, although test results obtained in this way are often questionable
if not misleading and generally difficult to interpret. Very often, the steel
rod of the testing equipment deviates from the vertical and leaves the relatively slender stone column without noticeable indication, thereby giving
room for incorrect interpretations. Scrutiny in checking the printouts of the
data collection system is the better, and most commonly used way to detect
deviations from the specified geometry of the constructed stone columns.
4.5 SUITABLE SOILS AND METHOD LIMITATIONS
In Figure 3.11, the application limits of the vibro compaction method are
basically restricted to areas A and B. Soils with grain size distribution falling into areas C and D cannot be compacted anymore by vibratory motions,
but their characteristics may be improved by the vibro replacement stone
column method, provided that their stiffness does not prevent the depth
vibrator penetrating further.
Degen (1997b) has provided useful comments for the applicability of the
vibro stone column method in these soils using the USCS classification system
(Table 4.3), which particularly take into account the plastic behavior of these
soils. To securely construct a stone column, the soils must have a minimum
strength, expressed by their undrained cohesion, which should not fall below
c u = 5 kPa, since otherwise it cannot provide sufficient containing pressure
for the column. If these soils extend to ground level, their softness generally
requires a competent gravel blanket of about 0.5 m thickness to be placed as a
working platform over the whole site. Into stiffer soils at lower water content
and cohesions of above c u = 50 kPa, even slender, powerful, high-frequency
depth vibrators may not be able to penetrate. Although their characteristics
may not need improvement, softer soils may necessitate the stone columns to
reach deep. In these cases, pre-drilling of the stiffer soils with suitable methods is advisable. Pre-drilling is often also necessary if a hard surface crust has
developed by drying out or through site traffic, or when stone and weathered
rock layers or other obstructions pose penetration problems.
In stiff soils with low water content, closely spaced vibro stone columns
at distances below 3d can cause substantial heave at ground surface during