Improvement of fine-grained and cohesive soils 177
of friction for slope stability are required. However, the vibrator when penetrating the ground induces lateral displacement of the soil. If the stone
columns are constructed too close to each other, excessive heave can be
generated. This can be overcome for treatment above the water table by
preboring at each vibro stone column location, and in this way area replacements of up to 50%–60% have been achieved.
Even higher replacement ratios were necessary for two port extension
projects in the United Kingdom. Both involved sand fill being placed behind
new quay walls where the presence of deep weak clays and silts required
special design considerations. While the specified short- and long-term settlement criteria could be achieved by soil improvement using vibro stone
columns at about maximum 40% area replacement, the quay wall stability
required the composite soil to be virtually wholly granular in character and
replacement ratios of 80% and 75% had to be performed.
The first project was based on nominal 1.2 m diameter stone columns
on 1.28 m triangular grid, resulting in a replacement ratio of a c = 0.8.
The larger-than-normal columns were achieved with the wet method by
high pressure water flushing and using Keller S300 vibrators. Intensive
investigations by CPT and piezocone had permitted the development of contours for the base and surface of the soft clays and silts across the area.
The scheme was then designed for the vibrator to be lowered at least 1 m
deeper than the contour base, with the stone columns to be performed to
at least 2 m above contour surface of the soft soils. vibro compaction was
then performed to the overlying sands up to ground level.
About 50% of the soil improvement works were performed at lower levels
behind the tubular pile wall between high tides. It was noted that even
when water flushing was used to wash out the required soil volume, the piles
were being displaced when the compaction was being applied to the stone
to achieve the specified high angle of friction. A practical approach and
sequence of construction combined with monitoring of the piles restricted
these movements to within acceptable limits.
This project involved the construction of about 10,000 columns/ compactions
to depths of up to 20 m. Back analysis of the stone consumption revealed that
about 98.7% of the target area replacement ratio had been achieved.
The second project was more complex since there were real stability
concerns and the vibro treatment had to be performed between tie bars
to even more variable soils, the extensive and thick soft clays and silts
sometimes being present to immediately beneath the working level above
high tide. In view of the experience of the first project two advance trials
were performed. The first was to confirm that the required stone column
diameter of 1.2 m and hence replacement ratio of 75% could be achieved.
The second was to examine the effect of the vibro stone column construction using high power vibrators to within 1 m of the sheet pile wall
with external support bund.
of friction for slope stability are required. However, the vibrator when penetrating the ground induces lateral displacement of the soil. If the stone
columns are constructed too close to each other, excessive heave can be
generated. This can be overcome for treatment above the water table by
preboring at each vibro stone column location, and in this way area replacements of up to 50%–60% have been achieved.
Even higher replacement ratios were necessary for two port extension
projects in the United Kingdom. Both involved sand fill being placed behind
new quay walls where the presence of deep weak clays and silts required
special design considerations. While the specified short- and long-term settlement criteria could be achieved by soil improvement using vibro stone
columns at about maximum 40% area replacement, the quay wall stability
required the composite soil to be virtually wholly granular in character and
replacement ratios of 80% and 75% had to be performed.
The first project was based on nominal 1.2 m diameter stone columns
on 1.28 m triangular grid, resulting in a replacement ratio of a c = 0.8.
The larger-than-normal columns were achieved with the wet method by
high pressure water flushing and using Keller S300 vibrators. Intensive
investigations by CPT and piezocone had permitted the development of contours for the base and surface of the soft clays and silts across the area.
The scheme was then designed for the vibrator to be lowered at least 1 m
deeper than the contour base, with the stone columns to be performed to
at least 2 m above contour surface of the soft soils. vibro compaction was
then performed to the overlying sands up to ground level.
About 50% of the soil improvement works were performed at lower levels
behind the tubular pile wall between high tides. It was noted that even
when water flushing was used to wash out the required soil volume, the piles
were being displaced when the compaction was being applied to the stone
to achieve the specified high angle of friction. A practical approach and
sequence of construction combined with monitoring of the piles restricted
these movements to within acceptable limits.
This project involved the construction of about 10,000 columns/ compactions
to depths of up to 20 m. Back analysis of the stone consumption revealed that
about 98.7% of the target area replacement ratio had been achieved.
The second project was more complex since there were real stability
concerns and the vibro treatment had to be performed between tie bars
to even more variable soils, the extensive and thick soft clays and silts
sometimes being present to immediately beneath the working level above
high tide. In view of the experience of the first project two advance trials
were performed. The first was to confirm that the required stone column
diameter of 1.2 m and hence replacement ratio of 75% could be achieved.
The second was to examine the effect of the vibro stone column construction using high power vibrators to within 1 m of the sheet pile wall
with external support bund.