178 Ground improvement by deep vibratory methods
The results of the second trial have been reported by Slocombe and
Smith (2008), where analysis of in-situ earth pressure cells suggested that
K values of about 1 could be developed against the sheet piles during
vibro compaction. Monitoring procedures were therefore put in place for
the proposed main quay combi wall contract works, comprising largediameter tubular piles with sheet pile infill. It was also concluded that
“by installing stone columns away from the wall the peak earth pressures
acting on the wall are lower than if the columns were installed towards
the wall.”
The works commenced with vibro treatment to the area in front and
behind the proposed piled anchor wall to the proposed tie bars. The ties
were installed, and the area then filled using clean sand to about high tide
level. Vibro stone columns were then constructed, with sand compaction
above, between the buried tie bars at two levels and at about 3.65 m centers. Every column location was very carefully set out to avoid damaging
the ties and, when the tie bars were subsequently exposed for tensioning, it
was confirmed that no damage had been caused.
Again, about 10,000 vibro probes (stone columns/compactions) were
constructed to depths of up to about 20 m on a triangular column grid of
1.32 m. Measurement of the stone take revealed that about 97.8% of the
a c = 0.75 target replacement ratio had been achieved. As an illustration of
the complexity of the works, the depths of treatment and elevations for the
tops of the stone columns resulted in groups of no greater than six adjacent
columns being of identical geometry and hence instruction.
A further phase of this port development was built where the soft clay
and silt was thin and easily removed by advance dredging. The area behind
the tubular pile main quay wall was then filled by placing hydraulic sand fill
in about 15 m depth of water. Vibro compaction was then performed prior
to the installation of tie bars using Keller S340 vibrators to achieve the
specified angle of friction and settlement performance. However, the quay
wall design required that the K value be limited to 0.5. Advance trials were
therefore performed to confirm how far behind the wall both high- and
lower-powered vibrators could be used. These revealed that better control
of the wall movements could be achieved by installing the closest line of
compaction using lower-power vibrators prior to using high-power vibrators further away.
4.7.5 Vibro stone columns for settlement
control behind bridge abutments
In the course of the construction of an approach road to an existing highway
in western Bavaria, the necessary embankments were built well in advance
to allow deformations to occur before any bridge and road building was due
to commence. These embankments typically have a height of up to about
6 m and rest on soft tertiary silts and clays of about 4.5–6.0 m thickness.
The results of the second trial have been reported by Slocombe and
Smith (2008), where analysis of in-situ earth pressure cells suggested that
K values of about 1 could be developed against the sheet piles during
vibro compaction. Monitoring procedures were therefore put in place for
the proposed main quay combi wall contract works, comprising largediameter tubular piles with sheet pile infill. It was also concluded that
“by installing stone columns away from the wall the peak earth pressures
acting on the wall are lower than if the columns were installed towards
the wall.”
The works commenced with vibro treatment to the area in front and
behind the proposed piled anchor wall to the proposed tie bars. The ties
were installed, and the area then filled using clean sand to about high tide
level. Vibro stone columns were then constructed, with sand compaction
above, between the buried tie bars at two levels and at about 3.65 m centers. Every column location was very carefully set out to avoid damaging
the ties and, when the tie bars were subsequently exposed for tensioning, it
was confirmed that no damage had been caused.
Again, about 10,000 vibro probes (stone columns/compactions) were
constructed to depths of up to about 20 m on a triangular column grid of
1.32 m. Measurement of the stone take revealed that about 97.8% of the
a c = 0.75 target replacement ratio had been achieved. As an illustration of
the complexity of the works, the depths of treatment and elevations for the
tops of the stone columns resulted in groups of no greater than six adjacent
columns being of identical geometry and hence instruction.
A further phase of this port development was built where the soft clay
and silt was thin and easily removed by advance dredging. The area behind
the tubular pile main quay wall was then filled by placing hydraulic sand fill
in about 15 m depth of water. Vibro compaction was then performed prior
to the installation of tie bars using Keller S340 vibrators to achieve the
specified angle of friction and settlement performance. However, the quay
wall design required that the K value be limited to 0.5. Advance trials were
therefore performed to confirm how far behind the wall both high- and
lower-powered vibrators could be used. These revealed that better control
of the wall movements could be achieved by installing the closest line of
compaction using lower-power vibrators prior to using high-power vibrators further away.
4.7.5 Vibro stone columns for settlement
control behind bridge abutments
In the course of the construction of an approach road to an existing highway
in western Bavaria, the necessary embankments were built well in advance
to allow deformations to occur before any bridge and road building was due
to commence. These embankments typically have a height of up to about
6 m and rest on soft tertiary silts and clays of about 4.5–6.0 m thickness.