Vibro compaction of granular soils 47
between 30 and 90 s depending on the characteristics of the soil, the
required degree of compaction, and the characteristics of the vibrator used.
Whatever the individual operational modes of the specialist contractors
may be, it should be remembered that effective compaction requires time
and that the unwanted lowering of the machine during the compaction
stage may give rise to increased power demand and to a false indication of
satisfactory compaction. Densification of the sand surrounding the vibrator causes a reduction of the pore volume, which must be compensated
either by extracting it from the material to be compacted, or by introducing imported suitable backfill into the bore from the surface. When no
imported sand fill is used and the sand surface is allowed to settle as a result
of densification, the surface settlement may reach up to 15% of the compaction depth depending on the original density and the degree of compaction
achieved. Independent of the operational mode, an effective regulation of
the water flow in the annulus is important. Falsely arranged side jets and
badly controlled jetting volumes often lead to the wrong assumption that
only by the addition of coarse backfill, which tends to sink more rapidly,
can too slow compaction be avoided.
When the cycle of lowering the vibrator to design depth and subsequent
stage-by-stage compaction is completed, a new compaction probe can be
commenced by repeating the procedure described. The compaction process
so carried out leaves behind a compacted sand column of enhanced density,
with a dense core followed by material of decreasing density as the distance
from the center increases. The diameter of such a column ranges typically
between 3 and 6 m depending on the characteristics of vibrator and soil. By
arranging the individual compaction points in a grid-like fashion, almost
any surface configuration can be treated to the desired depth, both from
the dry and offshore.
The distances between compaction points in an equilateral triangular
pattern are normally 2.5–5.0 m, and this can cover areas of up to 15 m 2 for
each compaction probe. In this way, up to 20,000 m 3 of sand of a favorable
grading can be compacted within a 10-hour shift with a single vibrator
operation. When twin vibrators can be employed, the production output can almost be doubled, and when special circumstances dictate (for
instance large volumes to be compacted offshore with difficult access to
site), a multitude of depth vibrators can be used simultaneously safeguarding accuracy of the compaction probe location and the mode of operation
(see Figure 3.8).
Typically, for large compaction volumes, the depth vibrators would be
suspended from a heavy crawler crane, which also carries an electric generator at its rear. For economic reasons it may be advantageous to operate more
than one vibrator from one base machine. In this way, up to six vibrators
have been used simultaneously, carried by a specially designed frame for
compaction over water (see Figure 3.8b). However, this kind of operation
is restricted for practical reasons to a compaction depth of about 25 m.
between 30 and 90 s depending on the characteristics of the soil, the
required degree of compaction, and the characteristics of the vibrator used.
Whatever the individual operational modes of the specialist contractors
may be, it should be remembered that effective compaction requires time
and that the unwanted lowering of the machine during the compaction
stage may give rise to increased power demand and to a false indication of
satisfactory compaction. Densification of the sand surrounding the vibrator causes a reduction of the pore volume, which must be compensated
either by extracting it from the material to be compacted, or by introducing imported suitable backfill into the bore from the surface. When no
imported sand fill is used and the sand surface is allowed to settle as a result
of densification, the surface settlement may reach up to 15% of the compaction depth depending on the original density and the degree of compaction
achieved. Independent of the operational mode, an effective regulation of
the water flow in the annulus is important. Falsely arranged side jets and
badly controlled jetting volumes often lead to the wrong assumption that
only by the addition of coarse backfill, which tends to sink more rapidly,
can too slow compaction be avoided.
When the cycle of lowering the vibrator to design depth and subsequent
stage-by-stage compaction is completed, a new compaction probe can be
commenced by repeating the procedure described. The compaction process
so carried out leaves behind a compacted sand column of enhanced density,
with a dense core followed by material of decreasing density as the distance
from the center increases. The diameter of such a column ranges typically
between 3 and 6 m depending on the characteristics of vibrator and soil. By
arranging the individual compaction points in a grid-like fashion, almost
any surface configuration can be treated to the desired depth, both from
the dry and offshore.
The distances between compaction points in an equilateral triangular
pattern are normally 2.5–5.0 m, and this can cover areas of up to 15 m 2 for
each compaction probe. In this way, up to 20,000 m 3 of sand of a favorable
grading can be compacted within a 10-hour shift with a single vibrator
operation. When twin vibrators can be employed, the production output can almost be doubled, and when special circumstances dictate (for
instance large volumes to be compacted offshore with difficult access to
site), a multitude of depth vibrators can be used simultaneously safeguarding accuracy of the compaction probe location and the mode of operation
(see Figure 3.8).
Typically, for large compaction volumes, the depth vibrators would be
suspended from a heavy crawler crane, which also carries an electric generator at its rear. For economic reasons it may be advantageous to operate more
than one vibrator from one base machine. In this way, up to six vibrators
have been used simultaneously, carried by a specially designed frame for
compaction over water (see Figure 3.8b). However, this kind of operation
is restricted for practical reasons to a compaction depth of about 25 m.