Vibro compaction of granular soils 83
sand is generally not well-compacted enough by vibro compaction requiring
either removal or subsequent surface densification by standard methods.
Very deep compactions, well in excess of 50 m, are known to have been
executed successfully in exceptional cases, either to mitigate earthquake
risks, to improve stability hazards of slopes, or in other similar cases.
In these instances, besides very heavy cranes, considerable experience is
required in safely operating the extended depth vibrators. Although deep
vibratory compaction is a very versatile method, in-situ trial compactions
may be advisable when unknown territory is entered.
The need to compact newly dredged sand in reclamation areas for the
safe foundation of quay walls, berths and similar structures often require
densification of the foundation soils through deep water. This can be
achieved from floating barges and pontoons or, in shallower water, from
special platforms. In offshore applications, the critical problem will always
be the accurate setting of the compaction locations, which can today be
mastered by satellite-supported positioning and inclinometer measurements. As a result of the complex surveying problems, the use of multiple
depth vibrators (more than four units have been used) may be advisable. It
is only heavy seas or swells and strong water currents that may prevent a
safe, secure performance of the vibro compaction process until a friendlier
environment prevails again.
Deficiencies in the material grading can also affect penetration of the
vibrator and successful compaction of sandy soils. A layer of cobbles or
gap-graded material of fine uniform sand with floating cobbles may prove
difficult for the compaction process; the cobbles can accumulate in front of
the vibrator point and obstruct further penetration. The use of air as the
flushing medium or changing the vibrator frequency can help to alleviate
the situation.
However, particle size distribution, in-situ density measurement, and
permeability are not always sufficient to define the suitability for vibratory deep compaction, nor are the in-situ measurements always easy to
interpret. The mineral composition and specific gravity of the sand deposit
can greatly influence the result of indirect density measurements. In this
context, and in contrast to silica sand, in carbonate sands containing considerable amounts of shell debris the evaluation of CPT results cannot be
translated easily into relative densities.
Various researchers have found that even relatively small percentages of
shell debris (10%–20%) (Vesic, 1965; Cudmani, 2001) in silica sand have
considerable influences on the CPT point resistance at the same density.
Bellotti and Jamiolkowski (1991) found a linear relationship between the
q c (silica)/q c (shell) ratio and the relative density D r as given in Equation 3.31.
Ratios between 1.5 and 3 have been reported in special publications.
q
q
D
c
c
r
(silica)
(shells)
1 0.015(
20)
= +
−
(3.31)
sand is generally not well-compacted enough by vibro compaction requiring
either removal or subsequent surface densification by standard methods.
Very deep compactions, well in excess of 50 m, are known to have been
executed successfully in exceptional cases, either to mitigate earthquake
risks, to improve stability hazards of slopes, or in other similar cases.
In these instances, besides very heavy cranes, considerable experience is
required in safely operating the extended depth vibrators. Although deep
vibratory compaction is a very versatile method, in-situ trial compactions
may be advisable when unknown territory is entered.
The need to compact newly dredged sand in reclamation areas for the
safe foundation of quay walls, berths and similar structures often require
densification of the foundation soils through deep water. This can be
achieved from floating barges and pontoons or, in shallower water, from
special platforms. In offshore applications, the critical problem will always
be the accurate setting of the compaction locations, which can today be
mastered by satellite-supported positioning and inclinometer measurements. As a result of the complex surveying problems, the use of multiple
depth vibrators (more than four units have been used) may be advisable. It
is only heavy seas or swells and strong water currents that may prevent a
safe, secure performance of the vibro compaction process until a friendlier
environment prevails again.
Deficiencies in the material grading can also affect penetration of the
vibrator and successful compaction of sandy soils. A layer of cobbles or
gap-graded material of fine uniform sand with floating cobbles may prove
difficult for the compaction process; the cobbles can accumulate in front of
the vibrator point and obstruct further penetration. The use of air as the
flushing medium or changing the vibrator frequency can help to alleviate
the situation.
However, particle size distribution, in-situ density measurement, and
permeability are not always sufficient to define the suitability for vibratory deep compaction, nor are the in-situ measurements always easy to
interpret. The mineral composition and specific gravity of the sand deposit
can greatly influence the result of indirect density measurements. In this
context, and in contrast to silica sand, in carbonate sands containing considerable amounts of shell debris the evaluation of CPT results cannot be
translated easily into relative densities.
Various researchers have found that even relatively small percentages of
shell debris (10%–20%) (Vesic, 1965; Cudmani, 2001) in silica sand have
considerable influences on the CPT point resistance at the same density.
Bellotti and Jamiolkowski (1991) found a linear relationship between the
q c (silica)/q c (shell) ratio and the relative density D r as given in Equation 3.31.
Ratios between 1.5 and 3 have been reported in special publications.
q
q
D
c
c
r
(silica)
(shells)
1 0.015(
20)
= +
−
(3.31)