Study on densification of the noncohesive soils by vibroflotation

1
Study on densification of the non-cohesive soils
by vibroflotation
ZENG Qingyou Geotechnical Department Tongji
University, Shanghai
2
Contents

• Introduction
• Model test
• In- situ test
• Conclusions

3
Introduction

• Vibroflotation(VF) with or without additional
  backfill has been widely and successfully used to
  treat weak sandy soils
• The VF method without backfill was only thought
  to be proper for treatment of medium and coarse
  sand with fine particles less than 10.

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Section 1 Model tests

• Larger size model test
• development and dissipation of excess
  pore water pressure induced by VF
• influence of the times of VF on
  densification effect.
• Smaller size model test
• real-time observe the change of
  arrangement and contact of sand grains during the
  VF process

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Soil description

• The soil selected for study is the hydraulic sand
  fill from Shanghai.

The sand was poorly graded According to
Chinese standard sieve test,the sand is defined
as fine sand.
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Model test facilities

• pore water pressure measuring sensors were placed
  on three planes with heights 40cm, 80cm, 120cm

froth- plastic (10mm thickness) and sponge
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Test process

• The model probe was released into the center of
  the tested soil and the process repeated as many
  as field placement procedures.
• Probe was driven in sand up to 100cm twice each
  time and 30 seconds for each.
• To ensure water pressure stabilization, a span of
  5hrs 30min was set during which the computer was
  continuously recording the water pressure.

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Cone penetration test(CPT)

• The CPT tests were made in two spots 20cm and 40
  cm from the VF point
• The CPT tests were repeated after every VF.

Top view of the soil bin
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Test Results

• Cone Penetration test(CPT) results
• Pore water pressure test results

10
Comparison of pre- and post- CPT results (20cm
from the Center)
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Comparison of pre- and post- CPT result (40cm
from the Center)
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Excess-pore pressure changing during VF

• Pore pressure increase at the beginning of VF
• Keep stable at a certain time

1.5
2
2.5
3
3.5
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Excess-pore pressure changing after VF

• Pore water pressure started to decrease when the
  VF finished.
• The rate of dissipation of pore water pressure is
  slow at the beginning but the rate of dissipation
  of water pressure increase quickly after 6min and
  13sec.
• The pore water pressure dissipated completely
  from about 30 min to 50 min.

14
Smaller size model tests
to real-time observe the change of
arrangement and contact of sand grains during the
VF process
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Picture of sands pre-VF and their orientation
frequency
0.35
0.30
0.25
0.20
Distribution frequency
0.15
0.10
0.05
0.00
-80
-60
-40
-20
0
20
40
60
80
orientation of the long axes of particles
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Picture of sands post-first VF and their
orientation frequency
0.35
0.30
0.25
0.20
Distribution frequency
0.15
0.10
0.05
0.00
-80
-60
-40
-20
0
20
40
60
80
orientation of the long axes of particles
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Picture of sands post-second VF and their
orientation frequency
0.35
0.30
0.25
0.20
Distribution frequency
0.15
0.10
0.05
0.00
-80
-60
-40
-20
0
20
40
60
80
orientation of the long axes of particles
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Picture of sands post-third VF and their
orientation frequency
0.35
0.30
0.25
0.20
Distribution frequency
0.15
0.10
0.05
0.00
-80
-60
-40
-20
0
20
40
60
80
orientation of the long axes of particles
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Comparison to before VF, after VF

• The long axes of sand grains tend to parallel to
  the horizontal plane
• the arrangement of the sand grains from
  out-of-order to in order.
• The contact type among the sand grains from the
  point to point to surface to surface
• The sand grains become more compact.

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Section 2 in-situ tests

• Trial sites and soils condition
• Trial parameters of VF
• Trial procedures of VF
• Field investigation procedures

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Trial sites and area

• Trial is conducted in more than eight sites, the
  area of each site is 20m by 30m, treated by
  conbination of follow methods
• Different nunber of vibroflot Single, double or
  tri-vibroflot
• Different spacing between vibroflot 2.5m, 3.0m,
  3.5m
• Different constructing technics power of the
  vibroflot, load time, load current etc.

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Soils Condition

• Hydraulic sands, silty to fine, uniform grain
  size, less than 5 clay,loosevery loose,
  saturated, 4.06.0m
• Groundwater level before compaction is about 0.7m
  below the fill surface

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Trial procedures

• Step1Penetration
• The vibroflot penetrates to the required
  depth by horizontal vibration and self-weight
• Step2compaction
• The vibroflot is extracted from the maximum
  depth in 0.5m intervals and vibrating at each
  position for a given time period (2025sec). The
  in situ sand is rearranged and the compaction is
  achieved while vibrating of the vibroflot in each
  interval as well as the sand flowing towards the
  vibroflot from surface around the vibroflot. If
  sufficient build-up ampere which reflects the
  compaction degree, the holding period will be
  terminated and the vibroflot rose to the next
  increment until surface.

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Schematic procedure of VF
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Field investigation procedures

• Cone penetration test(CPT)

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Comparison of Pre- and Post-VF (site C1A,
double-VF, Spacing 2.5m)
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Time effect of VF for fine silty sands
ps
(MPa)
0
1
2
3
4
5
6
7
8
9
10
0
-1
-2
depth(m)
-3
-4
-5
-6
Pre-VF
1d post-VF
9d post-VF
-7
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Conclusions

• The sand grains become more compact after VF
• .the strength of fine sand and silt sand has time
  effect after VF.
• According to model test and in-situ results
  analysis, VF has been proved to be satisfactory
  for the improvement of fine sand soil

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• Thanks!