The main goal of most soil improvement techniques used for
reducing liquefaction hazards is to avoid large increases in pore
water pressure during earthquake shaking. This can be achieved by
densification of the soil and/or improvement of its drainage
capacity.
Vibroflotation
Vibroflotation involves the use of a vibrating probe that can
penetrate granular soil to depths of over 100 feet. The vibrations
of the probe cause the grain structure to collapse thereby
densifying the soil surrounding the probe. To treat an area of
potentially liquefiable soil, the vibroflot is raised and lowered
in a grid pattern. Vibro Replacement (right,HB) is a combination of
vibroflotation with a gravel backfill resulting in stone columns,
which not only increases the amount of densificton, but provides a
degree of reinforcement and a potentially effective means of
drainage.
What Is Vibroflotation?Vibroflotation is aground improvement
techniqueused at considerable depth that by using a powered
electrically or hydraulically probe, it strengthens the soil. The
vibroflotation will compact the soil making it suitable to support
design loads. It involves the introduction of granular soil to form
interlocking columns with surrounding soil. The technique is used
toimprove bearing capacityand reduce the possibility of
differential settlements that might be allowed for the proposed
loads.Sometimes it is also referred as Vibrocompaction. The
compaction of soil can be obtained in soils as deep as
200feet.Therisk ofliquefactionin an earthquake prone area is also
drastically reduced.Vibroflotataion TechniquesVibroflotation can be
obtained by using three different techniques: Vibro Compaction
method- This method allowsgranular soilsto be compacted. This
method is only used to compact sandy soils. Vibro Replacement
method- The technique is used toreplace poor or inadequate soil
materialby flushing out the soil with air or water and replacing it
with granular soil. This can be used in various soil types such as
clay and sandy soils. Vibro Displacement method- This procedure is
used with no or small amounts of water used during the technique.
The probe in inserted into the soil and it will displace it
laterally as the new soil column is being formed and
compacted.Vibroflotation Advantages:Vibroflotation is one
affordable way to improve ground conditions when deep layer of
inadequate soil is found.The technique is so simple that willnot
require the delivery of additional materials or additional
equipment other than the probe and the equipmentthat has it
installed. The vibroflotation process can offer the following
benefits: When the process is done properly, it will reduce the
possibility of differential settlements that will improve the
foundation condition of the proposed structure. It is the fastest
and easiest way to improve soil when bottom layers of soil will not
provide good load bearing capacity. It is a great technology to
improve harbor bottoms On a cost-related standpoint, it helps
improve thousands of cubic meters per day. It is faster thanpiling.
It can be done around existing structures without damaging them .
It does not harm the environment It improves the soil strata using
its own characteristic No excavations are needed, reducing the
hazards, contamination of soils and hauling material out from the
site No need to manage table water issues, neither the permits
required to manage water discharge anddewatering issues. The
technique of vibroflotation can be adapted to each scenario and
site When vibroflotation is performed at a site, it will reduce the
possibility of liquefaction during an earthquakeHow Vibroflotation
Works?The process of vibroflotation is really simple as you will
see in this short description:The depth probe is located over the
compaction point. Flushing water or air is expelled trough jets in
the tip of the probe. These induced injected vibrations will
liquefy the soil temporarily allowing the probe a continuous
penetration under its own weight. Once the probe has reached the
strata or poor soil, the water and air injections is stopped. At
this point the soil is densify by the probe vibrations causing a
crater around the vibrator, that shouldbe backfilledwith granular
material. Once the process has been completed, the probe is slowly
withdrawn usually in satges of 12 inched. A cylindrical compaction
zone is formed around the probe, and the achieved degree of
compaction is indicated by an increase in oil pressure. The area
around the probe is backfilled with granular material that will
auto-consolidate, as the probe is begin brought up. The material
used to backfill should be free of silt,gravel orcrushed
stoneclassified material.
IntroductionLoose soils are a well documented problem, common in
the construction industry. Immediate settlements of 1 inch per foot
(depth) of loose sand can occur in worst-case scenarios.
Contractors have two main options: use deep foundations to bypass
the unfavorable layers, or use a compaction technique to improve
the site conditions (DAppolonia, 1954). Vibroflotation was first
used in Germany in the 1930s and first appeared in the United
States in 1948 when the Bureau of Reclamation studied the
possibilities for sand and silt compaction at the site of the
Enders Dam in Nebraska (DAppolonia, 1954).The term vibroflotation
is often used interchangeably with vibrocompaction in the
literature. However vibrocompaction is a broader term that
encompasses two different techniques. The first is vibroflotation,
which uses a vibroflot that vibrates horizontally. The second
utilizes a vibrating probe which vibrates vertically. It should be
noted that vibroflotation is used as a mechanism for implementing
vibro-replacement, a method which combines the technique of
vibroflotation with gravel backfilling in order to create stone
columns. This is a review of vibroflotation only.Vibroflotation
utilizes horizontal vibrations in conjunction with fluid to reduce
the interparticle friction of the surrounding soil. This allows the
material to densify and creates a column with improved engineering
characteristics, including an increase in strength and a reduction
in compressibility. Figure 2 displays the transition of soil from a
loose state to a dense state. The goal of vibroflotation is to
increase the relative density of a soil. This increase in relative
density results in reduced settlements as well as improved
resistance to liquefaction.
Figure 2: Densification of soil during vibroflotation(Bauer
Maschinen GmbH, 2012)
Several factors, including the equipment size and quality,
spacing and pattern, in situ material, vibroflot withdrawal
technique, backfill material, and workmanship greatly affect the
level of density achieved during vibroflotation (Brown, 1977).
Applicable SoilsThe suitability of site conditions is the most
important factor when considering vibratory compaction as a
solution. Most coarse-grained soils with fines content of less than
10% are considered acceptable for this method, i.e. sands, gravels,
and slags. A grain size distribution showing the range of
applicable soils is presented in Figure 3. Vibroflotation has been
found to work best for loose granular materials located below the
water table.
Figure 3: The orange area represents the grain size distribution
of soils suitable for vibroflotation (Bauer Maschinen GmbH,
2012)Clay layers, excessive fines content and organics can all
cause serious complications when attempting to improve a site with
vibroflotation. These materials generate excess pore water
pressures which greatly inhibits volume change and results in
preventing the granules to move into a denser state. To accomodate
some of these issues, "earthquake drains" is a possible solution
for sites which are susceptible to liquefaction (Rollins et al,
2003). The vibrations are also significantly damped in the presence
of these soils, and reduces the radial densification dramatically.
If the in situ material particle size is too great, the penetration
of the vibroflot is greatly hindered which increases the amount of
time needed to achieve adequate compaction. This can make the
technique very expensive (Brown, 1977).In order to accurately
depict the site characteristics through grain size distribution,
sieve tests are needed in order to assess a large number of
samples. A good alternative to this method is the use of the cone
penetration tests (CPT). The results from a CPT offer a continuous
soil profile at each location and measures variations through
correlations in soil strength, compressibility and hydraulic
conductivity if the piezocone is used (Massarsch, 2005).
Figure 4: CPT applicability (Massarsch, 2005)
Construction ProceduresThe vibroflot is inserted into the ground
and typically can be used to improve soil up to depths of 150 feet.
Vibroflotation utilizes water and the mechanical vibrations of the
vibroflot to move the particles into a denser state. Typical radial
distances affected range from 5 to 15 feet (Bauer Maschinen GmbH,
2012).The vibroflot is suspended from a crane and seats on the
surface of the ground that is to be improved. To penetrate the
material, the bottom jet is activated and the vibration begins. The
water saturates the material to create a quick sand condition (i.e.
temporarily liquefying the material), which allows the vibroflot to
sink to the desired depth of improvement. At that point, the bottom
jet is stopped and the water is transferred to the upper jet. This
is done to create a saturated environment surrounding the
vibroflot, thereby enhancing the compaction of the material. The
vibroflot remains at the desired depth of improvement until the
material reaches adequate density. The density of the soil is
measured by using the power input (via the electric current or
hydraulic pressure) as an index. As the material densifies, the
vibroflot requires more power to continue vibrating at which point
an ammeter or pressure gauge displays a peak in required power.Once
this point is reached, the vibroflot is raised one lift (generally
ranging from 1 to 3 feet) and compaction ensues until the peak
amperage or hydraulic pressure is reached once again. A figure of
the successive steps is provided in Figure 5. The peak power
requirement can be correlated to the density of the soil, so an
accurate measurement of the in situ density can be recorded.
Figure 5: Vibroflotation construction sequence (Bauer Maschinen
GmbH, 2012)As the procedure continues, a large crater is created at
the surface which must be backfilled, as seen in Figure 5. Roughly
5 cubic feet of backfill material is needed for every foot that is
compacted by vibroflotation (DAppolonia, 1954). Acceptable
backfills include gravel or sand with minimal fines content, or
material from onsite with a fines content of less than 6% (Bauer
Maschinen GmbH, 2012). Slag has been used in some cases, and can be
an economical option if there is a large supply available. The
gradation of the backfill is the most important factor controlling
the speed at which the backfill reaches the void created by the
vibroflot. The suitability number gives an index for the quality of
the backfill material. It is calculated as:
Equation 1: Suitability number (Brown, 1977)Where D50, D20, and
D10are the grain size diameters (in millimeters) at 50%, 20%, and
10% passing, respectively. Brown described suitability numbers in
the range of 1-10 to be excellent, while numbers greater than 50
are considered unsuitable (Brown, 1977).Coarser material generally
makes better backfill material, however, if the particle size is
too large, it can become stuck between the crater and the
vibroflotation apparatus, preventing it from reaching the desired
depth. It should be noted that vibroflotation generally does not
work for surface materials (uppermost 2 to 3 feet of material) and
instead a roller is needed to attain equivalent compaction.
Figure 6: Crater created due to vibroflotation method (Bauer
Maschinen GmbH, 2012)Close attention and observation of the process
is critical throughout the implementation of vibroflotation. If the
addition of backfill material is stopped or reduced, the vibroflot
may become starved. When this occurs, the vibroflot vibrates in the
hole without contacting the surrounding material, and thereby
reducing compaction effort. This can happen when a hole collapses
and cuts off the supply of backfill material to the vibroflot,
workers stop moving backfill into the hole, the probe is extracted
too quickly, or when the wash water flow is too great and prevents
the backfill from falling (Brown, 1977).Quality ControlDuring the
process, it is important to ensure that the technique is operating
efficiently and effectively so that low soil densities are not
discovered after completion of the site improvement. Like many
construction activities, quality control is very important during
construction. Several aspects can be monitored during
implementation, including penetration depth, penetration rate,
withdrawal rate, proper probe location, volume of added backfill,
backfill gradation, ammeter or hydraulic pressure peak, and
vibroflot operating frequency.Upon the conclusion of vibroflotation
activities, densities are usually checked to ensure that adequate
compaction was achieved. While the standard penetration test (SPT)
was the most used and available method for doing this, it gave a
poor measure of bearing capacity and relative density. Today, CPTs
are most commonly used for verifying relative density.
Relationships have been developed which correlate CPT results to
relative density.
VibroflotThe module which vibrates and compacts the surrounding
material is known as the vibroflot. Vibroflot dimensions and
vibrating capabilities vary by manufacturer and are often modified
by contractors to suit their intended purpose. As part of quality
assurance, it is important to verify that the vibroflot mobilized
is the one pretedermined, since minor variations can greatly affect
performance. Though dimensions vary, generally they are 7 feet or
greater in length, and rely upon an electric motor or hydraulic
power to generate the desired vibratory forces. The vibratory
forces are generated by a rotating eccentric shaft with frequencies
ranging from about 2000 to 3000 revolutions per minute. The
vibroflot is made up of two sections: the vibrator and the
follow-up pipe. The vibrator typically weighs on the order of
10,000 to 20,000 pounds and generates a centrifugal force of 43,000
to 70,000 pounds (Bauer Maschinen GmbH, 2012). There are multiple
water discharge points along the apparatus, one of which is near
the top (the upper jet) and one at the base (the bottom jet). The
follow-up pipe remains nearly stationary during operation and acts
as a rigid casing, providing protection to the power and water
supply. Figure 7 below provides a cross-section and typical
dimensions of the device.
Figure 7: Vibroflot cross section (DAppolonia, 1954)
DesignIt is necessary to plan the vibroflotation so that the
desired compaction and uniformity is achieved throughout the site.
Material compaction is measured in terms of relative density. This
is calculated as:
Equation 2: Relative density (DAppolonia, 1954)Currently, 80
percent is the general criterion for compaction; this level of
compaction is generally deemed acceptable for soils beneath
foundations. However, this number can vary considerably from
project to project depending on site and project requirements
(Bauer Maschinen GmbH, 2012).Spacing distance and pattern affects
the uniformity of the densified material. Typical patterns include
square, triangular, and line and spacing distances range between 5
and 10 feet. Typical spacing patterns are presented in Figure 8.
Rectangular and square patterns are used to improve soil beneath
spread footings or for small isolated areas of improvement.
Equilateral triangle patterns are the most efficient and are
commonly used in scenarios with large areas. It has been observed
that square patterns require about 5-8% more probe locations to
achieve densities equivalent to equilateral triangular patterns
(Brown, 1977).
Figure 8: Vibroflotation example patterns (Brown, 1977)Design
spacing distance is a function of desired relative density, grain
size distribution of the material, fines content, and power
capabilities of the vibroflotation device. The spacing is chosen so
that improved columns from each probe location overlap. The
effectiveness of the process decreases exponentially as the radial
distance from the vibroflot increases. To determine a suitable
spacing, an arbitrary number called an influence coefficient is
determined based on the compaction in relation to the radial
distance from the probe location. The influence coefficient is a
function of distance and relative density for one vibroflotation
probe location, and increases as the distance to the probe
decreases. For considered design patterns, the influence
coefficients are displayed around the probe location and represent
an equivalent value at the corresponding radial distance. The
critical point is determined based on the greatest distance from
the surrounding probe locations. The sum of the coefficients from
each of the probe locations must be greater than the required
minimum coefficient value. Figure 9 from the International Mineral
and Chemical Corporation Phosphate Plant case study shows the sum
at the critical point, A, is calculated as 4 + 4 + 4 = 12. For this
project, a minimum influence coefficient was determined to be 10,
based on achieved relative density. A value of 12 shows that this
design spacing is adequate (DAppolonia et al., 1953).
Figure 9: Triangular spacing pattern showing sum of influence
coefficients at critical point A (DAppolonia et al.,
1953)Correlating CPT Results to Relative DensityAs noted in the
section on 'Quality Control,' the CPT is the most common form of
verification testing used today. Data obtained from CPTs is used to
measure the relative density of the improved soil. Accurately
estimating relative density from CPT results is critical to
ensuring the vibroflotation process has achieved the desired
densification results.Research in large calibration chambers has
yielded several correlations between cone penetration resistance
and relative density. Two of these correlations discussed by
Robertson, et al. (1997) are be presented below. It is important to
note that most of the testing results used in the development of
these correlations was performed on un-aged, clean, fine to medium,
uniform silica sands. Site-specific testing is crucial in
developing representative relationships for soil at a site.
However, large calibration chamber testing is expensive and often
not implemented on projects, this can result in problems later
during the verification stage.The research completed through
verification testing has shown that cone penetration resistance is
controlled by soil density, sand compressibility, and vertical and
horizontal effective stresses. Sand compressibility has been shown
to be especially important. Robertson, et al. (1997) discussed a
review of calibration chamber test results that demonstrated a
lower cone penetration resistancefor high compressibility sands
compared to low compressibility sands fora constant relative
density. Figure 10 below shows the relationship between relative
density and cone resistance for sands of different
compressiblities.
Figure 10 : Relationship between relative density and cone
penetration resistance for sands of different compressibilities
(Robertson et. al, 1997)The first correlation is presented below
(Equation 3) is based on calibration testing of Ticino sand, which
considers the effects of compressibility and effective vertical
stress, was developed by Baldi et al. (1986).
Equation 3: Relative density and cone penetration resistance
correlation (Baldi et al, 1986)In this correlation, C0, C1and C2are
soil constants whose values can be seen in Figures 11 and 12. The
cone penetration resistance is represented by qcand is in units of
kilopascals, as is the effective vertical stress,'. This
correlation was used to develop Figures 11 and 12, which show the
relationship between vertical effective stress and cone resistance
for both normally consolidated and overconsolidated Tocino
sands.
Figure 11: Relationship between Dr, qc, and' for normally
consolidated Tocino sands (Robertson et. al, 1997)
Figure 12: Relationship between Dr, qc, and' for normally and
over consolidated Tocino sands (Robertson et. al, 1997)The second
correlation presented (Equation 4) was developed by Kulhway and
Mayne (1990).
Equation 4: Relative density and cone penetration resistance
correlation (Kulhway & Mayne,1990)Here qcis the penetration
resistance,v' is the effective vertical stress, Pais atmospheric
pressure, OCR0.18is the overconsolidation factor, QAis the ageing
factor and QCis the compressibility factor whose value ranges from
0.9, for low compressiblity sands to 1.09, for high compressiblity
sands.It is important to reiterate that the correlations presented
above are based on testing performed on un-aged, clean, medium to
fine, uniform silica sands. These correlations may be appropriate
to apply to reasonably similar soils, however, site-specific
calibration testing is crucial for producing accurate and reliable
relative density correlations from CPT results.
CostIn one case study, vibroflotation was found to be one of the
most economical remedial solutions compared to other common ground
improvement techniques. Concluded in 1999, the study examined
possible design solutions for Croton Dam, which was found to be
susceptible to damage due to earthquake shaking. Although, it is
located near Muskegon, Michigan, it is considered a high-hazard dam
so it must be designed to account for large ground accelerations.
The dam consists of two earth embankments, a gated spillway, and a
concrete and masonry powerhouse. The earth embankments are composed
of a hydraulically placed sand fill with concrete cores. A seismic
analysis of the embankments found that they were likely to liquefy
in the event of a magnitude 6 or greater earthquake. It was
determined that adequate strength to resist liquefaction could be
achieved by compaction (Uddin & Baltz, 2004). Table 1 displays
the possible remediation techniques examined in this study, along
with associated costs. Compared to the other two ground improvement
methods (jet grouting and compaction grouting), vibroflotation
proved to be the most economical solution for this case.
Table 1: Comparison of costs at Croton Dam (Uddin & Baltz,
2004)The three ground improvement techniques examined
(vibroflotation, jet grouting, and compaction grouting) were quoted
by contractors based on the maximum plausible area of improvement.
This was taken to be 70% of the maximum possible area of
improvement. While vibroflotation was judged to have the greatest
increase on material strength, it was also considered to cause
significant damage to existing surficial structures (Uddin &
Baltz, 2004).
Case Study: Densifying Sands Near Existing
StructuresVibroflotation for Ground Improvement(Sreekantiah,
1993)BackgroundThe Mangalore Chemicals and Fertilizers Company
Limited is situated on the West Coast of India in Mangalore,
Karnataka. The company desired to install new machinery at their
existing location. In order to install the new machinery, the very
loose granular soil which comprised the site had to be improved in
order to achieve adequate bearing capacity as well as to assure
that settlements of the soil where the new machinery was to be
installed would be within permissible limits.The company is located
on a site that originally consisted of agricultural land. In 1966,
the ground level elevation, which was originally 6 feet above sea
level, was raised through dredging. The soil used for dredging was
recovered during the construction of the New Mangalore Port on the
West Coast of India.Figure 13: Mangalore location (Sreekantiah,
1993)Site CharacteristicsThe subsurface soil on the site was
comprised of the soil that had been dredged in 1966. The profile
consisted of two layers: a clean sand layer underlain by a clay
layer with consistency that varied from soft to stiff throughout
the site.Standard and cone penetration tests were performed
throughout the site in order to the assess the subsurface soil
conditions. The clay layer was determined to be a marine clay with
varying consistency. The upper sand layer consisted of a stratum
varying from fine sand to sandy gravel with N values ranging from 1
to 16. Rock was found at depths ranging from 92 feet to 98 feet
below ground elevation.Statement of ProblemThe Mangalore Chemicals
and Fertilizers Company site posed a great challenge. The use of a
shallow foundation to support the machines that were to be
installed proved unfeasible. However, the use of traditional driven
piles was also not feasible because construction disturbance posed
a risk of damage to the nearby existing machinery at the
site.Solution and DesignBored compaction piles were one of the
alternatives considered for increasing bearing capacity of the
subsurface soil. However, difficulties were encountered during
construction of the bored piles. Additionally, the load tests
performed on the bored compaction piles proved unsuccessful. The
expected applied loads from the machinery proposed to be installed
on the site ranged from 229 pounds per square inch to 236 pounds
per square inch. By virtue of the limiting site conditions, ground
improvement by vibroflotation was determined to be the best course
of action.A thorough analysis of the subsurface soils was conducted
in order to determine whether vibroflotation would be a feasible
improvement technique. The upper sand layer was determined to
contain less than 10 percent fines making it ideal for improvement
by vibroflotation. An area of 79 feet by 20 feet was selected to be
compacted to a desired depth of 23 feet below ground
elevation.CEM-INDIA performed the work, electing a wet process
using a standard vibroflot with a 15.75 inch diameter, 26.5 feet
length and a weight of 6750 pounds. A square grid spacing scheme of
6.5 feet by 6.5 feet centre to centre was implemented. The
vibroflot was inserted at intervals of 6.5 feet to the desired
depth and was removed in 9.6 inch intervals. The criteria provided
for acceptance in the contract with CEM-INDIA was a maximum
acceptable settlement of 0.47 inches for a 6.2 feet diameter plate
under a pressure of 65 pounds per square inch. Additionally, a
static cone penetration test resistance of 22 pounds per square
inch had to be achieved post compaction.ResultsAfter compaction was
completed, the site was tested for the previously stated criteria.
Five of the eight load tests performed on the improved area
resulted in a settlement of less than 0.47 inches, while two load
tests resulted in a settlement of less than 0.60 inches and one
resulted in a settlement of 1.6 inches. The static cone penetration
test resistance of 2175 pounds per square inch was generally
achieved and demonstrated that between depths of 6.5 feet and 23
feet penetration resistance achieved a threefold to fivefold
increase post compaction, as can be seen below in Figure 14. Raft
foundations were selected for use on the improved site area.
Foundation settlement was monitored after the installation of the
new machinery and was determined to be within permissible
limits.
Figure 14: Mangalore CPT results (Sreekantiah,
1993)ConclusionsThe installation of new machinery on an existing
site posed challenges. The subsurface soil which was comprised of
granular soil underlain by a clay layer made the use of shallow
foundations unfeasible. However, the risk of potential damage to
nearby existing machinery due to disturbance caused by construction
made traditional driven piles also unfeasible. Vibroflotation
proved to be a successful technique for improving the ground on
site. It densified the soil which resulted in an increased bearing
capacity as well as reduced settlement to permissible limits.
Vibroflotation was the best technical and economical solution for
improving the loose granular soil without posing a great risk to
existing structures and machinery near the site.
Case Study: Vibroflotation for Densification of Hydraulic
FillApplication of Vibro Techniques for Infrastructure Projects in
India (Sharma, 2004)Background & Site CharacteristicsThe
Seabird Naval Base at Karwar in the Indian state of Karnataka
required the construction of a breakwater structure over a distance
of more than 3 miles. The existing seabed was composed of clay and
soft silt. In order to create favorable soil conditions for
construction of the breakwater, the existing seabed was dredged to
a depth of nearly 20 feet. Hydraulic sand fill obtained from nearby
borrow pits was used to the backfill the dredged area.
Figure 15: Project Seabird (Sharma, 2004)Statement of
ProblemAfter the hydraulic fill was placed, cone penetration tests
were performed to assess the conditions of the sand fill. The tests
revealed the need for compaction of the top 13 feet of the fill in
order to reduce potential settlements and mitigate liquefaction
potential.
Figure 16: Project Seabird soil profile (Sharma, 2004)Solution
and DesignVibro compaction is a technique commonly used to densify
hydraulic fills in offshore projects. For the construction of the
breakwater structure at the Seabird Naval Base an area of about 35
acres was selected to be compacted to a depth of about 13
feet.Keller Grundbau performed the work over a period of 10 working
months. The equipment setup included the use of four 49 foot long
vibrators suspended from a crane situated on a barge in order to
allow for the compaction of the soil beneath 33 feet of seawater.
The spacing scheme selected was a 9.8 foot by 9.8 foot center to
center grid.
Figure 17: Project Seabird setup (Raju et al., 2003)ResultsSeven
days after compaction was completed, cone penetration tests were
performed every 164 feet along the breakwater structure. The
results demonstrated that the 13 feet of compacted fill achieved a
twofold to threefold increase in penetration resistance compared to
the uncompacted values. Settlement was monitored after the
construction of the breakwater and range between 0.65 feet and 0.98
feet.ConclusionsOffshore densification of hydraulic fills can be a
challenging task. Vibro flotation proves to be an easily
implemented and economical technique for performing the necessary
ground improvement. The results from the use of vibro flotation at
the Seabird Naval Base in Karwar support this. Vibro flotation
densified the hydraulic fill beneath the breakwater structure which
increased the penetration resistance of the fill and reduced future
settlements.
Case Study: Compaction of Reclaimed Soil through
VibroflotationOn- and Offshore Vibro-Compaction for an Oil Pipeline
in Singapore (Wehr & Raju, 2002)BackgroundJurong Island is a
man made island that was formed from the combination of seven
islands southwest of the main island of Singapore. The installation
of a crude oil pipeline on the island mandated the densification of
the existing sandfill in order to reduce future settlements beneath
the pipeline as well as to stabilize the seaside slope of the sand
embankment adjacent to the pipeline jetty.Site
CharacteristicsJurong Island is an artificial island made of
reclaimed soil. The reclaimed soil consisted of medium to coarse
sand with fines contents less than 5 percent, making it ideal for
vibroflotation. The groundwater table was found approximately 16
feet below the ground level surface.Cone penetration tests were
performed both in the area where the pipeline was to be installed
as well as at the sand embankment at the pipeline jetty. Cone
penetration resistances ranged from 725 pounds per square inch to
1160 pounds per square inch with friction ratios of approximately
0.5 percent.Statement of ProblemBased on the results from the cone
penetration tests, it was evident that densification of the
reclaimed soil would be necessary in two areas. The area beneath
where the pipeline was to be installed required densification in
order to reduce potential settlements. The area of the sand bund
where the pipeline jetty was to be located also required
densification in order to reduce settlements and stabilize the
adjacent seaside slopes. The project posed a unique challenge in
selecting the ground improvement method to be used. The method
selected had to be of minimal disturbance to nearby areas because
of an existing gas pipeline situated approximately 6 feet to 10
feet below the ground surface. This was especially crucial in the
area where the crude oil pipeline and the gas pipeline were to
cross.
Figure 18: Jurong Island site layout (Wehr & Raju,
2002)Solution and DesignBeneath the proposed pipeline, an area of
nearly 2.8 miles in length and 65 feet in width was selected for
compaction to 70 percent relative density. Based on field trials,
an equilateral triangular scheme with 11.5 foot spacing was
selected. The area of the sand bund where the VLCC (very large
crude carrier) jetty was to be located required offshore compaction
spanning a distance of over 3200 feet in length and 65 feet in
width. This area was also compacted to 70 percent relative density
using the same 11.5 foot equilateral triangular spacing scheme. The
slope adjacent to this area had slopes of 1:3, vertical to
horizontal. A 13 foot equilateral triangular spacing scheme was
utilized at the inclined slopes in order to achieve ISO/IEC JTC
specifications for compaction of inclined slopes. A crane situated
on a barge was used for offshore compaction.ResultsPost compaction
cone penetration tests were performed throughout the improved
areas. The criteria requiring 70 percent relative density in all
areas of the project was achieved without any major issues.
Additionally, vibration measurements were carried out to assure
there wasnt any damage to the existing gas pipeline. Measurements
showed maximum particle velocities well within permissible
limits.ConclusionsThe densification of reclaimed soil both on and
off shore on Jurong Island was achieved through the use
vibroflotation. Vibroflotation was an appropriate method for this
project because of its relatively low cost, on and off shore
flexibility as well as its low disturbance level during
construction when compared with other techniques.
Case Study: International Mineral and Chemical Corporation
Phosphate PlantSand Compaction by Vibroflotation(DAppolonia et al.,
1953)BackgroundVibroflotation was chosen to improve the loose sand
deposits at the site of the International Mineral and Chemical
Corporation Phosphate Plant, located outside of Bartow, Florida.
This technique was selected because of its simplicity and its
economic superiority. The entire plant construction was projected
to cost roughly 12 million dollars, and included an improvement
area of 156,000 square feet. With compaction needed to an average
depth of 12 feet, the contractor was able to use vibroflotation on
1400 square feet per day, and cost about $0.10 per square foot of
vibroflotation improvement.Site CharacteristicsThe site was
composed of uniform vegetation, with no swamps present within the
boundaries of the project. Figure 19 displays a map of boring
locations and proposed building footprints. This figure also
displays the areas which were improved with vibroflotation. The
site is composed of a surface layer of very loose, clean sand
varying between 10 and 15 feet in thickness. SPT blow counts ranged
from 2 at the surface to 10 at the base of the sand layer. A sandy
clay layer with phosphate granules is located below the surficial
layer, and is locally called a matrix. This layer has an average
depth of 15 feet and is fairly dense with a high bearing strength.
Beneath the matrix, alternating layers of sand, gravel, and clay
compose a mix of stiff and soft strata. The presence of coarse
gravel in this lower layer gave rise to the possibility that the
SPT values could be artificially high.Figure 19: Phosphate plant
foundation layout (DAppolonia et al., 1953)Statement of ProblemIt
was determined that settlement could occur in the loose sand layer
and the soft clay layers, located at depths greater than 30 feet.
As stated in the site characteristics section, the uppermost
stratum consists of very loose sand. It was determined that the
average relative density of the sand in this layer was about 33%.
The site will be subject to vibrations due to train movement and
pumps located within the plant. Both of these sources almost
definitely ensure settlement. The compressible clay layers,
however, were only considered to be susceptible to a minor amount
of compression due to the small foundation stresses and the depth
of the compressible layers.Solution and DesignSeveral foundation
systems were examined to determine the best course of action in
minimizing the amount of settlement experienced by the structures.
Piles extending 50 to 60 feet to the gravelly clay layer were
considered, however it was determined that thepresence of the soft
strata at those depths would force the piles to utilize skin
friction over tip resistance. The frictional forces exerted on the
soft strata would induce consolidation, and result in unacceptable
amounts of settlement at the surface.Short point-bearing piles
driven to refusal in the sandy matrix layer were also considered.
However, it was determined that the stress exerted on the sandy
matrix would also adversely affect the clay layers below, resulting
in unacceptable settlement at the surface.It was determined that
spread and mat foundations on compacted sand would result in the
minimum settlement. Vibroflotation was selected to adequately
compact the sand at depths up to 15 feet. Based on the experience
of the engineers, the relative density goal was set at 70% or a dry
unit weight of at least 103 pounds per cubic foot. Three different
spacing patterns were examined, including the line, square, and
triangular pattern. Different spacing distances were also examined
for each pattern. It was found that spacing and pattern
combinations of 8 feet for the square and triangular patterns, and
7.5 feet for the line pattern all resulted in the same relative
density. Ultimately, a triangular pattern with a 7.5 foot spacing
was used. Compaction was conducted in one foot lifts, and an
average of 2.25 cubic yards of backfill material was added for each
probe.ResultsBenchmarks were placed around the site to record the
amount of settlement experienced before, during, and after
construction. Based on data acquired from the tests and boreholes,
in conjunction with the design building loads, the total settlement
was estimated to be 3.96 inches. However, at the time of completion
of the report (131 days after the conclusion of construction), it
was found that building had only settled 0.42 inches compared to
the projected 1.0 inch.ConclusionsIt is clear that vibroflotation
was an excellent solution for this scenario of very loose sands. It
greatly improved the site and yielded less settlement than
originally projected, while also offering a better alternative to a
deep foundation system. However, it should be noted that this case
study is from 1953; many contractors use updated technology and
projects today may require higher standards of relative density,
typically around 80%.
Case Study: Effectiveness of Vibroflotation on Silty
SandVibroflotation Compaction at Thermalito Afterbay (Harder et.
al., 1984)BackgroundThe construction of Thermalito Afterbay Dam in
Northern California was completed in 1967. In August of 1975 an
earthquake of magnitude 5.7 revealed an active fault that had not
been previously detected. As a result of the rupture of the
Cleveland Hill Fault during the 1975 earthquake, the Department of
Water Resources evaluated the embankments resistance to
liquefaction under a 6.5 magnitude earthquake. Their analysis
predicted that the silty sand layers in the foundation of the
embankment would liquefy entirely under these seismic conditions
and would result in failure of the dam. In 1979 storage restriction
of the reservoir was implemented in order to reduce risk of failure
until the seismic evaluation was completed. The need to restore the
dam to full operation was evident and therefore numerous remedial
methods for stabilization of the dam were considered at Thermalito
Bay.Site CharacterisiticsThe embankment was 8 miles long with a
maximum height of 39 feet. The foundation consisted of several
layers of different soils including clay, silt, sand and gravel.
The surface layer throughout most of the embankment was composed of
a clay and silt layer several feet thick. The silty sand layers
which were targeted for densification contained a median of 15
percent fines, with 30 percent of the samples containing more than
20 percent fines. The groundwater table downstream of the dam was
found at a depth of 5 feet to 10 feet.Statement of ProblemThe
seismic evaluation performed predicted that the silty sand layers
of the embankment foundation would liquefy entirely under an
earthquake of magnitude 6.5. Densification of these silty sand
layers was necessary to mitigate liquefaction risks. A primary
concern when selecting the remedial method to be used at the site
was assuring that the clay embankment would not be at risk of large
settlements or heaves during treatment.Solution and DesignA
vibroflotation testing program using 100 horsepower vibroflot was
performed on the embankment in order to determine the effectiveness
of vibroflotation in densifying silty sand. Two worksites were
selected to assure a range of conditions were represented in the
testing program. However, this vibroflot was not used to penetrate
the clay and silt surface layer. Instead pre-drilling with a 24
inch bucket auger was used to create holes that reached the silty
sand layers. These holes were then backfilled with sand before the
vibroflot was inserted. An equilateral triangular spacing scheme
was utilized with spacings ranging from 6.5 feet to 9.5 feet. Table
2 summarizing the vibroflotation test program and variables can be
found below.
Table 2: Thermalito Bay worksites table (Harder et. al.,
1984)ResultsStandard and cone penetration tests were performed
before and after the vibroflotation testing program was performed
at the embankment. SPT blow counts and CPT resistance at both sites
showed generally no change before and after compaction. SPT and CPT
results for worksite 2 can be seen below and demonstrate the
ineffectiveness of vibroflotation testing program.
Figure 20: Thermalito Bay CPT and SPT results (Harder et. al.,
1984)ConclusionsBased on the results of the vibroflotation testing
program at Thermalito Afterbay we can conclude that vibroflotation
is not an effective method for the densification of silty sands
below a cohesive soil cap. The failure of vibroflotation as a
technique in this case is most likely due to the relatively high
fines content of 15 percent in the silty sand layer. Generally,
vibroflotation is ideal for sands with less than 10 percent fines
content.
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