The International Information Center for Geotechnical Engineers

Vibroflotation

 A Review of Vibroflotation for Ground Improvement
 by Yanet Zepeda and Ian McCreery

 Vibroflotation being used in Dubai, UAE (Vibroflotation Group, 2014)

Figure 1: Vibroflotation being used in Dubai, UAE (Vibroflotation Group, 2014)


Introduction

Loose 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 (D’Appolonia, 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 (D’Appolonia, 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: Soil Densification (Bauer Maschinen GmbH, 2012)

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 Soils

The 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: Grain size distribution for applicable soils (Bauer Maschinen GmbH, 2012)

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)

Figure 4: CPT applicability (Massarsch, 2005)


 Construction Procedures

The 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)

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 (D’Appolonia, 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)

Equation 1: Suitability number (Brown, 1977) 

Where D50, D20, and D10 are 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)

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 Control

During 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.


 Vibroflot

The 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 (D’Appolonia, 1954)

Figure 7: Vibroflot cross section (D’Appolonia, 1954)


Design

It 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 (D’Appolonia, 1954)

Equation 2: Relative density (D’Appolonia, 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)

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 (D’Appolonia et al., 1953).

Figure 9: Triangular spacing pattern showing sum of influence coefficients at critical point (D’Appolonia et al., 1953)

Figure 9: Triangular spacing pattern showing sum of influence coefficients at critical point A (D’Appolonia et al., 1953)

Correlating CPT Results to Relative Density

As 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 resistance for high compressibility sands compared to low compressibility sands for a constant relative density.  Figure 10 below shows the relationship between relative density and cone resistance for sands of different compressiblities.

Capture

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).

eqn 1

Equation 3: Relative density and cone penetration resistance correlation (Baldi et al, 1986)

In this correlation, C0, Cand C2 are soil constants whose values can be seen in Figures 11 and 12. The cone penetration resistance is represented by qc and is in units of kilopascals, as is the effective vertical stress, σ'. 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.

NC

Figure 11: Relationship between Dr, qc, and σ' for normally consolidated Tocino sands (Robertson et. al, 1997)

NC and OC

Figure 12: Relationship between Dr, qc, and σ' for normally and over consolidated Tocino sands (Robertson et. al, 1997)

The second correlation presented (Equation 4) was developed by Kulhway and Mayne (1990).

eqn 2

Equation 4: Relative density and cone penetration resistance correlation (Kulhway & Mayne,1990)

Here qc is the penetration resistance, σv' is the effective vertical stress, Pa is atmospheric pressure, OCR0.18 is the overconsolidation factor, QA is the ageing factor and QC is 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.


Cost

In 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)

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 Structures

Vibroflotation for Ground Improvement (Sreekantiah, 1993)

Background

The 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 10: Mangalore location (Sreekantiah, 1993)

Figure 13: Mangalore location (Sreekantiah, 1993)

Site Characteristics

The 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 Problem

The 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 Design

Bored 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.

Results

After 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 11: Mangalore cpt results (Sreekantiah, 1993)

Figure 14: Mangalore CPT results (Sreekantiah, 1993)

Conclusions

The 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 Fill

Application of Vibro Techniques for Infrastructure Projects in India (Sharma, 2004)

Background & Site Characteristics

The 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 12: Project seabird (Sharma, 2004)

Figure 15: Project Seabird (Sharma, 2004)

Statement of Problem

After 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 13: Project seabird soil profile (Sharma, 2004)

Figure 16: Project Seabird soil profile (Sharma, 2004)

Solution and Design

Vibro 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 14: Project seabird set up (Raju et al., 2003)

Figure 17: Project Seabird setup (Raju et al., 2003)

Results

Seven 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.

Conclusions

Offshore 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 Vibroflotation

On- and Offshore Vibro-Compaction for an Oil Pipeline in Singapore (Wehr & Raju, 2002)

Background

Jurong 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 Characteristics

Jurong 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 Problem

Based 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 15: Jurong Island site layout (Wehr & Raju, 2002)

Figure 18: Jurong Island site layout (Wehr & Raju, 2002)

Solution and Design

Beneath 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.

Results

Post 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 wasn’t any damage to the existing gas pipeline. Measurements showed maximum particle velocities well within permissible limits.

Conclusions

The 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 Plant

Sand Compaction by Vibroflotation (D’Appolonia et al., 1953)

Background

Vibroflotation 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 Characteristics

The 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 16: Phosphate plant foundation layout (D’Appolonia et al., 1953)

Figure 19: Phosphate plant foundation layout (D’Appolonia et al., 1953)

Statement of Problem

It 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 Design

Several 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.

Results

Benchmarks 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.

Conclusions

It 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 Sand

Vibroflotation Compaction at Thermalito Afterbay (Harder et. al., 1984)

Background

The 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 Characterisitics

The 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 Problem

The 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 Design

A 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)

Table 2: Thermalito Bay worksites table (Harder et. al., 1984)

Results

Standard 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 17: Thermalito Bay CPT and SPT results (Harder et. al., 1984)

Figure 20: Thermalito Bay CPT and SPT results (Harder et. al., 1984)

Conclusions

Based 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.


 References

1.) Bauer Maschinen GmbH. (2012). “Ground Improvement by Depth Vibrator.” PI 80, Schrobenhausen, Germany.

2.) Bo, M.W., Arulrajah, A., Horpibulsuk, S., Leong, M., & Disfani, M.M. (2013). “Densification of Land Reclamation Sands by Deep Vibratory Compaction Techniques.” J. Mater. Civ. Eng. ASCE.

3.) Brown, R.E. (1977). "Vibroflotation compaction of cohesionless soils." J. Geotech. Eng. Div. ASCE, 103(12), 1437-1451.

4.) D'Appolonia, E., Miller Jr., C.E., & Ware, T.M. (1953). "Sand Compaction by Vibroflotation." Proc., ASCE, 79(200), 1-23.

5.) D’Appolonia, E. (1954). “Loose Sands- Their Compaction by Vibroflotation.” Symp. on Dynamic Testing of Soils, ASTM Int., 138-162.

6.) Harder, L.F., Hammond, W.D, & Ross, P.S. (1984). “Vibroflotation Compaction at Thermalito Afterbay.” ASCE, New Orleans, LA, 57-70.

7.) Lunne, T., Robertson, P.K., & Powell, J.J.M. (1997). "Interpretation in Coarse-Grained Soils." Cone Penetration Testing in Geotechnical Practice, Blackie Academic & Professional, London, 81-85

8.) Massarsch, K.R. & Fellenius, B.H. (2005). “Deep Vibratory Compaction of Granular Soils.” Ground Improvement: Case Histories, B. Indraratna & J. Chu, eds., Elsevier, Amsterdam, 539-561

9.) Raju, V.R., Wegner, R., & Veltriselvan, A. (2003) “Application of Vibro Techniques for Infrastructure Projects in India.” Proc. of the Indian Geot. Conf., Roorkee, India.

10.) Rollins, K.M., Anderson, J.K.S., McCain, A.K., & Goughnour, R.R. (2003). "Vertical Composite Drains for Mitigating Liquefaction Hazard." Proc. Thirteenth Int. Offshore and Polar Eng. Conf., Int. Soc. Offshore and Polar Eng., Honolulu, HI, 498-505.

11.) Sharma, Ravi (2004). “Seabird on Course.” Bharat Rakshak <http://www.bharat-rakshak.com/NAVY/Articles/Article18.html>

12.) Sreekantiah, H.R. (1993). “Vibroflotation for Ground Improvement - A Case Study.” Third Int. Conf. on Case Histories in Geot. Eng., St. Louis, MO, 949-954.

13.) Uddin, N. & Baltz, J.F. (2004). “Seismic Evaluation and Remediation of Croton Dam.” Waterpower 1999, ASCE. 

14.) Vibroflotation Group. (2014). “Vibroflotation.” <http://www.vibroflotation.com/> (Apr. 6, 2014).

15.) Wehr, W.C.S. & Raju, V.R. (2002). “On- and Offshore Vibro-Compaction for an Oil Pipeline in Singapore.” Proc., Int. Conf. on Vibratory Pile Driving and Deep Soil Compaction, Louvain-La Neuve, Belgium, 129-132.

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