stone columns for industrial fills
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Nicholson Construction Company
12 McClane Street
Cuddy, PA 15031
Telephone: 412-221-4500
Facsimile: 412-221-3127
Stone Columns for Industrial Fills
by
Martin G. Taube, P.E., P.G.Nicholson Construction Company, Cuddy, Pennsylvania
John R. Herridge
Nicholson Construction Company, Cuddy, Pennsylvania
Presented at:
The 33rd Ohio River Valley Soil Seminar (ORVSS)
October 18, 2002
02-02-135
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Stone Columns for Industrial Fills
Martin G. Taube, P.E., P.G.1
John R. Herridge2
Prepared for: The 33rd
Ohio River Valley Soil Seminar (ORVSS), October 18, 2002
ABSTRACT
Vibro technologies can provide quick and cost-effective solutions for areas of weak and unconsolidated
soils and industrial fills. These technologies, including vibro compaction, vibro stone columns, and vibro
concrete columns permit the in-situ improvement of soi l, while minimizing the generation of spoils that
might potentially require costly disposal. In many situations, vibro applications can be used to strengthen
or reinforce soils, control settlement, increase bearing capacity, and reduce the risk of liquefaction in
seismic areas.
A Pennsylvania refining facility installed a 115,000 barrel (BBL) above-ground asphalt storage tank at their
facility in Springdale, PA, just northeast of Pittsburgh along the Allegheny River. The tank is
approximately 134 feet in diameter, with a height of 48 feet. The site is underlain by industrial fill, fine-
grained soils, and gravel. Without improvement, the in situ soils were not adequate to support the proposed
tank without excessive settlement. Other foundation options included removing and replacing the upper 15
feet of soil; or installing traditional deep foundation elements. Both of these scenarios were deemed cost
prohibitive and the owner opted to install stone columns in order to reinforce and strengthen the in situ
soils. A total of 324 stone columns (15 feet in length) were installed in less than two weeks time. In
Crayford, Kent, U.K., two large, single-story industrial warehouse units were constructed over an in-filled
clay pit. A total of 494 stone columns were installed through the granular contaminated fill. Concrete
bottom plugs were installed at the base of each stone column to protect the underlying chalk drinking water
aquifer from downward migration of water-borne contaminants.
VIBRO TECHNOLOGIES
The original vibroflotation process was developed in Europe in the 1930s as an economical method of
densifying granular soils. Because of the development of more sophisticated equipment and greater
acceptance of ground improvement techniques in the U.S. geotechnical community, usage of vibro
technologies has increased in the past several years in terms of the number of projects, types of facilities,
and range of soil types being improved. Today, vibro technologies are commonly used to:
1 Business Development Engineer, Nicholson Constuction Company, 12 McClane St., Cuddy, PA 15031
2 Project Manager, Nicholson Construction Company, 12 McClane St., Cuddy, PA 15031
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Stiffen ground for spread footings, mat foundations, and floor slabs for commercial, industrial, and
residential buildings in soft alluvial ground and coastal plains.
Improve bearing capacity and reduce/control settlement
Facilitate port/coastal construction over existing hydraulic fills
Reduce the risk of liquefaction in seismically-active areas
Replace conventional deep foundation systems (pilings)
The vibro technologies are in general easily adaptable to a variety of structure types and can be installed
relatively economically. In many cases, by stiffening the ground, it is possible to reduce the thickness of,
or eliminate the need for structural concrete mats or grade beams . This benefit results from stone columns
being placed on tighter centers than conventional piling, and from the increased modulus of elasticity of the
ground between the stone columns.
The vibro technologies, including vibro compaction, vibro stone columns, and vibro concrete columns
(VCCs) are implemented by using either crane- or rig-mounted vibratory probes. Compaction, vibration,
and displacement are achieved by an eccentrically mounted weight powered by an electric or hydraulic
motor. A schematic of a vibratory probe is presented in Figure 1.
Figure 1. Vibratory probe configuration
Isolator
Eccentric Weight
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Vibro Compaction
Vibro compaction combines the action of the vibratory probe and saturation by water jetting to re-arrange
the surrounding soil particles to a denser, more stable state. The technique is applicable for loose, clean
sands and gravels above or below the water table. Sand backfill, from either on-site or off-site sources is
typically placed and compacted with the vibratory probe at the compaction location in order to compensate
for the reduction of volume resulting from the densification. Vibro compaction is used to increase bearing
capacity, decrease total and differential settlement, and reduce the potential of liquefaction.
Vibro Stone Columns
Vibro stone columns are installed to reinforce cohesive soils and to densify granular soils in order to
increase bearing capacity, decrease total and differential settlement, provide vertical drainage pathways to
increase the time-rate of consolidation settlement, and to reduce the potential of liquefaction. In contrast
with vibro compaction which is undertaken solely to compact granular soils, stone columns may beinstalled in granular or cohesive soils. Vibro stone columns are relatively stiff with respect to the
surrounding ground. Nonetheless, stone columns should not be viewed as rigid structural elements or
pilings. Traditional deep foundation elements effectively bypass the shallow, weak strata and transfer loads
from the structure to the underlying bearing stratum. Both the stone columns and the stiffened/reinforced
weak strata are loaded, with the proportion of the load carried dependent upon the relative stiffness of the
stone columns and the surrounding soils. Stone columns therefore reduce the stress differential induced to
the overlying foundation. Idealized stress distribution patterns for deep foundation systems and for stone
column systems are depicted in Figures 2 and 3.
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Stress
f - Mat/Pile Cap Pressurep Stress Induced to Piles
Figure 2. Idealized stress distribution pattern for deep foundation systems (piles )
f
p
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Stress
f - Mat/Pile Cap Pressurec Stone Column Stress
s Soil Stress
Figure 3. Idealized stress distribution pattern for stone column systems
Stone columns are installed by either replacement or displacement of the in situ soils (or by a combination
of these methods) using a variety of equipment and installation techniques that are discussed further below.
The authors consider the wet method as vibro replacement, because a significant volume of soil is removed
from the hole by jetted water, and subsequently replaced by stone. Dry methods create stone columns by
pushing the soil out laterally by the action of the vibratory probe and the authors equate the dry method
with vibro displacement. The vibro replacement and vibro displacement terms are used interchangeably by
various contractors. For clarity, the terms vibro replacement and vibro displacement will not be used
further in this paper.
f
c
s
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Vibro Concrete Columns
VCCs are structural elements that are installed through very soft soils (commonly peat or other organic
deposits) in order to transfer loads to more competent bearing strata. VCCs are used in cases that the soils
are so soft and unstable that installation of stone columns would be difficult, or where the stone columns
would not have sufficient lateral confinement to adequately carry vertical loads. VCCs are evaluated for
bearing capacity and settlement similar to traditional deep foundation elements .
VCCs are installed by inserting the vibratory probe through the weak layers and into the bearing stratum,
and then pumping concrete under pressure through a side feeder tube while withdrawing the probe a
distance of no more than 4 to 5 feet. The concrete is discharged at the bottom of the probe to form the base
of the concrete column . The probe is then reinserted through the concrete column 2 to 3 times in order to
form an enlarged, bulbous base. In granular soils, the probe re-penetration will increase the bearing
capacity of the bearing stratum. In cohesive bearing stratum, the probe re-penetration has minimal impact
on bearing capacity. Once the bulb has been constructed, the upper portion of the concrete column is
completed by injecting concrete through the feeder tube while raising the probe.
In order to transfer the load from the structure to the VCCs, it is common to install enlarged concrete heads
on top of the VCCs as well as grid-reinforced compacted aggregate layers (load transfer platforms).
Stone Column Installation Methods
Stone columns are installed using either top- or bottom-feed systems, either with or without jetted water.
The top-feed method is used when a stable hole can be formed by the vibratory probe. With the dry
method (top or bottom-feed), the probe is inserted into the ground and penetrates to the target depth under
its own weight and compressed air jetting. For rig-mounted models, pull-down further aides the downward
penetration. Pre-drilling is sometimes required to break/displace obstructions such as boulders or rubble or
to loosen stiff or dense layers to allow for probe penetration. Jetted water may also be used to loosen dense
granular layers.
For the top-feed method, the vibratory probe is either fully or partially removed from the hole depending on
the ground conditions and the characteristics of the probe, and stone is dumped into the hole. The stone is
placed in 2- to 3-ft lifts, and the probe is re-inserted following placement of each lift to compact the stone
and force the stone into the surrounding formation. Loaders with side-dumping buckets are preferred as
they minimize waste and allow for a more accurate measurement of stone volume as compared to front-
dumping buckets. Tapered chutes may be used for front-dumping loaders. The probe is held in place once
it has penetrated the previous lift of stone until the specified resistance criteria is met. The resistance
criteria is specified in terms of amperage for electrical models and hydraulic pressure for hydraulic models.
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The sequence of stone placement and probe compaction is repeated until the full length of the stone column
has been constructed. Figures 4 and 5 depict the dry top-feed method.
Figure 4. Dry top-feed method process schematic
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Figure 5. Dry top-feed stone column installation
In settings where the hole collapses or partially collapses when the vibratory probe is retracted, it is not
possible to construct high quality, continuous, stone columns using the dry top-feed method. The top-feed
method can typically be used to depths of 15 to 20 feet. However the depth to which a hole will remain
stable is dependent upon the soil type and strength characteristics , and groundwater conditions. The
bottom-feed method incorporates a tremie pipe that is mounted to the side of the probe to allow stone to be
introduced at the bottom of the hole without fully retracting the probe. One method of loading stone is to
use a traveling skip that is loaded at ground level, conveyed to the top of the probe and discharged into a
hopper. Without skip systems, stone may be loaded into the hopper (mounted at the top of the probe) by
means of a high lift, or by adding the stone once the probe has been inserted to full depth. Figures 6 and 7
depict the dry bottom-feed method.
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Figure 6. Dry bottom-feed method process schematic
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Figure 7. Dry bottom-feed stone column installation
The wet top-feed method of stone column installation involves jetting water as the probe penetrates to full
depth, reducing the water flow, and placing the stone (via top-feed methods). The stone column is
constructed using techniques similar to the dry top-feed method, except that a column of water is
maintained in the hole during stone placement. The wet top-feed method is employed in weak, unstable
soils to maintain a stable hole, to increase the diameter of the stone columns, and to remove fine materials
(clay, silt, and organic particles) from the hole. Significant efforts can be required to manage the runoff
generated from the wet method, especially in environmentally sensitive areas such as wetlands or where
contamination is present. Figures 8 and 9 depict the wet, top-feed method.
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Figure 8. Wet top-feed method process schematic
Figure 9. Wet top-feed stone column installation
Probe
Water
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Design Considerations
In order to assess the applicability of vibro stone columns for a given site and foundation system, it is
necessary to evaluate the performance of the unimproved ground and then determine if the stone columns
will achieve the desired results in terms of improved bearing capacity, densification, settlement reduction,
etc.
In the simplest terms, the preliminary design of stone columns can be accomplished as follows:
1) Estimate the settlement for the proposed loading conditions for the unimproved ground using
conventional settlement calculations.
2) Determine the reduction of settlement that is required to meet the design requirements. This
reduction factor which is expressed as a ratio of the amount of settlement of the unimproved soils
to the amount of settlement of the improved soils is referred to as settlement ratio, or
improvement factor. This concept was developed by Priebe.
3) Determine, based on contractors experience and published empirical data, if stone columns can
provide the required reduction of settlement. Typically, settlement ratios are between 2 and 3 (i.e.,
settlement can be reduced by a factor of between 2 and 3).
4) Determine the area replacement ratio (stone column area divided by the tributary area of the stone
column) necessary to provide the required reduction of settlement. The concept of area
replacement ratio for an equilateral grid configuration is illustrated in Figure 10.
5) Determine the stone column length, diameter and spacing. The stone column length is determined
from evaluation of the settlement calculations. Stone column diameter and spacing are determined
by contractor experience.
6) Assess the load-carrying capacity of the stone columns.
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Ac Cross-sectional area of stone column
Ao Cross-sectional area of foundation per stone column (hexagonal area)
Ec Deformation modulus of stone columns
Es Deformation modulus of soil
Kc Passive coefficient of earth pressure of stone column
Ks Passive coefficient of earth pressure of soil
d Stone column spacing
Figure 10. Design parameters and area replacement ratio concept
Prediction of the stone column diameter, which is a critical part of the design process, is based on
contractor experience. Column diameters are predicted empirically, based on the construction method (wet
or dry method, top or bottom-feed), vibratory probe characteristics, and characteristics of the strata in
which the stone columns will be installed. With a known required area replacement ratio, and a prediction
of the stone column diameter, the stone column spacing is simply calculated.
Typical diameters for stone columns using the dry method range from 24 to 36 inches, while diameters for
stone columns installed using the wet method are typically larger by a factor of approximately 20 to 40
percent.
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CASE STUDY: STORAGE TANK, SPRINGDALE, PA
A 115,000-barrel asphalt storage tank, 134 ft in diameter and 48 ft high, was to be constructed within an
existing, diked, containment area at a Pennsylvania refining facility, located approximately 10 miles
northeast of Pittsburgh. When full, the tank would impose loads on the order of 3,000 psf on the bearing
soils.
Soil borings were drilled within the limits of the tank foundation using hollow stem augers to assess the
subsurface conditions. Split-sp oon sampling was performed continuously to a depth of 10 ft and then at 5-
ft intervals to a depth of 35 ft. Representative soil samples were visually examined and classified in the
field. The boring depths ranged from 32 to 74 feet. Gradation tests were performed on several samples to
assist in the classification of the granular soils. From the geotechnical engineers detailed report, a
generalized subsurface profile was developed as shown in Table 1. Groundwater was encountered between
elevations 734 and 735.
El.750-745Loose to medium-dense industrial fill, predominantlysilt, including varying percentages of glass, brickfragments, cinders, slag and coal
N= 5-15
El. 745-735Loose to very loose, fine-grained native soils,
predominantly silt, clayey s ilt and sandy siltN= 3-8
El. 734-727 Well-graded, dense to very dense gravel N= 30-50
El. 727-680 Poorly-graded dense to very dense sand with gravel N= 30-50
Table 1. Generalized subsurface profile
Foundation Evaluation
Given the loose to very loose nature of the upper 15 ft of soils, the tank clearly could not be safely
supported on shallow foundations under existing conditions. The geotechnical engineer evaluated the
following treatment options:
Removal and replacement of the fill and silts
Ins tallation of deep foundations with a structural concrete mat
Installation of stone columns and a composite mattress
The cost of removing approximately 10,000 yd3 of loose to very loose material and replacing it with
compacted fill was estimated to be approximately $250,000.
Given the weight of the tank, piles would need to be driven at relatively close intervals to reduce the load
per pile to an acceptable magnitude and allow the reinforced concrete mat to span between piles.
Approxi mately 400 piles would have been required, spaced at 6-ft centers, resulting in loads of
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approximately 60 tons per pile. Furthermore, the piles would have been driven into the gravel layer and
possibly the underlying sands to develop sufficient capacity. The cost of this option was estimated to
exceed the cost of removal and replacement.
Stone columns have a proven history in the support of tanks, are ideally suited for silty and variable soils,
and would provide the densification and reinforcement needed to reduce both differential and total
settlements. The cost of stone column installation was estimated to be some 45% less than the cost of
removal and replacement, and the project could be completed in much less time. The schedule for the
stone column option was relatively independent of weather conditions, while the excavation and
replacement option would have been negatively impacted by heavy precipitation. Therefore, based on cost
and schedule considerations, and technical effectiveness, the geotechnical engineer recommended soil
improvement by stone columns, with a composite mattress.
Foundation Design and Construction
The intent of the stone column program was to provide a uniform, competent bearing stratum sufficiently
dense to keep tank settlements within acceptable limits. As such, an experience-based design was
developed that called for the installation of 30-in diameter stone columns, 8 ft on center, extending to the
top of the dense, gravel stratum. The stone column layout extended 20 ft beyond the tank footprint to take
into account edge effects. This design was anticipated to limit short-term settlements to within 1.0 to 1.5
inches.
Prior to stone column installation, 24 to 30 inches of fill was removed to allow subsequent placement andcompaction of the stone mattress that would support the tank. A 12-in thick lift of well-graded crushed
stone was then placed to provide a stable working platform for the geotechnical contractors rig.
The dry, top-feed method was used to construct the 30-inch diameter stone columns. At each location, the
probe penetrated to the top of the gravel formation. The probe was withdrawn sufficiently to allow a
charge of No. 57 crushed rock to be tipped from the surface to the base of the hole and then lowered to re -
penetrate and densify the stone. This procedure was repeated in 2-ft lifts up to the surface to complete the
column. A total of 325 stone columns with an average length of 15 ft were installed in nine working days.
During the stone column installation, the volume of stone per bucket and number of buckets per column
were continuously monitored by the geotechnical engineer to ensure that columns of adequate diameter
were being created. Pressure monitoring during re -penetration indicated that increases in hydraulic
pressure of 150 to 200 bars over the freely-suspended pressure were being achieved, verifying the
effectiveness of the construction process.
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Following stone column installation, the concrete ring wall was constructed, the grid-reinforced mat was
placed, and the tank structure was installed.
Testing
Hydrostatic testing was selected by the geotechnical engineer to evaluate the performance of the improved
bearing soils. Twelve, optical, differential leveling points were spaced equidistantly on the ring wall and
initial readings taken. Water was then added to the tank at the rate of 0.75 ft per hour. The tank ring wall
was periodically surveyed and the water height was manually measured as the tank was filled. A plot of
averagesettlement versus water height is shown in Figure 12.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 20 40 60
Water Height (feet)
AverageSettlement(inches)
Figure 11. Average settlement versus water height
Maximum settlement for the 12 individual monitoring points ranged between 0.48 and 0.84 inches, with an
average settlement under 48 ft of water of 0.60 inches recorded. Since settlement monitoring was
performed over a relatively short time, the geotechnical engineer used empirical relationships to estimate
total settlement over the life of the structure (Table 2).
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Time Period Factor Settlement
(inches)
1 month 1.0 0.75
1 year 1.2 0.90
10 years 1.4 1.0530 years 1.5 1.13
Table 2. Estimated long term settlements
Settlements recorded during testing were approximately 50 percent of the originally estimated initial
settlements and well below the accepted value for storage tanks of this size, attesting to the effectiveness of
the stone column/composite mat foundation system.
CASE STUDY: INDUSTRIAL WAREHOUSES, CRAYFORD, KENT, U.K.
The proposed site for two large, single-story, industrial warehouse units presented the project engineerswith three distinct issues:
1. The site was once a clay pit, filled in the 1950s with up to 16.5 ft of generally granular material
with an average Standard Penetration Test (SPT) N value of 2. The fill was separated from an
underlying chalk aquifer by a layer of gravel. Historically, fills in this area have been found to be
contaminated with heavy metals and organics, and site-specific environmental investigations
revealed asbestos and methane contamination. This raised concerns that the foundation work
could significantly increase the downward migration of contaminants into the aquifer, which
supplies drinking water to the combined Greater London grid (Ground Engineering, 2000).
2. The site was bounded on one side by an active railroad. Vibrations generated during foundation
installation had to be limited to a maximum peak particle velocity of 0.4 inches per second within
60 ft of the tracks low embankment, precluding the use of driven piles.
3. The floor slabs were designed to be separate from the main building frames and walls, which were
to be supported on VCCs into chalk, with pile caps and grade beams. The allowable ground
bearing slab load was 900 psf for Unit 1 and 800 psf for Unit 2. A maximum settlement of 1 inch
at these design loads was specified.
Slab Foundation Design
To overcome the potential for aquifer contamination and ensure construction vibrations were kept within
allowable limits, the consulting engineer specified the use of VCCs to bypass the weak fill layer and
dens ify the underlying gravel layer.
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VCCs were originally specified because they combine the load transfer capabilities of piles with the soil
densification and reinforcement capabilities of stone columns. In addition, VCCs produce no significant
volume of spoils and generate minimal vibrations.
The successful bid conformed technically to bid specifications and project constraints, but proposed the use
of concrete base-plugs in conjunction with conventional stone columns in lieu of the VCCs. Although
stone column treatment alone would have been effective from a load-bearing viewpoint, and would not
have increased the downward migration of contaminants, inclusion of the concrete base-plug provided the
additional level of comfort needed to satisfy the U.K.s Environmental Agency. The Agency was
concerned that stone columns penetrating the fill and gravel layers might hydraulically interconnect the
contaminated fill with the underlying chalk drinking water aquifer.
This innovative hybrid technology, proven in a previous application on a geologically similar industrialsite, saved 30 percent in slab foundation costs for this project by stiffening the underlying ground.
Construction
Granular blankets were placed over the two treatment areas (22,000 sf and 10,000 sf for the two separate
warehouse areas) to provide a stable working platform for the contractors standard, track-mounted
vibro/VCC rig. The vibro work was performed on a nominal 8 foot grid pattern.
At each column location, the probe was advanced through the fill to nominally penetrate the underlyingnatural sands/gravels (the end-bearing stratum for the hybrid columns). The vibrator was then withdrawn
approximately 2 to 3 ft while discharging low slump concrete. The concrete was then re-penetrated to
ensure adequate compaction of the bearing stratum and bulb enlargement. The probe was retracted as
concrete was continually pumped until a 5-ft thick (on average)concrete plug was constructed at the base
of the hole.
Following construction of the concrete plug, stone was introduced from the surface in discrete lifts and
compacted by the probe, forming a dense, 24-in diameter column, closely interlocked with the surrounding
soil.
The vibro treatment resulted in the on-grid installation of 337 columns for Unit 1 and 157 columns for Unit
2. Depending on the thickness of the fill, total column lengths ranged between 5 and 18 ft, with an average
length of 10 ft. In both treatment areas, additional columns were installed off-grid where localized soft
spots were encountered. At the completion of the work, the gravel blanket was rolled and covered with a
concrete mud mat before the ground-bearing slab was cast.
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Monitoring and Testing
During column construction, on-board instrumentation monitored:
Packing/ penetration pressure
Concrete pressure
Verticality
Depth
At the completion of the vibro program ten, 24-inch diameterplate load tests were conducted to three times
the working load to test the workmanship of the construction process. One, 40-inch diameter plate load test
was conducted to 1.5 times the working load to verify the performance of the stone columns/soil against
calculated settlements. The structures have performed within specified criteria for two years
CONCLUSIONS
For many sites at which industrial fills have been placed, vibro stone columns can successfully reduced the
total settlement, and increase the bearing capacity of the in -situ fill materials and soils , while reducing
construction costs and compressing the construction schedule. Vibro stone columns can significantly
reduce the affect of variability within industrial fills and act to homogenize the fill. Many designers view
stone columns as an insurance policy protecting the engineer and owner from weak layers that may not
have been detected or accounted for. With industrial fills, an added benefit of VCCs and stone columns
(installed with the dry method) is the elimination of significant amounts of potentially contaminated spoilsas compared to other foundation systems.
ACKNOWLEDGEMENTS
The authors would like to thank the following for their assistance/cooperation in preparing this paper:
Nicholson Construction Company (ground improvement contractor for the Springdale, PA project), Pulley
Engineering (geotechnical engineer for the Springdale, PA project), and Pennine Vibropiling (ground
improvement contractor for the Kent, U.K. project).
REFRENCES
Baumann, V., Bauer, G.E.A. (1974), The Performance of Foundations on Various Soils Stabilized by the
Vibro-compaction Method, Canadian Geotechnical Journal, Vol. 11.
Boley, D.L., Pagano, M.A., Swenson, E.J. (1994), Ground Improvement by Vibro- Flotation Techniques,
Geotechnical News, December.
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Elias, V., Welsh, J., Warren, J., Lukas, R. (2001), Ground Improvement Technical Summaries, Department
of Transportation - Federal Highway Administration Office of Infrastructure, Publication No. FHWA-SA-
98-0864R, Volumes 1 and 2.
Greenwood, D. A., Kirsch, K. (1984), Specialist Ground Treatment by Vibratory and Dynamic Methods,
Piling and Ground Treatment, Thomas Telford Ltd, London.
Hughes, J.M.O., Withers, N.J. Reinforcing of Soft Cohesive Soils with Stone Columns, Ground
Engineering, pp.40-49.
Priebe, H.J. (1995), The Design of Vibro Replacement, Ground Engineering, December, pp. 31-37.
Specifying Vibro Stone Columns (2000), Construction Research Communications Ltd., London, England.
Straight Flush for Vibro Poker (2000), Ground Engineering , July, pg. 27.
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