stone columns for industrial fills

7/31/2019 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.

stone columns for industrial fills

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