Method DescriptionsVibro Compaction

The method of soil improvement whereby granular soils are compacted using depth vibrators is known as

“Vibroflotation”.

Natural deposited soils as well as artificially reclaimed sands can be compacted to great depths.

The current depth record lies at 70 meters for reclaimed sands and at 53 meters for naturally deposited sands.

The intensity of compaction can be varied in order to achieve the desired effect depending upon the foundation

or ground improvement tpurpose.

Vibro Displacement (Stone Columns and Concrete Columns)

The utilization of Stone Columns can be split into two distinct areas; as foundation elements (‘stone piles’) or as

ground reinforcement.

As a foundation element (‘stone piles’), stone columns can be used for a wide range of building types from multi

story buildings to oil tank foundations. They can function under multiple foundation types including strip-, pad-,

slab- and single footings.

As a ground reinforcement technique, Stone Columns can perform liquefaction prevention, embankment

stabilisation, slope stabilisation, as well as other ground improvement applications via reinforcement and

drainage.

Here shall come introductory text about what we try to explain. Also a hint to go to articles (link) if someone wants

'method statements' which are not to be found here but under articles.

Vibro CompactionVibro compaction is a ground improvement technique that densifies clean, cohesionless

granular soils by means of a downhole vibrator.

The vibrator is typically suspended from a crane and lowered vertically into the soil under its

own weight. Penetration is usually aided by water jets integrated into the vibrator assembly.

After reaching the bottom of the treatment zone, the soils are densified in lifts as the probe is

extracted. During vibro compaction, clean sand backfill is typically added at the ground

surface to compensate for the reduction in soil volume resulting from the densification

process. The vibratory energy reduces the inter-granular forces between the soil particles,

allowing them to move into a denser configuration, typically achieving a relative density of 70

to 85 percent. The treated soils have increased density, friction angle and stiffness.

Compaction is achieved above and below the water table.

The improved soil characteristics depend on the soil type and gradation, spacing of the

penetration points and the time spent performing the compaction. Generally, the vibro

compaction penetration spacing is between 6 feet and 14 feet, with centers arranged on a

triangular or square pattern. Compaction takes place without setting up internal stresses in

the soil, thus ensuring permanent densification.

The use of clean sand backfill during vibro compaction allows the original site elevation to be

maintained. However, on sites where the planned final grade is below the existing grade,

lowering of the site elevation may be desirable. In these instances, the ground surface is

allowed to subside during the compaction effort.

Vibro compaction permits the use of economical spread footings with design bearing

pressures generally of 5 ksf up to 10 ksf. Settlement and seismic liquefaction potentials are

reduced. The required treatment depth is typically in the range of 15 to 50 feet, but vibro

compaction has been performed to depths as great as 120 feet. Examples of previously

performed applications include increasing bearing capacity, decreasing settlement and

mitigating liquefaction for planned structures, embankments, railways and roadways.

Vibro compaction rigs can be fully instrumented with an on-board computer to monitor

parameters during vibro compaction. Monitoring these parameters allows the operator to

correct any deviations in real-time during the construction process to keep the vibro

compaction within project specifications. Data from the Data Acquisition (DAQ) system such

as amperage and lift rate are recorded and displayed in real-time alongside specified target

values on an in-cab monitor.

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Vibro Compaction Vibro compaction is an established technique for stabilising granular soils such as loose sands, gravels and some hydraulic fills using vibroflots.

Download the full Vibro Compaction datasheet here.

Benefits

vibro compaction techniques are extremely well established and in suitable soil types can be used for construction of tank farms, ports and marine structures

vibro compaction is extremely effective for sand compaction, and to date BBGE has utilised the technique in the Middle East, North America and the Mediterranean

vibro compaction can provide fast, in-situ densification of loose sands to depths of up to 30 metres and is one of the most economical and sustainable ground improvement methods available

vibro compaction significantly reduces the threat of liquefaction in the event of seismic shocks

How vibro compaction works

The offset position of the eccentric weight housed in the vibroflot creates a horizontal vibratory action, which acts to compact loose granular soils (i.e. sand and gravels) into a denser condition, thus providing a significant improvement in the geotechnical properties of the treated ground.

On reaching the required design depth water jetting from the nose cone is reduced, and the vibroflot is slowly extracted with pauses at regular intervals to ensure satisfactory levels of compaction are achieved at each depth. Additional side water jets are often utilised to assist with compaction and to encourage further erosion of the soils around the bore.

The vibroflot is gradually withdrawn back to the surface where a zone of compacted ground is formed around the insertion point. Additional site won sand may also be added at the top of the hole to fill the cone of depression that is formed. The rate of extraction is varied to suit the conditions encountered on site and to ensure that the correct amount of densification is achieved for each project.

Construction of an "hidden" dam (Photo)To stabilize the hinterland against landslides.

PenetrationThe vibroprobe penetrates to the required depth by vibration and jetting action of water and/or air

CompactionThe vibroprobe is retracted in 0.5 m intervals. The in situ sand or gravel is flowing towards the vibroprobe.

CompletionAfter compaction the working platform needs be levelled and eventually roller compacted.

Vibro Compaction

Effects and TestCompaction of granular soils by depth vibrators is known as Vibro Compaction. The method is also known as “Vibroflotation”. Natural deposits as well as artificially reclaimed sands can be compacted to a depth of up to 70 m. The intensity of compaction can be varied to meet bearing capacity criteria. Other improvement effects such as reduction of both total and differential

settlements are achieved. The risk of liquefaction in a earthquake prone area is also drastically reduced.

The following diagrams illustrate the compaction process :

The principle of sand compaction (Vibroflotation):The compaction process consists of a flotation of the soil particles as a result of vibration, wich then allows for a rearrangement of the particles into a denser state.

Effects of Compaction

• The sand and gravel particles rearrange into a denser state.• The ratio of horizontal to vertical effective stress is increased significantly.• The permeability of the soil is reduced 2 to 10 fold, depending on many factors.• The friction angle typically increases by up to 8 degrees.• Enforced settlements of the compacted soil mass are in the range of 2 % to 15 %,

typically 5 %• The stiffness modulus can be increased 2 to 4 fold.

Test Pattern

On large projects the optimal compaction grid spacing has to be determined by test grids.

The compaction effect in the test grids should be as close as possible to the treatment in the later production areas.

In order to achieve this it is advisable to arrange the test grids close to each other.

The distance between grid A (3.10 m) and grid B (3.40 m) should be

Effects and Test Offshore and Land Based

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Vibro Compaction

Application of methodReduce foundation settlementsPrevent soil liquefaction during earthquakesIncrease in-situ density of land reclamation fillsIncrease shear strength to improve slope stabilityReduce water permeability to facilitate dewatering

Suitability of method

Design steps

1.In cooperation with the architect, structural engineer, and the owner, define design objectives, such as admissible settlement, max. differential settlement, design earthquake (Mw and amax ), shear strength.

2.Perform a site investigation. Select the field sounding and lab testing program so that it later can answer all design and QA/QC questions.

3.Calculate with unimproved soil parameters (Es , phi’) to see what settlement (or stability against earthquake liquefaction, or slope stability) would result without treatment.

4.From 3. above derive a sensible definition of what soil parameters need improvement, how much, and in which zones of the project such improvement is needed. Keep in mind that small differences in specified improvement level can mean huge differences in cost.

5.For proper QC conditions in the Technical Specifications, translate the target soil mechanical parameters (Es , phi’) into required sounding resistance values (NSPT, qc) that the contractor can work with.

6.Set up the QA/QC plan to supervise the ongoing compaction works.7.On large projects: Plan a test installation to allow the contractor to calibrate

is work method and grid spacing for the local soil conditions.

For a ground improvement project setting up the proper site investigation is of paramount importance. Since the end product is improved ground, one needs to know the BEFORE properties of this ground very well to establish the degree of improvement. -> Get expert advice already in this early phase.

For a “first guess” of the compaction grid spacing the following graph may be useful. However, the variations from this graph based on local soil conditions and compaction equipment differences are so large that a use of this graph for the purpose of designing and specifying a project is strongly discouraged.

Installation Process

PenetrateCompact baseCompactto full depthin steps to surface

Vibro Compaction Equipment

Sketch courtesy Betterground Ltd.

Quality control of installation1.Verify compaction point locations2.Verify treatment depth3.Control digital logs of Ampere over time and depth over time.

To the expert, ampere and depth over time logs reveal most of the quality control issues involved with Vibro Compaction.

Acceptance Testing1.Control settlements of compacted site area.2.Perform soil sounding (SPT, CPT, PMT, DMT)3.Load testing (Plate load, zone load)

Acceptance testing point 1. can be combined with soil sounding under point 2. to gain further insight in compactability of specific soil types. This is in particular relevant for large land reclamations.

Vibrocompaction methods

The method of soil improvement whereby granular soils are compacted using depth

vibrators is known as “Vibroflotation”. Natural deposited soils as well as artificially reclaimed

sands can be compacted to great depths. The current depth record lies at 70 meters for

reclaimed sands and at 53 meters for naturally deposited sands. The intensity of

compaction can be varied in order to achieve the desired effect depending upon the

foundation or ground improvement purpose.

Improvement effects are:

Increased bearing capacity

Settlement reduction under loads

Near-elimination of differential settlements for large foundations

Liquefaction mitigation

Prevention of lateral spreading

Prevention of settlements, due to rearrangement of Particles from impacts

Prevention of inundation settlements

K-value (permeability of soils) reduction

Compaction Process:

The compaction process, consists of a flotation of the

soil particles as a result of vibration, which then allows for a rearrangement of the particles

in a denser state

Effects of Compaction

• The sand and gravel particles rearrange into a denser state.

• TA significant increase in the horizontal to vertical effective stress ratio,

• The permeability of the soil is significantly reduced

• Increased friction angle.

• Settlements of the compacted soil mass (between 2% and 15%)

• Increased stiffness modulus.

The world’s larges Vibro Compaction project before Dubai’s Palm islands: 40 million m3 marine sand,

with compaction depths up to 40m. Hong Kong Penny’s bay Project (Disney Hong Kong) 2001-2003

Compaction trials:

For the selection of different patterns, holding times and methods, a field test is performed.

The methodical application of the technology in an optimal manner is an art.

The challenge of optimization lies in the multiple parameters that can be varied and the

narrow band in which those parameters need to be adjusted to deliver the desired results.

Some of the parameters that can be varied include:

• type of vibrator used,

• distance between compaction points

• hold time per depth interval

• water pressure,

• location and type of water jets

The Basic Compaction Procedure

Penetration:

By vibration and the flushing of water and/or air, the vibroflot penetrates to the desired

depth

Compaction:

The vibroflot is recovered a certain vertical distance after a verified holding time or buildup

of resistance from the compacted ground

Finish:

Immediate top layers may be leveled or impact compacted or roller compacted to ensure a

ready-to-build surface

Design and Specification

Foundation Design, utilising Ground Improvement

Natural and durable foundation solutions

As a first step, tour involvement starts with an understanding of our customer’s needs

in terms of structural requirements for their foundation design.

For the majority of building projects, ground improvement is designed to reduce

foundation settlements, while design for increased soil stability is usually not

required.

We routinely assist structural engineers in proposals for a structural foundation

design that is adapted to our ground improvement solution. Adapting the design in

this way allows often for substantial savings of overall foundation costs and for more

architectural freedom. However this adaptions should be made in an early stage of

planning to realize the full benefits of a foundation based on ground improvement as

compared to piles.

Our structural recommendations are accompanied by a thorough analysis of the soil

conditions in conjunction with a load analysis of the building. In many cases, fairly

standard foundation solutions (increasingly utilized worldwide) can be proposed.

After the initial study of all available soil and load information, in simple geological

conditions, with competent ground not deeper than 10 m, we often execute further

soil investigations using a very cost efficient and reliable sounding apparatus

(Dynamic Probe Heavy). The DPH is targeted on the determination of the depth of

competent ground as basis for the decision about the treatment depth of the Ground

Improvement foundation product.

The typical wide range of suitability of foundation products of Ground Improvement origin (here:

Stone Columns)

If the competent ground is deeper than 10 m or in poor quality soils the more

expensive soil analysis tools SPT and CPT have to be applied for good reasons.

Poor quality soils represent a significant risk not only for ground improvement

methods but also for conventional piling solutionsand need in any case a detailed soil

analysis.

As seen below (Fig.1), the foundation slab required for concrete piles is considerably

smaller than the equivalent slab required for a solution based upon ground

improvement methods. Despite this, the overall cost of a solution based on soil

improvement (i.e. stone columns) will be significantly lower providing the structural

design is compatible with the foundation design.

For this reason, our involvement preferably begins early in the design process.

During the initial stages of the foundation layout, the structural engineer should

evaluate the prevailing soil conditions for the suitability of stone columns (gravel

piles) and plan the foundation slab accordingly.

Our experienced support of the structural engineer but also of the project’s

geotechnical engineer in the evaluation of soil data and further soil investigation can

be essential. Since ground improvement is a rather newly developed foundation

solution we are prepared to give this support on all our projects

Finding solutions for challenging soil conditions

All ground improvement methods have their optimal range of application.We have a

deep understanding of these methods and can therefore select the right method or

the right mix of methods for any project.

The Dynamic Compaction process of dropping a weight to compact sandy soils is

most effective in the top 7 meters of a soil profile, while

Vibro Compaction is most effective for all granular soils deeper then 3 meters and

can be combinded successfully with surface compaction techniques.

The installation of Prefabricated Vertical Drains (PVDs) is one of the lowest cost

ground improvement methods available and has an extremely fast installation rate.

The sketch on the right demonstrates the installation of PVDs and illustrates how the

water drains from the clayey soil by flowing radially into the drains and then upwards

towards the surface.

The PVD System can be combined with Stone Columns where both, rapid water

drainage and instant slope stability increase are required.

In some cases ground improvement is not the main foundation system but acts in

support of another systems. Below, the main foundation system is a diaphragm wall.

Compaction of the sand fill in the so called active passive earth pressure zones of the

wall (hatched areas) increases the friction angle of the soil and thereby reduces the

force with which the soil on the active side (left) pushes onto the wall while it

increases the force with which the soil on the passive side (right) supports the wall.

This twofold effect of compaction allows for a diaphragm wall of smaller thickness

and in some cases this can even save the need for ground anchors to hold back the

wall.

Another, often overlooked, effect is that the wall can be installed with much less over-

profile, since the compacted sand can be excavated by the diaphragm wall

equipment more cleanly. Leaving a much smoother wall that almost needs no

chiseling or other finishing work to remove over-profile.

The following photos show the installation of stone columns in 1997 in Berlin for that

purpose.

Another application of ground improvement as an auxiliary technique is the reduction

of the water permeability of granular soils. The following photo from 1994 shows the

vibro compaction of sand inside an excavation to reduce the volume of water that

needs to be pumped as the excavation progresses downward.

Behind the person to the left of the vibroprobe, the dewatering wells are already

installed.

The pumping volume can be reduced up to tenfold if loose sands are compared to a

dense state.

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